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Brook sliding bias operation.
Brook sliding bias operation.
The Brook amp sliding bias class A amp is shown with the 1947
schematic at
* w w w .tubecad . com /2007/12/blog0128.htm
This has the pilot resistor R5 as a common cathode current sensing
resistance.
A two triode inverting dc amp amplifies the Ek and Vk at R5 and
applies a negative going grid bias voltage as Ek rises. C27 filters out
the ac content.
So as the Ek signal rises when the amp moves into class AB, the tubes
are biased
more towards class B, but without a serious rise in Ek as occurs in
cathode biased PP amps when a low load provokes a serious rise in Ek
across each Rk
which is bypassed.
The speed of rise in Eg is controlled by C27 and its total R feed.
A slow time constant is used.
In an SE amp, bias across Rk wil rise when the load is less than the
ideal for the amp
One might set up an SE amp so symetrical clipping occurs with an 8 ohm
load on the
output at maximum PO, let us say 25 watts, and 2% THD.
But with 4 ohms the waves clip off at the bottom with cut off distortion
and the PO max is reduced to say 16W at say 2% THD.
The Broskie SE tube amp with sliding bias has its schematic lower down
the above page.
It tries to keep Ek constant by dynamic shunting of ac peaks in current.
In effect, bias current is increased
The schematic has transistor used to turn on when +ve signals through a
4.7uF become large enough.
An IN4148 diode prevents Eb swinging lower than 0.7V
When cut off occurs due to a low value RL in an SE amp, positive going
current signals become larger than negative,
and the transistor begins shunting cathode current through itself and
the 240 ohms.
Effectively, bias current in the tube is raised, and the tube is pushed
into a
different operating point where Ea is lowered somewhat, and Ia raised,
and that will suit the lower value load, and a larger amount of clean PO
is
available.
If you could increase Ia to twice its value, and only lower Ea slightly,
the power into the output stage would nearly double.
Efficiency would however be lower than with the ideal load you designed
for and the
maximum PO with a lower load might only manage to be 1.5 times the
amount for
ideal nominal load for max PO.
OPT winding losses might be 10% with an 8 ohm load, but with 4 ohms the
winding losses leap to 20%,
and the extra PO is also reduced.
So probably its a simpler idea to use two OP tubes and fixed or cathode
bias for an SE amp to allow it to
produce adequate PO into a lower than ideal load for one tube.
Broskie only simulated the set up but his claims for increased PO
into low loads are exagerated imho.
I invented a similar proposal about 5 years ago or more for my 300W
amps.
The method is fully and totally described and detailed with proven
working schematics anyone is free to try at
* w w w .turneraudio . com .au/schem-300w-5-bias-stabilizer.html
In my scheme of biasing, the amp is idled at low bias, say 30mA with Ea
at say +470V,
with bias generated with cathode bias R&C networks in the traditional
manner.
Tube idle diss would then be 470 x 0.03 = 13.1 watts, ok for any octal
output tube
except 6V6, 6F6 etc.
As the working swings past the class A to AB threshold, the transistors
I use are
turned on by increasing positive going AC signals in current sensing R
and the
excessive signal current that would normally charge up the Ck is
bypassed
through the transistor. So Ek stays virtually constant.
The magic of my circuit allows the multiple tubes in a big amp like my
300 Watters
to all be biased with cathode bias which is much less trouble than
having multiple fixed bias adjust pots, and much much much less trouble
than having
only one value of grid voltage applied, which is very unreliable.
The amp then works to keep its Ek constant regardless of load and the
amp
has the same distortion as a fixed bias amplifier.
I have tested the amp and found this to be the case.
For class AB working with 2 tubes, fixed bias is the best way of
biasing,
and all you need is a balance indicator and a pot to get balance with
LED indication as
the schematic at my pages for the 5050 indicate.
* w w w .turneraudio . com .au/5050Integrated.html
Please forgive the hand drawn schematic for bias balance indication and
other
protection measures in the schematic dated April 2000 at the bottom of
the page.
Such simple bias management is a problem when you have multiple tubes.
I refused to saddle my customers with 24 bias adjust pots in stereo
system.
So I invented the dynamic bias stabilization method.
It works well.
In class A, the transistors do not function at all, and the signal
isn't changed from being ultra pure class A.
When the ac current shunting is going on,
the signal distortions are less than those caused by having the small
value
current sensing resistances present, and very much lower than if I were
to allow the Ek to rise when
high level signals are sustained without my measures against it.
Patrick Turner.
The Brook amp sliding bias class A amp is shown with the 1947
schematic at
* w w w .tubecad . com /2007/12/blog0128.htm
This has the pilot resistor R5 as a common cathode current sensing
resistance.
A two triode inverting dc amp amplifies the Ek and Vk at R5 and
applies a negative going grid bias voltage as Ek rises. C27 filters out
the ac content.
So as the Ek signal rises when the amp moves into class AB, the tubes
are biased
more towards class B, but without a serious rise in Ek as occurs in
cathode biased PP amps when a low load provokes a serious rise in Ek
across each Rk
which is bypassed.
The speed of rise in Eg is controlled by C27 and its total R feed.
A slow time constant is used.
In an SE amp, bias across Rk wil rise when the load is less than the
ideal for the amp
One might set up an SE amp so symetrical clipping occurs with an 8 ohm
load on the
output at maximum PO, let us say 25 watts, and 2% THD.
But with 4 ohms the waves clip off at the bottom with cut off distortion
and the PO max is reduced to say 16W at say 2% THD.
The Broskie SE tube amp with sliding bias has its schematic lower down
the above page.
It tries to keep Ek constant by dynamic shunting of ac peaks in current.
In effect, bias current is increased
The schematic has transistor used to turn on when +ve signals through a
4.7uF become large enough.
An IN4148 diode prevents Eb swinging lower than 0.7V
When cut off occurs due to a low value RL in an SE amp, positive going
current signals become larger than negative,
and the transistor begins shunting cathode current through itself and
the 240 ohms.
Effectively, bias current in the tube is raised, and the tube is pushed
into a
different operating point where Ea is lowered somewhat, and Ia raised,
and that will suit the lower value load, and a larger amount of clean PO
is
available.
If you could increase Ia to twice its value, and only lower Ea slightly,
the power into the output stage would nearly double.
Efficiency would however be lower than with the ideal load you designed
for and the
maximum PO with a lower load might only manage to be 1.5 times the
amount for
ideal nominal load for max PO.
OPT winding losses might be 10% with an 8 ohm load, but with 4 ohms the
winding losses leap to 20%,
and the extra PO is also reduced.
So probably its a simpler idea to use two OP tubes and fixed or cathode
bias for an SE amp to allow it to
produce adequate PO into a lower than ideal load for one tube.
Broskie only simulated the set up but his claims for increased PO
into low loads are exagerated imho.
I invented a similar proposal about 5 years ago or more for my 300W
amps.
The method is fully and totally described and detailed with proven
working schematics anyone is free to try at
* w w w .turneraudio . com .au/schem-300w-5-bias-stabilizer.html
In my scheme of biasing, the amp is idled at low bias, say 30mA with Ea
at say +470V,
with bias generated with cathode bias R&C networks in the traditional
manner.
Tube idle diss would then be 470 x 0.03 = 13.1 watts, ok for any octal
output tube
except 6V6, 6F6 etc.
As the working swings past the class A to AB threshold, the transistors
I use are
turned on by increasing positive going AC signals in current sensing R
and the
excessive signal current that would normally charge up the Ck is
bypassed
through the transistor. So Ek stays virtually constant.
The magic of my circuit allows the multiple tubes in a big amp like my
300 Watters
to all be biased with cathode bias which is much less trouble than
having multiple fixed bias adjust pots, and much much much less trouble
than having
only one value of grid voltage applied, which is very unreliable.
The amp then works to keep its Ek constant regardless of load and the
amp
has the same distortion as a fixed bias amplifier.
I have tested the amp and found this to be the case.
For class AB working with 2 tubes, fixed bias is the best way of
biasing,
and all you need is a balance indicator and a pot to get balance with
LED indication as
the schematic at my pages for the 5050 indicate.
* w w w .turneraudio . com .au/5050Integrated.html
Please forgive the hand drawn schematic for bias balance indication and
other
protection measures in the schematic dated April 2000 at the bottom of
the page.
Such simple bias management is a problem when you have multiple tubes.
I refused to saddle my customers with 24 bias adjust pots in stereo
system.
So I invented the dynamic bias stabilization method.
It works well.
In class A, the transistors do not function at all, and the signal
isn't changed from being ultra pure class A.
When the ac current shunting is going on,
the signal distortions are less than those caused by having the small
value
current sensing resistances present, and very much lower than if I were
to allow the Ek to rise when
high level signals are sustained without my measures against it.
Patrick Turner.
Re: Brook sliding bias operation.
Your circuit is equivalent to putting Zener diodes (about 40V) in parallel
with RkCk (750ohm||1000uF). Zeners would be simpler, but more difficult to
adjust threshold --- need to get variety of assorted 5W Zeners, connecting
them in series to the desired voltage.
In your circuit, if adjusting is required, it takes changing only two 10ohm
resistors (R9, R10).
Also, in your circuit the threshold goes down with the system warming up due
to beta rise and Vbe drop in the transistors. I am not sure whether this is
acceptable. To reduce the temperature effect, R11 and R12 shall be
eliminated. Thus the bias clipping will become "harder".
Zeners are more temperature stable than your circuit.
Regards,
Alex
"Patrick Turner" <info@turneraudio . com .au> wrote in message
news:47F0BD59.2D443A64@turneraudio . com .au...
> The Brook amp sliding bias class A amp is shown with the 1947
> schematic at
> * w w w .tubecad . com /2007/12/blog0128.htm
>
> This has the pilot resistor R5 as a common cathode current sensing
> resistance.
> A two triode inverting dc amp amplifies the Ek and Vk at R5 and
> applies a negative going grid bias voltage as Ek rises. C27 filters out
> the ac content.
> So as the Ek signal rises when the amp moves into class AB, the tubes
> are biased
> more towards class B, but without a serious rise in Ek as occurs in
> cathode biased PP amps when a low load provokes a serious rise in Ek
> across each Rk
> which is bypassed.
>
> The speed of rise in Eg is controlled by C27 and its total R feed.
> A slow time constant is used.
>
> In an SE amp, bias across Rk wil rise when the load is less than the
> ideal for the amp
> One might set up an SE amp so symetrical clipping occurs with an 8 ohm
> load on the
> output at maximum PO, let us say 25 watts, and 2% THD.
> But with 4 ohms the waves clip off at the bottom with cut off distortion
> and the PO max is reduced to say 16W at say 2% THD.
>
> The Broskie SE tube amp with sliding bias has its schematic lower down
> the above page.
> It tries to keep Ek constant by dynamic shunting of ac peaks in current.
> In effect, bias current is increased
>
> The schematic has transistor used to turn on when +ve signals through a
> 4.7uF become large enough.
> An IN4148 diode prevents Eb swinging lower than 0.7V
>
> When cut off occurs due to a low value RL in an SE amp, positive going
> current signals become larger than negative,
> and the transistor begins shunting cathode current through itself and
> the 240 ohms.
> Effectively, bias current in the tube is raised, and the tube is pushed
> into a
> different operating point where Ea is lowered somewhat, and Ia raised,
> and that will suit the lower value load, and a larger amount of clean PO
> is
> available.
>
> If you could increase Ia to twice its value, and only lower Ea slightly,
> the power into the output stage would nearly double.
> Efficiency would however be lower than with the ideal load you designed
> for and the
> maximum PO with a lower load might only manage to be 1.5 times the
> amount for
> ideal nominal load for max PO.
> OPT winding losses might be 10% with an 8 ohm load, but with 4 ohms the
> winding losses leap to 20%,
> and the extra PO is also reduced.
>
> So probably its a simpler idea to use two OP tubes and fixed or cathode
> bias for an SE amp to allow it to
> produce adequate PO into a lower than ideal load for one tube.
>
> Broskie only simulated the set up but his claims for increased PO
> into low loads are exagerated imho.
>
>
>
> I invented a similar proposal about 5 years ago or more for my 300W
> amps.
> The method is fully and totally described and detailed with proven
> working schematics anyone is free to try at
> * w w w .turneraudio . com .au/schem-300w-5-bias-stabilizer.html
>
> In my scheme of biasing, the amp is idled at low bias, say 30mA with Ea
> at say +470V,
> with bias generated with cathode bias R&C networks in the traditional
> manner.
> Tube idle diss would then be 470 x 0.03 = 13.1 watts, ok for any octal
> output tube
> except 6V6, 6F6 etc.
>
> As the working swings past the class A to AB threshold, the transistors
> I use are
> turned on by increasing positive going AC signals in current sensing R
> and the
> excessive signal current that would normally charge up the Ck is
> bypassed
> through the transistor. So Ek stays virtually constant.
>
> The magic of my circuit allows the multiple tubes in a big amp like my
> 300 Watters
> to all be biased with cathode bias which is much less trouble than
> having multiple fixed bias adjust pots, and much much much less trouble
> than having
> only one value of grid voltage applied, which is very unreliable.
>
> The amp then works to keep its Ek constant regardless of load and the
> amp
> has the same distortion as a fixed bias amplifier.
>
> I have tested the amp and found this to be the case.
>
> For class AB working with 2 tubes, fixed bias is the best way of
> biasing,
> and all you need is a balance indicator and a pot to get balance with
> LED indication as
> the schematic at my pages for the 5050 indicate.
> * w w w .turneraudio . com .au/5050Integrated.html
>
> Please forgive the hand drawn schematic for bias balance indication and
> other
> protection measures in the schematic dated April 2000 at the bottom of
> the page.
>
> Such simple bias management is a problem when you have multiple tubes.
>
> I refused to saddle my customers with 24 bias adjust pots in stereo
> system.
> So I invented the dynamic bias stabilization method.
>
> It works well.
>
> In class A, the transistors do not function at all, and the signal
> isn't changed from being ultra pure class A.
> When the ac current shunting is going on,
> the signal distortions are less than those caused by having the small
> value
> current sensing resistances present, and very much lower than if I were
> to allow the Ek to rise when
> high level signals are sustained without my measures against it.
>
> Patrick Turner.
Your circuit is equivalent to putting Zener diodes (about 40V) in parallel
with RkCk (750ohm||1000uF). Zeners would be simpler, but more difficult to
adjust threshold --- need to get variety of assorted 5W Zeners, connecting
them in series to the desired voltage.
In your circuit, if adjusting is required, it takes changing only two 10ohm
resistors (R9, R10).
Also, in your circuit the threshold goes down with the system warming up due
to beta rise and Vbe drop in the transistors. I am not sure whether this is
acceptable. To reduce the temperature effect, R11 and R12 shall be
eliminated. Thus the bias clipping will become "harder".
Zeners are more temperature stable than your circuit.
Regards,
Alex
"Patrick Turner" <info@turneraudio . com .au> wrote in message
news:47F0BD59.2D443A64@turneraudio . com .au...
> The Brook amp sliding bias class A amp is shown with the 1947
> schematic at
> * w w w .tubecad . com /2007/12/blog0128.htm
>
> This has the pilot resistor R5 as a common cathode current sensing
> resistance.
> A two triode inverting dc amp amplifies the Ek and Vk at R5 and
> applies a negative going grid bias voltage as Ek rises. C27 filters out
> the ac content.
> So as the Ek signal rises when the amp moves into class AB, the tubes
> are biased
> more towards class B, but without a serious rise in Ek as occurs in
> cathode biased PP amps when a low load provokes a serious rise in Ek
> across each Rk
> which is bypassed.
>
> The speed of rise in Eg is controlled by C27 and its total R feed.
> A slow time constant is used.
>
> In an SE amp, bias across Rk wil rise when the load is less than the
> ideal for the amp
> One might set up an SE amp so symetrical clipping occurs with an 8 ohm
> load on the
> output at maximum PO, let us say 25 watts, and 2% THD.
> But with 4 ohms the waves clip off at the bottom with cut off distortion
> and the PO max is reduced to say 16W at say 2% THD.
>
> The Broskie SE tube amp with sliding bias has its schematic lower down
> the above page.
> It tries to keep Ek constant by dynamic shunting of ac peaks in current.
> In effect, bias current is increased
>
> The schematic has transistor used to turn on when +ve signals through a
> 4.7uF become large enough.
> An IN4148 diode prevents Eb swinging lower than 0.7V
>
> When cut off occurs due to a low value RL in an SE amp, positive going
> current signals become larger than negative,
> and the transistor begins shunting cathode current through itself and
> the 240 ohms.
> Effectively, bias current in the tube is raised, and the tube is pushed
> into a
> different operating point where Ea is lowered somewhat, and Ia raised,
> and that will suit the lower value load, and a larger amount of clean PO
> is
> available.
>
> If you could increase Ia to twice its value, and only lower Ea slightly,
> the power into the output stage would nearly double.
> Efficiency would however be lower than with the ideal load you designed
> for and the
> maximum PO with a lower load might only manage to be 1.5 times the
> amount for
> ideal nominal load for max PO.
> OPT winding losses might be 10% with an 8 ohm load, but with 4 ohms the
> winding losses leap to 20%,
> and the extra PO is also reduced.
>
> So probably its a simpler idea to use two OP tubes and fixed or cathode
> bias for an SE amp to allow it to
> produce adequate PO into a lower than ideal load for one tube.
>
> Broskie only simulated the set up but his claims for increased PO
> into low loads are exagerated imho.
>
>
>
> I invented a similar proposal about 5 years ago or more for my 300W
> amps.
> The method is fully and totally described and detailed with proven
> working schematics anyone is free to try at
> * w w w .turneraudio . com .au/schem-300w-5-bias-stabilizer.html
>
> In my scheme of biasing, the amp is idled at low bias, say 30mA with Ea
> at say +470V,
> with bias generated with cathode bias R&C networks in the traditional
> manner.
> Tube idle diss would then be 470 x 0.03 = 13.1 watts, ok for any octal
> output tube
> except 6V6, 6F6 etc.
>
> As the working swings past the class A to AB threshold, the transistors
> I use are
> turned on by increasing positive going AC signals in current sensing R
> and the
> excessive signal current that would normally charge up the Ck is
> bypassed
> through the transistor. So Ek stays virtually constant.
>
> The magic of my circuit allows the multiple tubes in a big amp like my
> 300 Watters
> to all be biased with cathode bias which is much less trouble than
> having multiple fixed bias adjust pots, and much much much less trouble
> than having
> only one value of grid voltage applied, which is very unreliable.
>
> The amp then works to keep its Ek constant regardless of load and the
> amp
> has the same distortion as a fixed bias amplifier.
>
> I have tested the amp and found this to be the case.
>
> For class AB working with 2 tubes, fixed bias is the best way of
> biasing,
> and all you need is a balance indicator and a pot to get balance with
> LED indication as
> the schematic at my pages for the 5050 indicate.
> * w w w .turneraudio . com .au/5050Integrated.html
>
> Please forgive the hand drawn schematic for bias balance indication and
> other
> protection measures in the schematic dated April 2000 at the bottom of
> the page.
>
> Such simple bias management is a problem when you have multiple tubes.
>
> I refused to saddle my customers with 24 bias adjust pots in stereo
> system.
> So I invented the dynamic bias stabilization method.
>
> It works well.
>
> In class A, the transistors do not function at all, and the signal
> isn't changed from being ultra pure class A.
> When the ac current shunting is going on,
> the signal distortions are less than those caused by having the small
> value
> current sensing resistances present, and very much lower than if I were
> to allow the Ek to rise when
> high level signals are sustained without my measures against it.
>
> Patrick Turner.
Re: Brook sliding bias operation.
Alex wrote:
>
> Your circuit is equivalent to putting Zener diodes (about 40V) in parallel
> with RkCk (750ohm||1000uF). Zeners would be simpler, but more difficult to
> adjust threshold --- need to get variety of assorted 5W Zeners, connecting
> them in series to the desired voltage.
>
> In your circuit, if adjusting is required, it takes changing only two 10ohm
> resistors (R9, R10).
>
> Also, in your circuit the threshold goes down with the system warming up due
> to beta rise and Vbe drop in the transistors. I am not sure whether this is
> acceptable. To reduce the temperature effect, R11 and R12 shall be
> eliminated. Thus the bias clipping will become "harder".
>
> Zeners are more temperature stable than your circuit.
I first tried zeners with a revised ST70 schematic shown at my website.
The trouble with zeners is that they do not react to current. They
react to voltage.
So if a tube short circuits itself, Ek is held down but Ik goes high.
In the ST70 protection circuit I developed, series R between bottom of
zeners
and 0V sends an error signal off to an SCR, and amp meltdown is avoided.
The transistor dynamic bypass stabilizers use very non critical setting
for the
transistor bypass. It only reacts to ac current generated in the low
value sensing R.
So Ek can vary quite a bit before triggering the protect circuit
But if a hard sudden drum beat occurs, the high AB current is nicely
bypassed
and Ek barely moves at all, but with a zener, a considerable rise in Ek
has to occur
before it conducts at all.
The bypassing transistors actually do very little work
even with 3 ohms as the load and the amp run right up to clipping with a
sine wave.
They don't get very hot, and the non critical nature of my circuit
means that you don't have to worry about their thermal stability at all.
Try fitting my basic schematic into an old amp you might have,
and play around with values. You may see how well you can get the AC
current signal to regulate the DC!
I tried the scema of mine in a 300W amp with 12 x 6550 with ONE general
purpose TO3 power transistor each side of the PPL circuit
and it worked OK but I now favour having one TO220 devive per tube and
each tube having its own current sensor R.
Patrick Turner
>
> Regards,
> Alex
>
> "Patrick Turner" <info@turneraudio . com .au> wrote in message
> news:47F0BD59.2D443A64@turneraudio . com .au...
> > The Brook amp sliding bias class A amp is shown with the 1947
> > schematic at
> > * w w w .tubecad . com /2007/12/blog0128.htm
> >
> > This has the pilot resistor R5 as a common cathode current sensing
> > resistance.
> > A two triode inverting dc amp amplifies the Ek and Vk at R5 and
> > applies a negative going grid bias voltage as Ek rises. C27 filters out
> > the ac content.
> > So as the Ek signal rises when the amp moves into class AB, the tubes
> > are biased
> > more towards class B, but without a serious rise in Ek as occurs in
> > cathode biased PP amps when a low load provokes a serious rise in Ek
> > across each Rk
> > which is bypassed.
> >
> > The speed of rise in Eg is controlled by C27 and its total R feed.
> > A slow time constant is used.
> >
> > In an SE amp, bias across Rk wil rise when the load is less than the
> > ideal for the amp
> > One might set up an SE amp so symetrical clipping occurs with an 8 ohm
> > load on the
> > output at maximum PO, let us say 25 watts, and 2% THD.
> > But with 4 ohms the waves clip off at the bottom with cut off distortion
> > and the PO max is reduced to say 16W at say 2% THD.
> >
> > The Broskie SE tube amp with sliding bias has its schematic lower down
> > the above page.
> > It tries to keep Ek constant by dynamic shunting of ac peaks in current.
> > In effect, bias current is increased
> >
> > The schematic has transistor used to turn on when +ve signals through a
> > 4.7uF become large enough.
> > An IN4148 diode prevents Eb swinging lower than 0.7V
> >
> > When cut off occurs due to a low value RL in an SE amp, positive going
> > current signals become larger than negative,
> > and the transistor begins shunting cathode current through itself and
> > the 240 ohms.
> > Effectively, bias current in the tube is raised, and the tube is pushed
> > into a
> > different operating point where Ea is lowered somewhat, and Ia raised,
> > and that will suit the lower value load, and a larger amount of clean PO
> > is
> > available.
> >
> > If you could increase Ia to twice its value, and only lower Ea slightly,
> > the power into the output stage would nearly double.
> > Efficiency would however be lower than with the ideal load you designed
> > for and the
> > maximum PO with a lower load might only manage to be 1.5 times the
> > amount for
> > ideal nominal load for max PO.
> > OPT winding losses might be 10% with an 8 ohm load, but with 4 ohms the
> > winding losses leap to 20%,
> > and the extra PO is also reduced.
> >
> > So probably its a simpler idea to use two OP tubes and fixed or cathode
> > bias for an SE amp to allow it to
> > produce adequate PO into a lower than ideal load for one tube.
> >
> > Broskie only simulated the set up but his claims for increased PO
> > into low loads are exagerated imho.
> >
> >
> >
> > I invented a similar proposal about 5 years ago or more for my 300W
> > amps.
> > The method is fully and totally described and detailed with proven
> > working schematics anyone is free to try at
> > * w w w .turneraudio . com .au/schem-300w-5-bias-stabilizer.html
> >
> > In my scheme of biasing, the amp is idled at low bias, say 30mA with Ea
> > at say +470V,
> > with bias generated with cathode bias R&C networks in the traditional
> > manner.
> > Tube idle diss would then be 470 x 0.03 = 13.1 watts, ok for any octal
> > output tube
> > except 6V6, 6F6 etc.
> >
> > As the working swings past the class A to AB threshold, the transistors
> > I use are
> > turned on by increasing positive going AC signals in current sensing R
> > and the
> > excessive signal current that would normally charge up the Ck is
> > bypassed
> > through the transistor. So Ek stays virtually constant.
> >
> > The magic of my circuit allows the multiple tubes in a big amp like my
> > 300 Watters
> > to all be biased with cathode bias which is much less trouble than
> > having multiple fixed bias adjust pots, and much much much less trouble
> > than having
> > only one value of grid voltage applied, which is very unreliable.
> >
> > The amp then works to keep its Ek constant regardless of load and the
> > amp
> > has the same distortion as a fixed bias amplifier.
> >
> > I have tested the amp and found this to be the case.
> >
> > For class AB working with 2 tubes, fixed bias is the best way of
> > biasing,
> > and all you need is a balance indicator and a pot to get balance with
> > LED indication as
> > the schematic at my pages for the 5050 indicate.
> > * w w w .turneraudio . com .au/5050Integrated.html
> >
> > Please forgive the hand drawn schematic for bias balance indication and
> > other
> > protection measures in the schematic dated April 2000 at the bottom of
> > the page.
> >
> > Such simple bias management is a problem when you have multiple tubes.
> >
> > I refused to saddle my customers with 24 bias adjust pots in stereo
> > system.
> > So I invented the dynamic bias stabilization method.
> >
> > It works well.
> >
> > In class A, the transistors do not function at all, and the signal
> > isn't changed from being ultra pure class A.
> > When the ac current shunting is going on,
> > the signal distortions are less than those caused by having the small
> > value
> > current sensing resistances present, and very much lower than if I were
> > to allow the Ek to rise when
> > high level signals are sustained without my measures against it.
> >
> > Patrick Turner.
Alex wrote:
>
> Your circuit is equivalent to putting Zener diodes (about 40V) in parallel
> with RkCk (750ohm||1000uF). Zeners would be simpler, but more difficult to
> adjust threshold --- need to get variety of assorted 5W Zeners, connecting
> them in series to the desired voltage.
>
> In your circuit, if adjusting is required, it takes changing only two 10ohm
> resistors (R9, R10).
>
> Also, in your circuit the threshold goes down with the system warming up due
> to beta rise and Vbe drop in the transistors. I am not sure whether this is
> acceptable. To reduce the temperature effect, R11 and R12 shall be
> eliminated. Thus the bias clipping will become "harder".
>
> Zeners are more temperature stable than your circuit.
I first tried zeners with a revised ST70 schematic shown at my website.
The trouble with zeners is that they do not react to current. They
react to voltage.
So if a tube short circuits itself, Ek is held down but Ik goes high.
In the ST70 protection circuit I developed, series R between bottom of
zeners
and 0V sends an error signal off to an SCR, and amp meltdown is avoided.
The transistor dynamic bypass stabilizers use very non critical setting
for the
transistor bypass. It only reacts to ac current generated in the low
value sensing R.
So Ek can vary quite a bit before triggering the protect circuit
But if a hard sudden drum beat occurs, the high AB current is nicely
bypassed
and Ek barely moves at all, but with a zener, a considerable rise in Ek
has to occur
before it conducts at all.
The bypassing transistors actually do very little work
even with 3 ohms as the load and the amp run right up to clipping with a
sine wave.
They don't get very hot, and the non critical nature of my circuit
means that you don't have to worry about their thermal stability at all.
Try fitting my basic schematic into an old amp you might have,
and play around with values. You may see how well you can get the AC
current signal to regulate the DC!
I tried the scema of mine in a 300W amp with 12 x 6550 with ONE general
purpose TO3 power transistor each side of the PPL circuit
and it worked OK but I now favour having one TO220 devive per tube and
each tube having its own current sensor R.
Patrick Turner
>
> Regards,
> Alex
>
> "Patrick Turner" <info@turneraudio . com .au> wrote in message
> news:47F0BD59.2D443A64@turneraudio . com .au...
> > The Brook amp sliding bias class A amp is shown with the 1947
> > schematic at
> > * w w w .tubecad . com /2007/12/blog0128.htm
> >
> > This has the pilot resistor R5 as a common cathode current sensing
> > resistance.
> > A two triode inverting dc amp amplifies the Ek and Vk at R5 and
> > applies a negative going grid bias voltage as Ek rises. C27 filters out
> > the ac content.
> > So as the Ek signal rises when the amp moves into class AB, the tubes
> > are biased
> > more towards class B, but without a serious rise in Ek as occurs in
> > cathode biased PP amps when a low load provokes a serious rise in Ek
> > across each Rk
> > which is bypassed.
> >
> > The speed of rise in Eg is controlled by C27 and its total R feed.
> > A slow time constant is used.
> >
> > In an SE amp, bias across Rk wil rise when the load is less than the
> > ideal for the amp
> > One might set up an SE amp so symetrical clipping occurs with an 8 ohm
> > load on the
> > output at maximum PO, let us say 25 watts, and 2% THD.
> > But with 4 ohms the waves clip off at the bottom with cut off distortion
> > and the PO max is reduced to say 16W at say 2% THD.
> >
> > The Broskie SE tube amp with sliding bias has its schematic lower down
> > the above page.
> > It tries to keep Ek constant by dynamic shunting of ac peaks in current.
> > In effect, bias current is increased
> >
> > The schematic has transistor used to turn on when +ve signals through a
> > 4.7uF become large enough.
> > An IN4148 diode prevents Eb swinging lower than 0.7V
> >
> > When cut off occurs due to a low value RL in an SE amp, positive going
> > current signals become larger than negative,
> > and the transistor begins shunting cathode current through itself and
> > the 240 ohms.
> > Effectively, bias current in the tube is raised, and the tube is pushed
> > into a
> > different operating point where Ea is lowered somewhat, and Ia raised,
> > and that will suit the lower value load, and a larger amount of clean PO
> > is
> > available.
> >
> > If you could increase Ia to twice its value, and only lower Ea slightly,
> > the power into the output stage would nearly double.
> > Efficiency would however be lower than with the ideal load you designed
> > for and the
> > maximum PO with a lower load might only manage to be 1.5 times the
> > amount for
> > ideal nominal load for max PO.
> > OPT winding losses might be 10% with an 8 ohm load, but with 4 ohms the
> > winding losses leap to 20%,
> > and the extra PO is also reduced.
> >
> > So probably its a simpler idea to use two OP tubes and fixed or cathode
> > bias for an SE amp to allow it to
> > produce adequate PO into a lower than ideal load for one tube.
> >
> > Broskie only simulated the set up but his claims for increased PO
> > into low loads are exagerated imho.
> >
> >
> >
> > I invented a similar proposal about 5 years ago or more for my 300W
> > amps.
> > The method is fully and totally described and detailed with proven
> > working schematics anyone is free to try at
> > * w w w .turneraudio . com .au/schem-300w-5-bias-stabilizer.html
> >
> > In my scheme of biasing, the amp is idled at low bias, say 30mA with Ea
> > at say +470V,
> > with bias generated with cathode bias R&C networks in the traditional
> > manner.
> > Tube idle diss would then be 470 x 0.03 = 13.1 watts, ok for any octal
> > output tube
> > except 6V6, 6F6 etc.
> >
> > As the working swings past the class A to AB threshold, the transistors
> > I use are
> > turned on by increasing positive going AC signals in current sensing R
> > and the
> > excessive signal current that would normally charge up the Ck is
> > bypassed
> > through the transistor. So Ek stays virtually constant.
> >
> > The magic of my circuit allows the multiple tubes in a big amp like my
> > 300 Watters
> > to all be biased with cathode bias which is much less trouble than
> > having multiple fixed bias adjust pots, and much much much less trouble
> > than having
> > only one value of grid voltage applied, which is very unreliable.
> >
> > The amp then works to keep its Ek constant regardless of load and the
> > amp
> > has the same distortion as a fixed bias amplifier.
> >
> > I have tested the amp and found this to be the case.
> >
> > For class AB working with 2 tubes, fixed bias is the best way of
> > biasing,
> > and all you need is a balance indicator and a pot to get balance with
> > LED indication as
> > the schematic at my pages for the 5050 indicate.
> > * w w w .turneraudio . com .au/5050Integrated.html
> >
> > Please forgive the hand drawn schematic for bias balance indication and
> > other
> > protection measures in the schematic dated April 2000 at the bottom of
> > the page.
> >
> > Such simple bias management is a problem when you have multiple tubes.
> >
> > I refused to saddle my customers with 24 bias adjust pots in stereo
> > system.
> > So I invented the dynamic bias stabilization method.
> >
> > It works well.
> >
> > In class A, the transistors do not function at all, and the signal
> > isn't changed from being ultra pure class A.
> > When the ac current shunting is going on,
> > the signal distortions are less than those caused by having the small
> > value
> > current sensing resistances present, and very much lower than if I were
> > to allow the Ek to rise when
> > high level signals are sustained without my measures against it.
> >
> > Patrick Turner.
Re: Brook sliding bias operation.
> I first tried zeners with a revised ST70 schematic shown at my website.
>
> The trouble with zeners is that they do not react to current. They
> react to voltage.
> So if a tube short circuits itself, Ek is held down but Ik goes high.
> In the ST70 protection circuit I developed, series R between bottom of
> zeners
> and 0V sends an error signal off to an SCR, and amp meltdown is avoided.
>
> The transistor dynamic bypass stabilizers use very non critical setting
> for the
> transistor bypass. It only reacts to ac current generated in the low
> value sensing R.
> So Ek can vary quite a bit before triggering the protect circuit
>
> But if a hard sudden drum beat occurs, the high AB current is nicely
> bypassed
> and Ek barely moves at all, but with a zener, a considerable rise in Ek
> has to occur
> before it conducts at all.
Yes, that makes sense. The transistor circuit reacts to the
instantaneous current. The threshold can be higher than a quescient
current, while with the zeners it shall be right on the mark. With the
zeners you do not need 1000uF||750R resistors at all -- it will work
as fixed bias, which is, if I understand correctly, critical to tubes
replacement and B+ variation.
Regards,
Alex
> I first tried zeners with a revised ST70 schematic shown at my website.
>
> The trouble with zeners is that they do not react to current. They
> react to voltage.
> So if a tube short circuits itself, Ek is held down but Ik goes high.
> In the ST70 protection circuit I developed, series R between bottom of
> zeners
> and 0V sends an error signal off to an SCR, and amp meltdown is avoided.
>
> The transistor dynamic bypass stabilizers use very non critical setting
> for the
> transistor bypass. It only reacts to ac current generated in the low
> value sensing R.
> So Ek can vary quite a bit before triggering the protect circuit
>
> But if a hard sudden drum beat occurs, the high AB current is nicely
> bypassed
> and Ek barely moves at all, but with a zener, a considerable rise in Ek
> has to occur
> before it conducts at all.
Yes, that makes sense. The transistor circuit reacts to the
instantaneous current. The threshold can be higher than a quescient
current, while with the zeners it shall be right on the mark. With the
zeners you do not need 1000uF||750R resistors at all -- it will work
as fixed bias, which is, if I understand correctly, critical to tubes
replacement and B+ variation.
Regards,
Alex
Dynamic bias stabilization was Re: Brook sliding bias operation.
Alex wrote:
>
> > I first tried zeners with a revised ST70 schematic shown at my website.
> >
> > The trouble with zeners is that they do not react to current. They
> > react to voltage.
> > So if a tube short circuits itself, Ek is held down but Ik goes high.
> > In the ST70 protection circuit I developed, series R between bottom of
> > zeners
> > and 0V sends an error signal off to an SCR, and amp meltdown is avoided.
> >
> > The transistor dynamic bypass stabilizers use very non critical setting
> > for the
> > transistor bypass. It only reacts to ac current generated in the low
> > value sensing R.
> > So Ek can vary quite a bit before triggering the protect circuit
> >
> > But if a hard sudden drum beat occurs, the high AB current is nicely
> > bypassed
> > and Ek barely moves at all, but with a zener, a considerable rise in Ek
> > has to occur
> > before it conducts at all.
>
> Yes, that makes sense. The transistor circuit reacts to the
> instantaneous current. The threshold can be higher than a quescient
> current, while with the zeners it shall be right on the mark. With the
> zeners you do not need 1000uF||750R resistors at all -- it will work
> as fixed bias, which is, if I understand correctly, critical to tubes
> replacement and B+ variation.
>
> Regards,
> Alex
The idea of using zeners for cathode biasing tubes to give a form of
fixed
bias lacks the ability for adjustment.
I'd never use zeners to do it. The zeners are slightly non linear and
damn noisy.
You get zener coloured sound.
Fixed bias isn't fixed because its adjustable, but once its adjusted,
its fixed, right?
Makers have found making a negative grid supply cheaper and better than
having
say 35V at 50mA dropped across zeners for each OP tube. Kinda wasteful.
But let's go back to my page in the issue...
* w w w .turneraudio . com .au/schem-300w-5-bias-stabilizer.html
The first schematic is for a one pair of OP tubes and has 1,000uF and
750 ohm C&R cathode network.
R9 and R10 don't have a value shown, an ommision on my part which I must
fix soon,
but from the text they are both 10 ohms.
10 ohms is a good value to begin with.
If Ia = 30mA, the Vdc across R9&10 is =0.3Vdc. The bases of the BD239B
have 1k connected
between R9&10, and the base to 0V caps, C7, C8, could be 0.01uF,
giving a pole at about 16kHz because one does not want to have the
circuit work at above
this F.
The base current will be zero if the Vbe = +0.3V. Collector current also
is 0.0mA
But current will begin to flow at about Vbe = +0.6V, and the current
flow versus
10 ohm voltage transfer is not very linear, but its nonlinearity
complements the non linear tube current as it goes into class AB
operation from class A.
As the bjt draws base current during its turn on, the base current
causes a voltage across the 1K base resistors,
R11, R12. 1mA of base current gives a whole volt across the 1k.
Under normal operation, base current is kept low.
The collector current Ic = hfe x Ib
The max Ic is when bjts are turned on enough to pull nearly the full
cathode bias across the
R13 and R14, 47 ohms, so if Ek = 35V, max Ic = 0.744 amps peak.
If the hfe of the BD239B was 20, then max Ib would be 37mA, which
means 37V needs to be developed across the 1k R12, R13, and also across
R9 and R10,
and this is quite impossible for the tube to ever do,
The tube might manage 0.7A peak, giving 7V peak across the 10 ohms of
R9&10.
So adjusting R11&12 will determine just how smartly the bjt will turn
on.
The values I've shown work OK, but if you play with them and analyse on
your own,
you see what happens. I should not have to spell every tiny little
detail out.
With Ia dc = 30mA, the 10 ohm Vdc = 0.3Vdc, and the cathode dc could
rise
by 20mA before dc turns on the bjt, so a rise of Ek from 22.5Vdc to
37.5Vdc is possible without
the bjt turning on, but let's say we set the excess cathode current
detector to trip the protect circuit at 30Vdc, then we'd have some
variability
allowable in idle dc which would regulate the Ia.
The text below the schema says additional R from each base to 0V will
form a divider to reduce the Vdc at the base, and make the circuit
less prone to bypassing ac at a small peak current in excess of the
idle.
There should in fact be no AC shunting by the BJT where the audio
signals cause
less than + or - the idle current, and so R9 or 10 can be varied to
accomodate this principle.
If the bjts turn on at too close to the idle dc, you'll find the
Ek will FALL a bit, because the signal current in the tube is triggering
the
release of energy store in the 1,000 uF caps to 0V, and in fact
idle current and class A PO is actually increased.
The circuit works best when a load is chosen that produces
maximum class AB PO.
So in a "50W" PP amp meant for 4 ohms, one may find 60W is possible at 3
ohms
The transistors and their resistors attached should keep Ek no no than
10%
above the idle value when the amp clips into 3 ohms, ie, at 60W.
Between idle and 60W, the Ek should never fall slightly before slightly
rising
to the max at 60W clipping.
So these things have to be methodically checked when anyone tries this
circuit.
It only has a "handful of easy to get parts"
but there are at least 3 important things someone has to trim and get
right.
And diyers hate things like this, they want it easy,
because they hate learning, and hate spending time to learn.
I don't do easy.
I do hard,
I do mentally challenging.
Patrick Turner.
Alex wrote:
>
> > I first tried zeners with a revised ST70 schematic shown at my website.
> >
> > The trouble with zeners is that they do not react to current. They
> > react to voltage.
> > So if a tube short circuits itself, Ek is held down but Ik goes high.
> > In the ST70 protection circuit I developed, series R between bottom of
> > zeners
> > and 0V sends an error signal off to an SCR, and amp meltdown is avoided.
> >
> > The transistor dynamic bypass stabilizers use very non critical setting
> > for the
> > transistor bypass. It only reacts to ac current generated in the low
> > value sensing R.
> > So Ek can vary quite a bit before triggering the protect circuit
> >
> > But if a hard sudden drum beat occurs, the high AB current is nicely
> > bypassed
> > and Ek barely moves at all, but with a zener, a considerable rise in Ek
> > has to occur
> > before it conducts at all.
>
> Yes, that makes sense. The transistor circuit reacts to the
> instantaneous current. The threshold can be higher than a quescient
> current, while with the zeners it shall be right on the mark. With the
> zeners you do not need 1000uF||750R resistors at all -- it will work
> as fixed bias, which is, if I understand correctly, critical to tubes
> replacement and B+ variation.
>
> Regards,
> Alex
The idea of using zeners for cathode biasing tubes to give a form of
fixed
bias lacks the ability for adjustment.
I'd never use zeners to do it. The zeners are slightly non linear and
damn noisy.
You get zener coloured sound.
Fixed bias isn't fixed because its adjustable, but once its adjusted,
its fixed, right?
Makers have found making a negative grid supply cheaper and better than
having
say 35V at 50mA dropped across zeners for each OP tube. Kinda wasteful.
But let's go back to my page in the issue...
* w w w .turneraudio . com .au/schem-300w-5-bias-stabilizer.html
The first schematic is for a one pair of OP tubes and has 1,000uF and
750 ohm C&R cathode network.
R9 and R10 don't have a value shown, an ommision on my part which I must
fix soon,
but from the text they are both 10 ohms.
10 ohms is a good value to begin with.
If Ia = 30mA, the Vdc across R9&10 is =0.3Vdc. The bases of the BD239B
have 1k connected
between R9&10, and the base to 0V caps, C7, C8, could be 0.01uF,
giving a pole at about 16kHz because one does not want to have the
circuit work at above
this F.
The base current will be zero if the Vbe = +0.3V. Collector current also
is 0.0mA
But current will begin to flow at about Vbe = +0.6V, and the current
flow versus
10 ohm voltage transfer is not very linear, but its nonlinearity
complements the non linear tube current as it goes into class AB
operation from class A.
As the bjt draws base current during its turn on, the base current
causes a voltage across the 1K base resistors,
R11, R12. 1mA of base current gives a whole volt across the 1k.
Under normal operation, base current is kept low.
The collector current Ic = hfe x Ib
The max Ic is when bjts are turned on enough to pull nearly the full
cathode bias across the
R13 and R14, 47 ohms, so if Ek = 35V, max Ic = 0.744 amps peak.
If the hfe of the BD239B was 20, then max Ib would be 37mA, which
means 37V needs to be developed across the 1k R12, R13, and also across
R9 and R10,
and this is quite impossible for the tube to ever do,
The tube might manage 0.7A peak, giving 7V peak across the 10 ohms of
R9&10.
So adjusting R11&12 will determine just how smartly the bjt will turn
on.
The values I've shown work OK, but if you play with them and analyse on
your own,
you see what happens. I should not have to spell every tiny little
detail out.
With Ia dc = 30mA, the 10 ohm Vdc = 0.3Vdc, and the cathode dc could
rise
by 20mA before dc turns on the bjt, so a rise of Ek from 22.5Vdc to
37.5Vdc is possible without
the bjt turning on, but let's say we set the excess cathode current
detector to trip the protect circuit at 30Vdc, then we'd have some
variability
allowable in idle dc which would regulate the Ia.
The text below the schema says additional R from each base to 0V will
form a divider to reduce the Vdc at the base, and make the circuit
less prone to bypassing ac at a small peak current in excess of the
idle.
There should in fact be no AC shunting by the BJT where the audio
signals cause
less than + or - the idle current, and so R9 or 10 can be varied to
accomodate this principle.
If the bjts turn on at too close to the idle dc, you'll find the
Ek will FALL a bit, because the signal current in the tube is triggering
the
release of energy store in the 1,000 uF caps to 0V, and in fact
idle current and class A PO is actually increased.
The circuit works best when a load is chosen that produces
maximum class AB PO.
So in a "50W" PP amp meant for 4 ohms, one may find 60W is possible at 3
ohms
The transistors and their resistors attached should keep Ek no no than
10%
above the idle value when the amp clips into 3 ohms, ie, at 60W.
Between idle and 60W, the Ek should never fall slightly before slightly
rising
to the max at 60W clipping.
So these things have to be methodically checked when anyone tries this
circuit.
It only has a "handful of easy to get parts"
but there are at least 3 important things someone has to trim and get
right.
And diyers hate things like this, they want it easy,
because they hate learning, and hate spending time to learn.
I don't do easy.
I do hard,
I do mentally challenging.
Patrick Turner.
