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Balancing P-P O/P stages
Balancing P-P O/P stages
Balancing P-P O/P stages (long post!)
Cathode self-bias has an inherent balancing problem (but as, we know,
two resistors are much better than one!) With two resistors to either
side of the balance pot, with the slider grounded, the individual
cathode resistances to ground vary with balance pot setting. Thus,
measuring differential voltage across the cathodes does not help and,
of course, making them equal is quite counterproductive.
One goal is to get all DC bias out of the OPT primary (another is
equal drive to clipping - not addressed here but see below.)
It's easier to do this with a circuit that allows a centre-zero on a
meter at balance rather than switched current metering in the cathode
legs or a switched voltmeter across equal sensing resistors.
If the OPT had equal DC resistance primary sections one could put a
voltmeter between the two plates and, via the bias control (either
self bias or fixed bias), set it to zero for equal plate current.
Unfortunately, the OPT plate sections are generally not of equal DCR.
They are also very "B+ hot".
Anyway, I tried this recently on my "Scratch 10" amplifier (5AR4, 2 x
6v6 (2 x 470 ohm cathode resistors plus a 100 ohm balance pot), 6SN7
(with CC cathode cct), EF86) :
1. Measured the DC resistance of the two OPT primary sections (using
an AVO Model 8 analog multimeter. Note: digital DMM's can give errors
due to the high winding inductance.)
2. Added a series resistor to the lowest resistance side (plate to
OPT) to bring the total up to that of the other side (it is a small
resistance - see below)
3. Bypassed this resistor with a large capacitor (positive towards B+,
of course)
4. Placed AVO meter on a high DC voltage range across the plates
5. Powered up amplifier
6. Reduced meter DC voltage range until the unbalance deflection was
seen
7. Adjusted bias balance pot until differential voltage read zero.
8. Current in each O/P tube was now virtually equal.
Results
1. OPT coil resistance was was about 150 ohms and 175 ohms each side
of primary CT.
2. Added resistor to 150 ohm section - 22 ohms was to hand (close
enough)
3. Added bypass cap - 1000 uF, 16 VDC (-3dB corner frequency with 22
ohms is about 7 Hz, seems low enough.)
4. Put initial meter range on power up to 100 VDC full scale
5. Reduced meter range for balancing to 2.5 VDC full scale
6. Easily balanced the O/P tube currents with 100 ohm pot - much
easier than with interative voltage measurements across the OPT
primary sections as I had been doing.
7. Checked power output on 'scope and VTVM (across 8 ohms): OK, had
the same peak clipping (equal each side) and class AB X/O distortion
as before when overdriven at both 40 and 400 Hz.
I should add that I run an eclectic mix of NOS and well used O/P tubes
though this "bench" amplifier, from 6K6's to 6L6's (mostly using an
external variable B+ supply from 250 to 325 VDC), some tubes are quite
mismatched and a few sorry specimins below 50% emission - so it's
good to have a quick balancing act l
IMPORTANT! - there are some serious practical and SAFETY problems with
this method...
1. SAFETY: the meter is some 300 VDC above ground - not safe for the
average user
- might use a permanent, switched, diode-clamped, isolated, center
zero meter installed in the chassis but I'm not sure if meter
movements can take 300 VDC from coil to case - my guess is no!
2. There is a very large differential signal voltage across the two O/
P tube plates, i.e. to the meter:
-- must ensure that no residual hum, noise or signal is present while
taking measurements; perhaps pull out the phase splitter tube before
test power-up in most circuits.
3. You get a large meter kick on power-down if it is left connected on
low range:
-- if probes are used you will have removed the meter; if croc-clips
are used, remove or switch range back to over 100 VDC before power-
down
-- any meter must be out of cct. when amplifier is being used, also at
power up/down. A large AC range volt meter could be left in (I tried
this) but to what purpose? VU meter?
Question: Have I missed a simple, permanent "centre zero" method that
is entirely in the "safe" cathode area? I do not recall ever seeing
one and nothing comes to mind. Actually, I was going to apply the auto-
balance Blumlein "garter" bias but I'm a bit reluctant to concede the
20 VDC or so of useful B+ that it takes up. For this see
* w w w .tubecad . com /2005/May/blog0046.htm
Disclaimer: there are lethal voltages around tube circuits. Do not do
any of the above unless you are qualified by experience with tube
equipment design, build and repair. Don't use the above balance
method except on the bench under controlled conditions.
All comments welcomed.
Cheers,
Roger
Balancing P-P O/P stages (long post!)
Cathode self-bias has an inherent balancing problem (but as, we know,
two resistors are much better than one!) With two resistors to either
side of the balance pot, with the slider grounded, the individual
cathode resistances to ground vary with balance pot setting. Thus,
measuring differential voltage across the cathodes does not help and,
of course, making them equal is quite counterproductive.
One goal is to get all DC bias out of the OPT primary (another is
equal drive to clipping - not addressed here but see below.)
It's easier to do this with a circuit that allows a centre-zero on a
meter at balance rather than switched current metering in the cathode
legs or a switched voltmeter across equal sensing resistors.
If the OPT had equal DC resistance primary sections one could put a
voltmeter between the two plates and, via the bias control (either
self bias or fixed bias), set it to zero for equal plate current.
Unfortunately, the OPT plate sections are generally not of equal DCR.
They are also very "B+ hot".
Anyway, I tried this recently on my "Scratch 10" amplifier (5AR4, 2 x
6v6 (2 x 470 ohm cathode resistors plus a 100 ohm balance pot), 6SN7
(with CC cathode cct), EF86) :
1. Measured the DC resistance of the two OPT primary sections (using
an AVO Model 8 analog multimeter. Note: digital DMM's can give errors
due to the high winding inductance.)
2. Added a series resistor to the lowest resistance side (plate to
OPT) to bring the total up to that of the other side (it is a small
resistance - see below)
3. Bypassed this resistor with a large capacitor (positive towards B+,
of course)
4. Placed AVO meter on a high DC voltage range across the plates
5. Powered up amplifier
6. Reduced meter DC voltage range until the unbalance deflection was
seen
7. Adjusted bias balance pot until differential voltage read zero.
8. Current in each O/P tube was now virtually equal.
Results
1. OPT coil resistance was was about 150 ohms and 175 ohms each side
of primary CT.
2. Added resistor to 150 ohm section - 22 ohms was to hand (close
enough)
3. Added bypass cap - 1000 uF, 16 VDC (-3dB corner frequency with 22
ohms is about 7 Hz, seems low enough.)
4. Put initial meter range on power up to 100 VDC full scale
5. Reduced meter range for balancing to 2.5 VDC full scale
6. Easily balanced the O/P tube currents with 100 ohm pot - much
easier than with interative voltage measurements across the OPT
primary sections as I had been doing.
7. Checked power output on 'scope and VTVM (across 8 ohms): OK, had
the same peak clipping (equal each side) and class AB X/O distortion
as before when overdriven at both 40 and 400 Hz.
I should add that I run an eclectic mix of NOS and well used O/P tubes
though this "bench" amplifier, from 6K6's to 6L6's (mostly using an
external variable B+ supply from 250 to 325 VDC), some tubes are quite
mismatched and a few sorry specimins below 50% emission - so it's
good to have a quick balancing act l
IMPORTANT! - there are some serious practical and SAFETY problems with
this method...
1. SAFETY: the meter is some 300 VDC above ground - not safe for the
average user
- might use a permanent, switched, diode-clamped, isolated, center
zero meter installed in the chassis but I'm not sure if meter
movements can take 300 VDC from coil to case - my guess is no!
2. There is a very large differential signal voltage across the two O/
P tube plates, i.e. to the meter:
-- must ensure that no residual hum, noise or signal is present while
taking measurements; perhaps pull out the phase splitter tube before
test power-up in most circuits.
3. You get a large meter kick on power-down if it is left connected on
low range:
-- if probes are used you will have removed the meter; if croc-clips
are used, remove or switch range back to over 100 VDC before power-
down
-- any meter must be out of cct. when amplifier is being used, also at
power up/down. A large AC range volt meter could be left in (I tried
this) but to what purpose? VU meter?
Question: Have I missed a simple, permanent "centre zero" method that
is entirely in the "safe" cathode area? I do not recall ever seeing
one and nothing comes to mind. Actually, I was going to apply the auto-
balance Blumlein "garter" bias but I'm a bit reluctant to concede the
20 VDC or so of useful B+ that it takes up. For this see
* w w w .tubecad . com /2005/May/blog0046.htm
Disclaimer: there are lethal voltages around tube circuits. Do not do
any of the above unless you are qualified by experience with tube
equipment design, build and repair. Don't use the above balance
method except on the bench under controlled conditions.
All comments welcomed.
Cheers,
Roger
Re: Balancing P-P O/P stages
On Mon, 24 Mar 2008 15:16:36 -0700 (PDT), Engineer
<junk2007@rogers . com > wrote:
>Balancing P-P O/P stages (long post!)
>Cathode self-bias has an inherent balancing problem (but as, we know,
>two resistors are much better than one!) With two resistors to either
>side of the balance pot, with the slider grounded, the individual
>cathode resistances to ground vary with balance pot setting. Thus,
>measuring differential voltage across the cathodes does not help and,
>of course, making them equal is quite counterproductive.
>
>One goal is to get all DC bias out of the OPT primary (another is
>equal drive to clipping - not addressed here but see below.)
>It's easier to do this with a circuit that allows a centre-zero on a
>meter at balance rather than switched current metering in the cathode
>legs or a switched voltmeter across equal sensing resistors.
>
>If the OPT had equal DC resistance primary sections one could put a
>voltmeter between the two plates and, via the bias control (either
>self bias or fixed bias), set it to zero for equal plate current.
>Unfortunately, the OPT plate sections are generally not of equal DCR.
>They are also very "B+ hot".
>
>Anyway, I tried this recently on my "Scratch 10" amplifier (5AR4, 2 x
>6v6 (2 x 470 ohm cathode resistors plus a 100 ohm balance pot), 6SN7
>(with CC cathode cct), EF86) :
>1. Measured the DC resistance of the two OPT primary sections (using
>an AVO Model 8 analog multimeter. Note: digital DMM's can give errors
>due to the high winding inductance.)
>2. Added a series resistor to the lowest resistance side (plate to
>OPT) to bring the total up to that of the other side (it is a small
>resistance - see below)
>3. Bypassed this resistor with a large capacitor (positive towards B+,
>of course)
>4. Placed AVO meter on a high DC voltage range across the plates
>5. Powered up amplifier
>6. Reduced meter DC voltage range until the unbalance deflection was
>seen
>7. Adjusted bias balance pot until differential voltage read zero.
>8. Current in each O/P tube was now virtually equal.
>
>Results
>1. OPT coil resistance was was about 150 ohms and 175 ohms each side
>of primary CT.
>2. Added resistor to 150 ohm section - 22 ohms was to hand (close
>enough)
>3. Added bypass cap - 1000 uF, 16 VDC (-3dB corner frequency with 22
>ohms is about 7 Hz, seems low enough.)
>4. Put initial meter range on power up to 100 VDC full scale
>5. Reduced meter range for balancing to 2.5 VDC full scale
>6. Easily balanced the O/P tube currents with 100 ohm pot - much
>easier than with interative voltage measurements across the OPT
>primary sections as I had been doing.
>7. Checked power output on 'scope and VTVM (across 8 ohms): OK, had
>the same peak clipping (equal each side) and class AB X/O distortion
>as before when overdriven at both 40 and 400 Hz.
>
>I should add that I run an eclectic mix of NOS and well used O/P tubes
>though this "bench" amplifier, from 6K6's to 6L6's (mostly using an
>external variable B+ supply from 250 to 325 VDC), some tubes are quite
>mismatched and a few sorry specimins below 50% emission - so it's
>good to have a quick balancing act l
>
>IMPORTANT! - there are some serious practical and SAFETY problems with
>this method...
>1. SAFETY: the meter is some 300 VDC above ground - not safe for the
>average user
>- might use a permanent, switched, diode-clamped, isolated, center
>zero meter installed in the chassis but I'm not sure if meter
>movements can take 300 VDC from coil to case - my guess is no!
>2. There is a very large differential signal voltage across the two O/
>P tube plates, i.e. to the meter:
>-- must ensure that no residual hum, noise or signal is present while
>taking measurements; perhaps pull out the phase splitter tube before
>test power-up in most circuits.
>3. You get a large meter kick on power-down if it is left connected on
>low range:
>-- if probes are used you will have removed the meter; if croc-clips
>are used, remove or switch range back to over 100 VDC before power-
>down
>-- any meter must be out of cct. when amplifier is being used, also at
>power up/down. A large AC range volt meter could be left in (I tried
>this) but to what purpose? VU meter?
>
>Question: Have I missed a simple, permanent "centre zero" method that
>is entirely in the "safe" cathode area? I do not recall ever seeing
>one and nothing comes to mind. Actually, I was going to apply the auto-
>balance Blumlein "garter" bias but I'm a bit reluctant to concede the
>20 VDC or so of useful B+ that it takes up. For this see
> * w w w .tubecad . com /2005/May/blog0046.htm
>
>Disclaimer: there are lethal voltages around tube circuits. Do not do
>any of the above unless you are qualified by experience with tube
>equipment design, build and repair. Don't use the above balance
>method except on the bench under controlled conditions.
>
You already have resistors in the cathode circuit that could be used
to determine quiescent current balance directly. What you might do is
adjust static grid bias, as this does not affect AC gain of the
mismatched devices, and can be achieved using lower-power variable
resistors.
RL
On Mon, 24 Mar 2008 15:16:36 -0700 (PDT), Engineer
<junk2007@rogers . com > wrote:
>Balancing P-P O/P stages (long post!)
>Cathode self-bias has an inherent balancing problem (but as, we know,
>two resistors are much better than one!) With two resistors to either
>side of the balance pot, with the slider grounded, the individual
>cathode resistances to ground vary with balance pot setting. Thus,
>measuring differential voltage across the cathodes does not help and,
>of course, making them equal is quite counterproductive.
>
>One goal is to get all DC bias out of the OPT primary (another is
>equal drive to clipping - not addressed here but see below.)
>It's easier to do this with a circuit that allows a centre-zero on a
>meter at balance rather than switched current metering in the cathode
>legs or a switched voltmeter across equal sensing resistors.
>
>If the OPT had equal DC resistance primary sections one could put a
>voltmeter between the two plates and, via the bias control (either
>self bias or fixed bias), set it to zero for equal plate current.
>Unfortunately, the OPT plate sections are generally not of equal DCR.
>They are also very "B+ hot".
>
>Anyway, I tried this recently on my "Scratch 10" amplifier (5AR4, 2 x
>6v6 (2 x 470 ohm cathode resistors plus a 100 ohm balance pot), 6SN7
>(with CC cathode cct), EF86) :
>1. Measured the DC resistance of the two OPT primary sections (using
>an AVO Model 8 analog multimeter. Note: digital DMM's can give errors
>due to the high winding inductance.)
>2. Added a series resistor to the lowest resistance side (plate to
>OPT) to bring the total up to that of the other side (it is a small
>resistance - see below)
>3. Bypassed this resistor with a large capacitor (positive towards B+,
>of course)
>4. Placed AVO meter on a high DC voltage range across the plates
>5. Powered up amplifier
>6. Reduced meter DC voltage range until the unbalance deflection was
>seen
>7. Adjusted bias balance pot until differential voltage read zero.
>8. Current in each O/P tube was now virtually equal.
>
>Results
>1. OPT coil resistance was was about 150 ohms and 175 ohms each side
>of primary CT.
>2. Added resistor to 150 ohm section - 22 ohms was to hand (close
>enough)
>3. Added bypass cap - 1000 uF, 16 VDC (-3dB corner frequency with 22
>ohms is about 7 Hz, seems low enough.)
>4. Put initial meter range on power up to 100 VDC full scale
>5. Reduced meter range for balancing to 2.5 VDC full scale
>6. Easily balanced the O/P tube currents with 100 ohm pot - much
>easier than with interative voltage measurements across the OPT
>primary sections as I had been doing.
>7. Checked power output on 'scope and VTVM (across 8 ohms): OK, had
>the same peak clipping (equal each side) and class AB X/O distortion
>as before when overdriven at both 40 and 400 Hz.
>
>I should add that I run an eclectic mix of NOS and well used O/P tubes
>though this "bench" amplifier, from 6K6's to 6L6's (mostly using an
>external variable B+ supply from 250 to 325 VDC), some tubes are quite
>mismatched and a few sorry specimins below 50% emission - so it's
>good to have a quick balancing act l
>
>IMPORTANT! - there are some serious practical and SAFETY problems with
>this method...
>1. SAFETY: the meter is some 300 VDC above ground - not safe for the
>average user
>- might use a permanent, switched, diode-clamped, isolated, center
>zero meter installed in the chassis but I'm not sure if meter
>movements can take 300 VDC from coil to case - my guess is no!
>2. There is a very large differential signal voltage across the two O/
>P tube plates, i.e. to the meter:
>-- must ensure that no residual hum, noise or signal is present while
>taking measurements; perhaps pull out the phase splitter tube before
>test power-up in most circuits.
>3. You get a large meter kick on power-down if it is left connected on
>low range:
>-- if probes are used you will have removed the meter; if croc-clips
>are used, remove or switch range back to over 100 VDC before power-
>down
>-- any meter must be out of cct. when amplifier is being used, also at
>power up/down. A large AC range volt meter could be left in (I tried
>this) but to what purpose? VU meter?
>
>Question: Have I missed a simple, permanent "centre zero" method that
>is entirely in the "safe" cathode area? I do not recall ever seeing
>one and nothing comes to mind. Actually, I was going to apply the auto-
>balance Blumlein "garter" bias but I'm a bit reluctant to concede the
>20 VDC or so of useful B+ that it takes up. For this see
> * w w w .tubecad . com /2005/May/blog0046.htm
>
>Disclaimer: there are lethal voltages around tube circuits. Do not do
>any of the above unless you are qualified by experience with tube
>equipment design, build and repair. Don't use the above balance
>method except on the bench under controlled conditions.
>
You already have resistors in the cathode circuit that could be used
to determine quiescent current balance directly. What you might do is
adjust static grid bias, as this does not affect AC gain of the
mismatched devices, and can be achieved using lower-power variable
resistors.
RL
Re: Balancing P-P O/P stages
On Mar 25, 10:05 pm, legg <l...@nospam.magma.ca> wrote:
> On Mon, 24 Mar 2008 15:16:36 -0700 (PDT), Engineer
>
>
>
>
>
> <junk2...@rogers . com > wrote:
(snip)
>
> You already have resistors in the cathode circuit that could be used
> to determine quiescent current balance directly. What you might do is
> adjust static grid bias, as this does not affect AC gain of the
> mismatched devices, and can be achieved using lower-power variable
> resistors.
>
> RL- Hide quoted text -
>
> - Show quoted text -
I think you're suggesting a separate bias adjust like "fixed" bias -
it could work, too! But it needs a bias supply... then you might as
well go for full fixed bias regime and use 10 ohm precision resistors
in each cathode as a sensor with zero differential cathode voltage as
the aim. Gets a bit more B+, too!
I was trying to do this while using simple cathode bias. I conclude
that the balance measurement can't be done at the cathode.
Of course, you can calculate the currents if you know the cathode
resistors but:
1. You don't know them - the 100 ohms pot changes them differentially
each time you adjust it. That's it's job.
2. Even if you knew them (measure each time...) the balancing would be
iterative, a big pain! i.e. measure each voltage, calculate currents,
note error, change 100 ohm pot a bit, determine each new cathode
resistor, measure each voltage, calculate... etc.
I want a direct, one step, differential, centre zero set-up as we can
have for fixed bias. Think I'll just build a bias supply!
BTW, I'm not talking about AC drive balance, that's another topic...
I got lucky, my P-P class AB OPT was driven equally right up to
clipping at DC balance.
Cheers,
Roger
On Mar 25, 10:05 pm, legg <l...@nospam.magma.ca> wrote:
> On Mon, 24 Mar 2008 15:16:36 -0700 (PDT), Engineer
>
>
>
>
>
> <junk2...@rogers . com > wrote:
(snip)
>
> You already have resistors in the cathode circuit that could be used
> to determine quiescent current balance directly. What you might do is
> adjust static grid bias, as this does not affect AC gain of the
> mismatched devices, and can be achieved using lower-power variable
> resistors.
>
> RL- Hide quoted text -
>
> - Show quoted text -
I think you're suggesting a separate bias adjust like "fixed" bias -
it could work, too! But it needs a bias supply... then you might as
well go for full fixed bias regime and use 10 ohm precision resistors
in each cathode as a sensor with zero differential cathode voltage as
the aim. Gets a bit more B+, too!
I was trying to do this while using simple cathode bias. I conclude
that the balance measurement can't be done at the cathode.
Of course, you can calculate the currents if you know the cathode
resistors but:
1. You don't know them - the 100 ohms pot changes them differentially
each time you adjust it. That's it's job.
2. Even if you knew them (measure each time...) the balancing would be
iterative, a big pain! i.e. measure each voltage, calculate currents,
note error, change 100 ohm pot a bit, determine each new cathode
resistor, measure each voltage, calculate... etc.
I want a direct, one step, differential, centre zero set-up as we can
have for fixed bias. Think I'll just build a bias supply!
BTW, I'm not talking about AC drive balance, that's another topic...
I got lucky, my P-P class AB OPT was driven equally right up to
clipping at DC balance.
Cheers,
Roger
Re: Balancing P-P O/P stages
"Engineer" <junk2007@rogers . com > wrote in message
news:c5a5d96f-5774-4cc8-88f7-b5d113f41363@s19g2000prg.googlegroups . com ...
>
>
>[...]
>
>I want a direct, one step, differential, centre zero set-up as we can
>have for fixed bias.
>
>[...]
>
The circuit below comes to mind, but "did'nt try this at home..."
anyway ...
G1-------[.5M]----| |----[.5M]-------G2
| P1 |
|---[50K]----o---[10K]---o----[50K]---|
| N1 ^ N2 |
| | |
C1---0---[Rc1]----------o----------[Rc2]---o---C2
| | |
| GND |
| |
|-----------------( )-----------------|
BALANCE METER
Component values are random, no decoupling caps drawn.
F.e assume a 100% matched pair for a start, also assume
cathode C1/C2 voltages are 11V to GND, than with P1
in mid wiper position the nodes N1/N2 are 1V to GND.
As a result both grid to cathode voltages are -10V.
With no perfectly matched pair, but for equal cathode
currents, Vg-c can be shifted from -11V to -9.2V for
one tube while ATST but vice versa for the other.
What's the catch? AFAICS right now:
-sacrifice cathode circuit power to obtain balance control.
-red cheeked output tubes at potentiometer wiper lift off.
(add some protection diodes in series from N1/N2 to GND?)
-the network parallel to Rc1 or Rc2 is not of a constant
value, therefore resulting Rc1 and Rc2 are not constant.
however, a theorethical catch that is, not practical ...
-other catches?
Regards,
Jan
"Engineer" <junk2007@rogers . com > wrote in message
news:c5a5d96f-5774-4cc8-88f7-b5d113f41363@s19g2000prg.googlegroups . com ...
>
>
>[...]
>
>I want a direct, one step, differential, centre zero set-up as we can
>have for fixed bias.
>
>[...]
>
The circuit below comes to mind, but "did'nt try this at home..."
anyway ...
G1-------[.5M]----| |----[.5M]-------G2
| P1 |
|---[50K]----o---[10K]---o----[50K]---|
| N1 ^ N2 |
| | |
C1---0---[Rc1]----------o----------[Rc2]---o---C2
| | |
| GND |
| |
|-----------------( )-----------------|
BALANCE METER
Component values are random, no decoupling caps drawn.
F.e assume a 100% matched pair for a start, also assume
cathode C1/C2 voltages are 11V to GND, than with P1
in mid wiper position the nodes N1/N2 are 1V to GND.
As a result both grid to cathode voltages are -10V.
With no perfectly matched pair, but for equal cathode
currents, Vg-c can be shifted from -11V to -9.2V for
one tube while ATST but vice versa for the other.
What's the catch? AFAICS right now:
-sacrifice cathode circuit power to obtain balance control.
-red cheeked output tubes at potentiometer wiper lift off.
(add some protection diodes in series from N1/N2 to GND?)
-the network parallel to Rc1 or Rc2 is not of a constant
value, therefore resulting Rc1 and Rc2 are not constant.
however, a theorethical catch that is, not practical ...
-other catches?
Regards,
Jan
Re: Balancing P-P O/P stages
In article <47ef771d$0$14357$e4fe514c@news.xs4all.nl>,
"J.Koning" <mynamespacedbydots@xs4all.nl> wrote:
> "Engineer" <junk2007@rogers . com > wrote in message
> news:c5a5d96f-5774-4cc8-88f7-b5d113f41363@s19g2000prg.googlegroups . com ...
> >
> >
> >[...]
> >
> >I want a direct, one step, differential, centre zero set-up as we can
> >have for fixed bias.
> >
> >[...]
> >
>
>
> The circuit below comes to mind, but "did'nt try this at home..."
> anyway ...
>
>
>
> G1-------[.5M]----| |----[.5M]-------G2
> | P1 |
> |---[50K]----o---[10K]---o----[50K]---|
> | N1 ^ N2 |
> | | |
> C1---0---[Rc1]----------o----------[Rc2]---o---C2
> | | |
> | GND |
> | |
> |-----------------( )-----------------|
> BALANCE METER
>
>
>
> Component values are random, no decoupling caps drawn.
>
> F.e assume a 100% matched pair for a start, also assume
> cathode C1/C2 voltages are 11V to GND, than with P1
> in mid wiper position the nodes N1/N2 are 1V to GND.
> As a result both grid to cathode voltages are -10V.
>
> With no perfectly matched pair, but for equal cathode
> currents, Vg-c can be shifted from -11V to -9.2V for
> one tube while ATST but vice versa for the other.
>
> What's the catch? AFAICS right now:
> -sacrifice cathode circuit power to obtain balance control.
> -red cheeked output tubes at potentiometer wiper lift off.
> (add some protection diodes in series from N1/N2 to GND?)
> -the network parallel to Rc1 or Rc2 is not of a constant
> value, therefore resulting Rc1 and Rc2 are not constant.
> however, a theorethical catch that is, not practical ...
> -other catches?
I am glad to see someone finally post one way to solve this problem, as
I had been having trouble getting motivated to post my similar circuit.
My circuit operates along similar lines, but adds a couple of resistors
and rearranges the components, sort of flipping the circuit upside down,
to eliminate the second two objections, the possibility of red cheeked
output tubes, and the differing value of the networks paralleling Rc1 &
Rc2. The sacrifice cathode circuit power to obtain balance control
problem remains, however it is only a volt or two.
Regards,
John Byrns
--
Surf my web pages at, * fmamradios . com /
In article <47ef771d$0$14357$e4fe514c@news.xs4all.nl>,
"J.Koning" <mynamespacedbydots@xs4all.nl> wrote:
> "Engineer" <junk2007@rogers . com > wrote in message
> news:c5a5d96f-5774-4cc8-88f7-b5d113f41363@s19g2000prg.googlegroups . com ...
> >
> >
> >[...]
> >
> >I want a direct, one step, differential, centre zero set-up as we can
> >have for fixed bias.
> >
> >[...]
> >
>
>
> The circuit below comes to mind, but "did'nt try this at home..."
> anyway ...
>
>
>
> G1-------[.5M]----| |----[.5M]-------G2
> | P1 |
> |---[50K]----o---[10K]---o----[50K]---|
> | N1 ^ N2 |
> | | |
> C1---0---[Rc1]----------o----------[Rc2]---o---C2
> | | |
> | GND |
> | |
> |-----------------( )-----------------|
> BALANCE METER
>
>
>
> Component values are random, no decoupling caps drawn.
>
> F.e assume a 100% matched pair for a start, also assume
> cathode C1/C2 voltages are 11V to GND, than with P1
> in mid wiper position the nodes N1/N2 are 1V to GND.
> As a result both grid to cathode voltages are -10V.
>
> With no perfectly matched pair, but for equal cathode
> currents, Vg-c can be shifted from -11V to -9.2V for
> one tube while ATST but vice versa for the other.
>
> What's the catch? AFAICS right now:
> -sacrifice cathode circuit power to obtain balance control.
> -red cheeked output tubes at potentiometer wiper lift off.
> (add some protection diodes in series from N1/N2 to GND?)
> -the network parallel to Rc1 or Rc2 is not of a constant
> value, therefore resulting Rc1 and Rc2 are not constant.
> however, a theorethical catch that is, not practical ...
> -other catches?
I am glad to see someone finally post one way to solve this problem, as
I had been having trouble getting motivated to post my similar circuit.
My circuit operates along similar lines, but adds a couple of resistors
and rearranges the components, sort of flipping the circuit upside down,
to eliminate the second two objections, the possibility of red cheeked
output tubes, and the differing value of the networks paralleling Rc1 &
Rc2. The sacrifice cathode circuit power to obtain balance control
problem remains, however it is only a volt or two.
Regards,
John Byrns
--
Surf my web pages at, * fmamradios . com /
Re: Balancing P-P O/P stages
In article <47ef771d$0$14357$e4fe514c@news.xs4all.nl>,
"J.Koning" <mynamespacedbydots@xs4all.nl> wrote:
> "Engineer" <junk2007@rogers . com > wrote in message
> news:c5a5d96f-5774-4cc8-88f7-b5d113f41363@s19g2000prg.googlegroups . com ...
> >
> >
> >[...]
> >
> >I want a direct, one step, differential, centre zero set-up as we can
> >have for fixed bias.
> >
> >[...]
> >
>
>
> The circuit below comes to mind, but "did'nt try this at home..."
> anyway ...
>
>
>
> G1-------[.5M]----| |----[.5M]-------G2
> | P1 |
> |---[50K]----o---[10K]---o----[50K]---|
> | N1 ^ N2 |
> | | |
> C1---0---[Rc1]----------o----------[Rc2]---o---C2
> | | |
> | GND |
> | |
> |-----------------( )-----------------|
> BALANCE METER
>
>
>
> Component values are random, no decoupling caps drawn.
>
> F.e assume a 100% matched pair for a start, also assume
> cathode C1/C2 voltages are 11V to GND, than with P1
> in mid wiper position the nodes N1/N2 are 1V to GND.
> As a result both grid to cathode voltages are -10V.
>
> With no perfectly matched pair, but for equal cathode
> currents, Vg-c can be shifted from -11V to -9.2V for
> one tube while ATST but vice versa for the other.
>
> What's the catch? AFAICS right now:
> -sacrifice cathode circuit power to obtain balance control.
> -red cheeked output tubes at potentiometer wiper lift off.
> (add some protection diodes in series from N1/N2 to GND?)
> -the network parallel to Rc1 or Rc2 is not of a constant
> value, therefore resulting Rc1 and Rc2 are not constant.
> however, a theorethical catch that is, not practical ...
> -other catches?
Here is and ASCII art drawing of my version of the above circuit which I
mentioned in my earlier post, it fixes the potential problem with "red
cheeked output tubes at potentiometer wiper lift off", and makes the the
networks parallel to Rc1 or Rc2 equal in value so that the reading of
the "Balance Meter" is not affected by differential changes in cathode
circuit resistances between the two tubes.
The resistor values shown are for example only and must be changed to
reflect the needs of a particular amplifier design.
P1
G1------[.5M]----o--------[50K]--------o----[.5M]------G2
| ^ |
| | |
|---[30K]---(----------o----------)---[30K]---|
| | | |
| |---[5K]---o---[5K]---| |
| | |
C1---o------[Rc1]-----------o-----------[Rc2]------o---C2
| | |
| GND |
| |
|---------------------( )---------------------|
BALANCE METER
Regards,
John Byrns
--
Surf my web pages at, * fmamradios . com /
In article <47ef771d$0$14357$e4fe514c@news.xs4all.nl>,
"J.Koning" <mynamespacedbydots@xs4all.nl> wrote:
> "Engineer" <junk2007@rogers . com > wrote in message
> news:c5a5d96f-5774-4cc8-88f7-b5d113f41363@s19g2000prg.googlegroups . com ...
> >
> >
> >[...]
> >
> >I want a direct, one step, differential, centre zero set-up as we can
> >have for fixed bias.
> >
> >[...]
> >
>
>
> The circuit below comes to mind, but "did'nt try this at home..."
> anyway ...
>
>
>
> G1-------[.5M]----| |----[.5M]-------G2
> | P1 |
> |---[50K]----o---[10K]---o----[50K]---|
> | N1 ^ N2 |
> | | |
> C1---0---[Rc1]----------o----------[Rc2]---o---C2
> | | |
> | GND |
> | |
> |-----------------( )-----------------|
> BALANCE METER
>
>
>
> Component values are random, no decoupling caps drawn.
>
> F.e assume a 100% matched pair for a start, also assume
> cathode C1/C2 voltages are 11V to GND, than with P1
> in mid wiper position the nodes N1/N2 are 1V to GND.
> As a result both grid to cathode voltages are -10V.
>
> With no perfectly matched pair, but for equal cathode
> currents, Vg-c can be shifted from -11V to -9.2V for
> one tube while ATST but vice versa for the other.
>
> What's the catch? AFAICS right now:
> -sacrifice cathode circuit power to obtain balance control.
> -red cheeked output tubes at potentiometer wiper lift off.
> (add some protection diodes in series from N1/N2 to GND?)
> -the network parallel to Rc1 or Rc2 is not of a constant
> value, therefore resulting Rc1 and Rc2 are not constant.
> however, a theorethical catch that is, not practical ...
> -other catches?
Here is and ASCII art drawing of my version of the above circuit which I
mentioned in my earlier post, it fixes the potential problem with "red
cheeked output tubes at potentiometer wiper lift off", and makes the the
networks parallel to Rc1 or Rc2 equal in value so that the reading of
the "Balance Meter" is not affected by differential changes in cathode
circuit resistances between the two tubes.
The resistor values shown are for example only and must be changed to
reflect the needs of a particular amplifier design.
P1
G1------[.5M]----o--------[50K]--------o----[.5M]------G2
| ^ |
| | |
|---[30K]---(----------o----------)---[30K]---|
| | | |
| |---[5K]---o---[5K]---| |
| | |
C1---o------[Rc1]-----------o-----------[Rc2]------o---C2
| | |
| GND |
| |
|---------------------( )---------------------|
BALANCE METER
Regards,
John Byrns
--
Surf my web pages at, * fmamradios . com /
Re: Balancing P-P O/P stages
"John Byrns" <byrnsj@sbcglobal . net > wrote in message
news:byrnsj-2B2EEE.13170530032008@newsclstr03.news.prodigy . net ...
>[...]
>
> Here is and ASCII art drawing of my version of the above circuit which I
> mentioned in my earlier post, it fixes the potential problem with "red
> cheeked output tubes at potentiometer wiper lift off", and makes the the
> networks parallel to Rc1 or Rc2 equal in value so that the reading of
> the "Balance Meter" is not affected by differential changes in cathode
> circuit resistances between the two tubes.
>
>[...]
good thinking John, dark black cheeks at potentiometer wiper lift off...
>
> 0V P1 0V
> G1------[.5M]----o--------[50K]--------o----[.5M]------G2
> | ? |
> | ? |
> |---[30K]---(----------o----------)---[30K]---|
> | | | |
> | |---[5K]---o---[5K]---| |
> | | |
> C1---o------[Rc1]-----------o-----------[Rc2]------o---C2
> | | |
> | GND |
> | |
> |---------------------( )---------------------|
> BALANCE METER
So, what's next?
Red cheek version with balance(P1) and bias(P2) control...?
G1-------[.5M]----| |----[.5M]-------G2
| P1 |
|---[50K]----o---[10K]---o----[50K]---|
| N1 ^ N2 |
| | |
| v |
| [xxK]---| |
| P2 | |
| | |
C1---0---[Rc1]----------o-----o----[Rc2]---o---C2
| | |
| GND |
| /------------| BAL.
|-----------( )---------o S1
BIAS/BALANCE METER ----[xxK] --| BIAS
|
GND
...or black cheek version with balance(P1) and bias(P2) control?
P1
G1------[.5M]----o--------[50K]--------o----[.5M]------G2
| ^ |
| | |
| v |
| [xxK]---| |
| P2 | |
| | |
|---[30K]---(----------------o----)---[30K]---|
| | | |
| |---[5K]---o---[5K]---| |
| | |
C1---o------[Rc1]-----------o-----------[Rc2]------o---C2
| | |
| GND |
| /------------| BAL.
|-----------( )-----------------o S1
BIAS/BALANCE METER ----[xxK] --| BIAS
|
GND
Regards,
Jan
"John Byrns" <byrnsj@sbcglobal . net > wrote in message
news:byrnsj-2B2EEE.13170530032008@newsclstr03.news.prodigy . net ...
>[...]
>
> Here is and ASCII art drawing of my version of the above circuit which I
> mentioned in my earlier post, it fixes the potential problem with "red
> cheeked output tubes at potentiometer wiper lift off", and makes the the
> networks parallel to Rc1 or Rc2 equal in value so that the reading of
> the "Balance Meter" is not affected by differential changes in cathode
> circuit resistances between the two tubes.
>
>[...]
good thinking John, dark black cheeks at potentiometer wiper lift off...
>
> 0V P1 0V
> G1------[.5M]----o--------[50K]--------o----[.5M]------G2
> | ? |
> | ? |
> |---[30K]---(----------o----------)---[30K]---|
> | | | |
> | |---[5K]---o---[5K]---| |
> | | |
> C1---o------[Rc1]-----------o-----------[Rc2]------o---C2
> | | |
> | GND |
> | |
> |---------------------( )---------------------|
> BALANCE METER
So, what's next?
Red cheek version with balance(P1) and bias(P2) control...?
G1-------[.5M]----| |----[.5M]-------G2
| P1 |
|---[50K]----o---[10K]---o----[50K]---|
| N1 ^ N2 |
| | |
| v |
| [xxK]---| |
| P2 | |
| | |
C1---0---[Rc1]----------o-----o----[Rc2]---o---C2
| | |
| GND |
| /------------| BAL.
|-----------( )---------o S1
BIAS/BALANCE METER ----[xxK] --| BIAS
|
GND
...or black cheek version with balance(P1) and bias(P2) control?
P1
G1------[.5M]----o--------[50K]--------o----[.5M]------G2
| ^ |
| | |
| v |
| [xxK]---| |
| P2 | |
| | |
|---[30K]---(----------------o----)---[30K]---|
| | | |
| |---[5K]---o---[5K]---| |
| | |
C1---o------[Rc1]-----------o-----------[Rc2]------o---C2
| | |
| GND |
| /------------| BAL.
|-----------( )-----------------o S1
BIAS/BALANCE METER ----[xxK] --| BIAS
|
GND
Regards,
Jan
Re: Balancing P-P O/P stages
On Mar 30, 2:17 pm, John Byrns <byr...@sbcglobal . net > wrote:
> In article <47ef771d$0$14357$e4fe5...@news.xs4all.nl>,
>
>
>
>
>
> "J.Koning" <mynamespacedbyd...@xs4all.nl> wrote:
> > "Engineer" <junk2...@rogers . com > wrote in message
> >news:c5a5d96f-5774-4cc8-88f7-b5d113f41363@s19g2000prg.googlegroups . com ...=
>
> > >[...]
>
> > >I want a direct, one step, differential, centre zero set-up as we can
> > >have for fixed bias.
>
> > >[...]
>
> > The circuit below comes to mind, but "did'nt try this at home..."
> > anyway ...
>
> > G1-------[.5M]----| |----[.5M]-------G2
> > | P1 |
> > |---[50K]----o---[10K]---o----[50K]---|
> > | N1 ^ N2 =
|
> > | | =
|
> > C1---0---[Rc1]----------o----------[Rc2]---o---C2
> > | | =
|
> > | GND =
|
> > | =
|
> > |-----------------( )-----------------|
> > BALANCE METER
>
> > Component values are random, no decoupling caps drawn.
>
> > F.e assume a 100% matched pair for a start, also assume
> > cathode C1/C2 voltages are 11V to GND, than with P1
> > in mid wiper position the nodes N1/N2 are 1V to GND.
> > As a result both grid to cathode voltages are -10V.
>
> > With no perfectly matched pair, but for equal cathode
> > currents, Vg-c can be shifted from -11V to -9.2V for
> > one tube while ATST but vice versa for the other.
>
> > What's the catch? AFAICS right now:
> > -sacrifice cathode circuit power to obtain balance control.
> > -red cheeked output tubes at potentiometer wiper lift off.
> > (add some protection diodes in series from N1/N2 to GND?)
> > -the network parallel to Rc1 or Rc2 is not of a constant
> > value, therefore resulting Rc1 and Rc2 are not constant.
> > however, a theorethical catch that is, not practical ...
> > -other catches?
>
> Here is and ASCII art drawing of my version of the above circuit which I
> mentioned in my earlier post, it fixes the potential problem with "red
> cheeked output tubes at potentiometer wiper lift off", and makes the the
> networks parallel to Rc1 or Rc2 equal in value so that the reading of
> the "Balance Meter" is not affected by differential changes in cathode
> circuit resistances between the two tubes.
>
> The resistor values shown are for example only and must be changed to
> reflect the needs of a particular amplifier design.
>
> P1
> G1------[.5M]----o--------[50K]--------o----[.5M]------G2
> | ^ =
|
> | | =
|
> |---[30K]---(----------o----------)---[30K]---|
> | | =
| |
> | |---[5K]---o---[5K]---| =
|
> | | =
|
> C1---o------[Rc1]-----------o-----------[Rc2]------o---C2
> | | =
|
> | GND =
|
> | =
|
> |---------------------( )---------------------|
> BALANCE METER
>
> Regards,
>
> John Byrns
>
> --
> Surf my web pages at, * fmamradios . com /- Hide quoted text -
>
> - Show quoted text -
If the above graphics are distorted in your news reader (they are on
mine!), copy "as is" to Notepad and they become quite legible.
Cheers.
Roger
On Mar 30, 2:17 pm, John Byrns <byr...@sbcglobal . net > wrote:
> In article <47ef771d$0$14357$e4fe5...@news.xs4all.nl>,
>
>
>
>
>
> "J.Koning" <mynamespacedbyd...@xs4all.nl> wrote:
> > "Engineer" <junk2...@rogers . com > wrote in message
> >news:c5a5d96f-5774-4cc8-88f7-b5d113f41363@s19g2000prg.googlegroups . com ...=
>
> > >[...]
>
> > >I want a direct, one step, differential, centre zero set-up as we can
> > >have for fixed bias.
>
> > >[...]
>
> > The circuit below comes to mind, but "did'nt try this at home..."
> > anyway ...
>
> > G1-------[.5M]----| |----[.5M]-------G2
> > | P1 |
> > |---[50K]----o---[10K]---o----[50K]---|
> > | N1 ^ N2 =
|
> > | | =
|
> > C1---0---[Rc1]----------o----------[Rc2]---o---C2
> > | | =
|
> > | GND =
|
> > | =
|
> > |-----------------( )-----------------|
> > BALANCE METER
>
> > Component values are random, no decoupling caps drawn.
>
> > F.e assume a 100% matched pair for a start, also assume
> > cathode C1/C2 voltages are 11V to GND, than with P1
> > in mid wiper position the nodes N1/N2 are 1V to GND.
> > As a result both grid to cathode voltages are -10V.
>
> > With no perfectly matched pair, but for equal cathode
> > currents, Vg-c can be shifted from -11V to -9.2V for
> > one tube while ATST but vice versa for the other.
>
> > What's the catch? AFAICS right now:
> > -sacrifice cathode circuit power to obtain balance control.
> > -red cheeked output tubes at potentiometer wiper lift off.
> > (add some protection diodes in series from N1/N2 to GND?)
> > -the network parallel to Rc1 or Rc2 is not of a constant
> > value, therefore resulting Rc1 and Rc2 are not constant.
> > however, a theorethical catch that is, not practical ...
> > -other catches?
>
> Here is and ASCII art drawing of my version of the above circuit which I
> mentioned in my earlier post, it fixes the potential problem with "red
> cheeked output tubes at potentiometer wiper lift off", and makes the the
> networks parallel to Rc1 or Rc2 equal in value so that the reading of
> the "Balance Meter" is not affected by differential changes in cathode
> circuit resistances between the two tubes.
>
> The resistor values shown are for example only and must be changed to
> reflect the needs of a particular amplifier design.
>
> P1
> G1------[.5M]----o--------[50K]--------o----[.5M]------G2
> | ^ =
|
> | | =
|
> |---[30K]---(----------o----------)---[30K]---|
> | | =
| |
> | |---[5K]---o---[5K]---| =
|
> | | =
|
> C1---o------[Rc1]-----------o-----------[Rc2]------o---C2
> | | =
|
> | GND =
|
> | =
|
> |---------------------( )---------------------|
> BALANCE METER
>
> Regards,
>
> John Byrns
>
> --
> Surf my web pages at, * fmamradios . com /- Hide quoted text -
>
> - Show quoted text -
If the above graphics are distorted in your news reader (they are on
mine!), copy "as is" to Notepad and they become quite legible.
Cheers.
Roger
