Wot? No tube mixer??

Wot? No tube mixer??

Both Tynan and Eeyore seem to have disappeared.
Perhaps into a thermionic vacuum:-)

Does anyone know the state of play with the RAT
tube mixer?

--
Iain

Re: Wot? No tube mixer??

Iain Churches wrote:
> Both Tynan and Eeyore seem to have disappeared.
> Perhaps into a thermionic vacuum:-)
>
> Does anyone know the state of play with the RAT
> tube mixer?
>


Pass. Bryan seemed to be the most advanced.

Cheers

Ian

Re: Wot? No tube mixer??



Iain Churches wrote:

> Both Tynan and Eeyore seem to have disappeared.
> Perhaps into a thermionic vacuum:-)

Well ... Tynan disappeared first. I haven't posted here for a bit since
the place had collapsed into one of the usual pissing contests again and
I didn't see any interesting threads.


> Does anyone know the state of play with the RAT
> tube mixer?

I'm still game but I'm unlikely to spend much time on it without a
reason to do so commercially.

Graham

Re: Wot? No tube mixer??



"Eeyore" <rabbitsfriendsandrelations@hotmail . com > wrote in message
news:480CC1F9.A00E06B3@hotmail . com ...
>
>
> Iain Churches wrote:
>
>> Both Tynan and Eeyore seem to have disappeared.
>> Perhaps into a thermionic vacuum:-)
>
> Well ... Tynan disappeared first.

A couple of people suggested to me that the whole
thing was a troll, but it was such an interesting idea,
and made the great minds in RAT put on their
engineer's hats. So troll or not, it was worthy of
discussion.

> I haven't posted here for a bit since
> the place had collapsed into one of the usual pissing contests again

Isn't that about par for the course on Usenet groups generally?

> and
> I didn't see any interesting threads.

So start one:-) You are one of the better technically educated
people here!

Can I suggest a topic for you? A current limiter for tube heaters.
People are keen to use voltage regulation, which doesn't seem to
me to be so important. Heaters are specified at +/- 5% so
anything between 6V and 6V6 for a 6V3 heater is within tolerance,
and with a wirewound pot between two 'lytics one can get it spot on.
But heaters draw huge currents at switch on. Do you have a circuit,
using those funny black things with shiny legs, that could current
limit? Two circuits would be of interest. One at say 1.2A
(a pair of B9A's) and one at 3A (a pair of EL34's)

While RAT is quiet, I am using the time learning how to make
prof-quality PCBs ! (Gosh, is that a capital offence here?)

Regards to all
Iain

Re: Wot? No tube mixer??

On Mon, 21 Apr 2008 20:22:17 +0300, Iain Churches wrote:

<snip>
>
> Can I suggest a topic for you? A current limiter for tube heaters.
> People are keen to use voltage regulation, which doesn't seem to me to
> be so important. Heaters are specified at +/- 5% so anything between 6V
> and 6V6 for a 6V3 heater is within tolerance, and with a wirewound pot
> between two 'lytics one can get it spot on. But heaters draw huge
> currents at switch on. Do you have a circuit, using those funny black
> things with shiny legs, that could current limit? Two circuits would be
> of interest. One at say 1.2A (a pair of B9A's) and one at 3A (a pair
> of EL34's)
>
<snip>

Here's hoping the ASCIIart works...

+
----------,
| |
R1 heater
| |
| c
+------b TR2
| e
c |
TR1 b------+
e |
| R2
| |
----------'
-

+/- is from a 6.3v winding, bridge rectified & with a reservoir cap. It
should be about 7.8v in theory. TR1 is a small transistor such as BC107.
TR2 needs to carry the load so will need to be suitably rated. Both are
NPN.

R1 is only used to turn TR2 on. Value depends on TR2 hFE, but probably
about 150-220R.

R2 is calculated based on the current required. 0.6/I so for 1.2A use
0.5R. For 0.3A use 2R. For 3.0A use 0.2R. These won't be exact, but near
enough for jazz!

Note that because of the conversion from AC to DC you will need to have
more current capability in the transformer than you need for AC heaters.

The advantage over a lot of the heater circuits that I've seen is that it
only has about 0.8v drop, so the low supply voltage from a 6.3v winding
should be acceptable.

Now the disclaimer: I haven't actually tested this... :-)

--
Mick (Working in a M$-free zone!)
Web: * w w w .nascom.info * mixpix.batcave . net
Filtering everything posted from googlegroups to kill spam.

Re: Wot? No tube mixer??



mick wrote:

> +/- is from a 6.3v winding, bridge rectified & with a reservoir cap. It
> should be about 7.8v in theory.

Care to run your calculations for that 7.8V past us ?

Graham

Re: Wot? No tube mixer??

On Mon, 21 Apr 2008 23:45:48 +0100, Eeyore wrote:

> mick wrote:
>
>> +/- is from a 6.3v winding, bridge rectified & with a reservoir cap. It
>> should be about 7.8v in theory.
>
> Care to run your calculations for that 7.8V past us ?
>


Vac * SQRT2 - 2*Vfwd

I used the (approximate) formula:
6.3 * 1.4 - 2*0.5 = 7.82V

which allows 0.5v Vfwd for each rectifier diode. (Assuming a bridge rec
into a reservoir capacitor). I know this is often assumed to be 0.6V
each, but many rectifiers will be below this - particularly the larger
ones. You could use schottky diodes to keep it as low as poss, but I
doubt if you need anything more than a standard 5 or 6A bridge rec.

You also get a little more voltage as SQRT2 is 1.414, but who am I to
argue? ;-)

You only get 0.62 *Iac as Idc so watch the secondary current rating...

--
Mick (Working in a M$-free zone!)
Web: * w w w .nascom.info * mixpix.batcave . net
Filtering everything posted from googlegroups to kill spam.

Re: Wot? No tube mixer??



mick wrote:

> Eeyore wrote:
> > mick wrote:
> >
> >> +/- is from a 6.3v winding, bridge rectified & with a reservoir cap. It
> >> should be about 7.8v in theory.
> >
> > Care to run your calculations for that 7.8V past us ?
>
> Vac * SQRT2 - 2*Vfwd
>
> I used the (approximate) formula:
> 6.3 * 1.4 - 2*0.5 = 7.82V
>
> which allows 0.5v Vfwd for each rectifier diode. (Assuming a bridge rec
> into a reservoir capacitor). I know this is often assumed to be 0.6V
> each, but many rectifiers will be below this - particularly the larger
> ones. You could use schottky diodes to keep it as low as poss, but I
> doubt if you need anything more than a standard 5 or 6A bridge rec.

I use 1V typically for silicon rectifiers' Vf at rated current but as you say,
you could use Shottky diodes for this.


> You also get a little more voltage as SQRT2 is 1.414, but who am I to
> argue? ;-)
>
> You only get 0.62 *Iac as Idc so watch the secondary current rating...

Oh yes, that's very important.

Graham

Re: Wot? No tube mixer??



Iain Churches wrote:

> Can I suggest a topic for you? A current limiter for tube heaters.
> People are keen to use voltage regulation, which doesn't seem to
> me to be so important. Heaters are specified at +/- 5% so
> anything between 6V and 6V6 for a 6V3 heater is within tolerance,
> and with a wirewound pot between two 'lytics one can get it spot on.
> But heaters draw huge currents at switch on. Do you have a circuit,
> using those funny black things with shiny legs, that could current
> limit? Two circuits would be of interest. One at say 1.2A
> (a pair of B9A's) and one at 3A (a pair of EL34's).

What's the alleged advantage of limiting heater current ?


Graham

Re: Wot? No tube mixer??



"Eeyore" <rabbitsfriendsandrelations@hotmail . com > wrote in message
news:480D18D7.44F9567D@hotmail . com ...
´
> What's the alleged advantage of limiting heater current ?
>
Longevity

Iain

Re: Wot? No tube mixer??

On Apr 22, 12:17 am, "Iain Churches" <Iai...@kolumbus.fi> wrote:

> ´> What's the alleged advantage of limiting heater current ?
>
> Longevity

Iain:

Longevity in what way? I have a Dynaco ST70 (obligatory possession
IMO) that has many thousands of hours on it, is used nearly every day,
and has - as far as I can tell - the original Dynaco-branded Mullard
(6CA7 & 5AR4) and Sylvania (7199) tubes. On my Hickok 539B, they still
test at 130% of minimum-acceptable and the amp itself performs as well
as expected.

My Scott LK150 gets about the same use, it has been in my hands for
only about 5 years and about 3 thousand hours. I had to replace both
5AR4s on receipt - Chinese junque was in place - but the OEM Scott-
branded (GE) 6550s are still in place, as well as the Scott-branded
(RCA) 7199s. The 6550s test at about 160% of minimum.

Now, Dynaco is known to run tubes at about their maximum-before-
breakdown ratings, and the Scott is not far behind. So, of all the
amps out there that might be hard on tubes, these seem to be a good
start at that. Yes, I know that even a decent tube-tester is at best a
vague sort of test - but I am still puzzled at the claim that limiting
inrush current to the filaments will add significantly to longevity
all other things being equal. I would think that something that
accounts for local "excess" voltages now more and more common in the
US would be more effective? Further to this, I have seen my fair share
of bad output tubes, but relatively few of them with open filaments -
or is cathode stripping the concern?

Peter Wieck
Melrose Park, PA

Re: Wot? No tube mixer??



"Peter Wieck" <pfjw@aol . com > wrote in message
news:73de715e-6d5f-412d-865c-4c8517cdc412@u69g2000hse.googlegroups . com ...
On Apr 22, 12:17 am, "Iain Churches" <Iai...@kolumbus.fi> wrote:

> > ´> What's the alleged advantage of limiting heater current ?
>
>> Longevity

>>Longevity in what way?


Heaters. I have a few Tesla KT88s, which are quite superb. The
measure strong on the AVO MK IV, and sound very well indeed.
They are well-known for their fragility, and heater failure is the
most likely fault to occur. I would like to avoid this if possible.

Iain

Re: Wot? No tube mixer??



Iain Churches wrote:

> "Eeyore" <rabbitsfriendsandrelations@hotmail . com > wrote
>
> > What's the alleged advantage of limiting heater current ?
>
> Longevity

The simplest method would be a series resistor that gets swiched out by
a time-delayed relay. Any semiconductor based method will be complicated
if it has to cope with ac heaters for sure.

There's also the problem of having to produce a different voltage to
start with that's not compatible with existing transformer heater
secondaries.

Graham

Re: Wot? No tube mixer??



"Eeyore" <rabbitsfriendsandrelations@hotmail . com > wrote in message
news:480DCE41.51F03960@hotmail . com ...
>
>
> Iain Churches wrote:
>
>> "Eeyore" <rabbitsfriendsandrelations@hotmail . com > wrote
>>
>> > What's the alleged advantage of limiting heater current ?
>>
>> Longevity
>
> The simplest method would be a series resistor that gets swiched out by
> a time-delayed relay. Any semiconductor based method will be complicated
> if it has to cope with ac heaters for sure.

Most people use DC heaters these days.
>
> There's also the problem of having to produce a different voltage to
> start with that's not compatible with existing transformer heater
> secondaries.

If the heater current regulator is to be part of the design, one can specify
the heater winding to the appropriate voltage. This is not a problem.

If the regulation of the transformer is good, a properly specified
heater winding of 4.5V will give 6.3V DC with a 10 000µF
reservoir cap, and a lighter or heavier load will still fall within
the 5%.

The standard 6V3 winding would give 8.8V DC.

Iain

Re: Wot? No tube mixer??



Iain Churches wrote:

> The standard 6V3 winding would give 8.8V DC.

Not with rectifier voltage drops it won't. More like 6.8V.

Graham