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RE: Phil's Automatic Bias ckt.

Posted by coffee-phil (A) on March 6, 2010 at 21:39:37

In Reply to:
Designing Automatic Bias posted by Triode_Kingdom on
March 5, 2010 at 13:14:15:

Hi
Triode Kingdom,

I sketched a simple circuit which should work. I made
the following assumptions:

1. The amp is class A with no grid
current.

2. The average plate current of the output tube is to be held
constant.

With the assumption of no grid current the plate current and
cathode current (DC) are the same. The average voltage at the filament pins is
the cathode current times the resistance in the filament circuit (36 Ohms with
the heating voltage balancing pot in the middle).

The voltage analog of
plate current is integrated in the op-amp integrator using as a reference the
divided regulated supply of the op amp. There is a phase lead resistor in
series with the integrating capacitor for stability. There will be an other
pole in the loop from the .22 microfarad coupling cap and driver tube grid
resistor so the phase lead compensation most likley will be necessary. The
output of the integrator is level shifted and amplified with the common base
NPN transistor. The diode in series with the emitter is to raise the voltage
output of the integrator to avoid reverse biasing the electrolytic integrating
cap.

Later making the assumption the final tube plate current is ~ 70
mA I see that the voltage at the input of the op-amp is going to be about 2.7
Volts. It may be better to use a non polarized cap. With the low bias current
op-amps available today the integraring cap won't have to be huge so we can
probably get by with a film cap.

As shown the op-amp is a single supply
op amp. An op-amp such as the TLO71 can be used if we provid a negative supply
for it.

I haven't calculated the values or identified the op-amp and
transistor yet since I don't know if you are open to "sand" in your SET
amplifier.

If you like this concept I can help with the rest of the
design a bit.

By the way I think lighting the filiment of the output
tube with ultra-sonic AC is a good idea. I am thinking of building a triode
amp. If I go wit directly heated I was thinking of ultra-sonic AC to light
it.

Phil

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Follow Ups Full Thread

Topic - Designing
Automatic Bias - Triode_Kingdom 13:14:15 03/05/10
(14)

RE: Phil's Automatic Bias ckt. -
coffee-phil 21:39:37 03/06/10 (4)

RE:
Phil's Automatic Bias ckt. - Triode_Kingdom
22:07:30 03/06/10 (3)

RE:
Phil's Automatic Bias ckt. - coffee-phil
23:16:16 03/06/10 (2)

RE:
Phil's Automatic Bias ckt. - Triode_Kingdom
08:16:42 03/07/10 (1)

RE:
Phil's Automatic Bias ckt. - coffee-phil
23:13:27 03/07/10 (0)

RE:
Designing Automatic Bias - Thorsten 10:41:05
03/06/10 (0)

RE:
Designing Automatic Bias - coffee-phil 00:18:52
03/06/10 (0)

RE:
Designing Automatic Bias - VT4C211 22:28:22
03/05/10 (0)

RE:
Designing Automatic Bias - coffee-phil 20:13:54
03/05/10 (0)

RE:
Designing Automatic Bias - Michael Samra 20:03:08
03/05/10 (1)

RE:
Designing Automatic Bias - Triode_Kingdom 22:56:18
03/05/10 (0)

RE:
Designing Automatic Bias - Eli Duttman 17:37:22
03/05/10 (0)

Good
luck TK... - Allen Wright 14:58:37 03/05/10 (0)

RE:
Designing Automatic Bias - grhughes 14:46:13
03/05/10 (0)

Follow Ups

RE: Phil's Automatic Bias ckt. -
coffee-phil 21:39:37 03/06/10 (4)

RE:
Phil's Automatic Bias ckt. - Triode_Kingdom
22:07:30 03/06/10 (3)

RE:
Phil's Automatic Bias ckt. - coffee-phil
23:16:16 03/06/10 (2)

RE:
Phil's Automatic Bias ckt. - Triode_Kingdom
08:16:42 03/07/10 (1)

RE:
Phil's Automatic Bias ckt. - coffee-phil
23:13:27 03/07/10 (0)

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