J5 cathode. R30 cathode resistor sets current .38V/10.
J4 grid including 100K.
J3 mute - 0V drive grids to negative (warmup) ~-12V active.
J2 vneg -75V for 6550A
J1 HT supply
VBIAS about 0.38V
C3 is ceramic.
C2 is poly something MK-2 or MK-4 100V
J4 grid including 100K.
J3 mute - 0V drive grids to negative (warmup) ~-12V active.
J2 vneg -75V for 6550A
J1 HT supply
VBIAS about 0.38V
C3 is ceramic.
C2 is poly something MK-2 or MK-4 100V
So C3 removes the audio and goes into a LPT to compare with the reference. C2 is an integrating capacitor, and the output is buffered to generate the negative voltage to drive the grids. D2 trys to handle the case where the amp is operating in class B. It stops the grid voltage increasing by limiting the positive cathode current. The ratio of R1 to R2 control the minimum negative voltage the circuit will output.
I think I have a circuit of the bought in amp boards, but I cannot really publish it here. They have an integrating op-amp and then a level shifter. The only thing I did find with these it that they have poor -ve supply rejection so need a good filtered supply. Otherwise they work very well and do have an LED indication. I see they have the ST-70 upgrade kit too.
I think I have a circuit of the bought in amp boards, but I cannot really publish it here. They have an integrating op-amp and then a level shifter. The only thing I did find with these it that they have poor -ve supply rejection so need a good filtered supply. Otherwise they work very well and do have an LED indication. I see they have the ST-70 upgrade kit too.
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Looks nice, I even found the thread where it comes from, but can't find original poster.
Anyway, got a few questions: what does the "0,6V clamp" means and what is this "hfa"? And this 22 seconds?
Anyway, got a few questions: what does the "0,6V clamp" means and what is this "hfa"? And this 22 seconds?
Not him, but I believe the 0.6v clamp limits the action of the bias correction to +/- 0.6 mA as sensed in the 10 ohm cathode resistor.
22 seconds is the RC time constant of 470k and 47uF, filtering the bias control signal to well below the audio band.
I think the last thing (hard to read) must be the hfe spec of the transistors.
I don't see anything in this circuit that would prevent a sudden runaway bias condition, am I missing something?
22 seconds is the RC time constant of 470k and 47uF, filtering the bias control signal to well below the audio band.
I think the last thing (hard to read) must be the hfe spec of the transistors.
I don't see anything in this circuit that would prevent a sudden runaway bias condition, am I missing something?
No, there is no provision to counter a tube that's gone bad.
An alternative track solution is to keep the manual adjustment while monitoring the cathode current in order
to shut the amp down if current exceeds a certain value. ( and hopefully keep a memory of which tube that
caused the problem).
Good tubes in a good circuit will almost never experience "sudden runaway" aka redplating and will seldom
need bias adjustment once done.
So these circuits work by keeping the cathode bias current constant. So if the tube losses emission the grid will be adjusted to bring the cathode in line. If the tube goes more conductive then the bias is reduced. Most of these circuits limit the voltage range over what the grid can be driven with. They have a startup circuit to drive the grids full negative for 30s or so for the tubes to warm up.
If you want to check the tube is good a rotatory switch and a meter from each servo output can be used to check all is OK. The Led will glow if the bias is higher than -50V but that's all.
They also allow a higher grid series resistor than fixed bias mode as any tube leakage will be spotted and the bias reduced. This prevents thermal runaway. It means for example a 6550A with a 100K grid resistor is OK whereas the datasheet recommends 47K max which make driving them more difficult.
If the tube shorts then the 10R will blow and prevent further damage. As with all bias circuits if you see red plating something is wrong. I had a 6550A where the case had shorted to chassis. This caused the 10R to be about 2R and the measurement was then wrong causing red plating.
On the whole these circuits work well and overcome the short comings of fixed bias.
If you want to check the tube is good a rotatory switch and a meter from each servo output can be used to check all is OK. The Led will glow if the bias is higher than -50V but that's all.
They also allow a higher grid series resistor than fixed bias mode as any tube leakage will be spotted and the bias reduced. This prevents thermal runaway. It means for example a 6550A with a 100K grid resistor is OK whereas the datasheet recommends 47K max which make driving them more difficult.
If the tube shorts then the 10R will blow and prevent further damage. As with all bias circuits if you see red plating something is wrong. I had a 6550A where the case had shorted to chassis. This caused the 10R to be about 2R and the measurement was then wrong causing red plating.
On the whole these circuits work well and overcome the short comings of fixed bias.
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Yeah, I suspect most of the redplated power tubes I've had were due to mis-adjusted bias or an imbalance causing one tube to draw too much current. The onset of the over-current would probably be slow enough that the tubes would be protected in these cases.
bauduin0: Thanks for the explanation. Another advantage would be consistency in bias with line voltage variation, I see nearly +/- 5% bias current variation with varying line voltage.
The AB-Q is sensitive to the negative rail - but will correct after seconds. It also will couple the negative hum into the grids so needs the same supply filtering arrangement as a fixed bias solution. It would be in my view better to take the integrating cap from the output. Having said that I've used a couple and they do exactly what they should. It's also easy to upgrade an existing amp.
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A large amount of redplating is caused by ignoring the minimum rg resistor values. This might not cause
an immediate redplate but after a time and when some cathode material has accumulated on g1 it may
result in a gridcurrent/overcurrent event. Some tubes are more resistant, random effects plays a big deal.