Bias Circuit Questions

[psychepool]

I am building an amplifier with 6L6 pair Fixed Bias power.
It is a Diezel VH4 based amplifier, and the power amplifier is being produced with the same specifications as other amplifiers I have previously produced.
Power transformer with the same specifications, output transformer, same output tube (6L6GC pair), etc...

The poweramp unit was completed and for testing purposes, I temporarily connected the jack to the PI input and checked the operation of the power amplifier.
However, an intense blue light burned throughout the vacuum tube, and the dummy load's fan started spinning like crazy.
I checked the bias and found that it was between 80 and 100 mV.
I wanted to check the problem, so I wanted to check the voltage at other locations, but it seemed dangerous to leave the power on any longer, so I couldn't check it.

I think the first step to take is to reduce the bias to a normal level before doing any other checks.
The bias circuit I used is like this. Basically, it borrowed the circuit used in various amplifiers such as Plexi and JCM800.
However, my PT has 50VAC tap for bias, and the voltage-down resistor (R1) is adjusted accordingly.

bias_01.jpg

To lower the bias voltage, what other measures can be taken other than reducing the value of R1? I am asking this question because R1 is already a fairly low value and there does not seem to be much room to lower it further. Can I lower the value of R2 and increase the value of R3?

Is it okay to arbitrarily adjust the value of the circuit to suit the situation in this way?

Three other previous made amplifiers of the same configuration were able to adjust the bias to the appropriate range without any problems.
What causes problems in this amplifier manufactured with the same specifications?
The bias voltage itself can be adjusted by adjusting the value of the bias circuit, but if the problem occurs only in this operation when applying the same circuit, I am asking because I wonder if the cause may lie elsewhere.
In a power amplifier circuit manufactured with the same configuration, what should be checked when only the amplifier currently being manufactured has an unusually high bias?

The overall circuit of the power amplifier I made is as follows.

VH_lite_poweramp.jpg

[sluckey]

I suggest using the high voltage (345VAC) to supply your bias circuit. Change R1 to about 220K. Like this...

http://sluckeyamps.com/phoenix/phoenix.pdf

[psychepool]

I suggest using the high voltage (345VAC) to supply your bias circuit. Change R1 to about 220K. Like this...

http://sluckeyamps.com/phoenix/phoenix.pdf

I've often made like this.
I thought that using a separate 50V tap would be more stable, so recently I have been manufacturing power transformers with a separate 50V tap.
If I insulate the 50V line well and tie it somewhere, there won't be any problem, but since there is a separate tab exist, I would like to use it if possible.
Of course, if the bias cannot be corrected to the desired degree by modifying the resistance value, I will use the method you mentioned.

[pdf64]

… I thought that using a separate 50V tap would be more stable …

What’s your rationale for thinking that, what do you mean by ‘stable’ here?

The rectifier for the 12V supply is weird, looks like a voltage doubler; why do it like that if you’re using a custom transformer?

As a few general points, I think it’s a good idea to -

• power up via a light bulb limiter until a build / repair / major work is proved good

• check voltages at every pin of every valve socket are good before fitting the valves

• protect the mains transformer HT winding with HT fusing

[Stevem]

A great rule of thumb for a amp that uses 6l6 outputs with fixed bias is that before you put any of them in there socket, confirm that pin 5 has at least -52 vdc showing up.

[sluckey]

Of course, if the bias cannot be corrected to the desired degree by modifying the resistance value, I will use the method you mentioned.

Fender was fond of using a 50VAC bias tap for his bias circuit. But he used a 470Ω between the transformer and the diode. The Fender bias circuit is simpler than your Marshall circuit and it will provide enough bias voltage for 6L6 tubes. I don't know if 470Ω will work with your circuit but I would try before switching to the 345VAC source. Just pull the 6L6s out of the amp and measure the voltage on pin 5 of the empty sockets. If you can't get -50VDC to -55VDC then you need to either use the 345VAC source or use the 50VAC source with the Fender circuit. My choice would be use the 345VAC source and the better Marshall bias circuit.

[Stevem]

A good starting point should be off a leg of that 345 a 50k resistor into the diode and then a 27k bleeder heading to ground should get you into a the ball park.

I think the 50k needs to be a 2 watt, or split that 50 up into two 1 watters.

[psychepool]

I did a little more testing.

Thinking that the 50VAC tap of the power transformer itself might be defective, I checked the AC voltage of the tap in standby mode.
The voltage was measured to be within the normal range of 52VAC.

Next, I measured the voltage of the bias voltage output unit.
When I placed the trimmer in the center, the output voltage was within the normal range of -47V.

Everything seemed normal up to this point, so I turned on the B+ power.
It was quiet and no special symptoms occurred. The vacuum tube did not burn blue, there was no oscillation from the vacuum tube, and the fan on the dummy load did not spin.

"Why did it get fixed even though I didn't do anything?" I had no choice but to say this. I then plugged it into the jack connected to the PI input for a sound check.
Then the symptoms I mentioned at the beginning began to occur. Vacuum tube oscillation and intense blue light, high bias current, dummy load fan running like crazy, etc...
I quickly pulled the jack out and it became quiet again.

I checked the voltage by unplugging the jack and turning on B+ power.
The voltage chart looks something like this.

Voltage after B+ tap rectification: 428V

V4(Phase Inverter 12ax7)
pin1 285V
pin6 289V
pin2 22V
pin7 23V
pin3/8 33V

V5(Output Tube 6L6GC)
pin3 412V
pin4 427V
pin1/8 32V
pin5 -46V

V6(Output Tube 6L6GC)
pin3 425V
pin4 426V
pin1/8 31V
pin5 -46V

Bias Supply -47V


What is special is the situation when I put a multimeter to the vacuum tube pin to check the voltage.
When I touched the V5 plate (pin3) by multimeter, the same symptoms occurred, although not as severe as when plugged into the PI input.
When I applied a multimeter to pin 1 of the V4, the same symptom occurred to a milder degree.
No symptoms occurred on the plate of V6 or pin 6 of V4.
There were no symptoms on the screen grid (pin4) of both output tubes also.


Could this be a positive feedback?
I've experienced a lot of positive feedback while making amplifiers, but I I've never experienced positive feedback without an amplified signal from the preamplifier stage.

I'm going to do some tests as soon as I get home, but if no symptoms occur when I remove the NFB line, can I assume it was due to positive feedback?
If so, I think it can be solved by swapping the OT primary.


Please let me know if there are any other problems you can infer from the symptoms I mentioned. I can't think of any other cause at the moment other than positive feedback.

[psychepool]

What’s your rationale for thinking that, what do you mean by ‘stable’ here?

The rectifier for the 12V supply is weird, looks like a voltage doubler; why do it like that if you’re using a custom transformer?

As a few general points, I think it’s a good idea to -

• power up via a light bulb limiter until a build / repair / major work is proved good

• check voltages at every pin of every valve socket are good before fitting the valves

• protect the mains transformer HT winding with HT fusing

There is no basis whatsoever. I just thought that there would be less interference by using a different tab separate from B+.
Since the cost does not change in particular, I think it would be better to use a separate tap if possible, so I always order a transformer with these specifications to construct the circuit.

[Stevem]

Why are you reporting voltages on the pin 1 and 8 of your output tubes?
For example there is no physical pin 1 on a 6L6 and pin 8 should be at ground with fixed bias unless you have a1 ohm resistor in series to ground to read current.

[psychepool]

Why are you reporting voltages on the pin 1 and 8 of your output tubes?
For example there is no physical pin 1 on a 6L6 and pin 8 should be at ground with fixed bias unless you have a1 ohm resistor in series to ground to read current.

To measure bias, a 1 ohm resistor is connected between pin 8 and chassis (ground). (Pin 1 is tied together with pin 8.)

[WhopperPlate]

SteveM highlights a concern: if pins 1 and 8 are actually measuring 30v to ground with a 1 ohm resistor there is something wrong. Unless you are using pin 1 as a grid resistor mount, or tying it to the 1 ohm at pin 8 , there shouldn’t be anything there . Millivolts only with 1 ohm

[psychepool]

SteveM highlights a concern: if pins 1 and 8 are actually measuring 30v to ground with a 1 ohm resistor there is something wrong. Unless you are using pin 1 as a grid resistor mount, or tying it to the 1 ohm at pin 8 , there shouldn’t be anything there . Millivolts only with 1 ohm

I'm really sorry. After reading your reply, I finally discovered that I wirte the voltage incorrectly. It measured 31mV, not 31V. I apologize again.

[psychepool]

Since I have never experienced a positive feedback phenomenon in a situation where there is no preamplifier amplification signal, I was ruling out the possibility that positive feedback was the cause. However, when I looked at the symptoms, I came to the conclusion that it was likely positive feedback, so first of all, the NFB line I tried removing it.
Then, all the error mentioned disappeared and I heard clean sound with normal output.
It was clear that positive feedback was the cause, so I reversed the OT Primary line and reconnected the NFB line.
It also works cleanly. The output is normal, there is no noise, the Presence/Depth knob works well, and everything is perfect.

I have encountered the positive feedback phenomenon quite often when building amplifiers, but this is the first time that this phenomenon has occurred using only a power amplifier without a separate amplification signal such as a preamplifier.
In some cases, I didn't know that positive feedback was occurring until the entire amplifier was completed. I thought this phenomenon only occurred when a fairly strong signal was input to the power amplifier, but I think I gained some experience with this experience.
When positive feedback occurred during amplifier production, I did not think to calmly check the voltage, etc. due to the crazy loud noise, so I just repeated the solution by reversing the OT wiring and finishing it. Through this case, I got to experience for the first time how voltage and other actions occur when a positive feedback phenomenon occurs.

Thanks for your reply. I think I can now finish building the preamplifier with a comfortable mind.