Bias Voltage Issue

[imgumby001]

Good morning!! Hope this finds everyone well. I have been revisiting this little disaster from a few years ago and am stuck about where I was with the 6l6’s going into warp drive when the gain and volume are turned up. I have investigated several different options, however, I keep coming back to the bias pot. It seems like it may be under valued as I cannot reach the -30v value it lists on the voltage chart. The best I can do is about -32v. This being the case I have two initial questions; can I change the value of the bias pot to get me where I need to be, or, if not, is there an appropriate resistor in the circuit that can be changed to get me there? As I may be totally looking in the wrong place please feel free to set me straight if my understanding is wrong.

Thanks everyone!! All the help from this forum is more than appreciated!!

NR

[mresistor]

What do you mean by "warp drive"?   If you are trying to go less negative with your bias the power tubes will conduct harder.  With no signal applied and volumes turned down what are the voltages on pins 3, 4 and 5 and 8 on the 6L6GC tubes?

[shooter]

you have 1 ohm resisters, R24
with NO input signal what is the voltage across each resister?
that will be equal to the current each tube is drawing

[JPK]

Change the tail resistor. Can't remember exactly which one. I had the same issue with my AX84 November new build. Couldn't get bias current low enough. So I changed that tail resistor per an amp tech friend's advice to get more negative voltage (I think). More negative gave lower bias current with the adjustment pot. All from 10 years ago so it's foggy. I added a 10k resistor in series with the 33k tail resistor.

[imgumby001]

What do you mean by "warp drive"?   If you are trying to go less negative with your bias the power tubes will conduct harder.  With no signal applied and volumes turned down what are the voltages on pins 3, 4 and 5 and 8 on the 6L6GC tubes?

Sorry, red plate is what I meant.

V5-

3-405.2
4-387.1
5- -46.7
8-0

V6-

3-402.4
4-388.4
5- -46.6
8- 0

you have 1 ohm resisters, R24
with NO input signal what is the voltage across each resister?
that will be equal to the current each tube is drawing

I get no voltage across, or referenced to ground when measuring r24c or r24d, which I believe are the 100’s you’re referring to. This is fully fired, no input signal, and all the knobs set to zero.

Thanks fellas!! Your time and attention is very appreciated!!

[mresistor]

what is the voltage on pin 2 and 7 of the 6L6GCs?

[shooter]

yes, the schematic says 1R 2W
they should read in MILLI-volts across the resistor, so you need a decent meter that can read milli-Volts
IF they are open, or no ground reference, that's a bad thing

[Latole]

Don't put power tube unless you fix the issue

Schematic show 25 vac from Power Transformer for Bias voltage
1- Do you read 25 vac ?
2- You you read -32 VDC  on pins 5 on each 6L6

At -32 bias voltage,  tubes may work fine.
Are tubes are good ?

You can have little more bias voltage with smaller value with R55 , 22K
Or higher valu with R51, 15K
Don't expect too much with only 25 AC volts

[sluckey]

Don't expect too much with only 25 AC volts

The bias circuit is a voltage doubler. I expect lots. He already has -46.6 on pin 5.

[imgumby001]

what is the voltage on pin 2 and 7 of the 6L6GCs?

Pin 2- 3.6 v5 & v6
Pin 7- 10.7 v5 & v6

yes, the schematic says 1R 2W
they should read in MILLI-volts across the resistor, so you need a decent meter that can read milli-Volts
IF they are open, or no ground reference, that's a bad thing

Sorry Shooter. They measured .66 across each.

[imgumby001]

If anyone else sees this I’d like to add another question- this amp is listed as 25w but is only putting out approx. 1w ish. The circuit has a pentode/triode half power switch that not only does nothing, but also adds 120hz hum in one of the positions. Is it possible to remove this from the circuit? And if so, what would be the safest way to do that?

Thanks all!!!