Peavey VTM 120 Bias Voltage

[Helmholtz]

. Am I reading the suggestion correctly in that the zener would drop 60v out of the screen voltage supply?

Yep, look at his picture.
But seems 2.2k screen resistors might be sufficient.
Difference is that resistors cause more compression.

[tdlunsfo]

1k screen resistors and 32v zeners yield 40.4mA. 2k screen and 32v zeners give me 39.9mA. I think I'm going to stick with 1k resistors and buy some 42v/5w zeners. That should get me in the 60% dissipation ballpark.

[g1]

I think increasing the resistor in the screen node (R5) would also be a possible solution?

[tdlunsfo]

True. Only thing I got to test that with is a 10w/1.2k. I could give it a try in the other amp.

[Helmholtz]

Only thing I got to test that with is a 10w/1.2k. I could give it a try in the other amp.

A 10W resistor will do at idle, but probably not at full power as dissipation tends to increase with resistance.
You will need to measure voltage drop at max. output.

[tdlunsfo]

2.7k in R5 yields 44mA at 500v. I had a 130v drop at max volume so 6.6W. For those in the future with similar issues, the 100 ohm screens mounted to the output board are difficult to remove. The board is riveted to the chassis and the resistor legs are folded and soldered underneath the board. It's a lot easier to change R5 on the power supply board. Even when changing R5 it is mounted to studs, requiring a lot of heat and will lift traces if not careful. I'll probably end up with a combo of screen resistor changes and zeners.

[g1]

I'm still completely at a loss why the idle current should be so high with -56V bias voltage. Idling tubes even slightly hot is so very un-Peavey.
There was a similar thread and the conclusion was that the current measurement was inducing oscillation. When the method of measuring OT primary resistances and voltages (then V/R caculated) the current was found to be much lower. If you have the chance it's worth a try. Also removing the PI tube during the idle current checks.

[tdlunsfo]

I'll try that just to check. I've had my scope on it at times, but don't remember any oscillation. Will report back if I see anything.

[tdlunsfo]

Pulled the PI tube but no change. Confirmed with my watt meter and scope no oscillation. I also tried my other bias probe from tube depot that puts an 1 ohm on the cathode. Similar result. OT method yielded 58.2mA on left side(the side I've been monitoring) and 43.8mA on the other. Transformer dcR is 38 and 29.9 ohms respectively.

[Mars Amp Repair]

You could just tap the bias voltage from the B+ and use a divider network to bring it into a range that would allow a wider bias voltage. Since the grids do not draw much current, the divider network would not need very high wattage resistors or a zener diode w/serie resistor to accomplish this task. There are amps out there that do that. OR build a bias doubler circuit from the existing negative bias supply. Some Black-Star amps do that, too.
Glen

[catalin gramada]

As I remember this power stage works well with 6l6gc , and even with 6P3S with 100ohm into the screens - within screen power dissipation limit. I modded one once , installing a bias control circuit and a hefty bus rail with 1 ohm resistors in each cathode. Very simple to do.

[tdlunsfo]

These tubes have been bothering me. It just doesn't make sense that modern tubes bias so differently. I happen to look at the bottom of the boxes and these are labeled STR-387. I'm not familiar with this type of 6L6 but a quick internet search and some folks are saying these tubes are 35W? Anyone familiar?

[tedmich]

the Special Tube Request / STR 387 were supposedly just 6L6GCs made by Sylvania for Peavey and should bias as normal.
PV Forum has several discussions on the VTM bias stuff, on one Enzo (RIP) thought dirty sockets gave aberrantly high current readings.

[g1]

I see them elsewhere being referred to as 7581A. Do you have other 6L6's you can compare to, or another amp you can try them in and compare idle current with 6L6's?
That it were a tube specific issue rather than the amp running tubes hot would sure make a lot more sense to me.

[tdlunsfo]

I do have a Tweed Super sitting here. I'll try them in that and report back.