Trimpot/testjack on modded Peavey C 30

I bought a modded Peavey classic 30 a few years ago. I became interested in learning more about one of the mods...

It has an added chassimounted trimpot for biasing. Next to it is also a TRS jack for measurement during biasing.
What I don't really know is how to use this test jack. I guess there might be an adapter for a multimeter to buy somewhere, but cant find it anywhere. Do I have to build one myself? Can I expect the tip of the jack to be test point and the sleeve to be connected to ground?

Anyone that has seen or done this kind of mod that can help?

Stick a headphone plug in the jack. Measure DC Volts each side-terminal to chassis. (There's obviously two signal-points on a TRS. For headphone, left and right. In this case, tube 12 and tube 13, or how-ever they are numbered.)

To make sense of this, you need to see what cathode resistors are inside. Today 1 Ohm is common. But in past years 10 Ohms, 22r, and even 15.2r were used.

Thanks! I used an insert cable and measured V DC. Ring didn't show anything so it was probably a TS jack, not TRS.

Measured tip and sleeve and it was -18,2 V DC. Anyone who knows what V DC the tubes on a peavey classic 30 should be?

Haven't checked the value on cathode resistors yet. Have to figure out where and what and which they are first...

Then it sounds more like grid voltage, another way to measure "bias".

I think you need to find out more about this amp.

This page has some Classic 30 schematics. The first 2 schematics on the page show adjustable bias mods. Maybe yours, maybe not....

http://blueguitar.org/schems.htm

Thanks! That might be the mod, but haven't really looked in to it yet. That schematic says -14DCV to the bias, the amp is at -18DCV in my measurement. Don't know if mine is wrong biased or if  it's not the same mod.

Tried turning the trimpot today and the multimeter didn't react at all, still about 18 DCV.

A few years later...

I had another look at this amp. One of the power tubes is making a noise and had to change it. Was thinking of maybe changing them all instead of just one.

I had another look at the trimpot and the test point TS connector. My old DMM must have been broken, low on battery or someting. I dont have -18VDC at the test point, its about 138 -  145VDC. The trimpot is about 8.5K. What is this voltage reading? What am I measuring? And what should I aim for? I put the trimpot in center position now with about 4.2K to both sides: Voltage right now -141VDC.

Without pictures or a schematic every thing is a guess.

http://www.blueguitar.org/new/schem/peavey/c30schem.gif

Is the trim pot connected by R63, R64 bottom left of the schematic?
That would make some sense as a fixed bias adjustment point.
You voltages in post 3 and PRR's post in post  4

> My old DMM must have been broken, low on battery or someting. I dont have -18VDC at the test point, its about 138 -  145VDC.

If the amp has been working at *all*, your meter or the way you are using it is wrong.

There are an infinite number of "mods" and more ways modders can do it wrong. I agree you need to figure out what is really in there now.

I found the Classic 30 a very unreliable amp , I worked on  a few of them over the years ,including a few mods to try an improve reliabillity ,didnt really do much good in the end .

Classic 50 was a very similar amp but didnt have the tiny mains Tx of the '30' , it also had proper 6.3 volt heaters not the series arrangement found in the classic 30 . Heat is a massive problem for the classic 30 , after a couple of hours its hot enough to cook eggs on . Its also very easy to damage the link wires when you extract the circuit board , just having to flex things caused the wires to break on me ,and so your going back in again to fix up that  ,then more wires will break on you again 

I eventually started to refuse to service classic 30's in the end as its a no win situation for all involved .
Ive seen the tubes become noisey in it too , replaced with brand new fresh extra tough Russian equivalents ,same sputtering noise was back a few weeks later .