How to set bias on Peavey Valveking 112?

[morbidalex666]

I am really interested in performing the mod that "stokes" suggested here: http://music-electronics-forum.com/s...ead.php?t=5309

I have bought a matched pair of JJ 6l6gc's, 100k cermet pots and a multimeter.
Since I am new in the field of valve circuits, could someone provide me with a step by step guide of what measurements I have to take, from where I have to take them and in what range I can set the bias in (cold bias or hot bias)? Some safety rules wouldn't bother either. My soldering is pretty good by the way... Here is a schematic of the valveking's circuit:

http://www.blueguitar.org/new/schem/...veking_112.pdf

I've searching the web for ages and I can't find an answer. I reckon there are many experienced technicians in the forum so please help me.

[stokes]

As far as safety is concerned,you are dealing with potentially fatal voltages in a live amp,if you dont know your way around an amp chassis,stay out of the amp,it could kill you.
Assuming you are talking about the info needed to set the bias after you have made the mod,you need to check the plate voltage,that is taken at pin 3 of the power tube.You also need to know the plate current.There are a number of ways to do this.One way is to use your multi-meter set to read ma's.Connect the red lead to the CT of your output tranny and the black lead to the plate of one of the tubes.This will tell you the current across that side of the OT,so if you have a 2X6L6GC output,the reading is the current for that tube,if you have a quad of power tubes,the reading is for two tubes so you divide the reading by two to get the current thru one tube,assuming the tubes are matched.Then repeat for the other power tube(s).An easier way is to get a bias-rite type meter and you dont have to open the chassis to read the current and the plate volts.With these two numbers you get your idle watts dissipation,about 21watts at idle is safe.You get this by multiplying plate volts X plate current,for example 450v's times .045ma's equals 20 watts.You can go lower on the idle watts,but I wouldnt recommend going any higher.In any event keep an eye on the power tubes at idle and while playing and be sure the plates dont glow,if they do,you are too hot.

[Enzo]

I agree with stokes, if you don't know what you are doing here, stay out. The voltages in a tube amp can KILL YOU in an instant.

Measuring current across the output transformer as he described is a good way to make this measurement, but be very careful because if your meter slips and touches ground, you will at best burn out the meter fuse, and at worst the meter itself.

[tubeswell]

What'smore to what the others have said, if any part of the circuit, which is at a different voltage to the ground, is touched by you, there will be electrical current flowing through your body quickly trying to find its way to the ground, which could/will kill you. The higher the voltage, the more the current. So you can't afford ANY slip-ups whatsoever. (As well as that, nasty current can flow from filter caps, even long after the amp has been switched off, so you have to learn the necessary basic safety practices before working inside an amp)

[morbidalex666]

Thank you all very much for your help and your concern. I really appreciate it.
I have some questions though:
Where do I connect the red and black leads of my multi-meter in order to take the plate voltage from pin 3 of the tube?
What is the CT of my output transformer?
Is the plate of the tubes on pin 3?
Do you use any protective gear such rubber gloves, wooden benches etc?

Forgive me for being over-analytical but I really need to learn this stuff. My band and I live and play on a small island of Greece where we have zero technical support. A simple retube and consiquently rebias can cost me over 150 euros in shipping costs only, while risking potensial damage to the amp and not getting the job done according to what I prefer.

Thanx again.
Alex

[Enzo]

No offense meant, but this is what we were talking about. If you don't know what you are probing around at, then you can't be as safe as we'd like. If you cannot get tech support, why are you modifying the amp? The amps work fine as they come stock. SO then a tube change is a matter of plugging the new tubes into the sockets and playing the amp.

Unless otherwise specified, all voltage measurements are assumed to be from ground. In this amp ground is chassis for most measurements. COnnect your black lead to chassis and the red to pins 3.

CT on any transformer refers to the "center tap" of a winding. The one we are speaking of is usually red on American amps, but I cannot guarantee that, nor does that mean all red wires in the vicinity are connected to it.

Yes, pin 3 is the plate pin of a 6L6 or an EL34.


I work on an insulated surface - my bench top is wood with a carpet covering to protect the appearance of the amps I work on.

I do not wear rubber gloves, that would be difficult way to work I fear. But I also have been working on tube circuits for over 50 years, so I do know what I am touching.