Adjustable bias on Valveking

[msclford]

I'd like to mod my Peavey Valveking 112 for adjustable bias. Where is the best place for info on this type of modification? I have schematics for the amp, so if anyone has a solution or quick instructions, I'd really appreciate it.

[stokes]

I dont have a schematic for that amp,but to do the mod it is just a matter of changing one of the resistors in the supply to a cermet pot.Pick the value for the pot so the value of the resitor will fall in the middle of the pots range.For instance if you have a 50k resistor use a 100k pot,that way you have 50k in either direction.If you look at the schem for a Mesa MKIII you will see there is one resistor marked factory select,that is the one you would change in that amp.Hopefully that will give you an idea as to which one to change in your amp.

[msclford]

Here is a link to the schematic for the amp. The power section is on page ten, and the bias supply appears to be on page 8. If I'm not mistaken, you're saying that R205 should be replaced with a 50k pot? I've also seen people suggest putting a resistor half the value of the original in series with a pot. Is this a better or worse idea?
Also, (while I'm at it) can I use 5881's in this amp without modifying it in other ways?


Schematic

[stokes]

Thats the one,but I would recomend using the 100k pot.If you can get one I would also recomend a multi-turn cermet pot.It is a little easier to dial in the proper value.As for the lower value resistor in series with the pot,some people insist on using it in case the pot fails,so you still have some bias voltage.I cant say it is a bad idea,from a "safety" standpoint,but I dont think it is neccessary,especially if you use the cermet pot.A standard volume type pot doesnt like dc voltage and cant handle it well.If the pot is left in one position,as bias pots often are for long periods,the trace could burn out causing a loss of bias voltage and tube failure.A cermet pot doesnt have that problem,they take the dc voltage fine.In fact,I wouldnt recommend using anything but a cermet pot.A bias supply has no current to be concerned with,so there isnt much chance of the pot failing.I have done a lot of these bias mods in many amps over the years and never used the fixed resistor in series with the pot,and never heard of such a failure happening.If you want to use the fixed resistor it should go between the pot and ground. I wouldnt recomend using 5881's in this amp.At least not a real 5881.The amp is designed for 6L6GC's which will take about 450 volts on the plate.A true 5881 is only good for about 350 volts.

[msclford]

Too bad about the tubes ... I wanted to try out some of those Tung Sol reissues. Oh well, there are plenty of good 6l6's out there.

[morbidalex666]

I really liked this idea. Cant wait to try it myself.

[Enzo]

That is not why we put the resistor in there. it is not in case the pot fails. We put the resistor there so no one can dial the pot down to zero and kill the bias completely.

Replace R205 with a pot wired as a variable resistor. A 50k pot would be ideal here. A resistor added in series with the pot there would act as a lower limit. It protects against fumble fingers, not against part failures.

They have beeen making amps with plain old pots in them for bias for decades and they work fine. I have never had a regular trimmer pot fail in bias duty. The current through the pot is so vanishingly small. DC across a pot makes it noisy in an audio circuit, but as a bias adjust I don;t see the issue.

[stokes]

I have never seen a need for the fixed resistor in series with the pot,just going by what others have said thier reasons for using it.I dont adjust that pot unless I have a meter on it,so dialing the pot to zero isnt going to happen.If you leave dc voltage on some cheap pots it will burn the trace,even in a low current bias circuit,seen it happen in a Boogie MkII after about 5 years,that and the fact that it is easier to fit the smaller cermet pot in many crowded amps,is the reason I always use and recommend the smaller cermet pot.

[Enzo]

You miss the point stokes. YOU won't turn that control to zero, of that I am sure. Can we guarantee no one ELSE will do it once the amp is out of your hands? It is simple insurance that costs a couple pennies. Ever see a post here where someone confuses socket pins and has his meter on the wrong spot and is baffled why his bias control "isn;t working"?

[stokes]

I know what you mean,but can we make everything "idiot proof"?I figure a guy who turns his pot so far as to fry his tubes will surely find another way to foul something up.That is also why I never make the pot accessible from the outside,like a lot of people ask for,and always give the warning,dont touch the pot,ever.Like I said in my first post,I cant argue against the fixed resistor as a safety factor,just never saw a real need for it.