Bias Resistor

[skynyrd]

Anyone happen to know what resistor controls the bias on a Peavey Mace VT?

[pecorporation]

mace vt.pdf

[skynyrd]

Thanks for the shem Percorp, I am looking at getting a bias trim pot and the company that makes it doesn`t supply the resistor location to remove the resistor and install the trimpot. Looks like on the mace there are possibly 6 resistors that control the bias? Or am I reading it wrong?

[Enzo]

1R9 needs to be made variable. It is currently 65k, a odd value at that. I would replace it with a 50k pot and a 15k or 22k resistor in series with the pot.

[skynyrd]

Many thanks Enzo ! ! I could get a good quality bias pot and the resistor for alot cheaper than the premade rig I saw, will save me a bunch of money ! ! I`m guessing if I go with the 22K resistor that will give me a little finer range on the bias adjustment? This is going to help tremendously. When I get my mace done I know my buddy with his Heritage will be bugging me to death to do it to his as well,,lol. But thank you again Enzo you da man ! !

[Enzo]

The resistor prevents anyone from turning the4 bias all the way off. If an original resistor was 65k, then a 50k resistor and 15k pot will indeed gove you finer control when adjusting that simply using a 65k pot would.

And your buddy will be disappointed when he finds out you can;t make a Heritage adjustable. Not like that anyway. The Heritage uses cathode signal injection, the grids sit at a steady +15v, and the whole thing runs close to class B.

[skynyrd]

Thanks again Enzo, yep he will be bummed when he hears that,,lol. Tdoing the simple resister and pot mod I am ok with, I used to work on CB radios years ago so the simple stuff as long as I have a point in the right direction I am ok with it. I have been thinking about getting some of the amp repair books out there, and learning how to work on my own equipment. We have 2 techs in town and both stay 6 weeks behind with business, so have decided to venture into the DIY section and take care of my own.

[skynyrd]

I may hold off on the rebias mod, been thinking about it alot and while I am sure it will imrpove the tone of the amp it will also eliminate the plug and play aspect of the tubes. I may start trying different tubes with different break ups and see what happens. Take the simple road before I go and start messing with the voltages in the amp, have heard that replacing the screen grid resistors is a good place to start on upgrading that amp. May look into that as well.

[Prairie Dawg]

I may hold off on the rebias mod, been thinking about it alot and while I am sure it will imrpove the tone of the amp it will also eliminate the plug and play aspect of the tubes. I may start trying different tubes with different break ups and see what happens. Take the simple road before I go and start messing with the voltages in the amp, have heard that replacing the screen grid resistors is a good place to start on upgrading that amp. May look into that as well.

You can save the money and buy some liniment because you'll need it with your back with that monster.

[skynyrd]

Would it be do-able to replace the 65K bias resistor with a 60K resistor to open the bias up a tad yet still be able to keep it in the plug and play range safely?

[Enzo]

That would lower the bias voltage by about 1v or a little less.

[skynyrd]

Thanks Enzo, I would want to raise the bias voltage instead of lower it correct?

[Enzo]

Raising the bias voltage is a cooler setting, a lower current through the tubes. ALmost no one wants their amp modified for cooler bias.

My comment was that the resistor change you suggested would only result in a 1v or less change. Up or down was not my point at the moment. Form such a small amount of voltage change, I doubt you would hear any difference.

In my opinion, if you are going to the trouble to modify it at all, you really ought to make it adjustable through a useful range.

[skynyrd]

Thanks for clarifying that Enzo ! ! I would like to eventually go to a variable bias control, but with funds not where I want them to be and having to end up getting the bias probes and such it had crossed my mind to make minor changes to whats there so I don`t have to go all out. Even though understanding all out is whats going to produce the best end result I`m trying to operate within my budget so to speak. Maybe waiting and getting the equipment a piece at a time until I can do it right all at once will be best.