How to add bias adjustment to a Mesa 20/20 amp?

[crazytooguy]

Looks like the fixed bias is at -14V. Anyone do this? How did it work out? Being able to optimize the bias for EL84s seems to make sense to me, but maybe it's not worth the trouble? What do you think? Thanks!

[stokes]

It is worth the trouble,in my opinion.With an adjustable bias you can use any tubes you want,you are no longer locked into paying more for current production tubes because someone "rated" them to work safely in your amp.Doing the mod is fairly simple if you know your way around the inside of an amp.You must locate the resistor on the board that sets the negative voltage.You then replace this resistor with,I like the multi-turn cermet trim pots,a pot of about 2x's the value of the resistor you replace.For example,if the resistor is 10k,I would use a 25k cermet pot.Connect the wiper of the pot to one outside leg and use the other outside leg as the 2 connections.Set the pot in the middle,which will put you close to what you have and then you have enough range in either direction.If any of what I've said here is not clear,it is probably best to take it to a tech to do it.It shouldnt be too expensive,Ive done a lot for no charge when doing a cap job on Boogie amps.

[crazytooguy]

Thanks! It should be fairly simple. The bias is set with a 33k resistor in the power supply, so it delivers a -14V to the bias point. So I'll get a 50k cermet and slap 'er in there. I might put a 1 ohm resistor on the grounded cathode so I can easily measure the bias. The 20/20 is a simple amp, ripe for mods and experiments!

[stokes]

I'd go with 100k,you want at least 2x's,100k would be better.

[CharlieP]

I think you would be better off having a series resistor attached to the bias pot. I use a resistor and a 50k Weber bias pot to replace the 33k resistor of my VK112. What benefit of the multiple turns? There is plenty of adjustment with a standard bias pot of 50k or a 15k resistor and a 25k bias pot. Sensitivity of adjustment will be fine. 15k plus 25k is 40k overall. Well within the 33k range.

[stokes]

If you are replacing a 33k resistor,you want the 33k in the middle of your adjustable range,no?That would mean you need a 66k pot,100k is the closest standard value.The multi turn pot gives you a more stable adjustment,your bias voltage wont "jump" as much with a small turn.Using a fixed resistor in series with the pot just cuts down on your adjustable range,useless in my experience.

[CharlieP]

As I said, the sensitivity with a standard bias pot is fine and does not jump uncontrollably as you are stating and the resistor in series is a safety feature. I believe Enzo has recommended this method. Higher resistance gets lower bias not higher and a reasonable size resistor will not restrict biasing hotter. I have experienced this and can only tell you what I know to work. I have a 15k resistor and a 50k bias pot and I have all the range I will ever need and it is easy to bias. Same 33k replacement.

[stokes]

And I can only comment on what has worked for me and the many bias mods I have done.As for needing the fixed resistor in series,if you need the "safety factor",fine,use it,but if you dont turn the pot to zero,theres no need for it.I never said the bias voltage jumps "uncontrollably",but a multi turn pot makes for easier "fine tuning".