Adjustable Bias Mod info wanted for Simul-Class 295 PowerAmp

[japster]

Hi,

I'm looking to wire in an adjustable bias mod for the tubes in my Simul-Class 295 Power amp.

The Amp uses 2 x EL34s and 2 x 6L6GC's in both sides of the Stereo setup.

I don't want to be restricted to using Boogie Tubes and would rather decide my own sound.

Any Help appreciated.

 

[stokes]

Cant find a schematic for that amp,but most Boogies use the same bias supply.You need to find the two resistors that are wired in parallel in the bias supply.In the Mk series they are something like a 10k in parallel with a 6.5k or so.You would change the 10k for a 25k cermet pot.These values are not absolute,I'm just using examples off the top of my head.The idea is you change the higher value resistor for a pot of 2X's the value,that way your stock bias setting is about in the middle of the pots range and you have more or less equal adjustment either way.When doing this with an amp with the EL34/6L6 combo,you have to be careful when selecting the bias you want to use.There is going to be a wide range between the tube types bias.If you bias the EL34's too hot,it is likely your 6L6's will be way over the top.Your EL34's are still going to have to be fairly cold,or your 6L6's are going to red-plate.If any of this makes no sense,it is best to have a tech do the mod for you.It is an inexpensive and easy mod,shouldnt cost more than $25 or so.At least thats all I charge if I dont do it for free when I have an amp open for some other reason.

 

[talltxguy]

I had two techs look at putting an adjustable bias mod on my old 295. Between the simul-class and the lack of schematics, they didn't want to try it.

Something he did and something you may want to consider is having him change a few resistors so that the fixed bias is not so cold.

Even though I don't a 295 anymore, I'm interested to know if this can be done & wish you luck! Please keep us posted.

 

[stokes]

"Something he did and something you may want to consider is having him change a few resistors so that the fixed bias is not so cold"
You need to find a new tech.What he did was "adjust the bias",all he needed to do was use a pot instead of whatever fixed resistor he subbed.Dont know how long ago you had those two techs look at it,but until Schematic Heaven went down,that schem was easy to find.Any competent tech can easily do this mod with no schem if he has the amp in front of him.

 

[talltxguy]

I went to pick 295 up, but the tech wasn't there; so I browsed some music stores and found the Roadster 2X12 w/ warranty for $1,500. So I decided to let the 295. And I know the Roadster can have adjustable bias put on it.

I really loved the 295 though and wished one of those techs could have made it work. Not sure if they were being lazy or were just incompetent. But yeah, I hear ya Stokes...I won't be taking the Roadster to either tech - even if it's "easy" work for them. They can find $ from somebody else.