Howdy, N00b question here..

[csparks75]

I hate the way my Mesa Studio Caliber sounds with it's EL-84s...if I were to replace the fixed Bias with an adjustable one, would it be possible to throw some 6V6's in there? I know they share the same socket...

Thanks in advance!

[cbarrow7625]

Uh, actually they don't use the same socket. EL84 is a 9 pin & the 6V6 is an 8 pin large octal.

Did you mean EL34?

For that matter does anybody know a better, smoother sounding 9-pint power tube that can be used as a replacement for an EL84? I'm up to re-wiring if the pinout is different.

[csparks75]

Wow, I am a N00b! LOL

I can't stand my tone when using ELxx tubes...not sure why.

[stokes]

Mesa's "fixed" bias set-up is usually very cold,I have done the adjustable bias mod to quite a few Boogies and find that being able to bias and find the "sweet spot" for the power tubes is usually enough to make a big improvement.I would suggest doing the adjustable mod first and seeing how you like it then.It is possible that you just dont care for the EL84's,but you cant sub a 6V6 without wiring new sockets.
cbarrow,I dont know of another 9 pin tube,but I have found that a good NOS 7189A,which is a direct replacement for the EL84,is much smoother or warmer if you will,than the EL84's,especially the current production '84's.Worth a try.

[Regis]

I think it needs to be said that "fixed bias" and "adjustable bias" are the same thing. You adjust the bias to a certain point and then it remains fixed. AFAIK, boogies don't have a pot to adjust the bias, just a resistor setting it cold, like Stokes said.

Randy Aiken has a really good page on biasing here, and maybe someone else like Stokes or Enzo could explain in more detail about biasing and the differences between fixed and cathode bias.

[stokes]

"if I were to replace the fixed Bias with an adjustable one," This staement led me to believe he was about to do the adjustable bias mod,and might have some knowledge of biasing,although the fact that he thinks the EL84 and 6V6 are interchangeable says otherwise,csparks if you have more questions about this,just post them,I am sure you will get the help you need here.

[csparks75]

"if I were to replace the fixed Bias with an adjustable one," This staement led me to believe he was about to do the adjustable bias mod,and might have some knowledge of biasing,although the fact that he thinks the EL84 and 6V6 are interchangeable says otherwise,csparks if you have more questions about this,just post them,I am sure you will get the help you need here.

You are exactly right! I am thinking of the adjustable bias mod for all of my boogies (3). And I am new to the world of EE stuff and tubes...I have an excellent mentor, but am not an EE by trade. I've already found tons of good information here, and hope to do so in the future!

Thanks for the hand!

[stokes]

The adjustable bias mod is one of the best things you can do for a Mesa.Lets you take advantage of all the better sounding power tubes out there instead of being locked into using the crap they sell.Even if you still use the tubes they sell as being "graded" for their amps it is beneficial to be able to set the bias.I have seen different pairs of their tubes that are supposedly in the same bias range differ by as much as 10ma's.They are able to use such a wide margin because they bias the power tubes so cold they will still be "safe" even if the new set you use is 10ma's hotter than your last pair,and 10ma's can make a big difference in the sound.

[csparks75]

I can't seem to find a detailed document for the mod, can you help me with that? I think I know where it is on the schematic, I just want to be sure.

[stokes]

Looking at the schem I have,it is the 2xEL84 power section,I would want to build a seperate bias supply.The way that amp has the bias supply configured part of the negative voltage supply is used to feed the cathode of the PI.If you make that supply adjustable it appears you will be adjusting the neg volts going to the PI as well.Before I go any further,do you have a schematic?Do you have enough experience to take on a project of this size?I am not asking these questions because I doubt your capabilities,but this is going to be a long explanation,and I am not an engineer so I will have to do a lot of guesstimating on values here.I havent done this mod to this particular amp,so the approach I am going to take,will work,but is going to require disconnecting this supply from the power tubes but leaving it active for the PI supply,maybe there is someone here who has some experience with this particular amp and knows a simpler way to do it.Also you are going to have to add a couple of components,and if you dont have enough experience working on amps,I dont want you to get in over your head.