Class A question for studio .22.... self biasing?

[Mach1andy]

Does a class A amplifier's preamp section (5 12ax7) need to be biased? Some of the tubes on my amp aren't good so I replaced 2 of them V1 and V2. Does anyone see a concern by doing this?

Also, does anyone have suggestions for a replacement speaker for the amp? When I got it, it had a generic cheap speaker in it. I found an EV12L for $75. Is that a good match/price/sound?

Thanks!!!
Newbie[/i]

 

[LHanson]

Preamp tubes self-bias. For that matter, in general Mesas don't have bias adjustments. Check this out:

http://www.mesaboogie.com/US/Smith/ClassA-WebVersion.htm

For more info. I could be wrong, but I don't think the .22+ is class A.

 

[RR]

I could be wrong, but I don't think the .22+ is class A.

I too don't think Studio .22s are Class A.

Not sure what you mean preamp tubes if it needs to be biased. I never heard of it. But then again, I'm not a technician. You should be o-kay if you changed the V1 and V2 tubes.

Yes $75 EV12L I think is a good price. But Not sure how it would sound in Studio .22. I would prefer a Celestion like a Greenback especially through those EL84 tubes of Studio .22 But that my taste.

 

[RussB]

I ran an EVM12L in my 22+ for a decade or-so. That speaker is the shizzle in the 22+ if you love the overdriven tones...the cleans ain't so bad, either!

 

[Mach1andy]

My amp is a .22 not a .22+. It says 'class A power' on the back of the chasis...

 

[LHanson]

My amp is a .22 not a .22+. It says 'class A power' on the back of the chasis...

OK, that pretty much settles that. I'm guessing that any amp that Mesa sells that they call "class A" is going to be cathode biased, which means no bias adjustment. Of course, I'm not aware of any Mesa amp that can be adjusted without a soldering iron, anyway. That link above is a lot of fun, if you like that kind of thing. I sure learned a lot.

As well, preamp tubes are cathode biased, and aren't adjustable. They are also single-ended, so they don't have to be balanced or matched. The exception is the inverter tube, which can be found with the two triodes inside the tube are matched.

Mostly, find a few different preamp tubes, and experiment! You really can't mess up an amp by switching tubes around, assuming they are the same type, 12__7. Personally, I like a 12ay7 in the V1 of my .50+

 

[Bren10]

Okay,
Perhaps I can clear this up. I have a .22+ that I've rebuilt extensively. Also, I have quite a bit of experience working on electronics. Especially tube guitar amps. I'm not tooting my horn, I just want to give you some definitive answers that you can trust.

LHanson is right about this amp not having a Class A power section. Mine certainly does not. If yours is really old and has a vastly different design than I will stand corrected. But, I'm sticking to this theory for now.

Your preamp is, in a way, Class A. However, that is generally assumed since all preamps are this way(Generally Speaking). What I mean by "In a way" is that by definition, the single ended nature of your preamp circiut makes it Class A.

The term "Self Biasing" is a wierd one. I'm not sure what this means but I've never seen such a thing in any of the amps I've ever worked on from Fender, Marshall, Mesa, Rivera and so on. Here is the real answer to your question:
No (LHanson is right about this). Just put in new tubes. Evrything will be fine. FYI, Your Preamp tubes are "Cathode Biased". Because of this, you can't adjust the bias without changing some resistors.

Also FYI: Your power section, which is Class A/B, is what is called "Fixed Biased". This is done by aplying negative voltage to the Grid and is usually adjustable. Mesa doesn't let you adjust it because they want you to use tubes that are within a certain spec. These tubes are available from Mesa and Eurotubes to name a few. For best results you need to use these tubes. They are NOT "Self Biasing", whatever that is.

As far as the speaker goes, I vote for a Vintage 30 from Celestion.

The .22+ is a bad *** little amp. Congrats!

 

[Bren10]

LHanson,
I guess we were writing at the same time. Didn't mean to re-iterate.

I checked the Schematic. It's not Cathode Biased. I don't know why that would say "Class A" on the back. Oh well.

 

[RussB]

The 22 and 22+ are both fixed bias amps. Both run the crap out of the power tubes (mine ran JJ's at about 120% dissipation!)

But who cares? These are great sounding amps!

FWIW, I do not like a V30 in the 22+. The EVM12L is fabulous. So is the Mesa/Celestion C90. I have also used a Weber alnico "Blue Dog"...very, VERY nice. The stock "Mesa Vintage" 50 watter is good match for the 22 as well.

I have the Blue dog in the combo, and the EV in a Theile cab. The 22 drives the hell out of 'em!.

I'd like to hear a Celestion G12H30

 

[LHanson]

LHanson,
I guess we were writing at the same time. Didn't mean to re-iterate.

I checked the Schematic. It's not Cathode Biased. I don't know why that would say "Class A" on the back. Oh well.

Ya know, the old .50 cal with the EL84 quads said the same thing about "class A". How Mesa gets 50 watts out of 4 EL 84's running in class A is beyond me, and I'm sure Vox would like to know as well, since the AC30 is 30 watts.

But this is all splitting hairs. The .22 screams, from what I hear. There was a little problem with overheating the circuit board for some folks, so aim a fan at the back.

 

[Mach1andy]

Thanks for the info all!

 

[Mach1andy]

Right now my .22 studio is at the hollywood boogie shop getting looked at. I really like the amp but I've noticed a few weird things (when i go from the dirty channel to the clean channel there is like a continuance of distortion for about a half second before it gets clean). I don't really mind it that much but it could be annoying.

Also, I wanted them to put in a lead master. Call me a traitor of a 22+ but i really like this .22 and I just think it'd make a lot of sense to be able to control both volumes for $50 instead of getting a new amp and selling this one.

As far as the "class A' on the back, I will have to look when it comes back (monday/tuesday). I'm pretty darn sure it says it on the back but it seems like that may not be the case...

Are there any things that need to be addressed while its on the bench @ Boogie? I saw a post saying I should replace some capacitors and a few other things... The amp is probably 21 years old...