Fender amps - class A or B? - Jemsite
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post #1 of 13 (permalink) Old 05-20-2003, 03:49 PM Thread Starter
jpi
 
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Fender amps - class A or B?

I was wondering are Fender Reverb (Twin, Deluxe) and Vibrolux amps class A or B?

Thanks.
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post #2 of 13 (permalink) Old 05-20-2003, 04:04 PM
 
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They're Class AB
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post #3 of 13 (permalink) Old 05-21-2003, 02:39 AM
 
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...and I hear many Fenders can be modified to be pure class A.
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post #4 of 13 (permalink) Old 05-21-2003, 07:29 AM Thread Starter
jpi
 
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Quote:
Originally Posted by Mr Orange
...and I hear many Fenders can be modified to be pure class A.
And what is need to modify them to pure A? I'm especially intrested in Vibroluxes.
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post #5 of 13 (permalink) Old 05-21-2003, 11:04 AM
 
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A THD Yellow Jacket is the best option.

read up at THD's website. You can plug these things into any 6L6, EL34, 6V6, 5881 amp and it will convert it into a pure class a machine running off EL84's.

The best thing is to do the mod you simply take your old tubes out, and plug these in. Same to undo it and go back to your origional tubes. No rebiasing or anything.

You can do the mesa SimulClass thing and only replace 2 of the 4 tubes with the YJs or you can go all 4. If you have a 4 tube amp you can pull 2 tubes and have only 2 YJ's for about 15-20 watts.

I love mine.
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post #6 of 13 (permalink) Old 05-25-2003, 03:49 AM
 
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If I were you I wouldn't be so sure about Yellow Jackets. Even though THD claims them to run your tubes in Class A you can never guarantee that without knowing the exact circumstances. What those yellow jackets REALLY do is they convert your fixed bias amp into a cathode biased (aka self biased) amp which some people mistakenly call "class A" mode of operation. That is not true. Tubes (or transistors) are said to be running Class A when their quiescent operating point is located right in the middle of their transfer curve. This is the most linear and the most inefficient mode of operation for a tube. Idling tubes dissipate the same power as the tubes running at full blast (whereas in class AB amps idling tubes typically dissipate 70% of the power dissipated when cranked). But enough theory.

The point is, to guarantee that tubes are running class A you've got to know a whole bunch of parameters, first and foremost being plate voltage. This is something that they don't know when they make yellow jackets.

Any amp can be converted to true class A with the same tubes installed by bringing down the plate voltage, re-biasing the tubes and (sometimes) replacing the output transformer with a unit rated for a higher DC (bias) current. So it's not as trivial as just putting those jackets in, although they will definitely bring you closer to Class A.
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post #7 of 13 (permalink) Old 05-25-2003, 04:13 AM Thread Starter
jpi
 
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Quote:
Originally Posted by microdmitry
If I were you I wouldn't be so sure about Yellow Jackets. Even though THD claims them to run your tubes in Class A you can never guarantee that without knowing the exact circumstances. What those yellow jackets REALLY do is they convert your fixed bias amp into a cathode biased (aka self biased) amp which some people mistakenly call "class A" mode of operation. That is not true. Tubes (or transistors) are said to be running Class A when their quiescent operating point is located right in the middle of their transfer curve. This is the most linear and the most inefficient mode of operation for a tube. Idling tubes dissipate the same power as the tubes running at full blast (whereas in class AB amps idling tubes typically dissipate 70% of the power dissipated when cranked). But enough theory.
Don't confuse me with that tech talk, I don't (yet) understand any piece of it.

Anyhow those YJs didn't fell comfy when I read their website. I guess I'll go with true class A amps like Laney VC30 or Voc AC30.

Thanks.
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post #8 of 13 (permalink) Old 05-25-2003, 04:52 AM
 
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Here comes the surprise. Those amps AREN'T class A. And neither is Mesa Boogie "Class A" mode in their power amps. In fact I don't know any push-pull (non single-ended) mass produced guitar amps which have tubes running in true class A.

Now you have ask yourself, do you really NEED class A?
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post #9 of 13 (permalink) Old 05-25-2003, 06:15 AM Thread Starter
jpi
 
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Quote:
Originally Posted by microdmitry
Here comes the surprise. Those amps AREN'T class A. And neither is Mesa Boogie "Class A" mode in their power amps. In fact I don't know any push-pull (non single-ended) mass produced guitar amps which have tubes running in true class A.
Ummm... Why the hell do they then say that they're class A?
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post #10 of 13 (permalink) Old 05-25-2003, 09:33 AM
 
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Well, what is true?

I reckon EL84-based amps run Class A, but what do I know.

Vibrolux is stated to be one of the easier ones to mod to class A. Consult your local competent amp tech. Beware, there are many around who have ****Ú for brains or don't know the difference between a tube amp or a microwave oven.
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post #11 of 13 (permalink) Old 05-25-2003, 05:29 PM
 
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Quote:
Originally Posted by jpi
Quote:
Originally Posted by microdmitry
Here comes the surprise. Those amps AREN'T class A. And neither is Mesa Boogie "Class A" mode in their power amps. In fact I don't know any push-pull (non single-ended) mass produced guitar amps which have tubes running in true class A.
Ummm... Why the hell do they then say that they're class A?
Because it helps selling them. If you want a little more technical details, read up here: http://www.aikenamps.com/TI_Aiken_Q&A.html and here http://www.aikenamps.com/VoxAC30classA_2.html

VOX AC30 isn't class A as you can see.
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post #12 of 13 (permalink) Old 05-26-2003, 02:16 PM
 
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post #13 of 13 (permalink) Old 05-30-2003, 06:58 PM
 
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A with some
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