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Class A to AB ?

8/7/2003 7:07 PM
johan Class A to AB ?
Hello all  
just a little question..  
I know about the problems assosiated with biasing a class AB amp into class A, but what about going the other way?  
I have a little Gibson-GA40-knockoff and I am thinking of increaseing the cathodersistor a little to make it pass over into Class-AB territory( or at least sniffing the border..) for no other reason than convenience when replacing the tubes on the road and far away from my oscilloscope  
is there anything I should be aware of, other than crossoverdistortion and possibly a few more watts on that old speaker and output tranformer?  
thankfull for any answer..  
best regards  
8/8/2003 1:40 PM
johan, I'd be surprised if increasing the cathode resistance a small amount would have any significant increase on the power output. I'm not familiar with the particulars of this amp... what's the plate voltage, cathode voltage, and cathode resistance at (presently)? It's possible that you're already in within the city limits of class AB...  
what are the output tubes?  
Good luck!
8/9/2003 8:50 AM
no i am not to woried about any powerincrease either...  
..the plate voltage is 330volt, cathodevoltage 19volts and cathode resistor is 200ohms bypassed by a 50uF cap  
what i am woried about is if there is something I've there for example anything fundamentaly diffrent between O.T's designed for the different classes?...airgaps, offsets and there any risk of ruining the O.T if i rebias the amp into AB?  
I am siting on a lifetime supply of unmatched 6v6's of all makes and origins, and it would simply be easier if I didnt need a scope, tonegenerator and DVM when I change tubes....I know..I'm lazy...but I rather play my guitar than worry about the amp...  
8/9/2003 8:08 PM
Dave H.

The 6V6s are running at 14W each so itís probably a good idea to increase the cathode resistor. I wouldnít worry about the class. Itís most likely running in A/B now. If you were to split the cathodes and fit a separate 470ohm cathode resistor and 22u cap for each 6V6 you could sort your stock of 6V6s into matched pairs by comparing the cathode voltages.  
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