another tranny question

[Andy Le Blanc]

I'm always looking for ideas to explore for amp projects.

I've found a cheap power solution that will get me, after rectification:
280 v and 540 ma.. This looks to be in the right neighborhood for a large
current supply for operating in the class A region for the power side of an amp.

The question is, is a class A guitar amp worth it?

IE; A class A push pull circuit with say, KT88, wound up to 35w plate dissipation
each would mean a total of 70w at 250v and require 280 ma.

Its the difference between zero signal and max signal current that largely
determines class, it might be fun to get very close to class A but will it make
a good guitar amp?

[austinb]

I would think more like 15 watts if SE

[Phil_S]

Andy,
Admittedly, I'm a bit out of my depth here, and you seem to be quite well versed in this sort of thing. My gut reaction, though, is that class A amps operate at very low efficiency, maybe 25%. (Is "efficiency" the correct word?) I have been under the impression that you need to de-rate the power supply by a factor of 4 because of the efficiency factor. As a result, I am thinking that audtinb's comment about it being a 15W amp is probably not at all unreasonable. Oddly, I suspect (just guessing, no calc done) that you might get more out of one single ended KT88 because you will have more current available to drive the power section.

Are you going to tell us your secret source or just keep us hanging?

Regards,
Phil

[Andy Le Blanc]

The relative power efficiencies with practical circuits put class B around 60%.
Where class A falls around 35% to 45%. AB circuits are between these extremes.
Its common to state the 78.5% theoretical for class B to debunk class A,
but the apparent power isn't really the issue. It would be nice to hear a
class A push pull amp. Its exploring in the opposite direction, I've played
with enough older players (in their 70'S and 80's) who are looking for
something completely different in the tone of their instrument for jazz and
standards. The "turn that crap down", "if you have to turn up to sound good,
forget about" it crowd. Its a different requirement for tone, the watts ain't
gonna help.

The tranny's are off the shelf Hammond. You have to use separate units for
fil. and B+, but the total cost for the power side before shipping is $64 from
antique electronic supply, shipping is free till the end of the month too.
it puts the total cost of trannies below $200.