[Ian Iveson]

Ken wrote

Any heavy audio output gurus here? Need help bad. I've
discussed this on another NG, so here is my prob along
with answers to questions. Any help appreciated:

This may not be the ng for in depth audio questions, if
not, would appreciate a pointer. I have a Heath AA151 14
watt stereo amp, OT using screen taps, that probably
hasn't seen a good life. I can get 6 watts each channel,
then crossover distortion, BIG TIME, with or without scrn.
taps. Resistance check looks good. Subbed another 6BQ5
P-P OT, no taps, got 13 watts with a little sine wave
dist., but NO crossover dist. Does this sound like burned
OT's? Ken

Running one chan at a time, two sets of tubes used, bias
set properly, both chans. Everything looks good through
the phase splitter, wiring has been checked. I DID find a
problem there, though. One 6BQ5 grid had been shorted to
ground since the thing was built, so the sound must have
always been off. That's why I think an amateur put this
together. So I checked all the wiring, looks good. Yes,
BOTH showing the same distortion at the same power level
makes me wonder. Hate to trash those OT's though, looks so
good at six watts.

The distortion occurs just above the zero crossover point,
both pos. and neg. direction. The screen taps are correct
according to the resistance readings given on the
schematic. I did run the orig with no xfmr taps, applying
B+ to the screens, same dist. I recently worked on a Heath
AA100, 25 watts/chan., and the RMS watts was as stated,
plus some. I think when Heath says 14 watts, they're
talking RMS. I need to read up on this, any sites offer
this info? Ken


Mean power = rms voltage times rms current. Power is often
erroneously quoted as rms. Consider P = current squared
times resistance...if you square a square root you don't end
up with a square root.

A circuit diagram for your amp, including the output
transformer winding resistances, is available he

http://mcnally.cc/amps.htm

Ian

Cathode looks good to me, .05V signal, 15V DC for bias.
Ken