6BQ5 PP output distortion
"Ken"
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
Cathode looks good to me, .05V signal, 15V DC for bias. Ken
** You real sure the screen and plate tappings are not confused??
When you see that x-over distortion appear - I bet the cathode voltage has
risen well above 15 volts - indicating the tubes are heavily into class B
operation.
Could also be that the secondary windings are haywire, so the 16 ohm output
is labelled as 8 ohm.
Try using a test load of double the value you are now.
........ Phil
|