In article , 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
Cathode looks good to me, .05V signal, 15V DC for bias. Ken
Hi Ken,
It appears that you have checked the output tubes and ruled them out,
from other posts it sounds like you have also replaced the coupling
capacitors or at least checked them for leakage. While you haven't
checked for shorted turns in the output transformers, the fact that both
channels produce the identical 6 Watts, would seem to make it an
unlikely coincidence that both output transformers are shorted in an
identical way.
So what possibilities are we left with? One thing that can cause the
symptom you describe is a too high cathode resistor, have you measured
the value of the cathode resistor? Also you should measure the DC
voltage at the cathode without any signal, which I assume is how you got
the 15 volt reading, you should also measure it at full output power?
Do you know what the turns ratio of the replacement output transformer
that you used is? 6BQ6s are used at a wide range of operating points
and load impedances. IIRC the AA-151 output transformer has a primary
impedance on the low side, if your substitute output transformer had a
high primary impedance that could explain the results you are getting,
especially if the value of the cathode resistor has gone high. A quick
check for this effect would be to measure the power output with the
original output transformers into a 13 Ohm load connected to the 8 Ohm
output tap.
It is normal for some cathode biased class AB amplifiers to develop
crossover distortion of the type you describe when they are tested with
continuous tones. IIRC the AA-100 uses fixed bias rather than cathode
bias like the AA-151, and so isn't subject to this problem.
The AA-151 is rated at an output of 14 Watts per channel using the
³Heath Hi-Fi Rating² method. Does anyone know what the ³Heath Hi-Fi
Rating² method is? I have a vague memory that it may be similar to the
³IHFM Music Power² rating but with a lower distortion level?
Regards,
John Byrns
--
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http://fmamradios.com/