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In Reply to: hum louder in one channel posted by hurdy_gurdyman on September 23, 2002 at 19:58:18:
HGM,If the input section looks fine & you've changed all the PS caps and pre-amp tubes, still the same problem---maybe, it's the output transformers.
One could be weak, in the knees.
Follow Ups:
Now why did you have to ruin my day thay way? At least with the speakers I have the hum is low enough not to be a serious issue. Music sounds great through this little amp.
Dave
Pull the 6AN8 driver and see if the hum is still the same. If so, it's mismatched output tubes, heater-cathode leakage in same, or (sad to say) maybe output transformer. But I'll bet it's reduced, pointing to earlier stages.Oh yeah - hum balance pot have any effect?
I pulled the driver tube and the hum got louder. I reversed left and right output tubes and the hum stayed in the right channel. Finally, in desperation, I pulled the transformer out of a Heath UA-2 I have and put it in. No difference in hum. This is getting frustrating.
Dave
Oh yeah, the hum pot makes no change whatever. The left channel hum pot seems to do what it's supposed to do.
Dave
Since the hum is louder without the 6AN8 driver tubes it sounds like there is a fault with the output circuitry. I am looking at a schematic C20 cathode bypass is drawn backwards. Be sure C20 + positive end connects to the cathodes. If it has been installed backwards, discard and install a new cap.
I see all four output tubes share a common cathode resistor and bypass cap. I suggest the circuit be modified so that each channel has its own cathode resistor and bypass cap: Use 200 ohms/5 watts and 47uF for V6, V7 and another 200 ohms/5 watts and 47uF for V9, V10. This needs to be done even if it doesn't help the hum.
Also do a visual check for wiring errors. Awaiting your response....
I'm looking at my schematic now. It shows the negitive end of caps connecting to the cathode and plate of 6AN8. Should I connect the cathode caps with the positive towards the cathode only, or do the same with them and plate connected caps? I followed the schematic when I wired the caps in. Hate to think I screwed this up because some draftsman drew it wrong.
Thanks for the help. This could be the problem.
Dave
The caps on the 6AN8 are OK.
The cap C20 that I mentioned earlier is on the cathode of the output tubes.
Thanks! I will change it in the morning and see what happens.
Dave
The cap in question in my amp was in properly already. I changed it for a new one anyway, but no noticable effect.
I found what was causing the problem. There was some questionable looking solder connections on the ground rod (this unit has a wire rod that all grounds are connected to). The rod is heavily oxidized. Some of these bad looking solder joints were oxidized under the solder. I cleaned and used flux and resoldered. Problem solved! It looks like the person who built the amp was a good solderer, but someone did a few repairs somewhere along the line who wasn't very skilled. Amazing the difference a solid connection can make!
Thanks for all the help.
Dave :~)
As I suggested earlier it would be best to add separate cathode biasing for each channel.
I'm going to order some new electrolytics for the power supply. When I install them I'll rebuild what you have described. Doesn't sound like it'd be very difficult.
Thanks again.
Dave
One side of heater winding grounded? Check AC voltage on each side with hum pot centered... on second thought, the pot would have smoked if it was turned to the end in that case. Measure AC and DC voltages and you may find it anyway.
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