Electronics > Beginners
Tracking back 100Hz hum in a tube guitar amp
Andreax1985:
Hi all,
I have a small guitar amp which, even if nothing is plugged in and all preamp tubes are pulled out, has a low level 100Hz hum which bothers me. The hum level does not change if I increase master volume. Moreover, if I pull P24 signal header from the power amp section (see the attached schematics) the hum disappears. This puzzles me because the fact that the hum is there even if no preamp tube is rolled in makes me think it's coming from the power supply and not from the signal path in the preamp section: so, why if I pull the signal cable on the power amp the hum disappears? :-//
So I decided to track the hum back, with the help of a scope. I'd like to use this thread to show and discuss my progresses into this search. You find attached to this post the schematics of the amp as a pdf file.
This is the hum probed at speaker terminals (with all preamp tubes pulled out):
This is the ripple in B+1, (+/-5V at 100Hz):
And this is the ripple in B+2, (+/-100mV at 100Hz):
I can observe that in B+2 I have a bigger peak followed by a shallower one, as if a 50Hz oscillation is superimposed to the main 100Hz ripple. Don't know why.
What do you think?
soldar:
--- Quote from: Andreax1985 on February 01, 2019, 10:33:36 am --- I can observe that in B+2 I have a bigger peak followed by a shallower one, as if a 50Hz oscillation is superimposed to the main 100Hz ripple. Don't know why.
--- End quote ---
That is nothing to be concerned about. When using a bridge rectifier it is normal that there can be slight differences in positive and negative peak rectification. If the same circuit has devices with single diode rectifiers then those peaks will be slightly shorter. Also could be that some diodes of the bridge have slightly higher resistance. Etc. Many causes but nothing to worry about.
capt bullshot:
The sheer amount of different GND symbols in the schematic is confusing.
Looks like the EL84 tubes lose their GND (GNDb) when you pull P24. So no more power available to them, resulting in a silent output.
Andy Watson:
Pulling the pre-amp valves still leaves the B+3 power supply coupled through to the output section (via R79,17, C78,22). The output stage is differential and should ignore this common-mode signal, so: either the signal is too large for the output stage to handle or the output stage is not balanced. The fact that the problem is still present with the master volume turned down suggest that it is in-balance in the output stage that is causing the problem. Perhaps the valves are aging? Perhaps it was never that well balacned in the first place and you've only just noticed the hum. Perhaps the main power supply capacitors are drying out - giving rise to more common-mode ripple signal.
Andreax1985:
--- Quote from: Andy Watson on February 01, 2019, 12:50:02 pm ---Pulling the pre-amp valves still leaves the B+3 power supply coupled through to the output section (via R79,17, C78,22). The output stage is differential and should ignore this common-mode signal, so: either the signal is too large for the output stage to handle or the output stage is not balanced. The fact that the problem is still present with the master volume turned down suggest that it is in-balance in the output stage that is causing the problem. Perhaps the valves are aging? Perhaps it was never that well balacned in the first place and you've only just noticed the hum. Perhaps the main power supply capacitors are drying out - giving rise to more common-mode ripple signal.
--- End quote ---
The amp is new and I changed both filter caps and diodes in the rectifier bridge with higher quality components (Nichicon, Vishay...). I also bought different sets of EL84 power tubes (balanced pairs, or at least so the sellers said). I tried every possible combination of the tubes I had and, if they were mismatched, the hum increased otherwise the hum settled at a lower level but was definitely still there (as you can see from the waveform in the first pic).
I'd also like to add that when I switch on the amp, this is what happens:
i) 0-15 seconds: no hum.
ii) 15-20 seconds: hum quickly builds up to quite high levels.
iii) 20-35 seconds: the hum gradually subsides to zero level (not audible anymore). I wish it stayed like that forever.
iv) 35-45 seconds: the hum slightly builds up again and stabilizes at a low but well audible level (~7mV pk-pk at speaker terminals, see first scope screen-shot in the OP).
I'd like to prevent iv)
P.S. It has been humming since when I brought it home, I've always noticed it. Other amps in the shop, same model, hummed too. But what I want to do is to understand where the hum is coming from and fix it.
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