Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Toalpo on February 17, 2026, 10:23:30 am
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Hi,
I have some issue with a Fender HOT ROD humming in the clean channel. An FHR humming? no way.......... :laugh:
Firstly I though it would be those damn power supply IC caps but realized that the hum was actually wall-plug frenquency (50Hz here in EU). Rapidly thought about preamp heaters issues but nothing changed replacing tubes.
The hum is present with volume, drive and master set to min and is more notorious when Treble is at max. Since the first triode (V1A) is before the volume and drive pots, I thought the hum was getting in the signal path somewhere in the second triode (V1B). With that that tube unplugged (V1) the hum is dramatically reduced but still present. Also noticed, that when channel select is pressed to go into drive the hum dissapears but is back with more drive active. With the 2 preamp tubes out, there is no hum but when power amp in is connected the hum appears again. That took away my theory about V1B circuitry as culprit.
I was thinking about the channel trigering circuit but check TP32 and TP36 and are ok, and the trigering works fine. Yesterday the amp did some cracling and the hum dissapear and today after some cracling it came back. I am giving it a though about some issue with the Op-amps but any hints are more than welcome. If you have experienced something alike let me know, really appreciate and will let you know how this is resolved.
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Edited...not sure how to delete...
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95% of the problems with those amps comes from the low voltage PS (incl the 5W wire-wound cement resistors). This power supply supplies all the SS circuitry and the relays. When the PS goes bung, noise can get into the signal through the opamps etc
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Got it and it was not as exotic as I was expecting. I thought the 50Hz buzz was sniking in somewhere in the channel selector circuit. So in order to discard that possibility, I did disconnect the 39V_ac with no change at all. Giving it a though, op-amps were doing their job and TPs look good. I replaced preamp tubes and the PI one, but I did no think about the easy way out.....the power tubes. Sometimes, we make things so complicated, after replacing them the buzz is gone, feel so embarrased. At the end it was an easy one, thank you for all suggestions!