Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: punkykatt on July 19, 2012, 08:22:39 pm
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Hello all, got what looks to be a 5F6A/Plexi type head on the bench. The owner said he had someone build this for him awhile back and it always hummed when the Bass control is turned up. When he confronted the builder the builder kept giving him the runaround so he just put it in the closet.
Have any of you guys fixed a hum when the Bass pot is turned up? So far I have checked the tubes, subbed in another pot and reheated all the solder joints, checked the heaters and grounds. Any input is much appreciated. Punky
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Have you tried jumper wires to take the pot out of the circuit? And/or jumper from one pot terminal to the next? That might help you find "the spot". :dontknow:
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Poke your chopstix around the leads of pins 2 and 7 of V1 and V2.
consider shielded wiring from inputs, land grid stops on V1 and not on the jack
consider shielded wire to/from the tone stack.
If the amp sounds good, no apparent lack of power, I'd guess the bass pot is just passing some noise picked up in the preamp area. Large diameter speakers, over sized OT will also reproduce low frequencies that would other wise have gone unheard. the 5F6A I thought had 4 10's and a rather puny OT.
Some guys will "over build" or "over spec" parts not knowing they can induce other anomalies.
On that same thought look at the cathode bypass cap on V1 Is it HUGE? maybe a .62uf would cut back the freq spectrum here a bit.
Ray
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Ever try to swap out V2 (the cathode follower tube)?
Check the filter cap grounding on the preamp filters.
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There's certainly a ground loop created in the tone stack, I've already seen that in an AC30 with the bass pot as well. What's the ground scheme of the amp ? Any pics ?
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Thanks for the great suggestions Guys. Found the hum problem. The builder of the amp installed a 40/20/20/20 cap can under the eyelet board on the preamp side of the amp with HT wires running all over the place under the board. All the filter caps were grounded to the PT bolt (ground loop) I disconnected the preamp HT wires from the cap can and installed two separate 22uf/500v caps and grounded them to the input jack. With the location of the cap can in that area I had to reroute the wire from the two 270k mixing resistors on the board to the cathode follower from under to on top of the board. The hum is greatly reduced from what is was before. Punky