Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Diverted on May 06, 2025, 10:16:11 am
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Hello all,
Does anyone here have a schematic for this amp, or is otherwise versed in them?
Customer brought me one this morning, complaining no power and, previously, an issue with the footswitch where the channel light would not light. 2A fuse turned out to be blown.
Did a quick inspection, checked continuity on all transformer windings, tested tubes etc etc etc. All good.
Put in a new fuse, pulled the rectifier and started it out at 70 volts AC. The ammeter immediately read 3.8A. I turned it off before the slowblow fuse blew.
With no DC voltage, looks like I've either got a short in the power transformer or an issue in the relay supply. I will remove the transformer from the circuit and confirm its health. If it's OK, I'm looking at a relay issue I suspect.
My problem is I cannot find a schematic for this amp, so tracing this out is going to be difficult. And I am not versed in this circuit.
Any help or tips available? Thanks!
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Update:
I disconnected all secondary windings (HV, 5V, 6V and relay supply) from circuit and ran power through the transformer. Still getting the high current draw and the voltages are way low (at 70VAC I was getting around 40VAC across one half of the HV secondary, 1.5VAC across the 50V winding, etc. Followed up with out of circuit continuity tests and am seeing continuity between winding to relay and HV secondary winding. Ouch.
So I'm looking at a bad PT.
The owner said the relay/footswitch was giving him problems prior to the amp blowing the fuse. So I think that after I replace the PT I will have to reckon with a fault in the 50V-fed circuit, which contains relay, etc. I have little to no experience with relays and channel switching — I guess I will cross that bridge when I come to it. Meantime, any best practices for dealing with relay/switching issues?
Thanks.
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I found these in my "Trem" folder... haven't validated them against the real thing.
~S