Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Grantorino on January 10, 2026, 09:54:55 am
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Hello again EL34 world.
A 1997 Vox AC15 with no reverb send. Not a reverb tank issue, the RCA cables had a short and were replaced. Return path is fine, amp otherwise sounds great.
Background/Condition- Reverb send signal missing
- RCA cables were shorted
- Return signal increases with reverb pot as expected (tap test)
- Original R7 (10k reverb resistor next to TX1) was burned and was replaced
Evidence of previous work:
- Lifted/damaged PCB pads
- Traces restored using jumper wires (continuity tests okay to intended nodes)
- Several components altered from schematic (Reverb mod?)
Deviations from schematic:
- C30 (47pf mix cap) was replaced with two 150pf in parallel
- C17 (470pf driver) was missing and replaced with a 6800pf WIMA linking LK7/LK21 near V5 socket.
- These mods were removed and the amp has now been restored to stock schematic values - C30 restored to single 47pf 500v, c17 restored to 470pf 500 V, R7 replaced with 10k 2W
The amplifier was restored to stock according to the schematic.
Isolation tests:
- Reverb transformer TX1 removed
- Primary pads for TX1 jumped together
- All tubes except rectifier EZ81 removed, R7 still overheats immediately
This suggests to me the fault is downstream of R7 and is not TX1 or the tubes.
I have attached pictures of the board.
I'm seeking advice diagnosing this issue.
Thank you.
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All tubes except rectifier EZ81 removed, R7 still overheats immediately [/li][/list]
I would check/replace C1.
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All tubes except rectifier EZ81 removed, R7 still overheats immediately [/li][/list]
I would check/replace C1.
Thank you Sel. I was suspicious of C1, as R7 has a ground path through here. Having just tested the cap, I can confirm its within spec at 10.12uf, 2.9ESR, 0.1V loss. Seems to be no issue here.