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.
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The issue was traced to a single failed capacitor - C26, 470pf mica - pulling too much current through R7.
When replaced, R7 no longer overheated. Once the original Drake transformer was re-installed, the reverb circuit was restored.
Thanks again. :icon_biggrin:
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For context on pre-amp tube roles:
V5 provided the first two gain stages for the input signal. The signal coming from V5 was split between V2 and V3. Both triodes of V3 powered the reverb drive (send) circuit. One triode of V2 amplified the reverb return signal, the second triode was the modulator for the tremolo circuit. One half of V4 powered the tone controls, the second triode was used for gain recovery. V1 was the phase inverter (splitter) for the push-pull output stage.
(Taken from the VoxShowroom.com)
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The issue was traced to a single failed capacitor - C26, 470pf mica - pulling too much current through R7.
Glad you fixed it. However, I'm puzzled. You said above that you removed the reverb transformer. Doing so would totally remove any dc voltage seen at C26 and current through R7 would have been zero. Can you explain that?
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The issue was traced to a single failed capacitor - C26, 470pf mica - pulling too much current through R7.
Glad you fixed it. However, I'm puzzled. You said above that you removed the reverb transformer. Doing so would totally remove any dc voltage seen at C26 and current through R7 would have been zero. Can you explain that?
Thanks Sel - im glad I fixed it too!
To rule out the reverb transformer as the cause of the fault, I removed it and powered up the amplifier with a jumper wire between the primary pads of TX1, completing the circuit.
As R7 still overheated in this configuration, I decided the transformer was not the issue.
The primary winding DCR of TX1 measured around 900ohms or so, where the secondary measured around 37ohms. The secondary reading seemed high, so I had a replacement Hammond part on hand, but decided this was not needed.
Thanks again for being willing to help and offering advice.
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Makes sense now. Thanks for clearing that up.