Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Joel on March 06, 2026, 03:21:26 am
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Fine people of EL34World,
I have a strange issue I'm hoping someone can help me with.
When probing a circuit with a multimeter, what change could be introduced when measuring using the frequency (Hz) setting compared to ac or dc voltage settings? (Fluke 85 multimeter).
Situation: I have a noise issue in a new build. It's a Princeton Reverb style build, specifics aren't important right now. On the Reverb recovery triode, I get noise in the anode circuit. It manifests when turning the reverb return pot up. Amp is damn near silent in all other respects. Grounding input to reverb recovery pot - silent. I can ground the reverb recovery input (V3 pin 2) - no change to noise. Here's where it gets interesting. I am measuring voltages around this stage - all good. However, when I switch to trying to measure the frequency of the noise at the anode of the reverb return triode with my multimeter, the amp goes silent - ie, the noise goes away and the signal is good. Switch to measuring voltage - noise returns.
Why would the amp only run silent when I probe the anode of the reverb recovery triode with my multimeter set to measure frequency? What am I missing?
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Link to the schematic of amp in question. https://el34world.com/Forum/index.php?topic=32316.msg370757#msg370757 (https://el34world.com/Forum/index.php?topic=32316.msg370757#msg370757)
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Heisenberg? :laugh:
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the noise goes away and the signal is good.
likely meter is providing an AC "Path" to ground, try tacking a ~~~~~.001uF "across" the same point you had the meter leads
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Exactly. If you could determine the capacitance of your meter on freq setting you'd have your noise solution.
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the noise goes away and the signal is good.
likely meter is providing an AC "Path" to ground, try tacking a ~~~~~.001uF "across" the same point you had the meter leads
The frequency content of reverb drops like a rock over about 4kHz, so you can get pretty aggressive with the snubber cap before you start losing the stuff you want.
Also, I find a huge cathode bypass cap to be beneficial on the reverb recovery stage to limit h-K leakage hum. Attack that noise from both ends.
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Thanks for the suggestions. Much appreciated.
I found the solution in this thread: https://el34world.com/Forum/index.php?topic=17645.0. Grounding the reverb recovery cathode resistor and cap in a different place. I ran a wire to the RCA reverb input jack - it was the quietest spot.
Bypassing the anode cap to ground - I needed to use a cap so large to make an effect that it impacted the guitar signal. Effectively grounding almost all the audio, not just high frequencies. A larger cathode bypass cap (1000uF) didn't result in any change.