Solved. Weird measurement thing

[Joel]

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? 

[Joel]

Link to the schematic of amp in question.  https://el34world.com/Forum/index.php?topic=32316.msg370757#msg370757

[MORE_Guitar_Solos]

Heisenberg? 

[shooter]

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

[JPK]

Exactly. If you could determine the capacitance of your meter on freq setting you'd have your noise solution.

[stratomaster]

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.

[Joel]

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.