The Pump knob causes a sputtering sound, with no input, when it's turned up above 2 or 3. I've swapped tubes, and that doesn't make a difference. I replaced the 51V zeners, thinking they might be noisy, but that didn't help. Could the Vactrols cause this kind of thing? It doesn't really show up on my scope inside the pump circuit itself. I only see it on the plate of tube V2A. The clean channel is totally quiet, by itself.
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