fx loop issues
So I noticed, last time I tested the amp (prior to all my recent work with the v-mid EQ), that there was a problem with the effects loop. At low to moderate volumes, it seemed to operate normally, but with the master volume near max, the amp would seem to break into ultrasonic oscilllations.
This was before I got my new digital scope, so it was just my auditory assessment, based on past experience: everything would be operating as expected, getting louder as the volume was turned up, but then suddenly it would hit a point where the volume dropped precipitously, and the tone became sort of ragged and muffled.
Now that I have my scope, as you can see in the linked video, I can confirm my diagnosis. The amp breaks into oscillations at around 45.6 kHz.
So obviously, I am going to examine my layout -- a lot of wires and components are necessarily very close together, with my compact design. I don't have shield covers on the preamp tubes. There are a number of wires which run significant distances, which are not shielded in coax and perhaps should be: both, wires running to the fx loop jacks, and wires related to the power amp, in close proximity. I may try creating a small sheet of aluminum flashing, wired to ground, which I can move around inside and around the amp, and check if there are elements or regions which can be separated by a ground barrier to stop the oscillations.
However, since the oscillations are clearly affected by the master volume pot, this is a location where I could possibly insert a lowpass filter to block ultrasonic oscillations from any source. I'm looking at a two-stage cascaded passive R-C filter, with corner frequency around 13 kHz. This should provide pretty good attenuation by 45 kHz. R and C of 47 kOhms and 250 pF do the trick. Mr. Internet tells me the second stage should have 10x the R and 1/10th the C, to avoid excessive loading of the first stage. Anyway, I may try this, after doing the testing and examinations discussed above.
Also a possibility, I have "snubber" or degenerator caps across all the anode-cathode junctions of the 12AX7 preamp tubes, but I have none on the EL84. Since it is clearly involved with the oscillation syndrome, perhaps I should try a snubber there, as well.
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