Why did this fix my problem?

I first saw something similar occur in the 1970s with an ARC D-150 and an ARC preamp that had a bad regulator in the power supply.

The preamp would make some low frequency noise, the amp would amplify it, the AC line would sag, that caused the preamp to do it again, repeating the cycle. In that case the volume affected it as the problem was having to do with the phono stage.

Ralph,

Thank you for finally getting to the bottom of this. I did wonder for a while if the preamp was the problem because when I sent The amp in to Thor they could not get it to malfunction. The problem was it would only happen in one amp, the other one was fine, but eventually it did occur in the second amp at higher volumes. Kara originally offered to look at the preamp but by that time it was already on the techs bench and this is the solution that was agreed to. I will contact Kara and pass on your thoughts to her and get the preamp in to her for service. I was never comfortable with this solution. It just didn't make sense to me why the amps only had this problem in my system, but having virtually no technical skill couldn't get my head wrapped around why.

Thanks again,
Russ

Russ, the solution you have right now is a jury-rig. You need to get the preamp sorted out- and when you do, I think you will appreciate the improvement.

I sent the information off to Kara and she is going to run some calculations and contact me when she gets home from an audio show.

I do have one more query. Is it possible that the preamp is passing on this signal from a source? My Modwright-Logitech transporter is tube rectified. I have experienced woofer modulation in one speaker with certain tubes. The modulation moves to the other speaker when the tubes are swapped right for left. I just assumed I had a failing tube but these are tubes I was using when all of this started. As I have stated the amplifier modulation has never occurred since the 47k resistor was installed but I can still get the woofer modulation when specific tubes are placed in the transporter. Is it possible that this modulation was being amplified by the preamp and sending the amps in to motorboating? If this is the case does it still point to the preamp being the problem because the circuitry is designed to have such a minimal impact on the signal that it lacks the sophistication to correct this?

Huh! OK- yes, its very possible (and with this new information, very likely) that indeed the preamp is simply passing the LF noise along from the source. However there is a feedback mechanism that is also in play and its hard to know if that is the preamp or the CD player.

BTW, the preamp is not supposed to have anything in it that would 'correct' this!

If it were me, it would re-install the 100K resistors, and then run the preamp with the source, with the volume all the way up, and see if you can provoke it. Once it starts I would disconnect the source and see if it shuts up.

It seems highly coincidental (and a bizarre explanation) that you would have this phenomena in the CD player and it not also be the source of the problem! So right now, barring further information, in my mind the preamp is tentatively off the hook and the CD player should be under scrutiny instead!

FWIW I would not expect any LF noise from the digital system unless the tubes you are using are noisy/defective.