Why did this fix my problem?

Hifigeek1, The woofers oscillating is where I saw the problem in the first place. I thought it was DC leakage at first. I did lose 1 kt77 tube. I tried everything I could think of..swapped tubes in amp and preamp AND Modwright Logitech transport, cables, even brought out an old NAD integrated that I could use the preamp section to run the amps which is when I finally got it to settle down.

What I don't get is what did lowering the impedance do to make them compatible?

Al, Thanks for chiming in! I have read many of your posts over the years and your thoughts are always insightful. My amp doesn't have any xlr inputs though. I am assuming you read the review on the Crescendos in Stereo Times, I have read that also. There aren't any extra holes so this pair never had xlr inputs. These are only identified as TPA-60, not Crescendo. The amps are a push/pull design and Direct Coupled. That is about as much info as I have.

Gsm18439: The company is "The in house service company" at the address below. I am very happy with their service and prices. They quoted me a bench service price plus hourly. After my amps arrived and they got them on the bench they called, and we talked. They asked about talking to the maker of the preamp Kara Chaffee at deHavilland. I gave them her number and they worked out the details with her. She called me and said they were very knowledgeable and a solution was arrived that they both thought would work and it did! It did take a cpl of weeks before they even looked at it so I guess they keep busy.

http://www.stereorepair.net/highend.htm

Best regards,
Russ

How did lowering the input impedance from 100k to 47k stabilize the DC operating points?

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

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?

Thanks Ralph,

I will put the 100 ohm resistors back and see what happens.

Thanks again,
Russ