2. amps-preamps
  3. 1343529449

Why did this fix my problem?

I have a deHavilland mercury preamp and Thor TPA 60 EL34 based monoblocks. My amps input impedance was 100k ohms and the Mercury is a cathode follower which I understand needs a minimum 10k ohm load. I was having a problem with the amps going into wildly fluctuating bias and would have to shut them down. Upon restart the bias would hold for a short period and then the whole process would start over. After 6 months of sending the amps to Thor who found them in perfect working condition I sent to a repair company that specializes in vintage and tube equipment. After a week on the bench the amps never acted up. I put the tech in contact with deHavilland and they decided to lower the input impedance of the amps to 47k ohms. It has been six months and I not only have never had one problem, the sound of these 2 components is just glorious. Anyone know why this worked?
leatherneck1812
leatherneck1812 OP
116 posts
 
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?
atmasphere's avatar
atmasphere
11,862 posts
 
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.
leatherneck1812 OP
116 posts
 
Thanks Ralph,

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

Thanks again,
Russ
atmasphere's avatar
atmasphere
11,862 posts
 
Correction- 100K...
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