2. amps-preamps
  3. 128083-8369

Quicksilver preamp 1 channel noise

Background and Issue:

I purchased a used Quicksilver Line Stage preamp from Audiogon last year, it’s an older 6DJ8/6922 design. Everything is fine, it was using the tubes the fellow who sold it to me had in it (my fault for not getting new tubes). Sometime a month or so later the wife and I are watching TV listening through the stereo system and a sudden enormous low frequency bang erupts from the left channel and then that channel goes dead. I immediately turned off the amp and preamp and then powered everything back up and still left channel is dead. So I powered back down got some spare tubes put them in the preamp and it comes back to life (so I had a catastrophic tube failure)… except there is now a very strange low frequency hiss unlike any normal ambient tube preamp hiss, and it is not volume dependent. Again the hiss does not increase or decrease in amplitude when the volume control knob is adjusted. It is not tube related as I can swap tubes channel to channel and the problem stays put, and not change in any discernable way. So now the fun part. I have replaced 70% of the components in this god damn preamp (all told it’s actually an amazing preamp! It’s simple, sounds great and is designed to be serviced! Which is unheard of today, I have great respect for QuickSilver) with high quality same or better spec components (higher voltge/temp) and I still have the exact same damn issue!!!!! I am going out of my friggin mind!!!!! Also as stated earlier the hiss or distortion is very strange its kind of like coarse burlap being rubbed against coarse burlap. I have replaced all of the electrolytic caps with same value caps all Vishay or Nichon 105C (they were 85C) I replaced all of the diodes and rectifiers, even a small op amp and I have the exact same issue. There are a few more resistors to change out but damn! Also I have no schematic so sometimes I just have to pull components test the values verify the resistor bands vs the test and then order new ones and install them…it’s a lot of fun…(no its not!) If anyone can help me to isolate the issue, or has experience with a similar issue and could help me isolate the exact component(s) it would be greatly appreciated. Another clue, the noise/hiss is not present until the relays (tube warm up) kick in so I assume that it is in the high voltage side. PLEASE HELP, I am becoming manic.

Thanks
Jeff
jffyg
jffyg OP
53 posts
 
Czarivey, I dont know if they are bad? Because i dont have a schematic i have to remove each resistor and test then cross refrence as they are 6 band precision type. I wish i had a tracker tester. I then order the same value 1% at highest dissipation usually 1 to 2 watt depending on availability. There are no trim pots, all electrolytics have been replaced except 1 and its on order (but it visually looks good). 
czarivey
5,657 posts
 
Quicksilver on older preamps may have schematics... check with manufacturer or even hifiengine.

Auto-bias circuit and proper bias needs to be measured on faulty channel. I've seen Quicksilver would use Zenner diodes (some semiconductors in the mix) to provide auto-bias to tubes. Zenner diodes can go bad within time. I'm not sure if your preamp uses such. You also need to measure all output voltages for the faulty channel out of power supply. Start from B+ and compare it to healthy channel.  It clearly seems to me that you don't have the right offset quiescent voltage supplied to the faulty channel tube making an amp to oscillate on low frequencies.

If you DIY savvy, you should look for vintage units that have schematics.
There are plenty of very well built tube units done in 50's...70's with schematics -- just my $.02

jffyg OP
53 posts
 
Czarivey, thanks for the advice! Have done a limited search for schematic snd found none. I did replace diodes even though they tested good on my meter. If its not the the remaining resistors maybe its the damn socket itself, but resistance wise it measues like the good channel.

czarivey
5,657 posts
 
I would ask Mike(Quicksilver) to share schematics of ANY full function preamp they've ever built. You may find lots of similarities to your unit.
Also check conducting 'trails' over PCB around blown tube making sure that there's a continuity of applied voltages. Re-apply and/or retouch solder on entire board making sure that you don't have loose joints. I often go over with magnifying glass and find LOTS of cold or loose joints as a matter of fact especially in tube equipment.


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