2. amps-preamps
  3. 1367679356

DIY question on a preamp problem could use help...

Ok, love to hear from some of you who have good technical skill to help diagnose a problem. I purchased a used Lector Zoe preamp as an upgrade project. The unit has a lot of tube hiss/rush sort of noise through both speakers. The builder is no help as I have emailed them several times and they keep telling me to use a different tube. I have tried several tubes brand new and nos , but the same tube noise all the time. The preamp is very high gain as an FYI.

I am using two 6922 tubes. I have already modified the unit to sound far better with great results. None of the mods helped with the tube hiss that was present before and after my mods. I upgraded many caps and put in a nice Goldpoint stepped Attenuator with DACT remote. Lastly I bypassed the selector switch and went direct from the RCA input to the Goldpoint as I only use one source. I upgraded the signal wire and the end result is quite stellar. Problem is this tube hiss/rush noise that is constant through both speakers. I want to fix this and greatly reduce the noise. Where do I look?

Would reducing the gain help? Should I look at the resistors on the 6922 tubes? Change the value of any of these resistors to reduce gain? Not sure where to go next? The noise is not really impacted that much at all as I turn the volume up. The tube hiss stays pretty constant regardless of volume. The signal goes from the coupling caps right to the RCA outputs with no resistors after the coupling caps or on the RCA outputs to ground.

Any ideas?
grannyring

Showing 2 responses by mechans

mechans
3,048 posts
 
It is really frustrating when it acts just like a bad or simply noisy tube would almost exactly. I think his only choice is to check every component in the signal scheme that he mentions. He knows the noise is coming from a section of the circuit and see if any of those parts are bad or out of spec. What else can he do?
He didn't mention who sorted the tubes, factory sorting e.g., is next to useless. Some labs are much more sophisticated than others with differing strigent rules determining noisy or not tubes. RAM testing , for example is very stringent and is very expensive, even if he sometimes uses cheap tubes to start with. nI am not sure what his tube supplier is now for 6922s.
BTW really good NOS 6922s are painfully expensive, sold as is usually with just gain, transconductance, short and gas testing, but with no noise waranty !
mechans
3,048 posts
 
I did look at Rodger's (RAM) tube offerings. While expensive he does offer some rare NOS tubes that he tests the hell out of to select truly quiet ones. It might be worth the investment. I had thought that Upscale was doing good work but have only bought 1 pair of Mullard 12AX7s a decade ago and used them as an output stage of a CD player. They worked great in that capacity but that is hardly a true test.
I have a lot tubes as rolling turned into collecting. There is a big difference between noise generated by the tube's normal function and microphonics. Testing for microphonics is sort of useless in my book, because the sound very very rarely feedsback into the tube to cause a problem. So, unless you are tapping the tubes with a pencil while playing for some strange reason, microphonic tubes have little impact on the sound.
Noisy tubes are different. They make a noisy base signal that gets picked up and amplified. In a phono stage or other very high gain situation, this is obviously a big problem. Sometime the tube has both a noisy output and a distorted output but I don't want to write a novel here. The most common noise is from a near dead tube or one in the process of dying and it makes that tell tale hiss, which makes this case frustrating. Upscale should and I expect would have tested the tube enough to find hiss. Could they have missed it ?? I don't know. But substituting any other compatible tubes should have fixed the problem...
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