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DIY Pre amp Hiss

6l6gmike

6l6gmike

Active Member
Hello All. I did a little search on this subject but I couldn't find any real helpful advice. Most of the time it was about guitar amps.
So I have build another DIY preamp with a new schematic that came with my Dynaco Pas 3 TCLA from http://www.tubenirvana.net/pas3-main.html Preamp has a gain of 10, uses 4 double triode tubes.

  • The problem is I have is a very noticeable amount of hiss both channels. Like rain storm loud.
  • It does not go up with the volume the hiss level is the same at 0 or 11. The preamp does not have a noticeable amount of hum.
  • I tried a different/bench power supply with no results. I did leave in the old trans but unplugged the rectifier tube to remove it out of the B+
  • Tried AC heating the tubes. same with 60hz hum.
  • I can get the hiss to intermediately show up on my O scope trigger and the o scope says its 20mhz So not sure what would be generating 20mhz in it. Maybe that's why the output sounds like fm static since there must be something riding on the carrier.
  • I am using Russian 6sl7's and I tried different tubes but it didn't help.
  • This preamp is build with all brand new 1watt resistors (metal film, metal oxide, and carbon film) what ever is on sale on Allied. All coupling caps are Illinois yellow tubes. High voltage caps are a mix from new ones and ones 3 years old from another preamp project that saw very little use.
  • Power trans is an old vintage one of unknown creation but it works.
  • Pulled the last tube in both channels - no hiss. Put back in last tubes and pulled first tubes - some hiss. all four tubes gives me the amount of hiss of a snake pit.
All I can think is that it is a resistor somewhere causing this but I have never really had hiss that bad. Anyways that signal is difficult to capture with the scope and i tired the track it down in the preamp but it's hard to see on anything but the output. Let me know any thoughts or suggestions on tracking this down.

Thanks.
Mike.
 
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I tried 2 different tube amps and the preamp makes both of them hiss. One set of amps were Mk3's(which does not) and the other was a DG SE1(which I do have a level control on). If you turn off the preamp the hiss goes away after a couple of seconds and then it's dead quite on both amps.
 
Parasitic oscillation will sometimes manifest as hiss. Do you notice any change in the noise from waving your hands near the tubes or probing with the scope?
 
A noisy plate resistor could cause it.
A bad solder joint or a dry joint (no solder) might also contribute (in fact that could be the source of parasitic oscillation)
 
Thanks for the hints. I did notice for some reason all the tubes act like they are microphonic. One little tap anywhere on the pre amp set them off. Even on the phone stage when its not the input. Today I will re do ALL the solder joints and change the plate resistors. What else might be giving me parasitic oscillations?
 
It's going to be hard to help any further without pics and schematics. You could try to jumper a large cap between the grid and ground to short the input of each stage to isolate the problem to a stage.Discharge the cap between tests.
 
Problem solved! The micophonic sound was from a B plus lead that missed the soldering.
That was a good idea with the cap and probe. I was quickly able to find the trouble tube. It was a local nfb circuit that had a 1k cathode resistor soldered on to it. So I moved the resistor to to correct grounding spot and it fixed ALL the hiss. No more snake pit. I also found that I used 220k where the schematic said 220R. Must of been a rough day at work when I made that mistake. Thanks again for the help. I now have a few more tools for the nerd tools box
 
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