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Reducing gain level

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Hi All,
Well, I have a question that hopefully someone will answer seeing as how it is not for AN amps. Posting it here because it seems many off the fellows posting here have a good amount of electronics knowledge. Anyhow, I have a pair of KT88 based monoblock amps that are hi gain. Like the sound except the hi gain is causing some hum in my hi efficiency speakers. Can I reduce the gain level? Might require some tinkering to find best mix of gain / hum, but I can play around. Thanks for your time.

Mike

Follow Ups:

RE: Reducing gain level - mbhcid 17:49:36 03/04/08 (0)
Try a simple voltage divider consisting of two high quality resistors per side.
RE: Reducing gain level - GTCharlie 19:19:40 03/03/08 (2)
Hallo Mike. The "hum" you hear in your horns is due to insufficient filtering either in your filament supply or your b+ OTOH one way to reduce gain is to introduce local feedback to pre-stage, - it will reduce gain, and minimize distortion.
- RE: Reducing gain level - horneyMike 12:59:59 03/05/08 (1)
  Hi Charlie,
  Thanks for the advice. I did have the factory perform a revision of the filament supply and B+ voltage rails. The filament had previously been AC and is now on a DC level. The B+ level is supposed to be highly regulated; is retrospect that is an ambiguous statement huh? All this was supposed to bring the SNR to -99db. There is no feedback employed in the amps and I think I would care to not incorporate any into a tube amp. Would you suggest an additional cap, a bypass cap, around the cap already in the rectifier circuit? Any particular flavor you prefer? Thanks
  
  Mike
  - RE: Reducing gain level - GTCharlie 17:17:32 03/05/08 (0)
    Hallo Mike, I personally find "global negative feedback" (out of the OPT back to the cathode of the first stage) extremely difficult to do properly (it took me several months to realize I didn't know how to do it...) Local feedback, OTOH, can be quickly implemented by taking out the by-pass capacitor from the cathode of the input tube. This reduces gain, reduces distortion. And since the feedback is only within the very short path inside the tube, no fear of phase shift. Depending on the frequency of that "hum" it could come from various sources. But in all likelihood, an inadequate power supply filtering.

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