What makes noise/hiss; redux?

Sorry to beat a dead horse, but there is something and I'm not "getting it".
Thanks for bearing with me.

I'm on that Trainwreck Express variant again. There is hiss.

The V1 tube does matter, but I've got one 12AX7 (Siemens) that seems about
the same as a 5751. Clearly, some tubes hiss more, but it suggests to me
that I've got a pretty decent tube in there now, the Siemens.

I changed the 100K plate load R's (both sides) from 1/2W CF to 1W MF. No
appreciable difference.

I pulled V1, and yes, it is quiet, again suggesting the "problem" is with
the V1 section. There ain't much there. Input> V1A > Tonestack + Vol +
Bright Sw >V1B, in that order.

Beyond V1B, there is only a coupling cap and the grid leak for V2A. I can't
imagine I've got a problem there. Please tell me I'm wrong and I'll change
the C and the R.

The hiss rises and falls with the Volume control (nothing plugged in). The
hiss is also affected by the Treble control Treble off= less hiss, not
surprised by this. There is only one fixed resistor in the tone stack, a
100K CF between V1A plate > .02 cap > Bass pot wiper. Can't say if it's
noisy and it's a PITA change it.

Anyhow....the Volume pot follows the tone stack. Treble pot wiper goes to
outer leg of of the Volume pot. So, I reason that turning up the volume
amplifies whatever is in the tonestack. Noisy cap on the Treble pot? I
could change it, 500 pf to the outer leg.

Do I run a jumper over the tonestack, from V1A plate to the Volume pot? I'm
asking, does this bypass the tonestack, and eliminate any noise that might
be generated there? It seems that I need a coupling cap in the jumper, is
that right and what value is appropriate?

I've probed for ripple. It seems to all be in the 10-20mV range everywhere.
Not surprised -- hum is not a problem.

When I put the meter probe for voltage on the plate of V1B, the amp reacts,
but I'm supposing it is because I am injecting signal a that point? Probing
plate of V1A gives almost no change in output to the bench speaker (a sorry
affair that isn't nearly as good as a real speaker cab).

Thanks again. Your suggestions are always appreciated.

If you want to see a schematic, I will see if I can post one. Let me know.

Phil


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Relevant Pages

  • Re: What makes noise/hiss; redux?
    ... This amp has so much gain and is so close to oscillation even when it is working, that removing the gain sucking tone stack would just put you over the edge. ... Clearly, some tubes hiss more, but it suggests to me that I've got a pretty decent tube in there now, the Siemens. ... Treble pot wiper goes to outer leg of of the Volume pot. ...
    (alt.guitar.amps)
  • Re: What makes noise/hiss; redux?
    ... If your Express variant is fairly true to the Kelly 80 schematic, you don't have a resistor between the input jack and the grid of the first triode. ... I am pretty sure I need to replace V1 as it is also microphonic, so some of my hiss problem might be related to that failing tube. ... Treble pot wiper goes to outer leg of of the Volume pot. ...
    (alt.guitar.amps)
  • Re: What makes noise/hiss; redux?
    ... There is hiss. ... there is only a coupling cap and the grid leak for V2A. ... Anyhow....the Volume pot follows the tone stack. ... Noisy cap on the Treble pot? ...
    (alt.guitar.amps)
  • Re: Behringer UBB1002 Eurorack - pls help
    ... and turned the MAIN pot down, the hiss seems to have gone away. ... would really like to invest some time and learn some more about what ... There was a guy on here recently who was afraid to turn an input trim ...
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  • Re: Same old bull shit, insane delusions and blatant lies.
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