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Hi guys
Hi new to forum and amp building. Finished my first sIII amp end of last year, which has been working fine. There was a bit of noise coming from the amp so recently after reading the forum decided to change the 1M reference resistor to metal film 1 watt to reduce the noise. I have kept the grid blocking resistors carbon film at 68k.
Noise ( hiss ) reduced amp still producing great sound. Then decided I might want a bit more gain on TMB channel even with boost engaged. Have done the following mod.
removed R8
Changed VR1 to 1Meg 0.25watt Audio Taper
Changed R9 to 470R
Changed VR7 to 100k 0.25 watt Audio Taper.
While the gain has increased unfortunately I have also got a constant Crackling/buzzing noise When I keep the both volume and gain at 2 it is not an issue. Anything above that and the noise increases quite significantly. Additionally if I keep gain at 2 and reduce volume below 2 noise comes back. This happens with guitar cable plugged into amp or out there is no difference.
I have performed the Stage isolation test with the following results. When PI removed noise gone, replaced PI removed next v2 tube noise goes. Relaced V2 then removed v1 noise is there. I have moved all preamp tubes around to different position makes no difference . Looks like noise maybe coming from stage 1 ???
Next performed a chopstick test around stage 1 and 2 nothing stands out.
Did voltage checks with following results.
V1
1=152
3=1.0
6=152
8=1.0
V2
1=123
3=0.65
6=274
8=123
V3
1=210
2=55
3=80
6=218
7=56
8=80
Thought V2 looked a bit low!!!!
Maybe I have overdone it on the gain and this is causing problem. Any help in what might be causing the constant noise and possible remedy would be appreciated.
I am using 3 new JJ in the preamp stage and playing through Fender Telecaster.
Thanks
Mark
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