knows just how much it hisses/noise it makes.
These mods do reduce the hiss/noise in OD1/2 Orange & Red
to much more manageable levels, at least 75% reduction. It no
longer spits and hisses at me and is usable without a noise gate.
Although a gate is still helpful for feedback issues.
You'll find the original post for these mods 3 pages back, thanks
to Zozobra for saving a copy of this mod.
The gist of this mod is to replace the preamp plate resistors
which are carbon film to metal film as well as others coming
off ground. Also Screen Grid resistors were changed out as well.
Here we go:
Plate Resistors (These must be 1/2 watt Min):
R22 - 100K 1% Metal Film .5 W
R23 - 82K 1% Metal Film .5 W
R71 - 150K 1% Metal Film .5 W
R72 - 220K 1% Metal Film .5 W
R73 - 82K 1% Metal Film .5 W
R75 - 220K 1% Metal Film .5 W
R80 - 150K 1% Metal Film .5 W
(All Found @ Digikey, Vishay MBE0414 1 watt)
I used 1 Watt resistors in the plate positions and
they fit without problem, You'll get more noise
reduction the higher the wattage used.
Off Ground (These must be 1/2 watt Min):
R107 100R - 1% Metal Film .5 W
R118 47K - 1% Metal Film .5 W
R128 10M - 1% Metal Film .5 W
R130 10M - 1% Metal Film .5 W
(R107,R128,R130 used Vishay MBB0207 .6w resistors Mouser/Digikey)
(R118 used Vishay MBE0414 1 watt found at Digikey)
Screen Grid Resistors 2/3 Watt :
(All Found @ Mouser, Koa Speer SPR2/3)
R52 - 22K 2W
R67 - 10K 2W
R68 - 10K 2W
R69 - 10K 2W
R82 - 15K 3W
Used Koa Speer SPR2/3 which are Kiwame without the special
testing they use. Changing the Screen Grids will help with noise
but aren't necessary
One may wish to keep the carbon film resistors in the PI (R22,23) to
keep some warmth in the amp. I was looking for maximum noise reduction
as OD1 Orange/Red are my go to overdrive channels.
OD1/2 Orange is whisper quiet now and Red on par with other high gain amps,
quite unbelievable the difference in noise.
Also be forewarned, it can be a bit difficult to get the PCB back into the amp.
Takes a bit of PCB wrangling but it can be done.
