I did som tweaking to my DSL15h last Night regarding the lead channel: Clip C75, Clip C76, Clip C34, add R82 150k, that's what others successfully did. Additionally I tried to:
- make those changes switchable (mini switches on the rear side of the amp)
- Install a variable resistor/poti for R40, as different values are reported to sound best. I wanted to find my best R40 value stagelessly.
Bad result: lead channel has lots of high feedback when gain/volume knobs are turned to certain levels (e.g. From both at 9 o'clock on) The amp is just squeaking without any guitar plugged in. The Feedback disapears when either gain or Volume are reduced. Feedback also disappears when the R40 poti is turned to higher values, let's say above 4.5 kOhm (I used an open 5k linear poti for testing). The clean channel works as before and doesn't Show feedback or Noise. Preamp tubes all have been exchanged to ensure that it's not a microphonic preamp tube - no improvement.
So is it possible that I have to raise R40 (which limits V1B current and hence gain on lead channel) to a value that no one reported before, e.g. 5kOhm or more? Why?
What else could have gone wrong with the V1B circuit (tried Not to Touch anything Else)
Additionally, to understand the amp circuit better and to find any future errors, would anyone be so kind and helpfully send me the schematics to the DSL 15h?
Thanks a lot in advance, I Hope I didn't confuse to much of the technical details and of the english words.