After years of gigging with my trusty Model 75, I now have a pair RD-50s recently added to the family!
I've gone over them pretty thoroughly and looks like both have been repaired and/or modded along the way. I'm hoping I can get some opinions and suggestions on here as I bring them back to spec.
Amp 1 is an '81 Rev A. Two things that don't seem original:
- 470k resistors from cathode to ground on V2 and V3 (across pins 2 & 8). Or looking at it another way, 470k res. from collector to ground on Q5 and Q6. I measure ~65Vdc at the cathodes/collectors. An attempt at reducing bias?
- A .001pf cap on the limiter gain pot (RC3) across the input (C7 side) and the wiper. Treble bleed?
Amp 2 is an '82 Rev B with some obvious repairs:
- D9 replaced with a 1N3030 27v zener
- R54 replaced with 2 resistors totaling 7 ohms (3+4)
- R62 replaced with 560 ohm resistor.
Any opinions?
On Amp 1, I'm inclined to remove the 470k resistors but I'm wondering if this was perhaps a "good" mod and maybe I should leave it. Doesn't seem like this would be the right place to adjust the output tube bias though (I would like to be able to one day, but I want to get to it stable and close to stock first.)
On Amp 2, I have a 30v zener and 6.8 ohm resistors ready to install, but I'm wondering what to do about the 560 ohm res. for R62. Stock value is 470 ohms. Again, wondering if whoever did this knew what they were doing. The amp sounds ok, but a little dull.
Thanks!