Restoring a beat up RD-50 112, was worked on before I got it and just didn't sound as sparkley/lively as my other RD-50s. Seems to have a slight gritty note decay when played on the clean side. Overall it just doesn't have the crispness I am used to from these amps.
A cap job had been done, but some values were odd and one cap was installed backwards! Corrected and cleaned up all connections. Eliminated speaker, speaker cable, dirty pots/jacks/sockets as possible culprits (cleaned them all). Noticed I didn't have much bass control, turns out both yellow Mallory .22 caps were shorted. First time I'd seen that. Replaced both, good Treble and Bass control now on Clean and Dirty.
Swapped out the so-so Groove Tubes for a nice old pair of Sylvanias, seemed to make an improvement, but still not quite there.
Appreciate any advice where to begin to get this amp sounding right.
Thanks.
Crossover
It's probably crossover distortion. Common problem in this model.
Cheers,
Lars Verholt
While I'll defer to lars,
While I'll defer to lars, YES, it's probably crossover distortion. The problem being is that that circuit (to ME) being a cathode driven circuit means you have to think backwards as the control voltage is POSITIVE, not negative since it's a cathode driven circuit. I'm NOT versed in RD's so I'll defer to Lars but it MIGHT have the two halves not driving the circuit evenly, or one driving harder that the other. Lars HAS worked on a crap load of these so HE's the expert. I've just experienced some tube driven PI types tha one half of the PI wa wacko and gave some of these type. YOu just need to put a probe on each side of the driver circuit (IMHO) and see if the two side are driving evenly or UNDERDRIVING the final. PLEASE Lars, check in here and correct me. I've NOT worked on any RD's/ Mike.
Update
Thanks for the suggestions.
Went through the amp again, measured everything, replaced some borderline resistors just in case. Then decided to recheck all voltages based on the schematic, and sure enough the voltage to the power tubes was about 30% lower than spec (22 vs 30 VDC).
Traced the problem to the zener diode (D9) and a bad under-the-board trace connecting to it. The zener was original, looked fine and tested in spec, however I became suspicious since the 470K resistor (R62) that feeds it had been previously replaced. In my experience, when one goes, the other does too. Fortunately I have a supply of these diodes, so once I installed a new one and rebuilt the trace, the problem was solved and the amp sounded like it should.
Gave it to a friend of mine who has great ears to A/B test against his exceptional sounding RD 50 112 (that I recapped). He says that although the amp sounds much better than before I did the work, it still doesn't seem to have the bass response and top end crispness/clarity that his amp has. Swapped tubes, speakers, etc. to eliminate any other variables.
Can't think of anything else to check or replace except the 1692 transistors. I have some 2N6488s on hand, so guess that's my next step...
More as it happens.
Another Update
Got a chance to A/B the beat up 112 with my friends great sounding 112 with my own ears.
There is an upper end clarity and sparkle and some lower end richness that the beat-up amp is missing. Also seems to have a slightly nasal midrangey tone.
I swapped out IC-4 for a new LF353N opamp, and compared again. The differences now seemed more subtle.
Swapped out IC-1 and IC-3 while I was in there, noticeable improvement in overall volume and stronger reverb too. The tonal differences between the amps can just about be EQ'd out.
I'm going to see how balanced the original 1692 driver transistors are, and swap them out for a matched pair if it seems indicated.
IC swap
Are you using TL072 op amps? The RD50 tone critically depends on using these op-amps.
Replaced the original TL072s
Replaced the original TL072s with LF353Ns, they are a MM approved equivalent (mentioned on the schematics).
Op Amps
Monte Alums has an interesting discussion about the tone of ICs including the TL072 and the LF353 chips
https://www.monteallums.com/pedal_mod_parts.html#chipdescrip
I had a mish-mash of these because of a tech who worked on my 110RD in the 1990s and, about 3 years ago, I replaced them all with TL072 ICs to get "that sound". I also put a TL072 in one of my Boss SD-1 which improved the distortion no end.