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RD50 Mods and output Valve Oscillation

Hi all,

I recently re-capped my RD50 (Rev-B) I replaced all of the electrolytic capacitors. I upgraded C33&34 to 100uf, c35 to 20uf. I then did the BobW mods. The only other mods I have done are to bring the rest of the board to Rev E spec. That included diverting C23 to pin 2 on IC_2 from Pin 1 and changing R1, R21 and R22 to the Rev E values.

The result is that I now have a much nicer sounding amp with far less low end hum, it has a much nicer distortion on the limiter channel - previously it was very dark and muddy. The two channels are much more evenly matched in tone.

I do have a problem, however, at higher volumes or gain on the limiter channel. Above 3 or 4 on the master volume I start to hear a blooming in the low end on lower notes and then above 5 or 6 the output valves appear to be self-oscillating - I can see a blue ring appear when this happens. I have tried different valves, limiter and output, that I have spare and not much difference apart from the fact that with some different ECC83s the oscillation will start earlier or later on the volume dial which I guess is down to different amounts of gain in the valves.

I am a tube-modding newbie so please be gentle. I am amazed at the difference I have made in tone but would like to sort out the problems at the louder settings so any help would be much appreciated.


Some things to try

I have always used a 4uf cap to replace the 2uf in C35, based on a recommendation from Terry Loose. I see that others here have used 20uf sucessfully but it's easy enough to swap out and test if it helps.

Would also pull all the ICs, spray the sockets with contact cleaner and reseat. Maybe swap them around as well.

I'm sure you have already double and triple checked your work for stray blobs of solder, things touching things they oughtn't, etc.



I am wondering now if it is

I am wondering now if it is the R61 switch from 47k to 1k as recommended in the BobW mods - Ed Goforth suggests elsewhere that leaving stock can reduce hum so will try that and of course the IC work as suggested above by inertian



I have always left R61 as is, stock 47K.

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