MusicMan RD50 Mods - Oscillating Output Valves

[oktorockto]

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.

[Stevem]

Hello!
I have worked on tons of music man amps I have never had one that sounded muddy so I would suspect you have other issues going on in the preamp section that should have been addressed before starting any mods on the amp.

I have no idea what the Bobw mods are , so maybe you can post those up?

I suspect the root cause of your issue now is too much drive signal hitting the output tube plates.
If you put the amp back to stock and pump a test tone into the amp ( .150 MV should do ) you can hook up a meter set for AC volts across pin 5 of each output tube to read the maximum stock PI drive signal level and then compair that to the mod signal level.

Going by the tube spec charts for a EL34 / 6ca7 tube it looks like you do not want to see anything more then 28 to 32 volts of AC drive signal across the output tube inputs.

Rule one of modding amps is to make notes on how the amps signal levels are before doing the mods so you atleast have a baseline to reference back to!

Also note that these amps output stage are biased in class B, not class AB like most folks think.

You've picked a pain in the Ass amp to work and learn on with its SS preamp section!

[Stevem]

Oh, and by the way any output tube will Britten up and glow a blueish color inside its plate structure when it gets hit by signal, but if at idle it's glowing blue outside of its plate structure then that's a sign of a gassy tube that will fail in time .
The time frame for failure will be dependent on how hard the tube is used!

[Tony Bones]

Oh, and by the way any output tube will Britten up and glow a blueish color inside its plate structure when it gets hit by signal, but if at idle it's glowing blue outside of its plate structure then that's a sign of a gassy tube that will fail in time .
The time frame for failure will be dependent on how hard the tube is used!

Hmm... I've always taken it the other way around: blue glow deep inside the tube is an indication of gas, but blue glow on the inside surface of the bottle is ok, it's just the glass fluorescing.

[oktorockto]

Hi - thanks for the help. The BobW mods can be seen here https://imgur.com/a/Q9Ao364

I have sen it suggested that the 1K substitution for 47k at R61 may be the problem and leading to too much plate voltage so I will put that back to stock first. There is no problem with the clean channel. As for muddiness - a lot of people on forums have complained of muddiness in the limiter channel on the RD50 - it can be so different sounding to the clean channel. I believe this was the main aim of the revisions up to Rev E for this amp. The mods I have done have certainly brought the amp alive and reduced hum. I am going to check the ICs. I will update ASAP.

Edit: I didn't use the PAB with my mods.

[oktorockto]

Well, I returned R61 to 47k stock. Now the hum/oscillation doesn't start until the master is almost fully cranked, somewhere I'll probably not need to use it at. But now I have HF noise, like RF hum. I had used a cheap ceramic wire-wound resistor - idiot! Decent film resistor on its way which I reckon will sort things.

[Stevem]

Well I concider a noise hum to be something below 200 HZ , so is that what shows up on your scope if you hook it up to the amps output?

[oktorockto]

Sorry, I meant more of a noise than a hum after the resistor change. I have now changed the wire wound resistor to a carbon film resistor one and the amp is nice and quiet and no sort of oscillation/hum other than at absolute maximum volume and gain which I am unlikely to ever want anyway.

[oktorockto]

Replacing some out of whack resistors helped. Replacing the IC's stopped all the oscillation - sounds great now and very low noise floor even with gain.