I purchased a completely original rd50. Sounds great. Maybe a bit on the brighter side than I’m used to. It’s never been recapped but doesn’t sound like it needs it. Sticker dates 83/11 so maybe one of the last of these guys.
I was looking to just purchase caps and some of the transistors to have on hand for when the time comes. I have Rev E (https://musicmanamps.net/sites/default/ ... 650-rd.pdf)
I noticed that all other revisions only have a bypass cap on pin 3 of the limiter. Rev E also adds a bypass cap to pin 8.
Through all the revisions they are rated 2uf/25v
However when looking at my socket I actually see 20uf/25v installed for the pin 3 and 8 caps. This value is used elsewhere in the amps circuit. This isn’t the only difference from the spec as I have diodes in series where a standard failure point is on these.
So the question is. Do I just keep them at 20/25? Is there anything to gain from moving to 2/25? It’s an odd value but still available for purchase.
The sockets themselves do not hold tubes well. I’m not sure if I can just tighten the pins, but if I do have to replace the sockets, I’ll have to do these caps as well.
Music Man RD50 limiter cathodes
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Music Man RD50 limiter cathodes
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martin manning
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Re: Music Man RD50 limiter cathodes
Do you mean using 22u as opposed to 20u at 25V? I would go with a good quality modern 22u.
Tightening the pins is worth a try. Fitting retainers of some kind is an option.
Re: Music Man RD50 limiter cathodes
No I meant 2u/25v. Not 22u. The schematic lists 2u/25v. I thought maybe a typo, but all ~5 revisions list 2uf 25v.martin manning wrote: ↑Wed Oct 07, 2026 4:56 pm Do you mean using 22u as opposed to 20u at 25V? I would go with a good quality modern 22u.
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Stevem
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Re: Music Man RD50 limiter cathodes
Let your ears be your guide in terms of the bypass caps, audition both values, there certainly cheap enough to do so.
A 20 to 25 uf cap will gain wise boost all guitar frequencies equally, a 2 uf cap will make the lower mid’s and on up more pronounced
Don’t forget that any cap will have in general a 10% tolerance up or down in terms of its labeled uf value.
Those two diodes are for the regulation of the 16 volts plus and minus that powers the SS section of the amp.
If those 16 volt sources are not close to being the same then the amp may have a low level 120 hz hum to it, but for sure there will be a effect in the gain the SS section has.
Preamp tube socket wise, yes try to re-tension them with a scribe that you grind or file into an even sharper point so you can easily get into where you need to.
If the output tube sockets are loose and there are of the opposing C grip type then it’s best to just replace them with the better Marshall type sockets.
If pin 5 on either output tube socket wise goes open the tube even at idle will red plate and be destroyed, and especially with this amp because they run very high plate voltage.
A 20 to 25 uf cap will gain wise boost all guitar frequencies equally, a 2 uf cap will make the lower mid’s and on up more pronounced
Don’t forget that any cap will have in general a 10% tolerance up or down in terms of its labeled uf value.
Those two diodes are for the regulation of the 16 volts plus and minus that powers the SS section of the amp.
If those 16 volt sources are not close to being the same then the amp may have a low level 120 hz hum to it, but for sure there will be a effect in the gain the SS section has.
Preamp tube socket wise, yes try to re-tension them with a scribe that you grind or file into an even sharper point so you can easily get into where you need to.
If the output tube sockets are loose and there are of the opposing C grip type then it’s best to just replace them with the better Marshall type sockets.
If pin 5 on either output tube socket wise goes open the tube even at idle will red plate and be destroyed, and especially with this amp because they run very high plate voltage.
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Not screaming like the passengers in his car!
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martin manning
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Re: Music Man RD50 limiter cathodes
Ok, I'm with you now. If it already sounds bright then going to smaller caps will only make it more so, provided those 20u are still good. If you don't have a way to test them for capacitance you could try paralleling new ones and see if there is any effect.
Re: Music Man RD50 limiter cathodes
Seems the tube being loose was mostly the tube or at least the short pins on it, which are 2/3 the length of the TAD redbase I installed. A little wiggle, but it’s not coming out or moving at all with the shield on it.
It was marked with frosted letters “4U 7025 USA” with no other identifiers. I guess it goes in the pile.
Power tubes are a little loose but not bad.
I guess I’ll just get both caps and test them when it comes time to change them unless it’s just worth doing now for some reason?
They are 40 years old, but everything seems to be as it should be.
It was marked with frosted letters “4U 7025 USA” with no other identifiers. I guess it goes in the pile.
Power tubes are a little loose but not bad.
I guess I’ll just get both caps and test them when it comes time to change them unless it’s just worth doing now for some reason?
They are 40 years old, but everything seems to be as it should be.
Re: Music Man RD50 limiter cathodes
You can look at the effect of the cathode cap by plugging in the values here in this calculator.
https://www.ampbooks.com/mobile/amplifi ... alculator/
There is very little difference between 20uf and 22uf, both being flat response down to about 10hz.
A 2uf is about flat above 120hz, then starts to roll off around 110hz, and more dramatically around the low-E frequency of 82hz, where it is down about 10db.
If I were going to recap those, I'd just put in the 22uf@25v. They'll be fine. Those look original to me. Maybe take one end loose and let 'em sit on a power supply at 27v over night through a 10k resistor to reform them or see if they're leaking current. But at under a $1 each, I'd just replace them.
The early revisions were running that triode section in a weird configuration, with a 2.2M-ohm plate and no cathode resistor at all, so they didn't have a cathode resistor nor capacitor, since there's nothing to bypass.
Music Man amps liked to do weird things with tubes, like the cathode drive on the output section from a transistor totem pole.
It's been a long time since I've seen one of these hybrid amps. Back in the 1980's when I was a high-school kid fixing amps in a music shop, I used to see a lot of them. I vaguely recall the double-up zener diodes being a factory mode to make them more reliable, as the diode was shunting a lot of current.
I think they're normally on the bright side, but it might be worth investigating the rest of the preamp to see if something weird is going on. I recently came across a black-face Super Reverb that was grown tin-whiskers from the plating on the inside of one of the tone pots, which made the reverb channel sound way off. Weird stuff happens after all these years.
https://www.ampbooks.com/mobile/amplifi ... alculator/
There is very little difference between 20uf and 22uf, both being flat response down to about 10hz.
A 2uf is about flat above 120hz, then starts to roll off around 110hz, and more dramatically around the low-E frequency of 82hz, where it is down about 10db.
If I were going to recap those, I'd just put in the 22uf@25v. They'll be fine. Those look original to me. Maybe take one end loose and let 'em sit on a power supply at 27v over night through a 10k resistor to reform them or see if they're leaking current. But at under a $1 each, I'd just replace them.
The early revisions were running that triode section in a weird configuration, with a 2.2M-ohm plate and no cathode resistor at all, so they didn't have a cathode resistor nor capacitor, since there's nothing to bypass.
Music Man amps liked to do weird things with tubes, like the cathode drive on the output section from a transistor totem pole.
It's been a long time since I've seen one of these hybrid amps. Back in the 1980's when I was a high-school kid fixing amps in a music shop, I used to see a lot of them. I vaguely recall the double-up zener diodes being a factory mode to make them more reliable, as the diode was shunting a lot of current.
I think they're normally on the bright side, but it might be worth investigating the rest of the preamp to see if something weird is going on. I recently came across a black-face Super Reverb that was grown tin-whiskers from the plating on the inside of one of the tone pots, which made the reverb channel sound way off. Weird stuff happens after all these years.
Re: Music Man RD50 limiter cathodes
I could not find caps that cheap anywhere. I stuck with normal uf values in the spec and ended up close to $100 for all 14 caps plus a few spares. 10 20uf/50v caps where right around $50, I did buy vishay/sprague tva series for these so maybe there was a cheaper option out there.
Re: Music Man RD50 limiter cathodes
You can get 10 of these for $1.93. They're Panasonic, 105c rated.Bretly wrote: ↑Thu Oct 08, 2026 11:50 am I could not find caps that cheap anywhere. I stuck with normal uf values in the spec and ended up close to $100 for all 14 caps plus a few spares. 10 20uf/50v caps where right around $50, I did buy vishay/sprague tva series for these so maybe there was a cheaper option out there.
https://www.digikey.com/en/products/det ... NQQAvD_BwE
Thing is, radial caps are much, much higher quality and a fraction of the cost of axials.
They'll easily go on the socket, no problem at all. Sometimes it takes a little fidgeting to get them in a circuit board that was setup for a really long footprint axial. But if you can, it is cheaper and lasts a lot longer.
If you really want the axial package, these are decent 85c parts and 10 will cost you $8.93 ( 89 cents each)
https://www.digikey.com/en/products/det ... 0M/5411672
Re: Music Man RD50 limiter cathodes
I didn't write up the whole list, being that it is a voltage doubler power supply, you'll want to change those.
If you can figure out how to mount these, these are 10,000 hour @ 105c rated, 68uf @ 350v, very high quality parts, and they're $2.52 each in qty 1 units.
https://www.digikey.com/en/products/det ... 0S/3529045
they're even cheaper if you need 10.
The choices on axials are hugely more expensive and lower quality.
Big advantage to using the radials caps any place you can figure it out. There are adapter boards people have made up for just such situations. Be creative. I see nothing wrong with drilling a well-placed hole or two in the circuit board if it works and you can securely mount it.
If you can figure out how to mount these, these are 10,000 hour @ 105c rated, 68uf @ 350v, very high quality parts, and they're $2.52 each in qty 1 units.
https://www.digikey.com/en/products/det ... 0S/3529045
they're even cheaper if you need 10.
The choices on axials are hugely more expensive and lower quality.
Big advantage to using the radials caps any place you can figure it out. There are adapter boards people have made up for just such situations. Be creative. I see nothing wrong with drilling a well-placed hole or two in the circuit board if it works and you can securely mount it.
Re: Music Man RD50 limiter cathodes
I guess I was just under the impression that the Sprague Atoms were better caps compared to the IC’s as far as the axials go. I’m likely wrong as I really don’t know. There’s really no point debating that here.
As far as the radials, I had never considered them. One for the difficulty you mentioned and two, I want it to look the part. No offense but with how clean this is, if I pulled the chassis the first time and saw a bunch of radials and new holes drilled, I would be disappointed regardless of how it sounded.
I have a set of tad gold 70u/350v, so I am going to use those for the power supply.
As far as the radials, I had never considered them. One for the difficulty you mentioned and two, I want it to look the part. No offense but with how clean this is, if I pulled the chassis the first time and saw a bunch of radials and new holes drilled, I would be disappointed regardless of how it sounded.
I have a set of tad gold 70u/350v, so I am going to use those for the power supply.