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RD-50 Head: Strange New Squawking When Powering Off

Hey Everyone,

I hope I'm not repeating a question that's already been asked elsewhere....

I have a MM RD-50 head plugged into an 8 ohm cab. No issues until recently.

Recently, when I stop playing and go to power off the amp, the amp lets out kind of loud squawk-then-popping sound as it powers off. My amp didn't always do this, but it's been doing it for a few weeks now. It seems like it's getting louder, but I'm not 100% sure. The interesting thing is that I have my amp volume set very low.

Does anyone know what is causing this noise and/or how to fix it?

Thanks in advance.



Clogged Drain?

The smart guys Mike and Lars must be on vacation so here's a guess...perhaps the HV power supply is slow to drain when powered off. Have the HV power supply's 2 bleeder resistors been checked for correct ohms and inspected for cracks and/or broken solder joints? They are the 150k 2W resistors R59 and R60. Good luck. -mgriffin

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Or the HV Filter Capacitors

The HV filter Capacitors may be old and leaky. Best to replace the whole thing.


The squeak

Hello there,

the sound you hear could be coming from upstream the power tubes/transistors. Hard to tell. As pointed out in this thread, if the bleeder resistors (or just one of them) has stretched sufficiently the B+ doesn't drain fast enough to stop other weird sounds from coming through the speaker as the amp powers down. Pulling the phase splitter IC would tell you if the sound is originating in the power section or further up the chain. Does it matter which channel the amp is on when you switch it off?

Lars Verholt


Thanks, everyone, for all the

Thanks, everyone, for all the great suggestions. Here's the update:

Based on Lars' question "Does it matter which channel the amp is on when you switch it off?" I decided to test the Limiter channel to see if the amp also squawked when I turned off the amp while it was on the Limiter channel. Previously I had only been using the Clean channel, which is what I typically use when I play.

I switched to the Limiter channel, played for a while and then switched off the amp. The squawking was not present at all. The only sound the amp made when being switched off was a very, very acceptably faint sound like a Q-Tip very very lightly being tapped on a piece of cardboard one single time (that's the best way to describe it).

I then switched back to the Clean channel and, oddly enough, the squawking was no longer present there! On the clean channel I heard no sound during power off at all now.

For some reason, switching to the Limiter channel must've done something helpful because I've played through the amp several times (both channels) over the last few weeks since first trying the Limiter channel and there is no powering off sound now on either channel - including there being no "very faint single Q-Tip tap on cardboard sound" on the Limiter channel. The amp just silently turns off on both channels.

I'm very happy, of course, but also curious. I wonder what happened that fixed the issue. If anyone has any thoughts, I'd love to hear them. The weird thing is that, as you may recall from my original post, the issue had been getting progressively worse. Now, for it to be gone all of a sudden is strange (but very welcome!).

BTW, I had been using the same exact instrument, signal chain, and power source for this entire exercise.



Reviving this thread - Squawking When Powering Off

Was a final cause ever found for this behavior? My '82 110 RD50 does the same exact thing, squawks on Clean setting shutdown, silent on Limiter setting shutdown. Amp has been completely recapped, sockets cleaned, etc. Bleeder resistors measure in spec, may just replace them to be sure.

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