RM50 troubles

Hey guys,

Kc2eeb has been helping me a ton and assisted me in checking out all the basic connections and pin sockets working properly as well as verifying no open or blown fuses/resistors...All is fine there...

BUT...

Here's the problem in a nutshell:
Both tubes power up, both have plate voltage, only one tube will bias, the other reads a MUCH lower voltage but does adjust with the bias knob a small amount...So basically I can crank one up to 118 while the other goes to -1 ... ???

No, it's nothing like the tubes or anything like that...It's something more sinister and I just don't know what....I've been driving myself insane with this as I really need to get it up and running...

ANY ideas??? Thanks guys.

start looking for burnt grid resistors or plate resistors

most likely the resistor burnt "open" before it started to cook
and turn brown & smell

hence you'll need a meter to find the bad resistor (-- infinite OHMs)

that's what you get when you run big KT type tubes in the power
section :wink:

I checked R35 and 36 and they are fine...There are so many and I am not that familiar...Any suggestions of common suspects? BTW the tube in question is the one closest to the transformers if that narrows it down...

Thanks Gary!

Matt,
I've fixed a lot of these amps and the only time I have seen one tube bias but not the other was when a Grid or Screen resistor was bad. I won't have to show any signs of burning, it could be a short or a crack. Often it's one of the large 5W, 1K ceramics that have a hairline fracture. I'll bet that's it, particularly if it was just shipped.
Hope that helps,
Rob

Jaded Faith said:

Matt,
I've fixed a lot of these amps and the only time I have seen one tube bias but not the other was when a Grid or Screen resistor was bad. I won't have to show any signs of burning, it could be a short or a crack. Often it's one of the large 5W, 1K ceramics that have a hairline fracture. I'll bet that's it, particularly if it was just shipped.
Hope that helps,
Rob

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Very much appreciated...They seem to check out but I am gonna replace them tomorrow as it's cheap and easy...And the most likely suspect...

Kc2eeb gave me a bunch of methods to check as well...It's weird as everything is checking out but that bias won't work right....I'm hoping it's a strangely inconsistent resistor ....

If only the RM50 had MIDI! :evil: The you'd be able to connect to any current Apple computer with a MIDI interface and use their Time Machine back up application to return the amplifier back to the state it was in on a date before this happened! :shock:

Very strange that just yesterday I tried some el34 tubes in the rm50 head I got and could only get one to show a normal reading. The other tube really low. I put the 6l6's back in and they both bias fine. I assume bad tube but it seemed to switch sides on the low output one. Have to try some other el34 to see if it is the amp or the tubes.

Holy crap! It's fixed! I replaced the screen grid resistors with new ones and all is perfect!

Thank you guys...This amp was about to get its *** kicked....

I'll refrain from saying "I told you so."

Glad it's fixed. :lol:

Jaded Faith said:

I'll refrain from saying "I told you so."

Glad it's fixed. :lol:

Click to expand...

That's why I asked here...I know you guys know more than I and I appreciate everyone sharing their expertise so freely...

It's super cool to be able to save repair fees for simple repairs so I can buy more mods! Plus I like learning how things in here work and now I know a little bit more...