RM50 troubles

[Mattfig]

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.

 

[fuzzboat]

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:

 

[Mattfig]

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!

 

[Jaded Faith]

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

 

[Mattfig]

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

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 ....

 

[audiomidijace]

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:

 

[Mike361]

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.

 

[Mattfig]

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....

 

[Jaded Faith]

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

Glad it's fixed. :lol:

 

[Mattfig]

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

Glad it's fixed. :lol:

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...

 

[[email protected]]

Hello all!!! I just as of recently had this happen to my RM50,only 1 tube will bias the other is a crap shoot. So i am in need of some knowledge,PLEASE. Also does anyone have a schematic of a Randall RM50 combo. What size reistors and how many do I need ? Thanks!!