HELP!!! RM50 bias haywire!

[Jaymzrock]

I had my tubes running hot at 36mV, and wanted to take them down a bit. I plug in and notice they are at like 17mV. Okaaaay...I go to bring them up and it shoots straight to 48, 54, 46mV! Then I back off and it's jumping back down to 17mV. No readings in the middle at all! What could this be?

 

[Jaymzrock]

Does anyone know what's causing this? The pot perhaps? Has anyone heard of this in the past?

 

[crankyrayhanky]

I've heard of this in the past...hopefully someone chimes in with some help :?

 

[eyeball987]

Could be a tube going out on you.

I have a RM100 and can set the bias for each tube. I had one tube that just wouldn't hold and was bouncing all over. Replaced it and all was well again.

 

[Jaymzrock]

The jumpiness is happening on both tube/test points. I'm thinking of putting them back in the rm100 and Letting them roll for a week and see what happens since the rm100 is a known stable part. That way I can monitor each tube.

 

[kc2eeb]

Most likely it's the pot. Pull the chassis and make sure it hasn't been pushed too hard with a screw driver and the solder connections are intact.

 

[GuitarGuyLP]

If you look back a couple of weeks in this part of the forum I had the exact same problem with my rm50. It is a known problem, and I got the solution direct from Randall. I just have my playbook with me, if I had my computer I would post more details, but just look for the thread I started about rm50 bias.

 

[Jaymzrock]

I sure will. Thanks GuitarGuy!

 

[Jaymzrock]

Ok, so R35...1.8 to 3.3k. I have basic soldering skills from my job, but it's mostly wire bundle/connector repair. Is this resistor somewhat easy to access? I don't want to ruin my newborn amp but I also can't really afford a tech right now. Maybe a few pics if possible would be superhuman of you. If anything I'll step by step it with my camera for a future sticky.

 

[kc2eeb]

This is what the inside of an RM50 looks like.
http://mtsforum.grailtone.com/viewtopic.php?t=12785&highlight=lots+pictures+rm50+repair

Took these pics when I replaced the power amp board and O.T.

 

[Jaymzrock]

I think the resistor fix is the way to go if it's determined that the pot is good. However, I just swapped back the EL34's to the RM100 and the 6L6's back to the RM50 and all is kush in the world again. Crazy how 6L6's work fine in the RM50 but she gets all prissy with the EL34s. Thanks for all the help everyone!

 

[Mike361]

That is weird, are you sure the tubes are ok? Wondering if the 4 tubes in the 100 is covering for the bad ones. IDE have to try a different set of 34 before going further with repairs.

 

[GuitarGuyLP]

Basically changing the resistor changes the bias range. Right now some el34s are near the end of the adjustment range. Where the pot is not smooth, or linear. With the change you can bias most el34s nice, and easy.

It is not the easiest soldering job I have ever done unless you take the board out. I left mine in which is easier to damage the board that way, but I have a lot of experience with this type of work. The easiest way to run el34s in the rm50 is to request a pair that biases very hot so that a lower setting of the pot will get you biased perfectly. Same goes for kt77s. If you want great sounding cheap el34s get a pair of the Chinese made ones that ruby red-brands. I am pretty sure they are shuguang or something. They are usually $25-$30 for a matched pair, and sound killer!!

Hope this helps!!