RM50 - biasing issue

[tnjazz]

So I'm trying to bias my RM50 and I'm getting no reading on the second test point at all. The tube is lit and appears to be getting power but shows nothing on the meter. The first test point works as expected.

Anyone have any ideas on what to look for to troubleshoot? I haven't pulled the chassis yet or anything. The outside fuses are fine. I even swapped them and I still have the same issue.

 

[warmachine67]

Does it still make sound as expected when you play through it?

Did you change tubes from one kind to another or are you just checking bias?

 

[tnjazz]

It still plays but unless my ears are deceiving me it doesn’t sound particularly great. To get what I would consider gig volume I have to turn the master up to noon or slightly more with settings that peel paint in my Synergy rig, so I’m suspecting maybe it’s only running on a single tube or something.

 

[Mattfig]

Sounds like a single tube is running then.....I would try another known set of power tubes before doing much else....

 

[tnjazz]

Sounds like a single tube is running then.....I would try another known set of power tubes before doing much else....

Already did that with a brand new unused set. Same result.

I agree, it seems like only one tube is actually running in the amp. I'm taking it to my amp guy in a week or two anyway, but in the meantime I am still hoping it's something simple and someone here has seen it before.

 

[JKD]

Any recent popping / electrical arc type sounds or otherwise loud noises?

When you pulled teh tubes - did you see any brown/black residue around base/pins or socket?

I am thinking a Screen Grid Resistor is down....but someone more educated may know more.

 

[Mattfig]

Sounds like a screen grid resistor indeed....They are the big 5 Watt white ones on the board....Do you see or measure anything across the one not working?

Sorry, you're gonna have to do some surgery.....

 

[tnjazz]

Haven't cracked it open but will do that this weekend.

So if the resistor is gone, the tube will still light up and appear normal visually? Didn't know that.

 

[Mattfig]

Yes but won't get power to it beyond that light you see...

 

[tnjazz]

Thanks guys! I am a little too chicken to work inside of amps so I'll leave this one for my amp guy. I was planning to trade the amp to him in exchange for a mod anyway, but I wanted to rock out on it a bit in the meantime. Oh well…

Thanks again for all the info!