bias troubles RM50

[withmittens]

, so I am changing out tubes and i have JJ KT77 in there. My bias pot will not get me in where I need to be. it jumps from 28 to 60 Mv with no way to get in at like 40 or so.

Whats up? is it broken or is there a trick. never had this problem before....6L6's dial in fine.

closest I can get the KT77's is 34/32 Mv, then it jumps to 60's at the slightest touch.

 

[Jaded Faith]

You amp needs to have the bias circuit modified. You need to change R35 from 1.8K to 3.3K. You can then bias EL34's and KT77's perfectly without being right on the end of the adjustment range and having it jump around.

 

[JKD]

You amp needs to have the bias circuit modified. You need to change R35 from 1.8K to 3.3K. You can then bias EL34's and KT77's perfectly without being right on the end of the adjustment range and having it jump around.

Wow, I've been looking for this information for a while.....it's less jumpy on my MOD50, I wonder what value Bruce used in there = any idea?

 

[Jaded Faith]

I believe it's the 3.3K, as did Randall for most production runs. There are just a bunch out there with the wrong value installed. No spec change was ever made, it was simply a QC error.

 

[withmittens]

Thanks for that info, Rob. is that a pricey or difficult thing to do for the average amp guy?

 

[Jaded Faith]

No, It shouldn't be. That said, I'm often shock to hear how many techs won't touch these amps. I've recently had a couple amps come through the shop that some very respectable technicians would not touch. It should take 30-60 minutes all said.

 

[withmittens]

curious to know if the circuit board is resistor side up so that removal from the chassis is not needed. If I open it, will the circuit be right there and easy to get to. I don't see me being able to make a trip amytime soon...

thx

 

[Jaded Faith]

Yes it is.

 

[withmittens]

Thanks, Rob. I would love to get down to you but work is a killer. thanks so much for the info. My local alto Music, which is US Music's repair shop says he'd do it, althought they no longer work on the MTS. 3 weeks time with the amp.....


If I could wait a while I would bring it to you and look into one or two of those ala cart mods too.

damed adult life nonsense.......