rm50 bias issue

  • Thread starter Thread starter dawnofdreamx97
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dawnofdreamx97

dawnofdreamx97

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Sup fella's

Picked up a rm50 today , sounds pretty cool so far ... I popped in some ruby el34bstr power tubes and some chinese preamp tubes and went to bias and noticed something weird. To get the tubes up to 30-35 mv I am towards the end of the bias pot sweep. As i get towards the end the control is so sensitive that it jumps from about 22 mv up to passed 48mv with the SLIGHTEST little touch... what could this be ? tried a few different sets of power tubes and seems to happen on all of them. I've got it at about 38-40 right now, as that is about as close as i could get it. I just fear that it may move from transporting it... any help would be greatly appreciated... :rock:
 
I posted exactly same issue on MTSforum with one of my amps. It has a 'stickier' pot than the other two which compounds this problem a lot...it almost seemed to me as if a value in the bias circuit had change on newer production....

...unfortunately the only answer I came up with was to use a set of tubes that were in a different part of the pot sweep.
 
dawnofdreamx97":1s3xu7ap said:
Sup fella's

Picked up a rm50 today , sounds pretty cool so far ... I popped in some ruby el34bstr power tubes and some chinese preamp tubes and went to bias and noticed something weird. To get the tubes up to 30-35 mv I am towards the end of the bias pot sweep. As i get towards the end the control is so sensitive that it jumps from about 22 mv up to passed 48mv with the SLIGHTEST little touch... what could this be ? tried a few different sets of power tubes and seems to happen on all of them. I've got it at about 38-40 right now, as that is about as close as i could get it. I just fear that it may move from transporting it... any help would be greatly appreciated... :rock:
Click to expand...
The nice thing about it is you can check it when you get to your destination,can't say I ever had it go off that much but I always carried a meter with me cause it's just easy to do,I tend to check it every time I fire it up though :doh:
 
JKD":2tl44wo0 said:
I posted exactly same issue on MTSforum with one of my amps. It has a 'stickier' pot than the other two which compounds this problem a lot...it almost seemed to me as if a value in the bias circuit had change on newer production....

...unfortunately the only answer I came up with was to use a set of tubes that were in a different part of the pot sweep.
Click to expand...


thankyou man... that was very helpful...! :rock:
 
The jump is a bad pot. The 34's are gonna blow up anyway so just don't bother!

Options range of bias circuit is not quite right. It's ok to be at the end of the bias pot. But only if the pot is not defective.

Option: A pair of matched tubes that have a much higher current rating would allow the pot to have a lower setting.

But the indication is the pot is bad which is a bad thing. Suggest getting it repaired and while it's getting repaired have them replace the blown up 34's with 5881's :lol: :LOL:
 
Jeff Hilligan":7pluifnu said:
The jump is a bad pot. The 34's are gonna blow up anyway so just don't bother!

Options range of bias circuit is not quite right. It's ok to be at the end of the bias pot. But only if the pot is not defective.

Option: A pair of matched tubes that have a much higher current rating would allow the pot to have a lower setting.

But the indication is the pot is bad which is a bad thing. Suggest getting it repaired and while it's getting repaired have them replace the blown up 34's with 5881's :lol: :LOL:
Click to expand...

What he said.... :yes:
 
Jeff Hilligan":29rt0dbb said:
The jump is a bad pot. The 34's are gonna blow up anyway so just don't bother!
Click to expand...

Randall needs to source better pots! I have one that's iffy on a brand new amp from 2008 and the new production circuit board I got for my other amp has a defective pot full stop. I can't use most EL34 type tubes in it....luckily I have a spare...unluckily I don't feel like tearing the thing down again, the wiring harness makes it a PITA to move that board.

Never had this problem with either of my MOD50s....heck that pot even feels better quality..smooth turn, accurate settings with minor chages are easy :D
 
thanks for the info Jeff...Gotta get on the horn with randall tommorow... it shouldl be under warranty...
 
I know this is bumping a very old thread, but I don't want to have to go over all the details again. I just got a new RM50 head delivered yesterday. It has the same problem with biasing EL34s.

What I would like to know is, is there a replacement pot that can be installed of better quality? Is there a multi turn pot that can be used to allow greater biasing accuracy? Or is it easier to just use EL34s that bias hotter??
 
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