RM50 Bias Pot is SO touchy

[jc74868]

Swapped in some new tubes and went to bias the amp. Got it set, but it was extremely tricky. The adjustment is very sensitive. It took quite a few tries because every time I touched it with a screwdriver, the reading jumped very high or low. Anyone else experience this? Also, it seems so loose that I worry it may actually move and change bias just through vibrations from playing loudly or transporting.

 

[Mattfig]

RM50?

 

[jc74868]

Yup (originally a combo that I converted to a head)

 

[Mattfig]

OK, so you are handy with amps to a degree I assume...

If you check R35 near the bias pot it should be a 3.3k Ohm resistor, my guess is that yours is not...My wasn't either and that was the problem....It's been pretty documented over the years here...Safe and easy change if you can solder...

BTW, the tubes you swapped in...Were they EL34?

 

[jc74868]

I'll give it a check and see what I've got in there. Sounds like an easy fix if the resistor is the issue. I picked it up dirt cheap on craigslist. It looked mint, but since I gig fairly often, I swapped out the 6L6s immediately for some new Ruby 6L6GCMSTRs I had sitting on a shelf.

I've never tried EL34s. I had an RM100 for a short time when they first hit the market and ran 6L6s, and every amp I've ever had has been a 6L6 amp. I might try some EL34s or even some 6550s just to see what they sound like.

 

[Mattfig]

Just curious...Did ya get it fixed??

 

[jc74868]

Just curious...Did ya get it fixed??

I sure haven't. My entire rig is in a rented space about 45 minutes from home. Practiced yesterday and didn't think to open it up. Was gonna bring my multimeter with me to at least check to make sure it was still biased correctly, and totally forgot to.

 

[jc74868]

I did just pick up some EL34s on ebay for unbelievably cheap. Sometimes my OCD ebay scrolling pays off. I'll probably install them and take a look at the resistor soon (hopefully). I think I'm probably more of a 6L6 guy, but I'm curious to try an EL34 tone, and with the MTS, it's just so freakin easy

 

[eyeball987]

Every bias pot I have played with on the RM's (2 RM50's, 2 RM100's) have reacted the same. Doesn't take a lot of movement on the pot to move the bias reading a good bit.

 

[Rising Farce]

The bias pot on my early RM50C was extremely sensitive, to the point that dialing in precisely the number desired was virtually impossible. My (newer) RM50C and RM100 head are much more forgiving.

 

[Vin750]

I find that my rm50c bias pot is touchy as well. I am able able to still dial it by first getting it close and use just slight pressure on on the screw driver to hone in the values. I am going to check that resistor on mine when I get a chance as well.

 

[crazyeddie]

I've got the same problem with my RM 50...Haven't modded it yet because I'm still able to dial it in.