RM50 EL34 bias issue...is there a mod to fix it??

I just got my new RM50 head. It sounds killer!!! But I went to try my EL34s in there, and they bias near the end of the bias pot. I can't get them to be stable in the proper range. I have read a few other posts about this, but I have not seen anything about any mods that can correct this. If I had the schematic I am sure that I could figure out a solution, but that is a bit hard to find. I am looking for a mod to correct this, maybe a different value bias pot, a resistor in series, or a multi turn pot.

That sounds like a repair to me, not a mod. EL34's should bias just fine within the sweep of the bias pot, not on the extreme end. Does anything look wrong inside the amp near the bias pot? A blown resistor or something along those lines?

I re-read the thread that I read about this problem in

http://www.rig-talk.com/forum/viewtopic.php?f=26&t=45659

It sounds like a common problem. I am going to contact Randall, but I am not looking forward to dealing with this. I just got this amp yesterday, it is disapointing to have an out of the box issue. With how common this problem sounds like it is I will probably just replace the pot myself, and try to source out a higher quality pot. I also have some KT77s on the way. If those don't bias right I will get it repaired. So far the amp sounds killer!!! I will be checking the feedback circuit compared to my RM50 combo which is one of the older ones.

Thanks for the response!!!

I just got off the phone with Randall, and it sounds like it is a known problem of a wrong resistor value installed somewhere. I got the schematic, and I am going to get the details about the wrong resistor value. If I end up removing the bias board I will be looing into a multi turn pot to allow greater accuracy, but since it is just one resistor I should be able to do it without removing the PCB.

GuitarGuyLP said:

I re-read the thread that I read about this problem in

http://www.rig-talk.com/forum/viewtopic.php?f=26&t=45659

It sounds like a common problem. I am going to contact Randall, but I am not looking forward to dealing with this. I just got this amp yesterday, it is disapointing to have an out of the box issue. With how common this problem sounds like it is I will probably just replace the pot myself, and try to source out a higher quality pot. I also have some KT77s on the way. If those don't bias right I will get it repaired. So far the amp sounds killer!!! I will be checking the feedback circuit compared to my RM50 combo which is one of the older ones.

Thanks for the response!!!

Click to expand...

For what it's worth, I ended up pulling the board in the amp I spoke about with the problem and replacing the pot. The EL34s still bias up in the higher end of the pot range so it's quite sensitive but it fixed the sticking problem I was having.

Perhaps a better fix would be to add a resistor on a switch and change the ranging of the pot thereby creating an 'EL34 mode'....no idea if such a thing would introduce unacceptable noise into the bias circuit however.

I've no experience with multi-turn pots.

I think a switch would be great, a high low range on the bias adjustment would allow a lot more accuracy.

A Multi turn pot is just as the name implies instead of a 3/4 turn range or so it has usually 10 full revolutions to cover the full range. Complete overkill, but I am not sure if there is a 2-3 turn pot available. Looking at the schematic I think that the resistor in series with the pot (R36) that sets the range of adjustment might be what needs to be changed.

Two have two ranges you would just need a switch and two R36 resistors of different values. I could be completely wrong, and I don't really see the point of doing that mod, but that is the reason that a lot of other amps have a switch is to give you a less sensitive adjustment.

I did use some Deoxit fader lube on the pot, and that helped loosen it up a bit.

Just got the instructions for the fix. R36 change from 4.7K to 10K to get the proper range for the bias control.

Not 10K, that lowers the range. I am thinking that 1.5-2K will be perfect. I will post the exact value once I get it figured out.

I own 2 RM50'S and have never had any issues in biasing el34's kt66's or KT77's . Is the amp brand new?

This is a brand new RM50 head. I just got it on Tuesday. The resistor that I am changing sets the range for the bias adjustment. By going to a lower resistor value it increases the range.

That moves the bias point for EL34s away from the very end of the adjustment where the potentiometer is not very linear.

OK I just heard back from Randall, and they did some searching. The change is to R35 from 1.8K to 3.3K, or 3.6K I will put the original R36 back in, and change that one out tonight. I will report back once I have made that change so that I can pass on the proper information, but looking at the schematic increasing R35 will have the same effect as lowering R36. I just hope that R35 is a bit easier to change out without removing the PCB. R36 is a bit of a challenge especially with ROHS solder.

This is pretty much the same problem I ran into...unfortunately, I don't have the expertise to be implementing this fix, so I can either run the EL34s really cold, or just run the stock tubes. Kind of a bummer. I just wonder if I've damaged something with those massive spikes in the process.

R36 was put back to stock value, and R35 was changed from 1.8K to 3.3K, and now I can bias my EL34s perfectly without being right on the end of the adjustment range.

Re: Lead free solder.
Melt some standard 60/40 onto the lead free connection. The lead free will melt faster and then will alloy with the lead making removal much easier.
It will also make clean up with solder-wick easier too.

Bumping an old post for solid knowledge....Just fixed the bias on my RM50 using the 3.3 swap at R35....

sweet, thanks for that info. is it tough to get to? and is that all there is to it?

Easy to get to and simple...One of the few easy fixes to a problem I've encountered on here...lol...

Mattfig said:

Bumping an old post for solid knowledge....Just fixed the bias on my RM50 using the 3.3 swap at R35....

Click to expand...

Matt, I have the much smaller beige colored resistors, there are some scattered around the board. my 3.3 is much smaller than the resistor at R35-A. Can I assume this is a lower wattage/voltage spec resistor and may not last?

just f'd it up anyway...

burned out one of hte holes...