Is a Power attenuation toggle switch stock on RM50 Combo?

I just scored a RM50 combo loaded with a KH-1 and 1086 modules for $379. I took a chance with this from GC online. There was only one poor photo and very little written in the description. When I got it, I noticed that it had EL34 tubes (I think stock is 6L6), the stock Celestion speaker had been swapped for a Maverick speaker and there is a toggle switch on the back of the chassis near the transformers that is labeled "Hi" and "Low". I assume it is some kind of power attenuation switch. My RM100 combo does not have such a switch. Is the switch standard or did someone mod the amp?

It's modded.

Does this sound like anything that you might have done? It was shipped to me from a GC in Georgia. I am a little paranoid to power it up until I find out more about this mod! Should I be wary?

No, that's not my work nor any pro I can think of. I'd fire it up ASAP while it's under return policy. Worse case scenario, I have the last RM50 combo NIB here for sale.

Well, I fired it up! No smoke or fire yet! It sounds great...when it is working! The power keeps cutting in and out intermittently like the power tubes are going bad, yet the power tube status lights on the back of the amp are not indicating failure? The modules that came loaded in it work fine in my RM100 and my RM100 modules also cut out when placed in the RM50, so I don't think it is a problem with the modules. However, I can sometimes get the power to cut in and out by switching channels or by manipulating the controls on the modules. Kinda weird!

All of the chassis tubes were removed and placed in a seperate, well packed box prior to shipping. When I reinserted them, I did not touch the tubes with bare finger-I used a cloth rag to handle them. Is it possible that they may have been damaged or that I did not insert them all back in the same spot they came from? The bubble wrap they were packaged in was numbered "1-5", but I guess I could have accidentally mixed up the 12AX7s. Two of them were labelled "12AX7", while another was labelled "Phase Inverter". Does it matter where they go? Which is the correct position for the Inverter tube? Could it simply be a biasing problem?

Any advice would be greatly appreciated! I really want this to work out because I have been scoping out RM50s for a couple of years and this was the first one that came up in the right price range for me. I got it for $379 loaded with the KH-1 and 1086. I really cannot afford anything more at the momement.

Ricey, It doesn't matter what slots the 12AX7's are in. It could potentially change the way the amp sounds if you mix match the slots, but they should all work in any slot. (As long as they are all 12AX7's, and as long as the tubes themselves are in working order) Put the one that was labeled PI in the slot that's right next to the power tubes, but the amp should work either way.

As far as mix matching the power tubes. It could possibly make a difference, but the tubes would have the be really unbalanced for it to cause the amp to do what you're describing.

Do you have a mA meter? If so checking the power tubes is really easy. It's in the RM50 Manual. The manual is easy to find online.

I do not have a meter.

Well I'd loan ya' mine, but I'm betting you don't live in my neighborhood.

Yeah...I am in Buffalo. I am thinking that it mght be that whole corroded FX loop or boost jack thing that has been plaguing amps lately. I will break out the contact cleaner when I get home later and give it a try.

Ricey said:

Yeah...I am in Buffalo. I am thinking that it mght be that whole corroded FX loop or boost jack thing that has been plaguing amps lately. I will break out the contact cleaner when I get home later and give it a try.

Click to expand...

Very common, especially if an amp has had something plugged into the FX loop permanently or the amp has been stored a lot.

Spray contact cleaner onto a regular 1/4 plug then work it in and out of the loop sockets a few times always seems to do the trick for me.

VOILA! I sprayed contact cleaner on some short patch cables and plugged them into the FX loops and I also sprayed the cable plugged into the boost input jack and then plugged in a Boss FL-5 switch pedal. Both the amp and boost feature are now working fine. The only problem is that the amp still craps out if I unplug any of the cables, but at least I know the source of the problem.

Maybe the "Hi-Lo" switch is a "Triode/Pentode" switch.
Question for Rob. Is a Triode/Pentode switch feasible to do on an RM50?

Fan speed maybe?? Can you post a photo of the circuit showing the switch inside?

I appreciate your chiming in Bruce! I would love to post a picture of the circuit to get an opinion from the "creator" but I am not an amp tech and would be hesitant to start pulling the amp apart. Fan speed is an ineresting idea. I will try to see if I can hear any change in speed when I flip the switch. BTW, I also really appreciate that you designed a fan into the amp...it is a nice touch!

I could not hear any difference in the fan speed when switching between the Hi and Low settings. I have been afraid to play through the amp set to low, not knowing if the modder really knew what he was doing and fearing a potential blow out, but I guess I am going to have to give it a whirl and see if it is, in fact, some kind of volume attenuator.

It's quite possible that the switch is a "triode/pentode" switch. A tech would have to look at the circuit to know. The connections would be to the screen grid resistors and/or the connection to the screen voltage tap right after the choke in the DC supply circuit.

What does a pentode/triode switch do? Could engaging it result in any damage to the amp?

I have two types of amps with Triode/Pentode switches, one is a Seymour Duncan Convertible and an Ampeg VT 60.
It does NOT "disconnect" one of the tubes.
Without knowing some tube theory, it's a little difficult to explain in a short description. I'll try.
The RM50 uses output tubes that are pentodes.
Watch these two videos:
https://www.youtube.com/watch?v=Xo4nVjj4NSE
https://www.youtube.com/watch?v=Zh_51RP4V6I

When you disconnect the voltage to the screen grid, the amp will sound more compressed, earlier brake-up, more mids, and basically sound like a "smaller amp driven harder." You will either like the sound or hate it.
It does technically change the impedance of the output tubes but, in both of the amps I have, no change is required to match speaker impedance. The actual output wattage will go down.
If it was done correctly it shouldn't harm the amp. It may actually run cooler. If you like "loud and clear" you won't like it. If you like old "Tweed Deluxe" you very well might like it.

OK, I will give it a try. BTW, Tweed Deluxe is one of my favorite amps! I have been looking for a good Tweed inspired pedal to add that sound to my rig.

I would have a tech or someone who knows what they're looking at take a look at the amp first, if possible. Since I can't "see" it, that's the best I can tell you.