New MTS owner - RM50 head/Clean/Ultra XL mods

[trimix]

New MTS owner here, I bought an RM50 head with the Clean (black stripe) and Ultra XL (chicken head) modules installed. It arrived with a loose reverb tank that took out one of the JJ 6L6GCs, in turn popping the main and tube fuses on power up. After picking up a pack of fuses for each, I swapped in a pair of GT E34Ls and set the bias around 41. I also added some foam spacers under the reverb tank to prevent the circuit board from resting against the head shell and to cushion it's ride.
The Clean module sounds fairly flat but makes a distortion pedal sound huge. Checking out the schematic on this site I found it didn't match my module and the Gerber file is actually labelled "Blackface_Clean Gerber\modu_twin.brd" while being identical to the Blackface schematic. It appears there is no Clean module schematic available, but my module has the 150 ohm resistor in R1 already and I didn't plan on changing anything else.
The interesting bit is the Ultra XL module has the 150 ohm resistor in R1 as well, which I thought was a low gain mod only? It looks stock but there are some other changes which are obviously after market. C3 was changed to a .0047u as found on the Ultra, R31 was changed to a 33k as found on the Ultra (marked as R25 on the PCB, but it's the one behind the mid knob), and C14 appears to be an after market cap which I can't read the value on. I would guess it's been changed to a 100pF as those are the only three differences between the Ultra and the Ultra XL schematics, and what I have on my hands appears to be an Ultra XL that's been turned into an Ultra under the hood.
Questions - Do you normally find a 150 ohm R1 in the Ultra and the XL or the 470 ohm as shown on both schematics?
I'm going to change C3 to the .0015u as shown on the Ultra XL schematic and as advised in the mods section. I'm considering adding a switch to go from the Ultra mid values (R31 & C14) to the values from the Ultra XL. Anybody have experience putting a circuit like this on a single switch? I could just change it back to an XL, but it would be cool to compare the two at the flick of a switch... Thanks

 

[Julia]

You've got those E34L tubes biased really hot. They have the same specs as the JJ EL34. It's just has a little more headroom than the standard EL34.

http://www.jj-electronic.com/pdf/E34L.pdf

Both tubes have limiting value of 25W. Try biasing around 33 mV. That's about the 70% max range.

41mV is IMO too hot. That's 6550 range. 6550s are 35W tubes. I have my 6550s at 42 mV (62%)

FYI: my experience with E34Ls is not good. 3 months in the RM100 biased at 37 mV. 3 weeks in a Marshall where the tech said he biased them real hot to get rid of the crossover notch.

 

[trimix]

I just went off the RM50 manual's chart stating 35 - 45 was a good range for E34Ls, same range stated for the 6550s. I'll dial them back to the low side of the EL34 range as I have some Mullard pairs I pulled from my Sound City head that I want to record with.
If I get a reading under 1mV of difference between two tubes, that's a good match when evaluating pulls, correct?

 

[slashnstrings]

I too went by the RM100's chart 35-45, but i found 36 seems to work just nicely. Although I got a buddy amp tech and he said it should be more like 30-40 rating for the E34l's rather then 35-45. and he recommended 34, but 36 sounds better to me.

And i think its any reading under 4.8Mv between two tubes if memory serves me right. 5Mv and up bad match

 

[SacredGroove]

It sounds like the U/XL was converted to an Ultra. I'm surprised to see that C3 is .0047. It seems that modules from '07 to present have 150ohm resistors at R1-R3, from what I've noticed.

I've installed plenty of switches on my own modules for shifting the mids, as well as bass cut/boost. It's a little tedious, but not too bad when you do it a few times. If you do it yourself make sure to clear out all of the old solder, so your new connections are a s clean as possible. Also, use solid core leads from the switches, not braided wire.

I think I had my tubes biased at 37mV. They were JJEL34Ls in a VHT 2/50/2 poweramp.

 

[Julia]

Here's what I've come up with doing the calculations for tube ratings. The low end is 50% and the upper end is 70%.

Randall informs me the transformers are rated at 530 plate volts for bias calcs. So here goes:

EL34/6CA7/E34L/KT77/6L6/5881: 25W 24 mV - 33 mV
6L6GC/6L6WGT/6L6WXT+: 30W 28 mV - 39 mV
6550: 35W 33 mV - 46 mV
KT88: 42W 39 mV - 55 mV

 

[trimix]

Copied and pasted those values, thanks Julia! That should be stickied for newbs like me, separating the wheat from the chaff while searching around here has been educational, if somewhat tedious. The bad schematic for the Clean module being one example, the difference between the resistor labeled R31 on the Ultra XL schematic and the actual PCB labeling the same one R25 being another.
The .0047 cap was definitely an aftermarket mod, probably following the Ultra schematic on this site. There was flux on the PCB, looked like they soldered it from the component side without disassembling the module.

Thanks for the switch tips, I won't really know what I'm looking at until I disassemble the housing to get a look at the trace side. I'll probably float an internal switch on solid-core wire until I decide it's worth drilling the face plate to include it.

 

[SacredGroove]

You have to consider those pcb layouts are probably at least 4 years old. Things have changed in the last few years to the modules and I doubt any new pcb layouts will be avalable for download.

 

[trimix]

Yes, I noticed none of the signature series modules were included. It's a tricky thing revising a product over the coarse of it's lifetime, silver-face, black-stripe, chicken-head, and still you can't be sure what's under the hood. My own Ultra in Ultra XL clothing being an extreme example. So we listen...

 

[djkal]

Hi julia are the information for the tube biasing is also good for the rm50 combo? thanks for the info

 

[Julia]

I think so, but I'd make sure to check the plate voltage rating of the power transformer in it. Interesting that the RM50 manual and the RM100 manuals give the same values, so one can assume (which we know what that stands for) that they are the same.