The preamps I would use it with were both American and British flavored and seeing how I would never use it as a stereo setup, i thought it would be a great thing to mod one side to be able to run EL34s.
By the way, my 50/50 was version A made in '89 and needed some maintenance when I received it.
Starting with the basics, Mesa is a bit cryptic about the P.T. specs, so going from an equivalent replacement from TAD, I assumed that the specs are the same, so:
350-0-350 @ 400mA and 0-6.3 @ 5A.
Quick calculation for heater current:
- Stock with 4x6L6GC, 3x12AX7 ~= 4.6A
- With 2x6L6GC, 2xEL34, 3x12AX7 ~= 5.8A
So, the first conclusion was that the stock PT couldn't support the extra current if 2 tubes were EL34. For this reason and taking into consideration the space limitations inside the chassis, I decided to fit a Hammond 266M6 ( 3.15-0-3.15 @ 3A ), which was used to power the EL34 pair exclusively (Channel B).
Moving to the H.T. winding, according to my calculations it should be O.K. For 450V on the plates, per the schematic and calculating for 100% dissipation for both types (30W 6L6GV - 25W EL34) and then giving a 150% safety margin for operation, the current demands are actually lower in case there are 2x6L6GC, 2xEL34, 3x12AX7 instead of 4x6L6GC, 3x12AX7 in the amp. So, since the HT is enough as is, it should be OK with the EL34s also.
Regarding the choke, again Mesa is cryptic but as far as I could gather, they were using a choke similar to Fender 125C1A, so 4H @ 90mA, 105 Ohm DCR. The Hammond 194B was a perfect candidate.
Taken from TDSL Duncanamps, the screen draw + preamp draw in each case was:
4x6L6GC, 3x12AX7 = 2*(5.6...22) + 6*1.2 = 18.4...51.2 mA (... denotes a range)
2x6L6GC, 2xEL34, 3x12AX7 = (5.6...22) + (8...50) + 6*1.2 = 20.8...79.2mA
So again, even in worst case scenario, the draw should not exceed 80mA, so choke was theoretically approved. (There would be a problem with 4xEL34s according to my calculations but with only one EL34 pair it should be OK).
Moving on to the voicing concerns, in version C Mesa Boogie introduced a Deep mod from the factory to add more depth to the sound (which in reality involves only adding the choke, converting the first preamp cathode from 1K/0.15uF to 2K7/15uF and adding a 0.003uF 'resonance' cap in the Feedback Loop and a resistor after the 1st stage coupling cap). Since my version was A, i decided to implement the Deep Mod as well.
Regarding bias, it was made adjustable, with one trimpot setting the "overall" bias (affecting both the first 6L6 pair and the second EL34 pair) and a second trimpot used for fine-tuning the bias of the second EL34 pair. That means that the second pair can be also adjusted to 6L6 specs, making possible to use either 6L6 or EL34 on the second channel, as desired.
I also decided to install a Resonance pot (as stated version C already had added a 0.003uF cap in series with the presence pot), providing more resistance to the lower frequencies and thus lowering the feedback applied for those frequencies, making the amp sound bassier/fuller. I thought about making this adjustable by wiring a 1MA pot as variable resistance parallel to the 0.003uF and it works very well - at 0 it is the stock version (the cap is shorted) and at 10 it is like version C but the difference is that I can set the pot anywhere between. Normally a setting between 4-7 provides more depth without boominess.
Regarding the screen grid resistors for the EL34, i increased them to 1K from 470 Ohms to be more in line with the EL34 needs.
Lastly, i changed the Volume pots to switched type to provide some protection to the output transformers. In the 50/50 there is a single Standby switch which means that if an input is provided to a channel with no load connected and the volume pot is mistakenly set to other than 0, damage to the output transformer can occur. A switched pot seemed like an easy solution to avoid accidental rotation of the unused volume potentiometer. The presence pot was replaced by a stereo pot of the same value, which changes Presence for both channels and the same was done with the Depth pot.
All in all, without altering the Front Panel, useful functionality was added: protection for the unused channel and Depth control that was not available in the original.
To conclude, this was a really fun project that made the power amp perform and sound better than ever, offering great versatility compared to the stock version. At the moment it is equipped with JJ 6L6 (Channel A) and JJ EL34II (Channel B) and there are discernible differences. The EL34 pair has more aggressive and present mids, while the 6L6 side provides more bass and a smoother distortion.
