Mark IIB reverb etc, again

[Guitfiddle]

Just picked up.
Fully loaded simul-class EV 12L combo.
86xx serial number.
"JC 11-81" top inside edge of chassis.
RP-9B pre-amp board.
x100 power tranformer
Under power cord small case "rl"? "rb"? "ml"? Can't really make it out. I'll search some images.
"LJ 2/11/82" under chassis.
"HRGS" both sides of chassis.
Anyhow, thought I would ask a couple of questions here before I call MB on Monday. I need to call anyway because the EQ sliders are all broken off but still work. Probably ship the chassis for service and updates, effects loop mod?, etc. We'll see.

First the sound. Right now it doesn't sound so good. The cleans has no low note clarity; sort of blurry and not piano-like. The filter caps are getting ready to explode; ends cracking and almost pushing stuff out. All 30uf's test at 50uf. Amp has been dormant for years. This part I understand.

The reverb on "10" is almost non-existent. Pan is good, as are all connections. I traced the circuit and and do not see V4, the 12at7 driver, paralleled! This would account for the weak reverb. The blue hot wire goes to V4b plate, plate to tube cathode .001, and unbypassed 1K cathode resistor. That's the correct values for the IIb. But the plate, grid, and cathode are not paralleled. The whole circuit seems to match the IIB until I get to this area. This one has me baffled and I build simple tweed/Marshall/blkface tube amps. I'm a master level guitar tech but not master amp tech.

Also, the black marker tube array on bottom of chassis from V1-V4 is ax7, at7, ax7, at7. V1b goes right into V2a drive circuit. That shouldn't be an at7?

Amp is all original including 4xSTR 415 6L6's. Didn't have EL34's in the outside. Doesn't look like any of the tubes have ever been removed from the sockets!

By the way this amp has been on many hit recordings in L.A. during the 80's and 90's. The seller has been a guitar repair client for 17 years.

Thank you in advance for your help.

 

[Guitfiddle]

Ok. Found an image right here. It's mb under the power cord.

I can get more clean note clarity by plugging an 8 Ohm speaker into the 4 Ohm speaker jack.

V2: that would be the effects loop. Hence the 12at7.

Mesa schematics can be quite a bit different than actual amp.

Must call Mesa.

Still don't understand V4 not paralleled.

 

[Guitfiddle]

Update if anyone cares. Talked to Mike B at MB. It's a later Mark II and doesn't use all of V4 for the reverb driver. V4a is part of a cascode circuit on the high gain section. I would've preferred the stronger reverb model. But he gave me a couple of ideas to increase the reverb. Nuff said.

 

[dino2sf]

Send it to Mike B at Mesa have him do the effects loop mod and a cap job (it will cost $400 - $500 but well worth it) . It will make the reverb way better on both channels and it will greatly improve your lead channel. It will be as close as you can get to a Mark II C+ after the mod.

 

[Guitfiddle]

Yeah I'm considering sending it to Mike for the loop mod. If I saw one that was done I could do it myself but that info doesn't seem to be available. I believe it involves cutting traces. We talked and I can do everything else using their parts which are priced very reasonable. Did a couple of things to the reverb circuit that we talked about and it now has as much reverb as any Fender Blkface that I have. It can go surf. It's that good. Simply bypassed V4b 1K cathode resistor with a cap and changed the reverb level pot to a 200KL. In my stash I had an original 1980 250K pot that read 200k. He gave me several others ideas to change some resistor and cap values around the reverb tubes but those two changes did the trick. Retained the 12at7 driver because V4a is part of the cascode circuit. Tried several different #4 spring pans and the original #9 sounded the best. Now I can keep the amp because without good reverb it would be useless for me. Also need to change the 1Meg master volume pot as it reads 1.7Meg! Mike told me that might happen. I have original 1980 1M push-pull pots.

A concern I have about the design of this series is that the power tubes are mounted to the board and if a tube shorts out it can easily burn the board. If that happens I am capable of hand wiring the PI and power tubes.

Anyone ever install a pentode/triode switch like on the Mark IV? Right now the outside tubes are in triode.

Duh. Realized our Phoenix store is a Mesa dealer and we have a master level electronics repair guy. I run the repair shop at our Prescott location which is 100 miles north of Phx.