‘78 JMP 2204 1st Service/TAG Optimization (longish)

[Raoul Duke]

So a couple of months ago I traded a Z-Wreck Jr for a 2204. Sort of a random, opportunistic thing. Had an ‘82 2104 about 10 years ago and didn’t really like the way it sounded; but didn’t know much about the “how’s and why’s” regarding amplifiers - so I moved it along. I guess I was a little curious to see if I’d come to the same conclusion.

This amp sounded pretty good with its 6550s, stock grounding scheme, original electrolytics everywhere, and no mods to speak of other than some janky alarm cable-type shielded run between V1A and the HIGH input and a poorly installed IEC socket. Had some 120hz hum and hiss going on though - so I wanted to try and improve that.

Did a bunch of research on this circuit and made a plan to try to optimize what I have. Here’s the list - most of which I gleaned from TAG. Just wanted to throw this out there for anyone in a similar situation looking for some feedback on the multitudes of info “out there” on this circuit.

- First; measured every resistor, checked every diode, tightened every nut/bolt/screw. Every component was within spec. I guess those Piher resistors are sought after for good reason, lol.

- Replaced all electrolytics. The original Dalys were from 1975 and two had bulges in the tops while the third had a bulge at some point that had cratered and was starting to bubble like it was fixing to spew. Also had the original Frako bias caps where one was getting crusty at the positive end - so they got replaced.

- Added a 220k/3w bleeder to the reservoir cap.

- Converted to EL34 while the board was up. Had a matched set of old Winged “C”s in the stash, so they went in.

- Added power tube grid stoppers (5.1k)

- The impedance jumper switch was cracked - so I did the 1.5” fender washer with the SRF-113Z switch from AES trick.

- Then I dove into the grounding rabbit hole. Found a lot of info out there and learned a ton about the differences between signal & AC grounds, shielding etc. Read about Larry, hot shields, evolution of Marshall grounding etc. then settled on this blend of Larry and a diagram I found here where Martin Manning marked up a JTM layout for better grounding:
- grounded the screens and PI filter, reservoir, and PT CT together at the screens/PI cap can
- gave the bias its own ground between the Presence pot and standby switch
- grounded the tone stack and NFB just below the mids pot
- grounded V1&2 cathodes, preamp filter, gain and volume pots, and inputs by the input jacks
- removed the bus bar and cleaned all the pot to chassis interfaces

To further reduce noise I also
- ran shielded cable to the V1 triodes
- did the heater ACT
- cleaned everything best I could including under every ground lug

Kept two small changes that came with the amp:
- 10k grid stopper on V1A
- 12 AU7 in the PI

And added bias sensing resistors (that I spaced out on initially - but work fine now). Re-did the IEC socket wiring.

The voltages are all where they should be be based on the many Marshall schematics out there and the amp sounds fantastic with the El34s. The hum and hiss is pretty much gone as well - only slightly there when the gain and volume are dimed. The Low channel is a nice surprise also. The volume level isn’t close to the High and the tone stack needs a little fiddling with - but I was impressed with how good it actually sounds. Guess I’ll keep this one, lol.

“After” pics:

IMG_0448.jpeg

IMG_0449.jpeg

As always - I couldn’t have done this without all the great info and advice I get here at TAG.
Thanks guys! I appreciate all the help!

[Roe]

how does the 12au7 PI work? are the PI values adjusted for the 12au7?

[Raoul Duke]

All the PI values on the board are stock. The voltages didn’t seem out of the range and the sound seems fine. I’m still going to try some different preamp tubes and see how it goes. Right now I still have the tubes it came to me with in it:

V1: JJ ECC83S “gold”
V2: Psvane ECC83
V3: JJ ECC82

I did learn yesterday that this amp sounds better through a “cream back” type speaker though. Played it through the WGS ET65 in my 102 cab and that took the edge off in a really nice way.

BTW Roe, I owe you a “thanks”! Many of the posts I learned about this amp and Marshall’s in general from are yours - both here and on other boards. I appreciate the education!

And since I got you - any preamp tubes suggestions?

Thanks again!

[Reeltarded]

I like that you just did a bunch of good things and got a great result!

Cheers!

[Raoul Duke]

Thank you Sir!

I appreciate the positive feedback - I’ve learned many things from your posts here as well; specifically on Marshalls. And I’m also a huge fan of your non-sequiturs! Great fun stuff. I always look forward to learning from you and having a chuckle at the same time.

I’ve actually adopted one of your sayings; hopefully you don’t mind? I was demonstrating my recent Liverpool build for my wife the other day and got that wonderful ringing note spiraling into harmonic feedback thing going. My wife asked: “is it supposed to do that?” to which I replied “oh she’s just having a little tantrum; it’s part of her charm…” lol.

Thanks again!