2021 Monkeymatic Fiddy 800

[xtian]

First amp of the year! Building a JCM 800 2204, using a new PT from cedist.com and a NOS Schumacher 60w OT 495D (the last of the dozen I bought a few years ago). I ordered a Mojotone chassis, but it won't arrive for a week.

Final layout below.

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[layout updated 1/21/2021]

[M Fowler]

Nice should be fun.

I have 6 of those PT's, I use in my HRP 50w (Hot Rod Plexi) builds.

Mark

[pdf64]

Looking forward to this, as a 2204 / Dirty Shirley pre into a JTM50 power amp with switchable GZ34 / silicon rectification is my project for the new year
That radial ecap board mounting arrangement is very nice, though I’ve got axiels for mine now.
Rather than use the presence pot as a (key) chassis 0V return connection point, I prefer to keep my options open and bring that back to the board. But then I use alloy chassis so being able to torque up chassis connection fasteners may be more important than with steel chassis.

[ChopSauce]

Yet another inspiring build from yours...

That faceplate looks really beautifull but I'd like to see how it works with both the monkey and the name of the amp a (very small) bit more outwards...

Looking forward to this

Me too!

[xtian]

Don't forget the valence (front panel) of cabinet will partially obscure the left and right sides of the chassis!

[xtian]

Q: What's the first step to becoming an amp daddy?

A: Knock up a couple of cabinets!

You may now revoke my comedian's license.

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[xtian]

Layout updated above. I have to wait for Mojotone chassis to arrive for measurements--they didn't have a drawing to download.


What pico value caps do people like? I've had trouble with ceramic disc caps ringing or being microphonic, and with silver mica caps simply failing. What about multilayer ceramic (MLCC)? Film? They're all low-voltage tone caps, in this circuit.

[sluckey]

They're all low-voltage tone caps, in this circuit.

That 470pF that connects to the treble pot has a very healthy dc voltage on it. I'd get a 500v rated cap. I use silver micas (some mil spec NOS), others from Hoffman and AES and have not had any to fail.

[joeboo88]

Wow Aaron, nice cabinets! i can only dream of doing that. ( never done any wood working)
I have an Express without a cabinet for a year now, maybe you can send it up here to Canada

looking forward to your finished product

[xtian]

Layout updates posted at top of thread.

[xtian]

Eyelet board built and popluated.

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[xtian]

Mojotone chassis was meant to arrive Friday, but did not. I was despairing of losing a whole weekend, but FedEx delivered on Sunday!

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A tiny bit of reaming the square mounting holes is required for rivnuts. I'm using 1/4-20 rivnuts here. And the IEC cutout was a little small for my inlet unit, but a little work with a rat tail file solved the issue. I also 3D printed some covers for the unused cap can holes.

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[ChopSauce]

(how beautiful is this?!.)

Better too small the (pre-drilled) holes than too big...

[xtian]

Finished with the build-out. Happy to report no issues. Sounds very good! Very quiet at idle. No oscillation with all controls dimed.

I went pretty conservative with the treble peakers and bright cap, but maybe went too far (updated layout in first post shows current values).

I have two DPDT switches on the first two pots. The bright switch will be useful, I think, when I increase the cap's values for more brightness. The second switch has the V2a cathode bypass cap (boost). But I think this bypass will stay on permanently. So I have one unused switch. What are your favorite mods?

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[sluckey]

Very nice! A work of art! I love seeing your workmanship.