Monkeymatic Tamalpais #3 (5F6A/JTM45)

[xtian]

Starting on a 5F6A/JTM45 head. Not for purists (have I done anything according to the original?). Using OEM Fender HRD PT, one of my vintage Schumacher 60w 6K OTs with a pair of 6L6GCs, SS recto with stiffer filtering, Frondelli MV (per R.Robinette's page), separate V1 cathodes, Bassman/JTM NFB switch, adjustable fixed bias.

Cabinet is already built. Hope this creative take on housing the Tweed Bassman chassis will work--it has only two points to fasten the chassis. Pine box with maple front, stained, and just one coat of Tung oil.

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[xtian]

All that's left is wiring the boards into the chassis.

Instead of drilling holes to mount the boards, I used self-adhesive standoffs. Pretty handy!

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[joeboo88]

Wow Aaron that looks sweet. Can’t wait to see and hear how it turns out.

[dorrisant]

Very clean looking build!

[xtian]

Very clean looking build!

Of course! No wires in it to clutter up the presentation!

[ER]

It's bluetooth wireless!

I think I'd put the standby back on the front and put the MV on the back (if that's what's going on in the pics). The Frondeli is nice if you lose a wiper, but like most PPIMV it should be used full up as often as possible to approximate the stock circuit and only knocked back when/as needed. I also try and shy away from having any of the pots to close to any of the AC like the ON switch or the 6.3 of the lamp, plus the panel is already labeled for standby.

If you're playing live, you won't want to fumble around the back panel for standby, and with a drummer, the MV will most likely be a set-it-and-forget-it, if not full up affair. In the studio, you have the luxury to spend time dialing things. I had an old tweed bassman converted to a head back in the day and it wasn't overly loud with the right speaker combination, like a 1x12. My 2cents.

I almost did the same thing on a Silvertone 1484 I'm working on (the standby switches off the pre-amp signal because it has a doubler power supply) but I decided to put the MV in the ground switch spot instead since I'm putting a 3 prong on it and a true standby with a DPDT on the doubler B+ legs (fingers crossed).

Another thing to look at is trying to add switching where you can internally jumper and/or cascade the two bright/normal first gain stages, that could possibly get you some good drive and tonal flexibility in conjunction with the MV.

[xtian]

Thx, mate! No standby switches on my amps, man.

I'm running shielded (star quad) to the MV, so fingers crossed.

[ER]

Sorry! I mistook the NFB switch for a possible relocated standby. Now that I see it by the OPT jacks it all makes sense!

[xtian]

All done, and sounding great...after it took me a day and a half to figure out I had forgotten the artificial heater center tap. Duh.

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[xtian]

Here's the final schematic with all my mods.

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[Drumslinger]

Nice job man!

[fred.violleau]

Congrats on this great build! Any sound clip? I am curious to hear the differences between the bassman channel and the JTM45 on this build! How is the stiffer filtering? Tighter bass?

Thanks!

Fred.

Fred.

[xtian]

Fred, the only JTM/5F6A setting I have is the NFB switch, which chooses between 68K (5F6A) and 27K (JTM) NFB resistors. I just hear the 68K NFB as more gain--doesn't change the EQ much.

I'm using an ON-OFF-ON switch for NFB, and oddly, when I choose OFF, which means no NFB, my sound vanishes. Why would that be?

[didit]

I'm using an ON-OFF-ON switch for NFB, and oddly, when I choose OFF, which means no NFB, my sound vanishes. Why would that be?

End of crazy day, & ~13.5 seconds of thought before typing so wild thoughts. Looking at your schematic earlier and again just now left me head scratching vis grounding of the PI. My semi-blind mind's eye sees a path through the output transformer secondary. Perhaps, or perhaps not. And if not, perhaps someone else?

Best .. Ian

[sluckey]

Look at your schematic. That "x" below the presence pot is killing the PI tube current.