Monkeymatic Hammond Stout

[xtian]

Remember that Hammond L-100 I picked up and chopped down? I'm building an 18-watt to go in it, using the salvaged transformers, and Hoffman's Stout with Reverb schematic.

My control panel (input jack, Vol, Tone, Reverb, power switch) will mount on the top of the cabinet, about 24 inches from amp, using the Hammond's 6-pin connector as umbilicus.

[xtian]

Progress. I the power string and grounding arrangement of Hoffman's layout! Very nicely organized.

[kwijabo]

dood! that's great. ha - when i looked at the first set of pictures i thought to myself, "oh no he cut the circles in the wrong side of the board for the tubes." then i scrolled down and saw those were cut out for the caps. i like that creative solution.

[tictac]

Nice Job!

That's one of the best looking layouts (and wiring jobs) I've ever seen... reminds me of some of the Matamp builds...

TT

[xtian]

Thanks, guys! Finished control panel, photo attached.

[xtian]

Amp finished, mostly. Running well without reverb (didn't order reverb transformer until this weekend). Except, remote control panel is a huge headache. Hum and buzz.

Fixed the first main ground loop--my shielded wires were grounded only at one end, to the chassis, but at the other end the individual shields were all soldered together in a bundle. Loop!

Fixed the main cause of buzz--can't bundle the 120vac switch wires with the signal wires...even though the signal wires are all shielded. Lame.

Fixed more hum by process of elimination, basically eliminating EVERYTHING from my remote panel except the volume control. Finally, amp is quiet and sounds good.

Now I'll have to figure out how to add the reverb, tone and mix controls without reintroducing the hum. No more time to work, though--this week is going to be a beotch at my day job.

[xtian]

Almost done. Got the reverb transformer and RCA jacks installed. The reverb return is working fine. Plenty of sturm und drang when I touch the springs. But the reverb SEND is not working well--I can hear only a tiny little bit of reverb when playing guitar.

First unanswered question is whether the Hammond reverb tank I'm using is compatible with the Hoffman circuit. I'm using a JJ 12AT7 as reverb driver, and I measure 344vdc on the plates and 6.5vdc on the cathode. The plates are higher than the 300v max spec'd on the JJ datasheet, but less than the 350v mentioned on Hoffman's drawing. OK?

I guess I should inject a 1kHz 1vP-P sine wave and measure the output of the paralleled 12AT7. What output would you expect?

And if the output of my 12AT7 is good, does that mean I need a different reverb tank? And if so, what spec?

[JazzGuitarGimp]

Silly question, I know, but did you accidentally swap the reverb transformer primary and secondary?

[xtian]

No, don't think so.

I found my answer, I think. This page is very helpful: http://www.amplifiedparts.com/tech_corn ... d_compared

It shows my circuit exactly, a copy of the Fender Twin, with the output side of the reverb transformer spec'd at 8ohms. The article shows that I need a tank with input impedance of 8ohms, and they suggest this may measure 1ohm via DC resistance. MY tank measures 185ohms, and this seems to be a good explanation of why the reverb sounds very quiet when driven by my circuit.

[xtian]

I installed a Mojo reverb to Fender spec; works great. But I had to replace the 12at7 reverb driver with a 12au7, and V1 with a 12at7. I was getting a blatty cutoff distortion; must have been overdriving the 12at7 or the reverb recovery stage.

May have to install a voltage divider going into or out of the reverb circuit, but it's happy for now with the lower mu tubes, and sounds great.

[sepulchre]

Cool build, Honkin cab! The board looks great.

[xtian]

Remember this guy? I thought, "I'm never going to be able to haul this thing anywhere by myself." So I ditched the Hammond organ enclosure, cut the baffle down, and put it, along with the two 12" Hammond speakers, into a solid pine cabinet. Reverb tank is also mounted in cab. Then I moved the controls and jacks around to better suit a head, and also tweaked some values for less treble and more bass.

http://youtu.be/-gRZkAMla7M

[sepulchre]

Sounds great! Got a head cab plan?

[xtian]

No cab plan. Chassis is an odd duck, 12x10x2. And reverb tank is 17" wide. Got something in mind?

[sepulchre]

Well no. I build all mine custom to fit so odd size wouldn't be an issue. I was just thinking how the head and speaker cabs would match up, or could. It would require some extra wiring jacks for the reverb, eh? It would make an interesting build.