First project

[darefugee]

Hello forum. I'm new here and starting on my first amp build. I chose Mark Huss' Plexi 6v6, plan is to build it in a Hammond AO-29 chassis.

Question - Has anyone used the AO-29 chassis for their Plexi 6v6 build and if yes did you use the original Hammond turret boards? It looks like room in there is limited and it might be a problem for a lay-flat turret board.

If I'm the first to do this I'd like to draw a turret board plan based on the Hammond boards. What software is good for drawing the plan?

[10thTx]

Check out this thread. It might be useful to you:

http://ampgarage.com/forum/viewtopic.ph ... 73e81fc251

With respect, 10thtx

[10thTx]

I should add that one can parallel terminal strips quite close to do a paralleled style wiring such as on this JCM 900 to Plexi 50w conversion:

http://s28.photobucket.com/user/tubenit ... 0914315155

OR ............ you could buy some Hoffman board material, cut it very easily with a hacksaw & build parallel turrets very close to suit your need.

http://www.hoffmanamps.com/MyStore/perl ... =165421140

With respect, 10thtx

[darefugee]

Thanks. I looked that thread over and it's similar to what I have planned. I'm not going to shorten the chassis though, the standard length fits my Plexi faceplate pretty well so I think it'll fit a Marshall-esque cabinet:

[img764]http://i1285.photobucket.com/albums/a59 ... 8c5868.jpg[/img]

I'm hoping to off-board some of the power supply components to terminal strips between the power and output transformers, maybe enough to allow the rest to fit on only one of the original Hammond boards. We'll see.

[morcey2]

The only tip I'll give at the moment is to put the mixing resistors over directly on the V2 socket. I've built several plexi-style amps and every one that had the mixing resistors on the board picked up noise and a signal even when both volumes were turned down all of the way. With the mixing resistors tied to the socket with a short a lead between the resistors and the socket as possible stopped both the noise and the bleed-through signal.

Matt

[darefugee]

Thanks for the tip. I think you mean the two 470k resistors tied to V2 pin 2 on the Plexi 6v6 schematic, right? I'll be sure to put them on the tube socket and to feed them with shielded wire from the volume controls.

[ampfab]

Are you going to use the tube rectifier?
with that power transformer, if solid state rectified, the b+ will be quite high for 6v6s.
I built a similar project using a similar Hammond amp, the B+ needed to be dropped using a zener on the center tap of the power trans. in the end I used a pair of 6l6gc power tubes, larger output transformer, and removed the zener. I preferred the 6l6 tone and, against popular belief (and a easily found website's findings), the power transformer had no problem powering the 6l6GCs.
because of the limited space, I wired the circuit point to point.

[morcey2]

Thanks for the tip. I think you mean the two 470k resistors tied to V2 pin 2 on the Plexi 6v6 schematic, right? I'll be sure to put them on the tube socket and to feed them with shielded wire from the volume controls.

Yes, those are the two I'm talking about.

[menger66]

That looks alot like the hammond pt i used for my jtm45, if so you can exspect 425 plate voltage with 5ar4, mine powers 6l6s no problem.

[darefugee]

Finished putting it together and it works fine, but WAY louder than I expected. Thanks to all for the tips.

[Structo]

I did an amp using the Hammond PT.

I used a 5U4GB rectifier and I had very high voltages.

Not sure if your amp did this but on mine, the field transformer for the speaker also acted as a choke.

Eventually I had to add a high power zener resistor to drop the B+.

[darefugee]

That looks alot like the hammond pt i used for my jtm45, if so you can exspect 425 plate voltage with 5ar4, mine powers 6l6s no problem.

Yep, my plate voltage came in at 425 as well with the 5U4 rectifier. And it certainly has the power of a 6l6 amp, I can't turn it up past 2 in my den. It's louder than my Fender Twin, for God's sake. Guess I'll have to switch out the 6v6s for 6l6s and call it good.

[M Fowler]

It's amazing how good the old Hammond organ transformers are and how hard it is to drill those chassis.

[armillary]

Thanks for the great thread (and the threads it refs)! I have an AO-29 chassis downstairs along with the field coil speaker it came with, out of an early Hammond M2, from when I upgraded an M2 to an M3 for my son. That required an M3 chassis, plus a lot of work and scrounging for parts on ebay. That was a few years ago. My son never plays the thing.

So imagine my surprise this summer stumbling across these while toodling around. It was bulk trash pick up day, I could hear the trash truck on the next block, I didn't have room for one, was an hour from home and didn't have any help.

All I got was the picture. What would I have done with another M3 anyway?

[darefugee]

So I set the tube bias yesterday. I could not get it lower than 22.4 ma cathode current at 397 plate volts. That works out to 8.9 watts total - I'm guessing that the actual plate dissipation is lower because there's some grid current, right?

But I'm a little bothered that it wouldn't go any lower than that. When I turned the bias pot off the low stop about 1/4 revolution, the plate voltage went to 390 and the cathode current went to 26 ma I think it was, so the power went up over 10 watts. I set it back to minimum.

The amp sounds terrific. I played it about 4 hours yesterday and it works perfectly - the power trans never got what I'd call hot. It's warm, but not so warm that I couldn't hold my hand on it indefinitely. The 6V6 tubes are JJ 6V6S, which I've heard are pretty tough tubes. I'm inclined to leave it alone and call it ready for a cabinet. But do the bias figures cause concern to any of you more experienced guys?