Blackface preamp, Brown tremolo/vibrato

[dwhutchens]

I have aquired a pair of transformers from an Altec lansing 342B PA amp. The PT is about 355-0-355 with 5vac, 6.3vac ct and a 20vac ct tap that was used to make 12vdc for the preamp tubes. It also has a 60vac bias tap. The OT has 4, 8 and 16 ohm taps and a 70 volt tap. Tubes were 4 12ax7's, a pair of 7027's and a GZ34.

I want to use these for a new build. I have built a couple of plain push pull 6v6 amps. I want to try a 6L6 single channel with trem and reverb this time. The transformers are stand up mount so this will be in a head cabinet. I want to try a Princeton preamp grafted to a brown Vibroverb PI, output and trem section. I'll have 5 preamp tubes, a pair of 6L6's and a rectifier.

Questions:

1 I see that Tubedepot has a blank 17x6.5x2.5 aluminum chassis. Can I likely fit it into this or should I go bigger?

2 Can I hang the reverb tank from the top of the cabinet above the transformers? If so how much space do I need to leave between them?

3 Can I run all 5 preamp tubes off the 6.3vac tap or should I run the preamp tubes off 12vdc from the 20vac tap as the Altec did?

And finally, if someone is willing to look at my diagram for mistakes or omissions, I would really apreciate it. I know I'll have to tweak some values here and there.

Thanks,
David

[tboy]

look at my diagram for mistakes or omissions

You'll need to add a resistor between the reverb control's wiper and the grid of the mixer stage. Otherwise the dry signal will be attenuated by the reverb control. Fender used 470k.

[dwhutchens]

Thanks tboy. I see that resistor in both the Princeton and the Vibroverb drawings now. I fixed that on the drawing. It's those little mistakes that will have me scratching my head later.

The transformers are Peerless. The number on diagram for the PT is 6192a. I tried googling that but did'nt come up with any thing. Does anybody know where I might find the specs on that, like the filament current ratings?

Thanks again,
David

[GibsonLover]

I swear you must be looking at my notebook - I am building almost the exact circuit out of a Princeton. I liken the output circuit more to the Vibrolux than the Vibroverb though since it was a longer running model with bias modulated 6L6's. No matter. I'm using a Hammmond vertical mounted PT on a Princeton chassis, which is only 18-1/4" x 7" deep. It's a little tight, but do-able. With the transformer vertical mounted, I have more room directly underneath it to mount the filter caps on a turret board.

[GibsonLover]

Th input to the reverb driver could use a 1M to ground. Together with the 500pF coupling cap, you'll have the Fender 318Hz rolloff to avoid crashing your tank with a lot of low frequency energy.

[GibsonLover]

...and another thing... The input to the reverb recovery should have a 20K to ground. It's a standard Fender value to load the tank a little and keep it from sounding too wild.
C21 seems 'extra'. If you like a slow, stable LFO, check the Vibrolux Custom Shop version here (simple rewire): http://www.lynx.bc.ca/~jc/tremoloCircuits.html

[tubeswell]

...and another thing... The input to the reverb recovery should have a 20K to ground. It's a standard Fender value to load the tank a little and keep it from sounding too wild.

I think you meant 220k

[dwhutchens]

Hey GibsonLover, thanks for pointing out those resistors. I was finishing up that drawing at about 2am and everything was running together. I think tubeswell is right about that resistor being 220k. I originally had it in there, it's bypassed with a cap on the Princeton circuit, I can't make out the value of the cap though. I looked at my drawing and thought I had mistakenly duplicated the resistor and cap on the recovery tubes cathode because I had drawn it right beside it. It's drawn sideways above the reverb tank on the Fender drawings. I just missed the 1meg on the reverb driver alltogether, I see it on both Fender drawings now.

I just got the transformer covers painted. They were pretty rusty so I took them off and sandblasted them and painted them satin black. I've got the OT back together and I just found my misplaced screws for the PT tonight. I'm still up in the air about the chassis size. The Altec was in a 17.5" by 7.5" so I think its doable but I found that turretboards.com has some bigger sized chassis. I also have to decide whether to use two rows of terminal strips or make a custom eyelet board to mount the parts on.

Thanks again guys,
David

[dwhutchens]

Fresh paint! I got the old rusty transformers back together, now I just need a chassis, some tube sockets and some pots