Dumble-inspired 100 watt clean monster

[rfgordon]

Hey, guys,

Here's the amp I'm building for a young jazz/fusion wunderkind. As you can see by the front panel, the tone stack is very Dumble-esque. It's got an Ironsounds loop, spring reverb, and from the loop downstream it's very much like a BT Twin.

There's also an adjustable, footswitchable boost function on the back panel.

The client has the head cab with a loaner chassis, as well as the 2x12 speaker cab I built for it.

I used a Marshall style PT, and it puts 461 VDC on the plates. It's a huge, loud, glorious wall of sound!

Hopefully, when the client comes for it, I can get him to record a clip of it in action.

[DonMoose]

Good lookin'

Me, I'd probably have named it the UnderDrive Special. I'm a dork.

[Structo]

Interesting.

I'm a little concerned about the PT wires coming through the chassis hole without a rubber grommet in it.

With the open chassis ends, how do you mount the chassis into the cab?

[rfgordon]

Some guys use grommets, and I have in the past. Leo Fender never shipped a single amp with grommets, and I've been in lots of 40 year old Fenders whose tranny wires aren't frayed. Matter of taste, I suppose.

The ends of the chassis are bent downward, and the bolts come through the sides of the cab into those downward tabs. I first saw that approach on a Holland amp I worked on. Makes for an extremely simple chassis bend and mounting scheme. Also allows me to screw the chassis to the little plywood squares for a stand.

What can I say? 20 years in the govt makes me do weird things. Too many trips out to Frenchmans Flats....

[M Fowler]

Rich,

Nice amp you built there.

I have 30 years in the government and haven't made it to Frenchmans Flats yet.

Mark

[Structo]

Hmmm, I haven't been inside that many 40 year old Fenders but the few I've seen always had grommets.
Here's a 69 Bassman.
But hey, it's your amp.

[img768]http://i423.photobucket.com/albums/pp31 ... 270pic.jpg[/img]

[rfgordon]

Well, I was referring to Leo's (pre CBS amps). I reckon the emergence of the grommet in the CBS years is part of the aerospace industry influence. Then again, they could have been in there to start, and if they were made of natural rubber, the ozone produced by the PT would have long since eaten them up. So maybe I'm wrong all the way around.

Like I said, I have used them in some amps and not in others. I always deburr the holes, unlike companies that use punched steel chassis. The high-speed hydraulic punches can leave some pretty sharp edges. For those, it's faster, cheaper and makes more produciton sense to stuff a grommet in the hole than to deburr the hole's edges.

[briane]

Hey rf, whats the tube compliment?

I just finished up my version of the clean monster/screamer that I have been working on for a year and a 1/2. First version had the dual ef86, aka silver alligator. Not my cup of tea, the harmonics became grating on the ears after an hour or so.

Stripped and rebuilt with a ef86/parallelled 12ax7 pre (for clean), and that was the ticket for me. It runs a built in dumbleator, and dual tube PI, aka svt style, and dual KT88. Really a piece of work. Very clean in triode mode, and a good bit of grit (smooooth grit...) in pentode mode. So thats total of ef86, 12ax7 x 4, and dual KT88.

Just finally getting it where I want it, its been a long road to travel. I think with HI/low pass filters may be very SSS like.

[rfgordon]

Sorry I didn't take a tubes-up chassis pic.

V1 and the tone stack are pretty much straight Dumble, w/ 100k plates and 1k5 Rk/4.7mF caps on a 12AX7. I chose Sozos for their clarity. You can see that my component placement is non-D, as I like to keep my grid leads as short as possible. Sometimes I mount a tag strip on the board and connect the tone caps to the pot tabs. Not this time, though.

Next is a fairly BF verb circuit, with a 12AT7 driving the xfmr/tank, but with a 2m2 takeoff resistor. Then V3a for verb recovery.

At first, the Ironsounds loop was just in front of the PI, where they suggest. However, the V3b mixer/gain makeup stage was overdriving it badly. I left off the Rk on that gain stage for the sake of headroom.

So the loop circuit follows the verb recovery stage. The loop is much happier there, though I did have to use a pair of very short pieces of shielded wire, since the next gain stage would amplify any noise in the loop.

So then its on to a D-style master volume and a very BF Twin-like PI and power section with 4 6L6s.

This is the first amp I've built that's designed to make a big clean sound, since all my other guys are rock/blues/roots players. It did come out nicely, and just sounds huge! The guy really likes the way he can tweak it with the Deep and Rock/Jazz switches.

Oh, and there's a tone control on the reverb output. And I have the Rk on V1a wired through a 1k pot as a footswitchable, adjustable boost. In place of a second input jack, i used a mini toggle to lower input sensitivity.

I have made two changes since the pictures were taken. I turned V1a Rk around (to correct its orientaiton), and I ran a 1mF cap from the top of the verb xfmr to ground.

[Structo]

I have the Ironsounds loop in my D'Lite as well.

The only thing I dislike about it is it kind of depends on where the master is set. For example I run my reverb and delay pedal through the loop.
If I have the right amount of reverb mixed but then turn up the master, then the reverb goes up as well.

I probably should have installed the pots on the loop rather than the pad switch but I didn't know that it would do that. Oh well live and learn.
It's not that bad really.
I've heard you can upgrade the IC in those to a higher fidelity chip but I can't remember what it is now.

So did you use film caps (Sozo?) for your bypass caps as well?

So this amp does not have an OD channel?

[rfgordon]

Tom,

The bypass caps on the cathode resistors are just garden variety electrolytics.

The amp has not overdrive channel, though it'll grind pretty nicely with the preamp volume cranked.

I'm with you on the reverb. The best sound still comes from a non-reverb amp and a verb on the board.

[Structo]

OK, I see it now hiding over there at the end of the board.