What can i take from a dumble and add to a plexi?

[Homebelly]

I might be off on a bit of a goose chase here, but any ways.. no harm in asking.

I'm kinda sorta thinking about building a plexi inspired amp to use as a pedal platform. I've already built a JTM45 and a D*Style amp, and i've had my hand in another ODS style build as well as a 1987 build.

What i'm looking for is a big fat clean amp with a nice three dimensional sound. A lot of the plexi's i have played sound great cranked, but are kind of characterless when run cleaner. I'm wondering if there is a way to add to the circuit to create a nice tone some where between the plexi and a clean dumble.

I like the ability to mix the inputs on a plexi, as i like some of the tones hiding in there. And so i'm wondering what i could take from the dumble designs to add to a plexi.

I'm already looking at lead dress and so on, but i'm wondering about resistor and cap choises--i'm in no hurry to use CCR's every where-- and maybe other component value choices to help get a deep, fat three dimensional clean tone.

Any input greatly appreciated..

Cheers.

[stratpaulguy86]

Kinda like the Category 5 JB-100? A '68 plexi with a Dumble-style midboost? I too have toyed with the idea of marrying some of the aspects that I like from both amps. I would really dig a 50w plexi amp with a Dumble/Fender reverb circuit and a footswitchable gain boost.

[greekie]

One element I ALWAYS add to Marshall'ish amps that come through my shop: a 250K trimmer in stead of the two plate resistors on the phase inverter.

The stock Marshall design is really unbalanced, and kan overly stress one side of tubes, leading to redplating, etc.

The 400V B+ 50W JMP 2204 I have on my bench right now put out ~70VAC on one plate and ~50VAC on the other. That imbalance will introduce some nasty third order harmonics as well.

Jake

[10thTx]

I'm wondering if there is a way to add to the circuit to create a nice tone some where between the plexi and a clean dumble.

I built this Tweed Overdrive Special with a DPDT that when it switches the PAB on (bypassing much of the Dumble tone stack) ...... it engages a tone stack after the cathode follower. It's my favorite amp.

FWIW, I almost always play this with the PAB and the tone stack behind the CF.

Using that type of idea, one might be able to somewhat switch between a Dumble tone stack and a Marshall tone stack. You'd have to decide if you want a Marshall or Dumble type LTPI and power section? See DU-MAR schematic.

Just tossing out ideas for those who like to explore outside the standard Dumble ideas. You stated you wanted something "in between", so maybe this would do that?

with respect, 10thtx

[Structo]

One element I ALWAYS add to Marshall'ish amps that come through my shop: a 250K trimmer in stead of the two plate resistors on the phase inverter.

Jake

So you replace the two plate resistors with the 250K trimmer?

Is there any problem with reliability using a .25w trimmer there?

[jaysg]

So you replace the two plate resistors with the 250K trimmer?

...and do you tweak it for balanced voltage or mojo?

[phsyconoodler]

If you look at some d style amps they use a 10k balance trim pot inbetween the two PI plates but I have never seen a 250k pot used in place of the plate resistors,but it sounds like a cool idea.A 1/2 watt pot would be fine I would think.The plate resistors are usually only 1/2 watt.
that way you wouldn't have to get all crazy finding a matched triode for the PI.
I would think you balance it for the best tone whether it was balanced or not.Some guys say that a perfectly balanced PI sounds the best.I'm not convinced but sure willing to try.

[John_P_WI]

Some more food for thought from 10thtx :

ampgarage.com/forum/viewtopic.php?t=13360&postdays=0&postorder=asc&highlight=megaplex&start=0

or the "Ripper".

[10thTx]

Editable schematic and layout here:

http://www.el34world.com/Forum/index.php?topic=10776.0

With respect, 10thtx

[Roe]

One element I ALWAYS add to Marshall'ish amps that come through my shop: a 250K trimmer in stead of the two plate resistors on the phase inverter.

The stock Marshall design is really unbalanced, and kan overly stress one side of tubes, leading to redplating, etc.

The 400V B+ 50W JMP 2204 I have on my bench right now put out ~70VAC on one plate and ~50VAC on the other. That imbalance will introduce some nasty third order harmonics as well.

Jake

this also works:
- decreasing 1m on v3a to 390k (see park 1210)
- increasing nfb
- increasing tail resistor

[Harald.Nowak]

One element I ALWAYS add to Marshall'ish amps that come through my shop: a 250K trimmer in stead of the two plate resistors on the phase inverter.

The stock Marshall design is really unbalanced, and kan overly stress one side of tubes, leading to redplating, etc.

The 400V B+ 50W JMP 2204 I have on my bench right now put out ~70VAC on one plate and ~50VAC on the other. That imbalance will introduce some nasty third order harmonics as well.

Jake

3rd order? How comes? I thought an asymmetric signal has a non trivial amount of 2nd order harmonics (most notably and hearable the first octave) which are more nice than nasty - as btw all triodes in the preamp (except for the normally balanced PI) produce and is an imprtant part of the Dumble sound. Overly stressing one of the output tubes is of course without doubt a possibility and will lead at least to quicker aging of one side than usual (may be "cured" however by swapping the output tubes more often).

[Homebelly]

Wow
Thanks for all of the responses.
I've been looking into different "Plexi" circuits and i think i might settle on modifying the JTM45/100 super lead. I've never actually played one of these, my experience with plexi's has mostly been with JTM45's and JMP's. I love the JTM45 clean tones where it is just starting to break up and compress. The JMP's have always sounded a little flat to me until they are wound out, at that point they start getting way to loud for what i do. I could build one of those, but I'm not really a fan of master volume marshalls, they all tend to sound a little to thin and "tinselie" to my ear, thats what attracts me to D-Style amps, the OD just sounds like an extension of the amps clean tone.. must be the crystal lettuce.

I'm kinda sorta wondering about the possibility of adding a fourth 12AX7.
That way i could keep both channels of the plexi and jump them internally but keep the two volume controls, then use the extra tube as the HAD style OD..

I'm looking at a tubes up JCM800 chassis.
Its blank and will need cutting, but it means i can set out the layout how i want.

Any ways..
Cheers for all the drawings, links, help and hints so far.

[vibratoking]

must be the crystal lettuce

I love quotes. I'm going to trademark and market crystal lettuce. Something like health food for meth heads.

[Homebelly]

Since hearing it for the first time on the Henry Kaiser video i've though it would make a pretty cool name for an amp. If this bird i am researching every gets off the ground and flys, maybe i'll commercialize it and call it the "100 watt crystal lettuce."