Tweed preamp with reverb

[Mark Buckingham]

How hard is it to add reverb to a tweed-style preamp?

I'd like to build something like a tweed super, but I'd still like to have reverb with it...

I'm kind of thinking the whole 10pf/3.3m mixer thing would kill the tone of the preamp, so maybe there's a way I could do this without going down that route?

Thanks.

[Bruce / Mission Amps]

How hard is it to add reverb to a tweed-style preamp?

I'd like to build something like a tweed super, but I'd still like to have reverb with it...

I'm kind of thinking the whole 10pf/3.3m mixer thing would kill the tone of the preamp, so maybe there's a way I could do this without going down that route?

Thanks.

Not ready yet but on my drawing board and in development is a new Mission Amps product that will allow a builder to put reverb in any tweed Fender amp... possibly even a tweed 5F6A Bassman.
Mostly for my 5E3, 5F4, 5E5A and 5E7 customers, I think it can be modified to work with others.
Yes that mixer thing is the problem but fortunately, many tweed amps have, or can have the potential to produce, much more gain then you'd think.
This will not be a hard, splashy black face tube reverb but I have been experimenting more and more with Power Mosfets and small signal FETs with really good sonic results when it comes to things like reverb and vibrato and I feel this is the way to go... especially since it is an effect not the amp tone.
This project is a simple Power Mosfet driven transformer reverb circuit with a couple FET recovery stages, reverb output level and a reverb tone control, all built in an outboard chassis mounted to either a 9" three spring tank, which sounds really good for some reason, or if it will fit the cabinet and speaker, a standard tank.

[Loogie]

I have been experimenting more and more with Power Mosfets and small signal FETs with really good sonic results when it comes to things like reverb and vibrato

I'm very interested in doing that. How will you implement the tremolo? Will you modulate the preamp side or the output side? I've also wondered about how a hybrid system would sound. Transistor driver, tube recovery -- or vice versa, I suppose.

It would seem to me that at the very least tremolo would be a good candidate for silicon treatment.

[Mark Buckingham]

Bruce- on the mixer thing... Are you basically saying that there's enough signal there to say, drive the 12at7 in the fender circuit without killing the non reverbed tone?

Also- I like your mosfet idea... For me, the "not a splashy kind of reverb" thing is a plus, not a drawback. I just like a little bit of airy-ness.

[Bruce / Mission Amps]

Keep in mind this is a new reverb project, even though we do have it up and running in the Aurora amp, and I might change it at any time!!

The power MosFET's input is really high zed and has almost no effect on the triode it is connected to.
All the power MosFET has to do is swing about +12vac through the the standard reverb OT to drive the reverb tank well enough to sound good... however, the reverberated signal level out of the tank is PUNY!! ... like <10mvac or so.
Recovery of that tiny signal is is simply two series J201 FETs over to a reverb level pot and reverb tone control, then up to the two mixing resistors before the phase inverter.
All that recovery gain is why reverb sections are noisy, hummy and can be built with a lot of built in AC filament hum... it takes a lot of gain to get that puny tank signal up where it can be used.
The trick after the gain boosting is the values of the two mixing resistors in front of the phase inverter.
It takes about 26vac-36vac to drive the 6V6 power tubes hard enough to over the top, tweed Deluxe clipping so you don't want to kill the dry signal just for the sake of "reverb".
The reverb signal needs to be way less then the dry signal.
The two mixing resistor values are picked where the path from the dry signal is not too low and the reverb resistor is not too low in resistance, which would load down the dry signal but give powerful reverb.

It's a compromise.... full level reverb in this circuit is about where a good working black face reverb amp has it's reverb level set to about 3.5 to 4.5 max... if you know what that sounds like.
I don't see many real players using their black face Fender amps with reverb set to much higher then 2.5 to 3 anyhow so it seems like a good trade off for a relatively easy, two hour bolt on reverb.

Drippy wet surf music is out with this setup! There will not be enough there to get that giant, 22" crash cymbal dropped on ice cold water, reverb.

Again.. I have a lot on my plate right now but this is being tweaked out and we're very close to having it where it sounds good without effecting the dry signal enough to negatively impact the classic tweed tone.

[Bruce / Mission Amps]

I have been experimenting more and more with Power Mosfets and small signal FETs with really good sonic results when it comes to things like reverb and vibrato

I'm very interested in doing that. How will you implement the tremolo? Will you modulate the preamp side or the output side?
It would seem to me that at the very least tremolo would be a good candidate for silicon treatment.

***************************
Preamp!
The tremolo thing ought to be very easy because all you would do is lift the cathode bypass cap off the first 12AX7 triode (the one that drives the phase inverter) and couple the LFO (low freq oscillator) AC with a .1uF- .22uF cap from the tremolo circuit level control to the cathode of that triode.
If you need the extra gain by bypassing the triode biasing resistor, you could also lift the parallel combo of the cathode bypass cap and biasing resistor from ground with a few hundred ohms resistor and feed it at that junction.
Either way would increase and decrease the gain of that stage greatly by altering the bias from full on, to near cutoff of that triode... like a volume control being turned up and down slowly or faster.
The FET trem circuit's LFO would only have to swing 2vac to 3vac PP to do it!

[Mark Buckingham]

Thanks for the explaination, Bruce... Sounds like a very good circuit.

[Mike Marsh]

Reverb in a Tweed Super

I've used the Weber's Add A Verb Chassis to build (with Quality Components) the Fender Stand alone Circuit. It fits fine in the Super Cabinet and sounds awesome!

[Mark Buckingham]

So... When you switch the reverb out, does the amp basically sound the way it did before you added the reverb unit?

Thanks!

[Mike Marsh]

Yep. Hear it for yourself, borrow a Fender Stand alone from a friend who has one. The circuit is identical it's just in a different form factor. You can buy a kit or build from parts. Mike

[Mike Marsh]

if you think you hear any loss of high end (from added capacitance of the parallel circuit in the line) you can easily add True Bypass switch. Much better than trying to add the mixer and series resistance circuit.

[clyde]

Bruce,

That sound's like just what I've been looking for. I've got a stock MA 5e3 that's begging for reverb. Can you keep us posted with further developments and/or availability?

[Bruce / Mission Amps]

Well I've been playing with this again and the modulated bias supply for the power tubes (although not super super deep)... has great melodic pulse to it and I think I like it much better then modulating the preampstage, which sounds a little forced and mechanical.

[Bruce / Mission Amps]

Yes, we are trying to see if it is possible to use a 9 pin adapter to take some dry signal out to the aux reverb box and then reinsert the reverberated signal back in the same tube socket.

Bruce

[RC99]

Any updates to this? Very interested.