adding a switchable gain stage to super twin reverb

[wagdog]

If you've ever had the pleasure of playing through a super twin reverb, you would know that the "distortion" control adds a hideous, horrible buzz (actually buzz would be a compliment - this is worse) to your tone. It's bad, really bad. Makes you wonder what fender thought people were going to do with that sound.

Knowing I would never need it, I took out the components for the distortion control, thinking I'd use the extra triode at some point.

I'm at that point. I'm thinking of adding a switchable gain stage in, right after the first 12AX7 triode (V1A) and using the now unused triode of the 6C10 (V3C) and the now unused distortion pot (100k). I would need to add a DP/DT switch, or maybe even mrelays (probably just go w/the switch).

Here is what I'm thinking:



The new gain stage is just a starting point, I'm pretty sure that would work and would need to adjust from there as far as tone.

Would appreciate any tweaks on the second gain stage and any thoughts on the concept overall.

Thanks,

Wag

[Pedro Vecino]

This can run (probably need appropriate values to obtain a good overdrive) but if successful, will need another master working together to compensate volumes if you want to activate the overdrive remotely.
I would like to know how the third preamp step has been eliminated in this amp because at first glance looks like a fairly complex/dependent circuit.
Can you put a schematic with the disconnections that has done?
If you want, you can use this base: http://www.bymcomunicacion.com/pvecino/john/super.jpg
Regards

[wagdog]

This can run (probably need appropriate values to obtain a good overdrive) but if successful, will need another master working together to compensate volumes if you want to activate the overdrive remotely.

I was thinking that using the existing 100k overdrive pot would be enough for my needs. I'm not much of a switch channels mid-song kinda player (use pedals for that). So I was thinking I'd just use a dp/dt switch mounted to the amp (maybe take out the 2'nd input jack).

I would be interested in any thoughts on getting a decent overdrive sound out of it.

I would like to know how the third preamp step has been eliminated in this amp because at first glance looks like a fairly complex/dependent circuit.
Can you put a schematic with the disconnections that has done?
If you want, you can use this base: http://www.bymcomunicacion.com/pvecino/john/super.jpg
Regards

Here is what I did to remove the distortion circtuit:



I will admit that I don't fully understand how that distortion circuit actually worked (and if by working we mean that it sounded awful, but did do something).

However, the amp works great with the circuit taken out, as I did above. I have V3A pins 2, 3 and 11 currently grounded.

Something else I was thinking about was not doing this as shown in the drawing in my first post, rather switching in V3C before V1A, i.e. putting the switching point right after the junction of the two 33k resistors and V1A pin 7.

I'm also thinking of experimenting with the value of the 750uF bypass cap on the cathode of V1A - that value is really high.

[Pedro Vecino]

Thanks, Wagdog.
I d´ont understand the circuit around V3A, but omitting the value of the potentiometer (100K) and the components in series to provide a reference to ground, to the negative voltage and HT may not sound the same as originally.
I think it would be interesting to know well the operation of this circuit. If someone would explain it would be very grateful. Not interested to know how the overdrive works (a very bad thing) but whether the system used is correct to maintain the original response removing that triode (very interesting).
Using switches and comparing it could be determined by ear (the other components removed may have a negligible effect or not.). I have a SuperTwin, but 400 miles away and I can not test it.
I made a small scheme by setting the start point (A), which has in series:
100K resistor (pot at minimum) > 2K2 and diode in parallel> inductor > diode to ground
100K resistor (pot at minimum) > 2K2 and diode in parallel> inductor> 22K to negative voltage*
100K resistor (pot at minimum) > 2K2 and diode in parallel> inductor> 470K to HT (290V)
(*) OFF DISTORTION > -30V
..... ON DISTORTION > -11V
Another question that intrigues me (although not related to the elimination of the triode) is the effect of the presence of negative voltage on V3C grid (orange line) and his grounded cathode.

Sorry for the intrusion on your topic but I thought you might be also interested.
Thanks

[wagdog]

Thanks, Wagdog.
I d´ont understand the circuit around V3A, but omitting the value of the potentiometer (100K) and the components in series to provide a reference to ground, to the negative voltage and HT may not sound the same as originally.

I honestly couldn't tell you if it sounds the same as it's impossible for me to test it without putting parts back. It does sound good now, so maybe that circuit, even though it was "off" was loading down the signal (whether that was a good or bad thing depends on the players taste).

I think it would be interesting to know well the operation of this circuit.

I agree with you. I can kind of see how it works a little, just guessing, that when on, v3a is running as a cathode follower, and the distortion pot controls how much of that signal is mixed back into the signal from the plate of v3c. How that turns into the super twin brand of distortion, and what purpose the diode and inductor play, I have no idea. I just know it's horrible and not usable.

I do wonder if this is how the designer of the amp pictured it working, or what they were thought of the tone. It's so bad, and so unusable... there must be a story behind it!

Another question that intrigues me (although not related to the elimination of the triode) is the effect of the presence of negative voltage on V3C grid (orange line) and his grounded cathode.

Sorry for the intrusion on your topic but I thought you might be also interested.
Thanks

That is interesting, and I hadn't noticed it. And no apology needed - I'm interested as well.

FWIW, I tried putting in this gain stage today (the top version). It was WAY to much gain - to the point of oscillations and squealing. After messing around for a couple of hours trying to get a decent overdrive tone, I pretty much gave up and put the amp back to its stock configuration (without the overdrive).

I even ended up moving the screen supply back to the UL taps in the OT (not actually UL as I understand it). The final mod I did keep was using the switch on the overdrive knob to lift the NFB - that actually livened up the sound quite a bit and I can a decent tone at low volume (my actual long term goal for this amp!).

Anyway, definitely interested in the theory and operation of the overdrive too.

[Alex R]

Yes one stage will add a whole lot of gain, and not with a very distortion-friendly EQ. I would recommend adding a filter in line with the signal before the grid, like a 2.2meg resistor coupled with a 120p or 270p cap - experiment a bit, the idea being to cut the signal a little bit and roll off very low frequencies slightly - and bleeding off some highs with maybe a bypass cap on the plate resistor - try 0.001uF or 500pF. You might need to trim the signal down a bit further with a resistor to ground from the grid, like 220K-ish, or maybe after the coupling cap on the way out of the stage. Getting a good distortion sound needs attention to clipping levels - not too much - and to EQ - not too many lows, and bleed off some highs if it's too, kind of, Marshally.

It is very hard to get good cleans and good distortion out of the same amp. Sometimes I think it would be better if everyone stopped trying!

[wagdog]

It is very hard to get good cleans and good distortion out of the same amp. Sometimes I think it would be better if everyone stopped trying!

Too true!

Thanks for the suggestions! I will probably give this another shot next weekend.

[cyclecamper]

I picked up a Super Twin chassis with the intention of putting it into a new cabinet to make a bass head out of it, much like the Studio Bass. Kind of like 2/3 of an SVT, with real power and the ultralinear output it's much more suitable for bass than guitar as it comes from Fender. Of course, that's the model without reverb.

If you want to have an easier time of understanding the ill-conceived distortion knob, it helps a lot to look carefully at the schematics for the Super Twin With Reverb, Super Twin (without reverb, the schematic title is just '180 watt guitar amp'), and the Studio Bass.

The chassis for all 3 is identical. The Super Twin and the Sutdio Bass parallel up both sides of a dual-triode to push the rotary-graphic EQ, whereas with reverb they needed to use the other half, and also made one of the dual-triode sockets take a somewhat more rare triple-triode instead. Look at the Studio Bass, which does not have the distortion knob. That's one good way to disable it, but of course turning it off is just as good. Unless you want to use that stage for something else.

On the other hand, it is a horrible waste of a perfectly good tube stage. The stock distortion stage takes its output from the cathode side instead of the plate, I assume so that the diode to ground and the footswitch are not running high B+ voltages.

This kind of distortion has no point of onset, so there is no clean/dirty line to play around with crossing over. You can't change the grit and bite in response to your pick or finger touch or guitar volume control, so the stock 'distortion' is not at all emotive.

And it wastes a perfectly good gain stage which the amp could definitely make better use of.

Making things worse, the distortion circuit is after the reverb. Anybody who's messed with rack effects or recording knows to put the ambiance effects after the tone effects.

If you want to be able to switch in an extra gain stage with volume control, it should ideally be between the normal tone controls and the EQ. That allows you to have independent control over the tone of the distortion versus the tone of the guitar. For instance, if you want to turn down the treble of the distortion but leave the guitar's basic treble tone relativley unaltered, first you boost the guitar's treble, then generate the distortion, then runthe combined distortion and guitar tone thru the EQ to cut some treble. The result is that the guitar had treble boosted then cut and sounds about the same, but the distortion was generated between the treble-boost and the treble-cut, so the distortion only sent thru the treble-cut and got its most irritating buzziness removed.

[wagdog]

Wow, old thread. Do appreciate your insight.

FWIW, I've since gutted the super twin reverb and built up an ab763 style twin from the carcass - MUCH nicer sounding amp now. Still loud, and still heavy though.

Interestingly, a buddy (bass player) picked up a fender studio bass. I immediately recognized the circuit. I redid his caps and retubed it and man, what a huge, great sounding bass amp it is. The cab is pretty shot though so he's having split into a separate head and cab.

[cyclecamper]

Wow, old thread. Do appreciate your insight.

FWIW, I've since gutted the super twin reverb and built up an ab763 style twin from the carcass - MUCH nicer sounding amp now. Still loud, and still heavy though.

Interestingly, a buddy (bass player) picked up a fender studio bass. I immediately recognized the circuit. I redid his caps and retubed it and man, what a huge, great sounding bass amp it is. The cab is pretty shot though so he's having split into a separate head and cab.

It would have been nice to make your twin conversion switchable between 2/4/6 output tubes. Did you make it with 4 6L6s or 2? I assume you change the speaker load to minimize the mismatch the tubes see. Like if you run only 2 6L6s you should put on a higher impedance speaker load.

The Studio Bass is identical to the Super Twin without reverb except the capacitors in most of the EQ are different and the distortion knob circuit of course. Much nicer being able to use a variety of cabinets, there was nothing that convenient about the studio bass combo anyway. But it does make a lot of heat, and I didn't want a fan unless absolutely necessary, so I made the cabinet about 2 inches taller than usual, I put in 4 large openings in the bottom, and the front grille actually passes air. I made tha cabinet deeper so the back panel upper opening has lots of room from the chassis, and notched the top back to allow easy access to the back panel. The oval air inlets in the back are enormous. The back panel has an internal board which forces the incoming air to go back to the tubes. Here's a pic, trying to decide on the piping to frame the front.

[cyclecamper]

Here:

[wagdog]

It would have been nice to make your twin conversion switchable between 2/4/6 output tubes. Did you make it with 4 6L6s or 2? I assume you change the speaker load to minimize the mismatch the tubes see. Like if you run only 2 6L6s you should put on a higher impedance speaker load.

I'm running 4x6L6. Using the same (UL) OT, however the UL leads are taped off and the screens are getting their voltage from the proper (ab763 proper) location. The conversion I did was more of a gutting and rebuilding the entire circuit. . At the time, I did the math on the OT and determined there was not much of a mismatch between 4x6L6's, the OT and a 4ohn speaker loud. I wrote down the measurements, somewhere, lol.

I did try running the amp w/2x6L6 (actually ran it for a while w/4x6V6's, if you can believe that) and the vol difference was not that great.

It's a twin. It's big, it's heavy and it's loud.