Fender Super Twin Reverb with Dumble-Style Mods...continued

[ic-racer]

Here are the pictures of the insides before I put it all back together.

This first is a picture of the relay board, before and after conversion.

[ic-racer]

More pictures

[ic-racer]

Faceplate shots. The one with the switches was kind of a 'Photoshop mockup' suggesting what I hope for with a sticker to go over that area.

[Bob-I]

Just a comment on Fender's work... WHAT A RATS next!!

[Bob-I]

Just a comment on Fender's work... WHAT A RATS next!!

[ic-racer]

Just a comment on Fender's work... WHAT A RATS next!!

(I know you really mean "CBS" and not Leo Fender )

Thats for sure! Its so tempting to just gut the whole thing and just start from scratch on a project like this.


I am very happy with the way this project turned out. I think that a Dumble mod is really fitting for the Super Twin for these reasons:

1) Super Twin was one of the last point-to-point Fender amps made
2) Super Twin has little or no "collector" value and is inexpensive on the used market
3) Dumble circuits don't rely on output tube distortion and this is in keeping with the Super Twin's output section.
4) There is a lot of room in the chassis
5) It comes with a footswitch and jacks
6) The pots/switches/jacks/sockets/hardware are all high quality
7) After you take out the EQ you have plenty of room for toggles or additional pots or I/O jacks without drilling any holes.
The power amp section does not have any goofy extra caps or resistors and can be used 'as-is'
9) it has a separate heater supply just for V1 and V2

Final (...well these things are never really finished ) sound clips to follow.

[jaysg]

Just a comment on Fender's work... WHAT A RATS next!!

I believe this is Ed Jahn's handiwork. It's not true that it's near the end of the line for eyelet board amps. The 80's series II are all eyelet board as are the big bass amps Jahns did. Iirc, the first PCB tube amps are the Red Knob amps and the Super 60, which is about 10 years after.

Ed Jahns gets a pass for developing the Tung-Sol 5881.

If we're ever lucky enough to learn the details of the SSS amps, the Super Twin is an ideal platform for the SRV freaks who just gotta have that 150W Steel String Singer.

[ic-racer]

Just a comment on Fender's work... WHAT A RATS next!!

I believe this is Ed Jahn's handiwork. It's not true that it's near the end of the line for eyelet board amps. The 80's series II are all eyelet board as are the big bass amps Jahns did. Iirc, the first PCB tube amps are the Red Knob amps and the Super 60, which is about 10 years after.

Ed Jahns gets a pass for developing the Tung-Sol 5881.

If we're ever lucky enough to learn the details of the SSS amps, the Super Twin is an ideal platform for the SRV freaks who just gotta have that 150W Steel String Singer.

Thats right it is the 'second-to-last' eyelet board 'Twin' style amp (I forgot about those "II's" )

Tweed->Blackface->Silverface->Super Twin->Super Twin Reverb (1976-1980)->Twin Reverb II (1981-1987)->PCB "The Twin"


But now you have me concerned with the PI (and so the project keeps going... ). I found some posts on that Electronics forum ( http://music-electronics-forum.com/showthread.php?t=159 ) and it was suggested that those 0.22 uf coupling caps, just after the 12at7 buffer won't have a good transient response.

I noticed the Mesa 400 bass amp (six 6L6 also) uses 0.1uf as a final coupling. I was also wondering about putting a .022uf in parallel with the .22uf. Something easy to try...

[ic-racer]

I couldn't help but searching for more info on Ed Jahns. It turns out that the infamous distortion circuit of the Super Twin was patented by Jahns.

Too bad this is the worst sounding guitar distortion ever conceived. Before I started this current project, I drew up the schematic of the ignoble distortion circuit with the intent of understanding it and tweaking it. It turns out it was just easier to convert it over to a Dumble ODS circuit.

Anyway here is the original Jahns patent and the Super Twin distortion circuit.

[ic-racer]

Here is a post that indicates that an ultralinear output section has a Dumble precedent (at least in a clone).

http://ampgarage.com/forum/viewtopic.php?p=28178#28178

[jelle]

I do not know if I'd bother to change the PI/cathode buffer system. Those CBS EE's were pretty good at making those distorting CKT's performing more linear. They did a great job on some parts of the circuitry but screwed up with the distortion stuff IMHO

I wouldn't mess with those 220's unless you have to much bass. you could try replacing them with 100 or 50nF but again I would not mess with it. You might consider to release the NFB a bit to gain highs in case required but the mids could get harsh. Are you happy with the sound?

jelle

Just a comment on Fender's work... WHAT A RATS next!!

I believe this is Ed Jahn's handiwork. It's not true that it's near the end of the line for eyelet board amps. The 80's series II are all eyelet board as are the big bass amps Jahns did. Iirc, the first PCB tube amps are the Red Knob amps and the Super 60, which is about 10 years after.

Ed Jahns gets a pass for developing the Tung-Sol 5881.

If we're ever lucky enough to learn the details of the SSS amps, the Super Twin is an ideal platform for the SRV freaks who just gotta have that 150W Steel String Singer.

Thats right it is the 'second-to-last' eyelet board 'Twin' style amp (I forgot about those "II's" )

Tweed->Blackface->Silverface->Super Twin->Super Twin Reverb (1976-1980)->Twin Reverb II (1981-1987)->PCB "The Twin"


But now you have me concerned with the PI (and so the project keeps going... ). I found some posts on that Electronics forum ( http://music-electronics-forum.com/showthread.php?t=159 ) and it was suggested that those 0.22 uf coupling caps, just after the 12at7 buffer won't have a good transient response.

I noticed the Mesa 400 bass amp (six 6L6 also) uses 0.1uf as a final coupling. I was also wondering about putting a .022uf in parallel with the .22uf. Something easy to try...

[jelle]

Hmmmm, server bug? I deleted double post...

[ic-racer]

I do not know if I'd bother to change the PI/cathode buffer system. Those CBS EE's were pretty good at making those distorting CKT's performing more linear. They did a great job on some parts of the circuitry but screwed up with the distortion stuff IMHO

I wouldn't mess with those 220's unless you have to much bass. you could try replacing them with 100 or 50nF but again I would not mess with it. You might consider to release the NFB a bit to gain highs in case required but the mids could get harsh. Are you happy with the sound?

jelle

Back when it had the original distortion there was a very bad sounding thing that is hard to describe. When you plucked muted bass strings the sound was very thin and bad sounding. Now that every single component of that original distortion circuit is out, there is still a very, very small hint of that thin sound when the muted strings are plucked. It could be my imagination also (seriously!). Anyway, it seemed to be even less pronounced when I fiddled around swapping out the PI tubes.

The more I think about it , putting a smaller cap in parallel with the 0.2uf will probably cause some weird phase thing. So I thought of 'blueprinting' the PI circuit with original value, fresh, coupling caps and also putting some Vishay/Dale resistors on the cathodes and plates.

I had made a point to NOT replace any of those little blue coupling caps, like one would do to save the 'vintage vibe' of an amp. But this amp does not have any 'vintage vibe!' So, no real reason to keep those 30 year old blue guys in there. (I will save them in a bag though).

From many years of experience with this amp (I have owned it from new), it really does not undergo any significant tonal metamorphosis when cranked through a Power Soak or Power Break. So, I want to work with this characteristic to see if I can get the 'cleans' on this amp to be like SRV mentions, that just keep getting louder, without distorting more.

[jelle]

I do not know if I'd bother to change the PI/cathode buffer system. Those CBS EE's were pretty good at making those distorting CKT's performing more linear. They did a great job on some parts of the circuitry but screwed up with the distortion stuff IMHO

I wouldn't mess with those 220's unless you have to much bass. you could try replacing them with 100 or 50nF but again I would not mess with it. You might consider to release the NFB a bit to gain highs in case required but the mids could get harsh. Are you happy with the sound?

jelle

Back when it had the original distortion there was a very bad sounding thing that is hard to describe. When you plucked muted bass strings the sound was very thin and bad sounding. Now that every single component of that original distortion circuit is out, there is still a very, very small hint of that thin sound when the muted strings are plucked. It could be my imagination also (seriously!). Anyway, it seemed to be even less pronounced when I fiddled around swapping out the PI tubes.

The more I think about it , putting a smaller cap in parallel with the 0.2uf will probably cause some weird phase thing. So I thought of 'blueprinting' the PI circuit with original value, fresh, coupling caps and also putting some Vishay/Dale resistors on the cathodes and plates.

I had made a point to NOT replace any of those little blue coupling caps, like one would do to save the 'vintage vibe' of an amp. But this amp does not have any 'vintage vibe!' So, no real reason to keep those 30 year old blue guys in there. (I will save them in a bag though).

From many years of experience with this amp (I have owned it from new), it really does not undergo any significant tonal metamorphosis when cranked through a Power Soak or Power Break. So, I want to work with this characteristic to see if I can get the 'cleans' on this amp to be like SRV mentions, that just keep getting louder, without distorting more.

Those blue paktrons are great sounding polyester caps. I would never consider replacing those.
If you decide to rip them out, send them to me!

As far as replacing the resistors in the pi section...I would not bother. You'll find almost no tonal effect there, IMHO.
As for a loud clean sound, I'm confused. You write the amp stays clean when dimed. Why bother?

As for the thin sound when the strings are plucked...that could very well be parasitic coupling due to layout issues and lead dress. (Rats nest... ). I haven't studied the pics in detail but those relays for Pab and channel-switching will give trouble when placed so close to eachother. As you may know, it's not just the combination of a schematic and good components that makes a great amp. Good luck!

jelle

[ic-racer]

As I was making the order of the new caps and resistors I was having second thoughts about it. Perhaps I will just package the amp back together and play it for a while.

In terms of keeping old caps, I actually replaced some of the power supply caps but then changed them back to the originals. The originals are very high quality industrial strength Mallory capacitors; they lasted this long, and it just seemed to me they would outlast the cheaper looking replacements. There was nothing to indicate the originals were bad.