Fender Super Twin Reverb Recap - Help Identifying a Couple Capacitors

Hi all,
I'm looking for help in identifying a couple electrolytic caps in my Fender Super Twin Reverb amp... One is labeled 7851A 235, and the other is labeled 7836A 235.
I've had the amp for a while and am finally starting to update the thing. I feel comfortable doing the caps as a starting point, but these 2 aren't labeled as something I recognize (and Google isn't much help)
Any help would be greatly appreciated!

What does the schematic spec them as?

I see these listed in a "kit":

3 F&T 22uF-500V (Main Filters - Decoupling)
2 F&T 220uF-300V (Main Filters)
2 F&T 100uF-100V (Bias Filters)
2 Axial 4.7uF-450V (Decoupling)
3 25uF-25V (Cathode Bypass)
2 4.7uF-50V (Cathode Bypass)
NOTES: Does not include the 750uF/3V or 150uF/6V cathode bypass capacitors. Some Super Twin models (without reverb) will not use all of the cathode bypass capacitors provided.

2.3uF caps from the 36th and 51st weeks of 1978. I don't recall seeing any 2.3uF electrolytics in a Fender amp and would go by the schematic. Maybe they were being used together in parallel instead of a 4.7uF? Do they appear to be original to the amp? Where are they in the amp/what's their function in the amp?

Quote:

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Originally Posted by Don

2.3uF caps from the 36th and 51st weeks of 1978. I don't recall seeing any 2.3uF electrolytics in a Fender amp and would go by the schematic. Maybe they were being used together in parallel instead of a 4.7uF? Do they appear to be original to the amp? Where are they in the amp/what's their function in the amp?

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They look to be original... I've taken a photo and labeled the caps and nearby V1A/Inputs. I'm not so good at matching up the schematic with the actual circuits, so I apologize for that.

https://www.thefenderforum.com/forum...a-d38a187babf2
https://www.thefenderforum.com/forum...e-d805befb76ba

Quote:

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Originally Posted by Rick_Fender_1

They look to be original... I've taken a photo and labeled the caps and nearby V1A/Inputs. I'm not so good at matching up the schematic with the actual circuits, so I apologize for that.

https://www.thefenderforum.com/forum...a-d38a187babf2
https://www.thefenderforum.com/forum...e-d805befb76ba

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Ooops! I tried to paste an image and it didn't work.

https://we.tl/t-S0EHGcgSHU

Quote:

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Originally Posted by Rick_Fender_1

Ooops! I tried to paste an image and it didn't work.

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Here's a WeTransfer link... Not sure if the forum allows attachments(?)
Thanks
Rick

https://we.tl/t-S0EHGcgSHU

Quote:

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Originally Posted by Rick_Fender_1

https://we.tl/t-S0EHGcgSHU

Here's a WeTransfer link... Not sure if the forum allows attachments(?)
Thanks
Rick

https://we.tl/t-S0EHGcgSHU

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Here is the schematic...

https://we.tl/t-hgpClOSOlc

It looks like a subscription is required to see the pics.

Quote:

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Originally Posted by Don

It looks like a subscription is required to see the pics.

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https://www.thefenderforum.com/forum...c-eebfa75fc318
Thanks for giving this your attention! I found an image host that doesn't need a registration...

I outlined the components in the circuit detail.jpg image. Not sure what part of the amp that area is, so I included an image of the whole amp in the hope that it will help.

I've never learned much about analogue electronics, but have some experience in digital, and am comfortable with soldering/desoldering etc.

https://i.ibb.co/3k7SQfX/Super-Twin-Reverb-Circuit-Detail.jpg

https://i.ibb.co/QQ5BPkv/Super-Twin-Reverb-Circuit.png

https://i.ibb.co/2MJtRF8/super-twin-...180w-schem.jpg

Kind Regards,
Rick

Quote:

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Originally Posted by phantomman

What does the schematic spec them as?

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Thanks for your reply! Sorry I haven't responded quicker, but I'm a little embarrassed about my lack of knowledge regarding analog circuits. I've been trying to figure out how to reply, as I have made a list of caps based on doing a visual inspection.
All the caps that have the markings I don't recognize are shown in the Circuit-Detail.jpg image link below... I outlined them, added the labels, and knocked out the background. Also included the whole chassis image, and a thematic I downloaded...


https://i.ibb.co/3k7SQfX/Super-Twin-...uit-Detail.jpg

https://i.ibb.co/QQ5BPkv/Super-Twin-Reverb-Circuit.png

https://i.ibb.co/2MJtRF8/super-twin-...180w-schem.jpg

Kind Regards,
Rick

Quote:

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Originally Posted by Bill Moore

I see these listed in a "kit":

3 F&T 22uF-500V (Main Filters - Decoupling)
2 F&T 220uF-300V (Main Filters)
2 F&T 100uF-100V (Bias Filters)
2 Axial 4.7uF-450V (Decoupling)
3 25uF-25V (Cathode Bypass)
2 4.7uF-50V (Cathode Bypass)
NOTES: Does not include the 750uF/3V or 150uF/6V cathode bypass capacitors. Some Super Twin models (without reverb) will not use all of the cathode bypass capacitors provided.

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Hi Bill,
Thanks for replying to this! I really appreciate the help!
I posted a few images in some other replies, showing the components, with labels that I'm not familiar with, and Included the schematic and image of the whole chassis.
Not very good with analog circuits. I used to work on digital electronics as a tech, building Sequential Circuits synthesizers, but am not familiar with tube amps etc.
Kind Regards,
Rick

The schematic is too small to provide much help but the "235" seen on the components is the EIA code for Mallory and I'm betting that the other numericals are date codes. I think Dan nailed the values.

Those links are smaller than the originals... here's a larger schematic...

https://ibb.co/KsnqB5N

Thanks!
Rick

Rick, I own a Super Twin Reverb from around 1977 and it's a very complicated circuit. It's the only Fender amp I own that I took to a professional tech for service. I printed out the schematic on 4 sheets of paper and taped them together so I could read it more clearly. It uses a couple of tubes that are getting hard to find, especially the 6C10, which I've seen for $100. I have one spare.

I don't see any 4.7uF caps anywhere on the schematic. I do see the 750uF/3V cap that the kit you referenced does not include, but I didn't see the 150uF/6V cap.

Yeah those compactrons are getting brutal on the wallet. Grab them if you can find them.

Quote:

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Originally Posted by phantomman

Yeah those compactrons are getting brutal on the wallet. Grab them if you can find them.

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A person online has published a diagram of how to put together a converter that uses 2 12ax7 tubes in place of the compactron. (using 3 of the triodes). I think that same tube was used in the Super Champ (I could be wrong) and possibly some Ampegs.

https://i.imgur.com/RSTMrFo.png

Yeah that's what stopped me from searching for an original (Rivera era) Super Champ. My Dad had several of those twin triodes but he sold them all off before I discovered I might need them. He did save me several sleeves of my beloved black-plate RCA 6L6's though.

Quote:

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Originally Posted by phantomman

Yeah that's what stopped me from searching for an original (Rivera era) Super Champ. My Dad had several of those twin triodes but he sold them all off before I discovered I might need them. He did save me several sleeves of my beloved black-plate RCA 6L6's though.

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One of my buddies buys huge quantities of old tubes, hoping to find a few desirable ones to resell. I think he goes to estate sales, auctions, etc.

The compactron was also used in old TV sets, so there's probably a bunch of used ones in some repairman's stash.

Yeah they were popular with Curtis-Mathes and some later Zenith sets.

Quote:

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Originally Posted by phantomman

Yeah they were popular with Curtis-Mathes and some later Zenith sets.

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We had a Zenith from the 1980's. I should have checked the tube compliment before hauling it to the curb. Or maybe they were solid state by then.

Quote:

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Originally Posted by Rick_Fender_1

Those links are smaller than the originals... here's a larger schematic...

https://ibb.co/KsnqB5N

Thanks!
Rick

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Rick, at a minimum I would replace the filter caps and resistors in the doghouse. My amp was idle for a few years and when I fired it up again, I got a "fireworks" show.

https://i.imgur.com/ExW8Ttm.jpg

Catastrophic smoke-test failure second only to Chernobyl!

:wow

The Super Twin Reverb was an odd amp. One channel, high and low inputs, volume, treble, middle, bass and presence controls, and a 5 band "equalization and harmonic balance" circuit activated by footswitch. Pulling on the volume switch activates bright mode. Master volume control and a "distortion" circuit controlled by a pot that when fully counter-clockwise clicks to off. When on, the distortion feature can also be activated by a footswitch, with a volume boost slider switch on the front of the pedal. Reverb can be activated by a single button footswitch.

The amp has pretty clean headroom. The distortion feature sounds pretty bad, and I never use it. I don't know the purpose of the 5 band eq circuit, other than to dial in a tone beyond what the treble, middle, bass and presence controls provide. As best I can tell, when the 5 band eq circuit is activated, the normal tone controls are still active also. (I may have to double check that with the manual, which I found online).

https://i.imgur.com/VZ0RgbY.jpghttps://i.imgur.com/czv9LE6.jpghttps://i.imgur.com/nC7eLyu.jpg

Quite an impressive rat's nest of wiring.

https://i.imgur.com/rNQpHm7.jpg

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Originally Posted by Michael Smith

Quite an impressive rat's nest of wiring.

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It's why I never felt guilty about charging customers $40/hr for my bench fee.

:hi:

Here's what the owner's manual says about the Equalization and Harmonic Balance controls. "This group of five controls can produce wonderful and weird effects far beyond the capability of most amps". I don't know whether I would use the term "weird" in my product literature, but this was 1977.

The control layout *seems* to imply a rudimentary graphic equalizer. It's certainly not a parametric equalizer (not enough flexibility within the bands).

Quote:

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Originally Posted by phantomman

The control layout *seems* to imply a rudimentary graphic equalizer. It's certainly not a parametric equalizer (not enough flexibility within the bands).

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I think that's right. The manual says: "Equalization and Harmonic Balance controls for maximum "cut" and "boost" and Harmonic emphasis at selected frequency bands."

I don't know what they mean by Harmonic emphasis. Being able to cut or boost implies an active circuit, but the schematic is too complicated for me to determine whether that's correct. It could just be a marketing gimmick to add 5 knobs to the control panel to make it look like you are getting more for your money.

The Super Twin Reverb listed for $745 in April, 1977. That's $3,799 in today's dollars.

I usually just leave them all at "0" and use the passive tone controls I'm familiar with.

Try playing with them and see how they affect the amp's overall tone and timbre.

Quote:

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Originally Posted by phantomman

Try playing with them and see how they affect the amp's overall tone and timbre.

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I may try that tomorrow. I will set all of the passive tone controls at "noon", and then fiddle with the other controls. I was thinking that since they are foot switchable, perhaps they were meant to switch between rhythm and lead playing.

Right now the amp is out in my live room paired with a 2 x 15" cabinet, because I've had it's 2 x 12" cabinet in my control room with my "new" Bandmaster Reverb.

Edit: Surprisingly, there are a number of YT videos of people playing the Super Twin Reverb. It looks like the passive and active tone controls are interactive. Looking around on Reverb for the current ones for sale, mine is the only one I've seen with the Utah square magnet speakers, although some of the amps on Reverb have non-original speakers. Mine appear to be original based on the 1976 date code.

https://i.imgur.com/3vPp5xf.jpg

+1

I'd say they're the OEM speakers. The only other factory driver I've seen in that model was the Gauss-Cetec PS 12.

Quote:

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Originally Posted by phantomman

+1

I'd say they're the OEM speakers. The only other factory driver I've seen in that model was the Gauss-Cetec PS 12.

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I may put in a set of foam ear plugs and my shooting ear phones, and turn it up to 10 and see what happens, lol. Although with the modifications I think have been done to it, it is producing 60 Watts RMS output, based on my measurements, which could very well be incorrect.

I remember jamming with some guys at a remote farm years ago. We were playing in the barn and I had my 135-watt UL Twin Reverb dimed. The other guitarist playing full-stack Marshall 100 Super Lead Plexi kept begging me to turn down. Even on "12" those eight V30's couldn't drown my pair of JBL D120F's.

Well I did mess around with the harmonic balance and equalization controls this afternoon, and they are interactive with the "normal" tone controls. They do cut or boost certain frequencies, so they are a graphic equalizer of sorts, as was suggested above.

That said, I really don't see the need for them, as the normal tone controls are sufficient to dial in a tone.

The tech who converted my amp to a be switchable between 25/100 Watts output apparently didn't consider the bias. At the "25" watt setting I am getting 60 Watts of idle plate dissipation, using four 6L6 output tubes. That is around 50% of maximum for 4 6L6's. At the "100" watt setting idle plate dissipation is at or above 100% of maximum. I'm not sure what he really did to the amp, other than install a switch and a couple of resistors as a voltage divider.

So I will only use the lower wattage setting, and I added a label on the rear panel for future owners.

https://i.imgur.com/Gfivdpy.jpghttps://i.imgur.com/gXgCZKT.jpghttps://i.imgur.com/D9lvNzQ.jpg

Good idea, but for future owners?
Or for a few years down the road when you wonder why that switch is there and what the heck does it do? I've got things I've built years ago and I can't remember what I did, (or why), and why are extra switches, jacks, pots stuck in odd places?
I told a fellow I had a Peavey 4 button footswitch he could have, when I found it I remembered I had converted it to run a relay board switching the effects for our Space Echo.

Quote:

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Originally Posted by Bill Moore

Good idea, but for future owners?
Or for a few years down the road when you wonder why that switch is there and what the heck does it do? I've got things I've built years ago and I can't remember what I did, (or why), and why are extra switches, jacks, pots stuck in odd places?
I told a fellow I had a Peavey 4 button footswitch he could have, when I found it I remembered I had converted it to run a relay board switching the effects for our Space Echo.

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Well, when I'm gone, if someone has the misfortune to buy this amp, they might need this information. At a minimum, it's a reminder to me to leave it on the low power setting. It does sound pretty good with plenty of clean headroom.

It does seem a bit much to be lugging around when a 45-lb Pro Reverb could essentially perform the same task and be much less taxing on your, um......constitution.

:D

Quote:

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Originally Posted by phantomman

It does seem a bit much to be lugging around when a 45-lb Pro Reverb could essentially perform the same task and be much less taxing on your, um......constitution.

:D

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These days, I'm lucky if I can lug a Carvin Vintage 16 up and down 17 steps to my second story studio, and that's with the stock speaker replaced with a lightweight Jensen. I have to keep one hand on the handrail due to wearing bifocals and afraid I'll miss a step, lol.

Why not get yourself about seventeen more cats then yoke 'em all together on a single harness designed to haul amps?

:biglaugh: