Funky distortion circuit on this fender 'super twin' amp

Tube or solid-state, this section goes to eleven!

Postby magikker » 13 Aug 2007, 19:16

One of the reasons I am keeping this amp is that it is a great test for pedals. You plug a pedal into it and you get the sound of the pedal out. You're not getting the pedal pushing the amp into overdrive you are getting the pedal's overdrive.

Sooner or later I want to put together a series of pedal demos and this will be the amp of choice.
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Re: here's the soundclip...

Postby guyg » 13 Aug 2007, 19:27

indyguitarist wrote:http://www.indyguitarist.com/soundclips2/fender-super-twin-soundclip-mp3.mp3

omg.. it sort of sounds like a misbiased ff to me.
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Re: here's the soundclip...

Postby indyguitarist » 13 Aug 2007, 22:32

guyg wrote:
indyguitarist wrote:http://www.indyguitarist.com/soundclips2/fender-super-twin-soundclip-mp3.mp3

omg.. it sort of sounds like a misbiased ff to me.


yeah, it does... not a very good sounding distortion...
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Postby magikker » 14 Aug 2007, 00:17

what that sound clip before or after the mods?

because mine doesn't sound that good. :D
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Postby indyguitarist » 14 Aug 2007, 01:22

magikker wrote:what that sound clip before or after the mods?

because mine doesn't sound that good. :D


that sounds good?! :lol:

That is before any mods, and including old tubes. :D
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Postby magikker » 14 Aug 2007, 01:25

Good is subjective.. you ought to hear how bad mine sounds. It has always sounded broken to me... though it's been to a couple of tech over the years and they've all told me that it is supposed to be that way.
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Postby CS Jones » 14 Aug 2007, 05:59

Good demo Brian. Interesting. The clean sounds quite nice actually. The distortion is about what you would expect given that V3a setup. It's a little surpising too that you're not getting more troublesome noise with the distortion engaged given that stage's current gain.

I think you're on the right trail with the mod. That stage seems like a real misfire. Lossy (signal wise), Leaky (grid) and Lopsided (both with the power supply centerline and the available signal swing from the grid to the cathode). It's getting it's gain at that stage just from the tube and it's using the highest gain tube available. Even a *slight* amount of gain (it looks like just a 2 volt swing upward) will send it into saturation. What's interesting is that it's even able to do clean (which I liked).

I couldn't tell what values to use if you decide to self bias that section. It's something you'd have to decide for yourself because it's so subjective. Pick a *reasonable* static current. 500ua, 1ma. etc? and build around that. I don't know. It depends on how you want it to respond. Do you want a high signal output? A *stiff* current transfer? A big current differential from grid voltage to plate current?etc. again, I don't know. Whatever you decide set your bias network up based on that. Drop the plate voltage to the middle of the supply so you can get a clean output swing. Frequency wise that point in the circuit is pretty much a slave to the frequency response of everything which has come before it so you've got to decide if you're going to go even farther back and dink with those values too. That stage's particular frequency response can be adjusted in the bog standard ways. Both at the cathode and the plate. Pick your poison. A lot of options to choose from though. Phew... on and on. There's a safe way to do it but no right way to do it.

I don't know if I'd dump that buffer circuit at V3b so readily though. Maybe just a readjustment of that too.

What a deal you got though! $275. Most excellent for you!
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Postby Ed G. » 28 Sep 2007, 05:18

soulsonic wrote:I remember in the mid-90's you'd see these for sale all the time in the Trader paper for $250-$300.... same goes for Dual Showman Reverb heads.

I think the super-clean abilities of the amp make it a real good choice for using lots of pedals. Maybe a little chilly, but not bad. I've gotten lots of good tones with pedals and a 100w Super Bassman. And the Super Bassman is a really good example of a bad CBS Fender circuit, but for bass and some guitar it sounds fine to me. Just don't expect any wicked crunch or overdrive from it and you won't be disappointed. These CBS Fenders are perfect for people who want a JC120 sort of tone but can't force themselves to use a solid state amp.


I used to have a Bassman 135...all volume, little soul.
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Postby kinAxis » 01 Nov 2007, 03:15

A friend of mine down the street (In Windsor Ontario) has one that I did a cap job on; I really like it for the extreme clean headroom, I can certainly see why Steel players dig it. Very punchy as 180 tube watts SHOULD be!
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Postby jrc4558 » 02 Nov 2007, 05:01

It is a clipping/limiting circuit built around a simple cathode follower. The bias is shifted with the control, driving the tube into cutoff.
Whats really weirt to me is the drivers for the output tubes... My god, who designed this?:O
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Re: Funky distortion circuit on this fender 'super twin' amp

Postby DrNomis » 15 Jan 2018, 06:22

Last year I managed to acquire a Fender Super Twin amp chassis that was mounted in a wooden cabinet for a Blackface Fender Bassman head, how much did I pay for it?....Au$300.00 all up, here's a pic of it:
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Genius is not all about 99% perspiration, and 1% inspiration - sometimes the solution is staring you right in the face.-Frequencycentral.
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Re: Funky distortion circuit on this fender 'super twin' amp

Postby teemuk » 06 Feb 2018, 16:44

Zombie thread resurrected I see... :D

Yes, this is definitely a weird circuit since cathode follower likely isn't a first choice when you want to produce "tube distortion". This one "brickwall" clips very asymmetrically and very "hard", with abrupt clipping onset. Aside the signal asymmetry it is pretty much everything else than what the popular tube lore suggests is "nice" kind of tube clipping. The "blending" feature can adjust between clean signal (no harmonic distortion) and distorted signal (lots of harmonic distortion) and - while not similar to "Gain" control - allows to control magnitude of the distortion effect and amount of additional harmonics in the signal. Yes, with this technique quite "soft" clipping is produced with modest drive settings.

In history's perspective the design is more sensible. It is pretty much Fender's first attempt to deliberately implement a distortion effect to their amps, and probably when Ed Sanner devised this the thought process must have been that distortion = distortion. Yep, they just wanted to put a circuit there that distorts because they were competing (and losing in it) with other guitar amp manufacturers who made amps that distort. Unfortunately Ed Sanner must have thought that any distortion suffices and that all distortions sound alike. Or something along those lines. No, no "cascaded gain", no pre -or post enhancement EQing, no things that actually make the distortion sound decent instead of like farty off-tune radio station. No, it didn't make the Fender sound like a Mesa/Boogie or Marshall amp. Not at all. Their second attempt was much better.

But, IMO, Ted Nugent could really rock that effect. With his chops that poor distortion circuit actually sounds damn great.

Whats really weirt to me is the drivers for the output tubes...

There's a sextet of 6L6 tubes and each triplet features only 33K grid bias resistance! It would have been weird if Fender would have tried to drive them directly with a mere 12AX7-based LTP.
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