Funky distortion circuit on this fender 'super twin' amp

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

Funky distortion circuit on this fender 'super twin' amp

Postby indyguitarist » 09 Aug 2007, 15:10

http://www.indyguitarist.com/schematics ... r-twin.pdf

I'd be interested in hearing your guys' opinions on this circuit...
I'm actually a bit stumped too. I'm not familiar with what the distortion control is doing to achieve distortion, the tube stage does not have a plate resistor, cathode cap and resistor not going to ground... funky!!

thoughts?

bw
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Postby JHS » 09 Aug 2007, 17:43

Extract the dist-circuit from the schem (this cathodyn stage) and then take a look at it.

The dist-pot shifts the working point of the tube into the non-linear region. IMHO the stage works acc. to the dist-setting as a limiter too.

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Postby magikker » 09 Aug 2007, 23:01

I own a super twin reverb... it is the single worst sounding distortion ever. EVER!
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Postby soulsonic » 09 Aug 2007, 23:04

Yeah, that era of amps has a really bad reputation for some crummy designs. Usually the best thing to do with them is gut them for parts.
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Postby bajaman » 09 Aug 2007, 23:10

Yep - one of CBS Fender's worst amps - but can be easily modified to twin reverb specs, and has a 5 band tube graphic (rotary knobs) equalizer too.
As for the distortion circuit - total crap
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Postby magikker » 09 Aug 2007, 23:28

Yeah, not the best design in the world by anyone's standard. Oh, wait, I take that back. I forgot about one group of people who absolutly love this amp. Pedal steel players.
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Postby soulsonic » 10 Aug 2007, 00:09

magikker wrote:......I forgot about one group of people who absolutly love this amp. Pedal steel players.


Yet another reason not to like them. :lol:
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Postby bajaman » 10 Aug 2007, 01:47

If my memory serves me correctly, it was a Paul Rivera design too.
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Postby indyguitarist » 10 Aug 2007, 17:06

JHS wrote:Extract the dist-circuit from the schem (this cathodyn stage) and then take a look at it.

The dist-pot shifts the working point of the tube into the non-linear region. IMHO the stage works acc. to the dist-setting as a limiter too.

JHS


I'm feeling like a serious newb on this one. :lol:

The signal appears to come out of the anode @ v3a into the .047uf cap and then into both v3b AND the hi pass filter going to the gain control which is a .0047uf and a 68k. The opposite lug on the gain control is going to the cathode cap and resistor on v3b (?) which are connected to the inductor and a 2.2k resistor to ground.

What function does this inductor do here?

On v3a there is a 3.9k to ground before the grid there... I wouldn't think the signal would be THAT strong after the eq so as to need this resistor that small... thoughts? No cathode cap or resistor there either, and a 47k plate resistor (?)

I'm still not seeing any sort of of plate resistor for v3b - is this a different type of tube circuit that I'm just not familiar with?

Thanks again for the "schooling"!

:D
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Postby magikker » 10 Aug 2007, 17:35

Well it's a different type of amp. It's complicated. Very. I'm getting to the point were I can understand the smaller fenders but this thing is above my head. It's a beast...

Ok... one thing to consider about this amp. It's very clean. You can crack it and it is still clean. To my memory, (been a while since I used it) it never goes into the kind of break up that you'd expect from a tube amp.

I think there might be a trick to the distortion circuit. It sound terrible at reasonable volume... but if you pull the boost knob, you get the full 180 watts. Maybe it sounds better like that? I can't imagine that anyone would purposely design this sound into the amp. Maybe if you turn it on while it's cranked it would sound better. ??? You guys really need to hear some sound clips to understand what that knob sounds like.
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Postby indyguitarist » 10 Aug 2007, 18:27

magikker wrote:Well it's a different type of amp. It's complicated. Very. I'm getting to the point were I can understand the smaller fenders but this thing is above my head. It's a beast...

Ok... one thing to consider about this amp. It's very clean. You can crack it and it is still clean. To my memory, (been a while since I used it) it never goes into the kind of break up that you'd expect from a tube amp.

I think there might be a trick to the distortion circuit. It sound terrible at reasonable volume... but if you pull the boost knob, you get the full 180 watts. Maybe it sounds better like that? I can't imagine that anyone would purposely design this sound into the amp. Maybe if you turn it on while it's cranked it would sound better. ??? You guys really need to hear some sound clips to understand what that knob sounds like.


:lol: I'll have to make a quick soundclip next week... as magikker says, it is beyond description how BAD it is!
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Postby magikker » 10 Aug 2007, 21:05

Wait, you got one too? I didn't know anyone else owned one of these things. Man, I feel like there is finally someone else that feels my pain. Just don't tell me you play pedal steel.
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Postby magikker » 10 Aug 2007, 21:13

for the curious, I found this on youtube.... http://www.youtube.com/watch?v=tmo3xYafadY

I'm still looking for a clip of the distortion circuit.
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Postby indyguitarist » 10 Aug 2007, 21:53

magikker wrote:for the curious, I found this on youtube.... http://www.youtube.com/watch?v=tmo3xYafadY

I'm still looking for a clip of the distortion circuit.


honestly, I think it's a decent sounding amp for a 180 watt behemoth!
Besides the distortion that is.
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Postby soulsonic » 10 Aug 2007, 23:31

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.
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Postby magikker » 11 Aug 2007, 00:46

When I say I'm glad there's someone else that shares my pain. I mean that in a lot of different ways. It might be the heaviest combo ever built. It is also really hard to explain the thing to some one... I really can't discribe the sound of the distortion. Nor do many people understand me when I say it is an Ultra Linear design.

It really is an interesting amp. It's something that I'm going to hold on to forever because it is pretty different as far as amps go.

And the thing does handle distortion really well.
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Postby CS Jones » 11 Aug 2007, 05:34

It's a follower circuit with an over designed bias network built around the negative supply biasing the power tubes. I say over designed only because it seems like a lot of work to get a distortion capability which by all accounts seems to be pretty crappy sounding.

Building on what JHS mentioned...
The plate takes a 300 vdc charge and sits at AC ground through it's PS node filter cap. Like any follower there won't be any gain and a slight attenuation. Based on shifting VB3's negative grid voltage it'll distort though. With "distortion off" the diode is reverse biased which sends a large enough negative voltage on to the grid so that a full signal swing is well under the cathode's upper limit. When this voltage gets raised to a certain level it looks like it works to oppose the tie in (through the 470k R) to the + rail so that the diode gets a forward biasing voltage and starts to conduct. The current flow stablizes the voltage at whatever the diode conduction limit is and this raises the grid bias up closer to where the cathode sits. Much closer. The static K voltage without distortion is + 3.5V. This would shift with the new bias conditons (to what, I don't know - it's hard to say) but not so much that the new "distortion on" grid bias with input signal couldn't reach it. It looks like it's meant to pit the rail voltages against each other until a point is reached where the diode kicks in and raises the grid bias up to a level where doinking around with the variable resistor starts to impact the grid/cathode interaction creating more or less distortion at the input. This signal gets "followed" out of the cathode circuit to the volume pot slightly smaller but with drive potential.

The stage before this doesn't need a huge amount of gain. Actually it can get by with very little because even a small signal voltage would cause an ugly (even uglier?) distortion had they not added the ground lift network "under" V3B starting at the coil. The grid of V3B is connected to the bottom of the Cathode RC network, through the 100k distortion pot. The signal goes from there into one end of the Volume pot and then divided down. This whole network sits on top of another network which looks like it's meant to allow for the clean headroom needed for "distortion off" settings along with the bias shift adjustment needs for "distortion on" setting which I mentioned above.

The coil looks like a two way street. Input one way, feedback the other. Filtering high frequency content either way; just keeping frequency crud off the grid and letting the bypass cap at the cathode do it's usual thing but with the higher frequencies cut out.

A word of warning Brian. I haven't built an amp in probably 5 years. I haven't l even looked inside one and very rarely even think about them anymore so these are just "at a glance" observations and I am rusty to say the least. I haven't even completed a pedal from scratch in over 2 years now. I just really wanted to more say "hi" to you.

So, hi Brian. :)
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Postby indyguitarist » 11 Aug 2007, 10:00

CS Jones wrote:It's a follower circuit with an over designed bias network built around the negative supply biasing the power tubes. I say over designed only because it seems like a lot of work to get a distortion capability which by all accounts seems to be pretty crappy sounding.

Building on what JHS mentioned...
The plate takes a 300 vdc charge and sits at AC ground through it's PS node filter cap. Like any follower there won't be any gain and a slight attenuation. Based on shifting VB3's negative grid voltage it'll distort though. With "distortion off" the diode is reverse biased which sends a large enough negative voltage on to the grid so that a full signal swing is well under the cathode's upper limit. When this voltage gets raised to a certain level it looks like it works to oppose the tie in (through the 470k R) to the + rail so that the diode gets a forward biasing voltage and starts to conduct. The current flow stablizes the voltage at whatever the diode conduction limit is and this raises the grid bias up closer to where the cathode sits. Much closer. The static K voltage without distortion is + 3.5V. This would shift with the new bias conditons (to what, I don't know - it's hard to say) but not so much that the new "distortion on" grid bias with input signal couldn't reach it. It looks like it's meant to pit the rail voltages against each other until a point is reached where the diode kicks in and raises the grid bias up to a level where doinking around with the variable resistor starts to impact the grid/cathode interaction creating more or less distortion at the input. This signal gets "followed" out of the cathode circuit to the volume pot slightly smaller but with drive potential.

The stage before this doesn't need a huge amount of gain. Actually it can get by with very little because even a small signal voltage would cause an ugly (even uglier?) distortion had they not added the ground lift network "under" V3B starting at the coil. The grid of V3B is connected to the bottom of the Cathode RC network, through the 100k distortion pot. The signal goes from there into one end of the Volume pot and then divided down. This whole network sits on top of another network which looks like it's meant to allow for the clean headroom needed for "distortion off" settings along with the bias shift adjustment needs for "distortion on" setting which I mentioned above.

The coil looks like a two way street. Input one way, feedback the other. Filtering high frequency content either way; just keeping frequency crud off the grid and letting the bypass cap at the cathode do it's usual thing but with the higher frequencies cut out.

A word of warning Brian. I haven't built an amp in probably 5 years. I haven't l even looked inside one and very rarely even think about them anymore so these are just "at a glance" observations and I am rusty to say the least. I haven't even completed a pedal from scratch in over 2 years now. I just really wanted to more say "hi" to you.

So, hi Brian. :)


Hi Clay, and a BIG THANK YOU!!!!
That type of circuit through me for a loop a bit - I haven't came across that before. I got a pretty good deal on it, $275 in immaculate condition, original speakers and everything. My hope was that I could do a couple of tweaks to use for my own personal rig (I won't be doing any DIY how-to's on this one). The other guitarist in my band has a 65RI twin and I love it as far as the RI's go.

Looks like I can just disconnect the particular tube stage in question and bypass it and get a more "twin like" tone. Played my first gig with it tonight... it loves pedals! :)

Hope you've been doing well-
Brian
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Postby indyguitarist » 11 Aug 2007, 21:23

Here's what I'm wanting to do:
http://www.indyguitarist.com/schematics ... in-mod.gif

What are your guys' thoughts? I'm hoping I don't have a ton of clipping between stages so the voltage dividers may need tweaked a bit.

soundclips coming soon...

bw
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here's the soundclip...

Postby indyguitarist » 13 Aug 2007, 18:37

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