Twin Reverb pull-gain... how does it work?

[SSLtech]

Referring to this schematic here:

http://www.schematicheaven.com/fenderamps/twin_reverb_sf_100_schem.pdf

There's two 'MVS' switch poles, each with a number 5 in a triangle... one JUST above V4B, the other just above the reverb tank dotted rectangle.

So on my amp, this doesn't do anything. -I wouldn't normally worry, but I'm about to sell it, and the buyer wants EVERYTHING to work. -Heck, I don't like the sound of what happens when you pull that switch... I don't really see how anybody COULD like the sound it makes... but do I agree with the buyer that if I want a fair price, the unit really ought to be in full, correct working order... after all, it's how I would like to be handled by a seller.

Replacement 2-pole pull switch log potentiometers with splined shafts ares apparently made of 'unobtanium', and -measuring the circuit, I found that the upper pole in the drawing (the one which shorts out the 1MΩ resistor) is operating... but no noticeable change in gain, and no 'distortion' sound. the lower-drawn pole doesn't seem to short in either position.

So I tried swapping the poles, thinking that the OTHER pole did more...

Still no change in gain, no 'distortion' sound.

I'm afraid that I don't see what these are supposed to do... does the circuit require BOTH poles to open/close simultaneously to operate properly?

Everything else works correctly; Reverb, tremolo, both foot-switches, both 'bright' tone-switches, tone stacks, and the amp sounds big and strong... -bigger and stronger in fact than the brand new "1965 reissue" twin reverb which I just put next to it.

Can anyone explain the circuit to me, or give me some pointers? -If I can't get a 2-pole-pull-switch-splined-shaft-log-pot of the correct value, should I maybe use a 6VAC 2-pole relay, powered through the remaining working pole from the heater supply? -Would that work? I really DON'T want to start making this unit excessively 'inauthentic', though.

The wise counsel of experienced tube guitar amp gurus is appreciated!  

Keith

 

[sahib]

Keith,

I am certainly not a valve guitar amp guru. Also for some reason I could not open your link but I checked the Fender web site.

The way the switch is shown both contacts open and close at the same time. The one connects to the secondary of TR4 feed backs certain amount of signal determined by the 1K and 470K divider network. The one shorts the 1M resistor really alters the high frequency cut-off. That is simply a hi-cut network. I have never used a twin so I don't really know what the original effect should be.

If you can not get the original switched pot your only option is to use a DPDT relay and turn it on/off using the working contacts of the pot switch,

 

[SSLtech]

That was my guess at first, and the pole which WAS working was the HF corner-frequency pole (the "1MΩ-becomes-0Ω" pole), so I swapped the two pairs of wires,expecting the gain to change, but no corresponding HF rolloff shift.

But after I powered it back up and tried again, STILL no gain change.

That's when I decided I probably didn't understand the circuit, and elected to ask for help.

-Maybe the second pole has now failed also? -stranger things have happened, I suppose.

Keith

 

[craptical]

Hm... Isn't this a case of "oxide inside switch noone ever pulls or should pull" (as pointed out earlier...), did you try the spraying-and-push-pulling-a-million-times-method?  ;D  You could (don't try this at home!) croc-clip the poles to verify function, perhaps? You probably thought of this already, but it seems wierd that a switch that never sees any use should break down. ???

Anyway, good luck!

/Dave

 

[Gus]

From the link it looks like the "top" switch changes the plate loading of V4B open 1m in series with .01uf and 12K(not doing much),  Closed as the schematic shows 12K and .01uf Loads V4B more gain reduction.

The other triangle changes the loading of the reverb driver transformer one way transformer to transducer other way transformer to transducer in parallel with 1K

The way it is drawn the added 12K .01uf load on V4B for lower output and the reverb transformer being loaded harder via the 1K added load (and maybe decaying faster).  I would think the two switches are meant to be switched together

It is not extra distortion it is reducing gain and it looks like it only works on the vibrato channel.  It looks like it will not do anything on the normal channel.  I am guessing distortion(vibrato channel only) is when the switch is to the right and V4B is unloaded. Clean is when V4B is loaded harder reducing its gain and the reverb output gain is reduced by the added 1K

 

[Bryson]

Why a splined shaft pot? Fender amp knobs use a set screw.

http://taweber.powweb.com/store/potsord.htm

http://www.smallbearelec.com/Detail.bok?no=619


The switch just passes more dry signal to the reverb/dry mixer triode.
Another way to achieve this (in regular Fender **** Reverb amps) is to bypass the 3.3meg resistor with a 1meg (or whatever) resistor - and maybe a cap in series if lo cut is desired.

 

[SSLtech]

Why a splined shaft pot? Fender amp knobs use a set screw.

Bugger me... so they do!

My apologies, it's been "amp-day" in the shop today... amorris and I have been inside two little Vox amps, a Roland, an Ampeg, a Galien-Kreuger and the Fender. I'm more than a little mixed-up!

Regarding the switch, I'll try exercising and spraying, and see if anything changes.

Thanks!

Keith

 

[Gus]

Did you test it on the normal or vibrato channel?  Try the vibrato channel

 

[SSLtech]

I definitely tried both. -And measured a bad switch.

Will finish it off tomorrow, when I get back in to work

Keith

 

[johnR]

I used to own one of these, but I disabled that switch because the distortion sounded so horrible. I've never been able to work out exactly how that circuit worked either, but I suspect it's using the signal from the reverb drive stage to boost the gain, then pulling the level down after V4B. I remember the reverb level used to drop when the switch was engaged, and the distortion increased. It was thin and buzzy and muffled at the same time - very strange. Does your buyer know what it would sound like if the switch worked?

 

[abbey road d enfer]

Keith,
the way this boost operates is horrible:
A fraction of the reverb drive signal (from the secondary of the reverb xfmr) is injected into V4B (the mixing stage that sums the Vibrato channel and the reverb recovery) via a 470k that goes to the grid.
In order to make it sound better (?) the CBS engineers put a shelving low-pass (the 12k-0.01u combo) on the anode of V4B.
Only the roll-off section of the dual switch works, so you may hear a difference on the reverb sound.
But obviously sincethe other section is duff, the boost signal is not injected.
After reversing the switches, you should have heard the boost.
And since it uses the reverb drive signal, it operates only on the Vibrato channel.
This "feature" of the amp is so popular that many have been removed; you may find one of these pots lying in the drawer of a conservative amp tech...

 

[PRR]

What Abbey said. It was a good normal Fender amp. Someone noticed the reverb driver signal is loud and distorted.... "free gain and flavor!"

The time-honored "pure" path from V2B plate to V4B grid has gain of about 1/12.

The path through V3 TR4 and 470K has gain of near 1/4.

The 12K +0.01u cuts V4B gain in the highs, not the lows, to complement the bass-cut of the 550pFd+1Meg at V3 grid. Top-boost, distort, top-cut makes mellower dirt. I think the 12K actually puts gain back near normal, but with V3 and V4B straining to make 10V peak at V4B plate. We need like 8V peak here to force the output stage to full 100W, so the strain is audible. 

You could also imagine V3 as a micro-Champ SE transformer color-amp, which can be brutally slammed and then attenuated to get tube-abuse sounds while the quad 6L6 work at non-lethal acoustic levels. It isn't really the same, but it only cost a few bucks to try.

> doesn't do anything.

Did you PLAY it? Or just strum a few bars? I see you confirmed a bad switch, oh well. However when working but not worked HARD, the difference may be very mild.

The schematic says "some versions". If it is stuck in "pure" mode, I say scratch-off the "Pull for nasty" label. Millions of amps copy other Fender designs; over 39 years I can only think of a few who copied this reverb-tap hack (and one of them is another Fender).

 

[okgb]

Correct me when i'm wrong but i thought some those twins
simply bypassed the large Meg resister that shunted signal into the reverb
from inbetween tube stages

 

[SSLtech]

Okay, back to the amp...

So I test-wired in another 2-pole switch, (not part of the pot) and it works correctly.  Then I re-checked (measured) the original switch and still one pole works correctly.

I re-wired the pull-switch in circuit again (with the poles once more swapped from their original assignments), and now I have boost.

I should probably conclude that I must have done something wrong in my original 'swap-wiring'.

Yep, as I think I mentioned earlier, it sounds utterly offensive to my ears. It's like a cheap fuzz box which isn't actually fuzzing very well, or something. -It plainly works as it should in terms of circuitry.

Thanks so much for the circuit explanations to one and all... NOW it is clearer to my eyes how it functions.

I might try the 'switch-cleaner' option to see if I can revive the dead pole, but if not, it'll probably just stay as-is.

It certainly looks pretty enough, though!

Keith

 

[sahib]

Good stuff Keith.

Nice selection of amps there.

 

[SSLtech]

Heh, Thanks!

-You should see the OTHER amps, out of shot!

;D