Silverface Fender Super Six Reverb issue

[Amwatts]

Hi folks ,
This one has me stumped ... I have a 70's Master Volume Pull Super Six Reverb in for service,

All issues it came in for have been resolved apart from 1 - the Normal Channel is about 1/2 the volume of the Vibrato Channel, It's doing my head in.
Voltages on both channel preamps are identical, New filter caps on cathodes, all components have been tested and are within tolerance , the wax buildup on the board has been heated and cleaned of all dirt, no voltages are leaking across the board through dirty wax... Preamp valves have been changed with known good ones , all solder eyelets and all tube sockets resoldered . Every component between input to the 220k mixing resistors has been tested and checked on both channels , yet the volume difference is still there .

I don't have a scope but I have traced all the signal with my fluke and there is no signal leaking to ground ... The Normal channel and vibrato channels sound great , full with heaps of headroom , it's just the huge volume difference

NB Schematic for SF Vibrasonic/Quad Reverb/MV Twin/Super Six Reverb are all the same

Any ideas ? This one has me stumped, of the hundreds of BF and SF vintage amps I've serviced I've never come across this

[Matthews Guitars]

Why don't you have a scope? It's such an essential and useful tool, and so many affordable ones are on the market these days, there's no reason not to have one.

You can't effectively trace an audio signal with a voltmeter. Or measure its level. A scope is NECESSARY. Get one.

I suspect you've got a leaky capacitor in the audio path.

[Amwatts]

You're right , I had a great scope for many years, It was a loaner that I thought the owner had forgotten about , but he asked for it back a few months ago and I've been avoiding buying one to replace it .... slack I know , but I think it's time ...

[Amwatts]

I suspect you've got a leaky capacitor in the audio path.

no it isn't unfortunately, I went through every component and tested every cable, resistor , pot , cap , jack all the way from the input jacks to the 220k mixing resistors and still no change. Decided to spend a day on it , mystery amp wins.

[Stevem]

Get some wire and swap the output ( pin 6 ) of V1b with the output of V2b, does the issue still remain with the normal channel?

[romberg]

The fender super six schematic I'm looking at:

https://www.thetubestore.com/lib/thetub ... ematic.pdf

Seems to be similar to the AB763 blackface circuit in that the vibrato channel has one more gain stage (v4b) than the normal channel. So, I would expect the vibrato channel to be louder than the normal channel just because it has more gain. So, I'm guessing that the volume difference is more extreme than this would account for?

Mike

[Stevem]

That 4th tube in any Fender black or silver face tube amp with reverb is a mixer stage.
It does not add gain to the tremolo channels output because that tremolo channels output goes thru a 3.3 meg resistor first .

Of course this is not 100% carved in stone since V1a/b and V2A/b might not be as strong as one another and on top of that each channels volume pots have at least a 10% tolerance to them .

[romberg]

The first half of V4 is the reverb recovery amplifier. But the second half is a gain stage. It is there to compensate for the loss in the voltage divider after the second stage which is a 3.3m over a 470k.

This voltage divider knocks the signal down to 470k/(3.3m + 470k) or about 0.125. Then this signal hits the third gain stage. To bring the signal back to unity (the same value it had after leaving the second stage) we need a gain of 8 (0.125 * 8 = 1). But the third stage using a 12ax7 probably has a gain more like 60. Thus the vibrato channel has say 0.125 * 60 = 7.5 times more gain than the normal channel. Which is why most guitar players like this channel over the normal one even if reverb and vib is off .

Mike

EDIT: The situation is not quite as extreme as my above napkin math would indicate. I forgot about the trem intensity pot following the third gain stage (mine can be lifted from ground via a switch when not in use so I forget about it).

Anyway the trem intensity pot can load down the output of the third gain stage a bit. Lets assume the output impedance is 100k so a 50k pot will load down the signal by a factor of about 1/3.

So, the real difference between the two channels is 7.5 * 0.33 = 2.5. So there is your twice as loud . This all is of course very dependent on the gain of the 12ax7 used in V4 along with other factors.