Fender Twin Reverb Master Volume 135w dist. / early break up

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mkiijam

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Joined
Aug 25, 2017
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Twin Reverb. Not sure the year, but is the 135w master volume with pull switch.

Vibrato channel breaks up / distorts ( mostly asymmetrically clipping with the top half of the waveform being the most affected)

I have replaced ALL electrolytics, power and bypass. Swapped / subbed in new tubes. Replaced 100k load resistors in that circuit.

Data I'm working with:

1. The reverb tank is missing, seems like from the schemo this shouldn't cause any issues???
          a. Maybe a should stop right here, cause it would cause this funky clipping somehow?
2. The vibro channel can only take about 300mV before it starts to clip (Attached is a JPG of the schematic with waveforms and voltages)
http://Twin Reverb. Not sure the year, but is the 135w master volume with pull switch.

Vibrato channel breaks up / distorts ( mostly asymmetrically clipping with the top half of the waveform being the most affected)

I have replaced ALL electrolytics, power and bypass. Swapped / subbed in new tubes. Replaced 100k load resistors in that circuit.

Data I'm working with:

1. The reverb tank is missing, seems like from the schemo this shouldn't cause any issues???
          a. Maybe a should stop right here, cause it would cause this funky clipping somehow?
2. The vibro channel can only take about 300mV before it starts to clip (Attached is a JPG of the schematic with waveforms and voltages)
https://imageshack.com/i/plF4Q81vj
 

Attachments

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mkiijam said:
1. The reverb tank is missing, seems like from the schemo this shouldn't cause any issues???
          a. Maybe a should stop right here, cause it would cause this funky clipping somehow?
Click to expand...
With reverb pot turned down it shouldn't matter.
But fixing issues like this works best with a process of elimination.
- You can pull out V3 (reverb driver tube) to eliminate it as a possibility
- Turn the vibrato intensity all the way down
Then you could swap the leads from the treble pots to see if the problem stays or switches with the front end.

That kind of process. Eventually you will find the area of the circuit that is the problem.

2. The vibro channel can only take about 300mV before it starts to clip (Attached is a JPG of the schematic with waveforms and voltages)
Click to expand...

Are the drawings of the waveforms showing that there is no clipping before the 3.3Meg reverb mixing resistor, but there is clipping at the grid of V4b? And then the clipping is worse on the plate of v4b?
 
I've always found Twin reverbs sensitive to output tube balance/ bias settings and even tube positioning, I think the bias arrangement is to some degree responsible . 
With marginal driver and output tubes you might be lucky to  get 50% of rated power , also check that the preamp tubes are the right kind ie,12AX7 ,12AU7 or 12AT7 in the proper positions .
Its not really an amp known for its distortion , but Steve Jones of the Sex Pistols did use it with a Les Paul custom and plenty of pre gain to get his sound .


 
> *6*mV input.  Are we being trolled?

G-amps Full-UP should overload the output with 20mV (or 6mV) at the input.

In most normal non-headbanger use, the gain will be turned-down at one or more points. The *input* stage should accept 300mV-500mV "clean". Later stages full-up will distort this; turn it down!
 
Sorry, I didn't word my issue very well.  (I have pulled V3 and the issue is still there.)

The 300mV PP is at the grid of V4B,  where is looks like it should take about 20 vac pp like the normal channel.

The plate of of V1B and V2B put out about 20vac pp before clipping.

 
ok that's easy, there is an extra gain stage in the vibrato circuit.

where are your tone controls set?

put them all wide open and see what happens to both channels.

weird as it may seem, i have noticed that the vol pots in the vib channel come on quicker than the normal channel.  did they screen pots to do this? i do not know. and i do not have the data for this weridness based on 3000 amps, just a handful so take it with a  grain of Unique.  :D  be sure to vote.  ;D

 
CJ said:
ok that's easy, there is an extra gain stage in the vibrato circuit.

where are your tone controls set?

put them all wide open and see what happens to both channels.

weird as it may seem, i have noticed that the vol pots in the vib channel come on quicker than the normal channel.  did they screen pots to do this? i do not know. and i do not have the data for this weridness based on 3000 amps, just a handful so take it with a  grain of Unique.  :D  be sure to vote.  ;D
Click to expand...

The tone controls and channel volume are up stream of the issue. Regardless of how I set them, the clipping still happens at what seems a low level.

Does somebody have a twin? Does it clip at "3-4" on the vibro channel with a Gibson humbucker as the source?
 

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