Distortion only on reverb channel

[sonofmickel]

Fender Super Reverb clone, B+ is 445.
Both channel volumes on 6 using single coil strat.
Normal channel sounds good.
Reverb channel is very distorted.
have tried:
Rolling back the guitar volume cleans up the signal.
Changing Input tubes(swapping with normal channel), Reverb tubes-no change, still distorted.
Bypassing reverb circuit cleans the signal but volume is lower (jumper output V2 plate to the input of the .1 coupling cap before the 220k mixing resistor).
Voltages:
V1. 233, 0, 1.8, 245, 0, 1.8
V2. 236, 0, 1.7 243, 0, 1.8
V3. 439, 7mv, 8.6 439, 7mv, 8.6
V4. 244, 2mv, 1.9 235, 2mv, 1.9
V5. 439, -35, 0.3 387, -38, 20
V6. 240, 58, 92 239, 61, 92

V5 I'm not getting a cathode voltage from pin 3? Cathode voltage on pin 8 is high too(should be 5V). Also V5 Plate 220k is not dropping enough voltage, pin 1 should read closer to mid 200V's.
I'm a Fender newb...looks like voltage does not flow until the vibrato switch is activated! Facepalm...with vibrato switch activated V5 pin 1 bounces between 250 ish and 439. V5 pin 3 cathode voltage bounces also. Still distorting early on reverb channel.

I'm still thinking the reverb circuit is where the circuit is gaining up.

I'll keep digging.

[sonofmickel]

Some build pics.
This was built by a friend waaay back in 2011-12. I have modded it and brought it back mostly to stock. I left 4.7uf bypass caps and changed some of the tone caps. Does anything scream "fix me!"? Should I gut it and rebuild with better parts?

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[Stevem]

You need to trace out the signal flow level between channels from one gain stage to the next, and this can be done even with just a volt meter, you do not need a O scope.

If you set a meter to a low ac volt setting and hook it to a the down stream / output side of a coupling cap then you can read signal level.

Set the controls on each channel about the same ( remember each pot has a 10% or more tolerance) and input a signal level of about .150 MV which will be about -10 do, then work your way back from the two 220K mixer resistors before the input to the PI section while noting the signal levels found.

The 4th preamp tube is the reverb recovery amp and the mixer stage for the vibrato / Tremolo channel and its output down stream to the PI is supposed to be unity, if it is then keep going back up stream until you find the stage that is reading a hotter signal level then the normal channel.

Once that stage is found then test out the stages plate load resistors and cathode resistors for being the correct value and do the same for the cathode bypass caps.

Note that V3, V5 and V6 will have nothing to do with this hot signal level that you have, so leave them out of the picture!

[pdf64]

What happens to the nasty distortion if the tube in V3 is removed?

[sonofmickel]

Removed V3, no change.

Not sure of your Multimeter AC measuring technique?
What are you doing exactly?
Please explain the technique with an example measurement for myself and future readers.

[pdf64]

A Vdc survey of at least the preamp tubes would be a good next step.

[sonofmickel]

Virtual Design Construction survey?
Not sure what a VDC survey is?
I took voltage readings though, they are in the first post.

[pompeiisneaks]

VDC= Volts DC
VAC = Volts AC

Every amp is going to be a bit different due to tubes, components etc, but common AC voltage readings I have seen on my amp were something like

input to grid of first tube, shoot for 50-100mVAC Then check post coupling cap on the anode of that and you s hould see maybe 1-5 VAC. Next check the grid of the next stage, and you might lose a small amount due to the tone stack etc. (I'm not looking at this specific schematic) But then again at the post coupling cap of that next stage's anode you should see another gain, maybe 10-20VAC. Usually post PI I think you'll often see 40-60VAC.

Yours might vary quite a bit, the bigger thing is to see at least the same voltage output if it's a cathode follower or other near unity gain stage, or significant increase if it's a normal gain stage. A 12AX7 is going to amplify more than all the other 12A*7's each being different, lower levels of amplification.

~Phil

[Stevem]

He posted a VDC survey across all 6 preamp tubes already.

[pdf64]

Oops, sorry, I was reading on a smartphone and didn't make sense of that.
OK, if I'm reading that right, the grids of V3 and V4 are screwed up, should be at 0Vdc; a good next step would be to work out why they're not.

[sonofmickel]

Took new readings on V3, V4 with reverb tank and footswitch hooked up to the amp.
Still reading 7 mvDC on the grids of V3(pin 2,7).
V4 now reads 0 on pin 2
V4 voltage bounces between 1.4 - 2.1 mvDC on pin 7.

[pdf64]

Yes, modern silver mica caps have a rep for dc leakage.
V4 should have no relationship to the trem circuit.
Maybe the grid’s 0V reference is bad, eg loose retaining nut on the rca jack, where the 220k grid leak is fitted?

[sonofmickel]

Why would I get bouncing voltage on the V4 "mixer" tube grid?

[sonofmickel]

Replaced the 500pf feeding the reverb driver.
Now getting only 5 mvDC on pin 2,7. Is that progress?

[sonofmickel]

Sorry guys, I messed up with my multimeter readings!
I misread the my auto switching meter when reading V3 and V4 grids, the meter switched to mv and I didn't notice and just ASSUMED it was Volts!

So the real millivolts on are now in the voltage chart at the top.

I am now noticing that the pots on the normal channel turn like they are full of grease vs the reverb channel pots all turn very easily.
A few posts above mentioned that the pots may be of vastly different values, this may turn out to be the case. Reverb channel volume is very distorted at 6, Normal channel is clean at 6, could it be a wrong taper pot?

Again, I apologize for the incorrect reading of my multimeter!
I am trying to do this too fast, slow down and do it right.