Fender ProTube Twin-Amp reverb feedback

[jim p]

Do you have a schematic for this amplifier? I was looking at the Pro Reverb schematic at the Fender web site and see that this is an all tube amp including the reverb drive and recovery circuits with an 8 ohm input tank, nice. From what you have posted the reverb will start to feedback with no signal into the amplifier just turn up the reverb level pot. By looking at the schematic for the Pro Reverb if the signal level out of the reverb is high enough it could feedback across R76 to its input and then oscillate (squeal). This is probably at a frequency greater then you need for the range of the guitar. One way to prevent this would be to limit the bandwidth of the recovery amplifier the 12AX7 V5-A. If you add a 300pf 500volt cap with 10k ohms in series across the plate load resistor R71 this will roll off the maximum frequency from the recovery at approx 5 kHz and may stop the oscillation. Because this oscillation happens with no mechanical excitation (no input to amplifier) I kinda don’t think it is a microphonic tube that is the cause.

[Mick]

Do you have a schematic for this amplifier? I was looking at the Pro Reverb schematic at the Fender web site and see that this is an all tube amp including the reverb drive and recovery circuits with an 8 ohm input tank, nice. From what you have posted the reverb will start to feedback with no signal into the amplifier just turn up the reverb level pot. By looking at the schematic for the Pro Reverb if the signal level out of the reverb is high enough it could feedback across R76 to its input and then oscillate (squeal). This is probably at a frequency greater then you need for the range of the guitar. One way to prevent this would be to limit the bandwidth of the recovery amplifier the 12AX7 V5-A. If you add a 300pf 500volt cap with 10k ohms in series across the plate load resistor R71 this will roll off the maximum frequency from the recovery at approx 5 kHz and may stop the oscillation. Because this oscillation happens with no mechanical excitation (no input to amplifier) I kinda don’t think it is a microphonic tube that is the cause.

Dude, I'm afraid to even look at a capacitor!

If it comes down to modifying the circuitry the way you describe, then I'll take it to someone like you that knows what an ohm is.

[mrmudcat]

Where the he..............hec is DR. TUNG:

[jim p]

Dude, I'm afraid to even look at a capacitor!

If it comes down to modifying the circuitry the way you describe, then I'll take it to someone like you that knows what an ohm is.

Well thought it might help. On the chance changing a tube could have an effect changing V3 may have an effect. This is used as a cathode follower for the clean signal to the reverb and the output. If its cathode impedance was higher then normal it could cause a problem like this. Hope this helps.

[Mick]

Okay, I tried swapping out all of the preamp tubes, one at a time.

No joy...

I'm about 99.999% positive that it's not a microphonic preamp tube causing the feedback.

So, I pulled out the reverb tank...



...but I didn't see any obvious problem with the tank, like a loose wire, broken spring, or whatever. So I cleaned the RCA connectors where the cable goes into the tank, then put it back in the amp.

I still have nasty feedback when the reverb is turned up.

Could the reverb transformer be causing the feedback? Is there some way to test it to see if it's within specs?

What should I try next?

[oldguy]

I think Jim P gave a fair evaluation of his prognosis after perusing the schematic of the amph. I would suggest taking it to an amph tech to confirm (or disprove) his theory as to the cause of the feedback.

[Mick]

I think Jim P gave a fair evaluation of his prognosis after perusing the schematic of the amph. I would suggest taking it to an amph tech to confirm (or disprove) his theory as to the cause of the feedback.

Pay someone that knows what they're doing to fix it, eh? That's quite a concept you got there!

Nawt yet... Methinks I'll try to verify that all the existing components are within specs before I throw in the towel.

I checked the tubes.
! checked the tank.

What's next? Maybe pull the chassis and have a look inside? How do I do that without electrocution? How do you drain capacitors safely? Lick the ends?

[Mick]

C'mon people! Doesn't anyone have any helpful suggestions?