Fender Deluxe Reverb AB868 Squeal

[BungleSim]

Alright, so a customer brought in his late '70s Silverface Deluxe Reverb. It has the pull-boost function and the 5U4GB rectifier, so I gather that's the AB868 version. When you turn the Reverb knob up past 4 or 5 a piercing feedback comes from the speaker.

Here is the schematic (but I think nearly any Deluxe Reverb schematic will do): http://www.davidsonamp.com/sf/images/de ... bab868.gif

I rolled tubes through every position, obviously concentrating on the reverb driver and recovery tubes, but no dice. Still squeals like crazy. I opened it up and now I've got it on my bench.

*Removing the reverb tank from the circuit does not change the squeal.

*Removing V3 (12AT7 reverb driver) from the circuit does not change the squeal.

At this point I have a brand new 12AX7 in V4 (reverb recovery) and none of the tubes are microphonic (V3 and reverb tank still removed). The 0.003uF coupling cap before the Reverb pot was microphonic so I replaced it with a 0.0022uF poly cap for the time being. I have removed the 10pF cap leading to the second half of V4 and the 500pF coupling cap leading to V3 (though without V3 in the circuit I wonder why it would matter at all...). The squeal persists but it does not happen until about 6 or 7 on the Reverb knob now. I have verified all of the resistor values between the reverb return and the second half of V4.

The wire connecting the 470K coming off the Reverb pot's wiper to the grid of the second half of V4 is microphonic. I have tried shielding it with a GND wire wrapped around it and connected at one end but that hasn't done anything for the squealing.

I haven't found any of the capacitors in the circuit, other than those mentioned, to be microphonic. Only the 0.003uF and 10pF caps were microphonic and replacing them did nothing for the squeal. I have also disconnected the boost feature to no avail.

The squeal definitely sounds like a parasitic oscillation and as you turn the Reverb knob up you can hear the Q of the noise coming from the speaker change. Right before it squeals it hits a resonant peak.

Any thoughts? Any and all help is appreciated.

[tictac]

I've seen this happen before and it was the reverb pan itself...

Took some foam insulation tape with adhesive on one side and taped a few

pieces to the tank and the problem went away....

TT

[BungleSim]

I've seen this happen before and it was the reverb pan itself...

Took some foam insulation tape with adhesive on one side and taped a few

pieces to the tank and the problem went away....

TT

Thanks but the squeal is happening without the tank even connected.

[Stevem]

That's the result Fenders faster build rate on those amps and the lack of proper wire layout and sheilding!
If the customer does not use that dam boost function just rip it all out, the amp will sound better for it anyway and save the customer 2 hours in trouble shooting and layout rework time!
The amp may have some power supply filters going south and now the gain stages are not un-coupling from each other like they should,or in the worst case the oil turret board has now become conductive and is bleeding signal to a stage it should not!

[rfgordon]

The lead dress in these Fenders is really bad. If you shorten the leads going from the board to all the preamp tubes, you'll probably take between 1 and 2 feet of wire out of the amp. The boost circuit injects signal from the reverb transformer output back into the signal flow. Thus, even if the boost is not engaged, poor lead dress can have that signal couple into another part of the circuit.

I would disconnect the boost from the front of the amp and see if that solves the problem. If it does, and the client wants the boost function kept, try replacing that lead with shielded cable.

[pdf64]

A failed V4 cathode bypass cap is a likely cause of such symptoms.
It demonstrates why it's poor practice for cascaded stages to share cathodes, in that the arrangement isn't resilient to the cap developing high ESR, allowing a positive feedback path to form.

[BungleSim]

The lead dress in these Fenders is really bad. If you shorten the leads going from the board to all the preamp tubes, you'll probably take between 1 and 2 feet of wire out of the amp. The boost circuit injects signal from the reverb transformer output back into the signal flow. Thus, even if the boost is not engaged, poor lead dress can have that signal couple into another part of the circuit.

I would disconnect the boost from the front of the amp and see if that solves the problem. If it does, and the client wants the boost function kept, try replacing that lead with shielded cable.

I agree that the lead dress is a total mess in this amp. I did disconnect the boost circuit from the amp completely, though, and it did not improve the issue.

[Stevem]

If you place the 12at7 from the reverb driver to replace the 12ax7 reverb recovery socket does the issue subside some?

[Stevem]

Also if you lift that bypass cap off of V4 does that change the issue ?

[pdf64]

The squeal definitely sounds like a parasitic oscillation

Parasitic oscillation usually means that a regular signal is required for the instability to manifest, with the amp being stable under static conditions, eg the oscillation 'rides' on sections or all of the applied signal waveform.

A squeal is usually just a standard free running type of oscillation.

You can just quickly 'tack' a good cap across the existing cathode cap to verify the high ESR hypothesis.

[BungleSim]

If you place the 12at7 from the reverb driver to replace the 12ax7 reverb recovery socket does the issue subside some?

The squeal changed slightly in tone but I would not say it was any less severe than with the 12AX7.

Also if you lift that bypass cap off of V4 does that change the issue ?

No.

You can just quickly 'tack' a good cap across the existing cathode cap to verify the high ESR hypothesis.

I have an ESR meter and I checked all of the power supply caps and cathode bypass caps. The power supply caps were still good, surprisingly, but all of the cathode bypass caps were dry. I lifted them each, one by one, to see if they would affect the squeal but no dice.

What I'm going to propose to the customer is that he allows me to give the entire amp a re-capping and to clean up some of the lead dress. At this point I'm wasting my time trying to find that one part (if it is just one part) and would rather clean house, so to speak, and get the amp sounding great again.

Thanks for everyone's help and suggestions. Fingers crossed my next post will be celebrating the amp being fixed.

[Stevem]

If you pull out V2 is the issue still there?

[BungleSim]

If you pull out V2 is the issue still there?

Yes.

[pdf64]

If you touch a good cap across the V4 cathode resistor, does it stop the squeal?
Enquiring minds wish to know!

[BungleSim]

If you touch a good cap across the V4 cathode resistor, does it stop the squeal?
Enquiring minds wish to know!

Forgive my ignorance, but wouldn't disconnecting the (potentially) bad cap get rid of it, too?