RI 65 Twin Reverb with excessive feedback

[cjlectronics]

I'm working on a reissue Twin reverb Schematic is here that goes into a squealing feedback at 2 on the volume pot. Here is what I have done so far:
1. Both Vibrato and Normal do the same thing.
2. I have searched this forum up and down and performed a few of the suggested things like turning the reverb up a little and then banging on the amp to see if I can get it to slip into the feedback... this resulted in NO change. It did NOT force the amp into a feedback so I don't think I have a loose connection.
3. I have swapped out V1, V2, V3 (with a 12AVT), and V4, but no real change. The amp went into feedback with these replacement tubes.
4. Something of significance is that the feedback only happens when the treble knob is increased above about 3 ON BOTH Channels. If the treble is turned way down it will not feedback.
5. Also the bright switch when engaged causes a horrible hiss and grappling sound in both the Normal and Vibrato channel. In the vibrato channel it pushes the amp into feedback and squealing instantly.

I have poked around with my DVM using an 8 ohm dummy load and following the schematic to measure the test points. My measurements don't jive with the TP values? I have read the notes over and over again trying to verify I have the test conditions setup right. Are some of the voltage values on the schematic an AC measurement and some are DC measurements? For example, TP15 at V4B measures 250vdc, where do they get 4.2v on the schematic?

Hopefully someone can shed some light on this problem. Oh, I have also pulled the pc board and inspected the solder joints, they look good. Has anyone else seen this problem?

Thanks,
CJ

[52 Bill]

What you seem to be describing is not an acoustic feedback problem that would respond to a physical stimulus, but a high frequency oscillation.

Check the amp for broken ground connections, bad lead dress, and maybe even bad filter caps in the pre-amp stages.

[Prairie Dawg]

I'm working on a reissue Twin reverb Schematic is here that goes into a squealing feedback at 2 on the volume pot. Here is what I have done so far:
1. Both Vibrato and Normal do the same thing.
2. I have searched this forum up and down and performed a few of the suggested things like turning the reverb up a little and then banging on the amp to see if I can get it to slip into the feedback... this resulted in NO change. It did NOT force the amp into a feedback so I don't think I have a loose connection.
3. I have swapped out V1, V2, V3 (with a 12AVT), and V4, but no real change. The amp went into feedback with these replacement tubes.
4. Something of significance is that the feedback only happens when the treble knob is increased above about 3 ON BOTH Channels. If the treble is turned way down it will not feedback.
5. Also the bright switch when engaged causes a horrible hiss and grappling sound in both the Normal and Vibrato channel. In the vibrato channel it pushes the amp into feedback and squealing instantly.

I have poked around with my DVM using an 8 ohm dummy load and following the schematic to measure the test points. My measurements don't jive with the TP values? I have read the notes over and over again trying to verify I have the test conditions setup right. Are some of the voltage values on the schematic an AC measurement and some are DC measurements? For example, TP15 at V4B measures 250vdc, where do they get 4.2v on the schematic?

Hopefully someone can shed some light on this problem. Oh, I have also pulled the pc board and inspected the solder joints, they look good. Has anyone else seen this problem?

Thanks,
CJ

Interesting stuff. I had a Crate a while ago that'd do the same thing, tracked it down to an improperly assembled reverb tank. Have you tried it with the reverb tank unplugged?

[cjlectronics]

Thanks for the replies,

The amp isn't that old so I wouldn't think the caps are bad yet???

Now bad lead dress and bad ground could be a possibility. I'll check it out

The amp is sitting on my bench with the reverb tank disconnected. So, I din;t think the reverb is an issue.

[52 Bill]

The amp isn't that old so I wouldn't think the caps are bad yet???

If it uses those tiny Illinois brand 22@500v caps, I've seen a number of them go bad early on.

[melvin]

There are both AC measurements and DC measurements on the schematic. With your DVM, I think you'll want to check the DC or "VDC" measurements. The most telling ones can be the plate voltages. You may have to look to the reference point in the power supply to see what they are supposed to be. (For reference, the non-reissue schematics are sometimes easier to read.)

The AC voltages are usually labeled with VAC or "V peak to peak" and are really only useful when the amp is already working somewhat normally, in which case you use a waveform input and an oscilloscope at the test points. Not that the oscilloscope wouldn't be helpful in this situation, but comparing the DC voltages to the schematic may give you a clue to what's going on in the amp without the need for an input, etc.

As 52 Bill has already suggested, I would suspect a faulty ground in the preamp. On the reissues this is sometimes just a wire clipped to a connector on the board. Not to be trusted.

[melvin]

Having looked at your schematic, it seems all of the voltages without units (mV or V) are the DC voltages. The others may be RMS voltages because there are also pk - pk voltages labeled.

[Don Moose]

Think those are related to the 'lytics with the counterfiet electrolyte that buggered the computer industry for a while (3 - 5 years ago, IIRC)?

[Prairie Dawg]

Think those are related to the 'lytics with the counterfiet electrolyte that buggered the computer industry for a while (3 - 5 years ago, IIRC)?

I'm going to be real interested in hearing how this problem ends up getting itself fixed.

You've pretty much eliminated everything that I would have thought of. Only thing I can think of that you haven't done is change out all the electrolytics. My whole take on them is to establish a baseline of known good parts to work from, and you're already inside.

[MWJB]

voltages in the ovals are AC, voltages in the rectangles are dc.

Schem shows voltage supplied at "X" to V4b's plate is 393vdc, a 12AX7 in a typical Fender triode returns approx 66% of that on it's plate so I'd expect 259vdc...close enough.

The twin has a 4ohm load, you'll get different readings with a 8ohm dummy load.

[cjlectronics]

I have ordered the 22uf/500v and they should be here in a few days. I'll reply back with my findings.

I have checked the ground cables and they all look good. I've seen the IDC connectors from the potentiometer board cause problems with the wires not seated well. I've pressed all of those in and they look fine as well.

I'm starting to lean toward the bad electrolytic caps like 52 Bill recommended. I don't know if this is significant but when I turn the amp off the high voltage drains immediately. I remember on other fender amps I had to drain the high voltage with a 100k resistor to GND. Do these newer RI amps have bleeder resistors or are they designed to drain the caps quicker?

CJ

[52 Bill]

Simple test for the caps is to parallel a good one across the one in question and see if the problem stops. I think it's an ESR problem with the caps.

Be careful when doing this test. Attach the new one with the power off and then power up the amp. Shut down the amp and let the charge drain off before removing the new cap.

I've seen these caps fail early in a few of the Blues Series as well as some of the reissue blackface amps.

[pdf64]

Don't forget the cathode bypass electrolytics on the pre-amp. The shared cathode resistors in several places can lead to this type of problem when the bypass caps go bad.
Also try some different 12AX7 tubes, they can have a weird coupling between their sections. Peter.

[MWJB]

"I have checked the ground cables and they all look good." Actually measure resistance from the grounded end of the component to the chassis, e.g. grounded ends of cathode resistors, pots etc.

If you leave the standby inplay mode when you power down, the 220K smoothing resistors at the first filter section will drain the caps.

[Enzo]

If you think your lytic filters are going soft and loss of decoupling is allowing oscillation, look at the B+ nodes with a scope. If you see signal there, then there is a loss of decoupling.