'65 Fender Vibro Champ squel at volume 10 and 1 only

[cjlectronics]

I am working on a Vibro Champ AA764 that squeals only in the 10 position and 1 position of the volume pot. All other positions it plays GREAT!! Came in the shop with no output and power tube was a 6L6 (wrong tube). Replaced output tube with a 6V6, replaced the OT with a drop in replacement from Antique Elec., and replaced the 25uf electrolytic cap on pin 4 of the preamp tube. (V1).
The amp came alive after replacing these items. It is loud and sounds good except for the whine at 10 and 1 of the volume pot.

The attached schematic will demonstrate the items I have swapped for a known good part or replaced. Also, voltages are listed. All 3 B+ tubes have been substituted for known good caps. The vibrato works too. I disassembled and cleaned the volume pot thinking that at the two extremes of the pot the pot wasn't making good contact. That didn't help. Then I replaced the volume pot. That didn't help. I substituted caps in the TONE circuit. Still same result. I've tried all that I can think of. Any suggestions from the experts?

CJ

[The Dude]

Have you tried reversing the wires on one side of the OT? If out of phase, your negative feedback will be positive causing oscillation.

[Chuck H]

This^^^^^^^^^^^^^^

Also, if the above doesn't work out, was the amp factory stock when you got it? I ask because there could be a lead dress issue. Especially the NFB lead to the board, which should be routed AWAY from the preamp as well as practical.

[Justin Thomas]

Carefully check ALL your grounds - loose pot nuts, the brass plate if it has one, chassis grounds, any wires going to "solder blobs" on the chassis, etc. The wires, give em a tug. They can LOOK solid but pop right out of a solder blob. And make sure your input jacks are up to par. Clean em if ya gotta.

Justin

[m1989jmp]

I am working on a Vibro Champ AA764 that squeals only in the 10 position and 1 position of the volume pot. All other positions it plays GREAT!! Came in the shop with no output and power tube was a 6L6 (wrong tube). Replaced output tube with a 6V6, replaced the OT with a drop in replacement from Antique Elec., and replaced the 25uf electrolytic cap on pin 4 of the preamp tube. (V1).
The amp came alive after replacing these items. It is loud and sounds good except for the whine at 10 and 1 of the volume pot.

The attached schematic will demonstrate the items I have swapped for a known good part or replaced. Also, voltages are listed. All 3 B+ tubes have been substituted for known good caps. The vibrato works too. I disassembled and cleaned the volume pot thinking that at the two extremes of the pot the pot wasn't making good contact. That didn't help. Then I replaced the volume pot. That didn't help. I substituted caps in the TONE circuit. Still same result. I've tried all that I can think of. Any suggestions from the experts?

CJ

Have you tested the old OT if it's still working properly? The way you've written your post makes me think you just replaced both output tube AND OT without testing.

These gentlemen ^^ are right about swapping OT secondaries though

[cjlectronics]

Thanks for the replies.

I have not tried swapping the leads on the OT mainly because the supposed drop-in replacement has the same color wires as the original OT. I will try this.

I did not test the original OT mainly because I don't know how to test it other than a resistance check. Can you test the OT with an inductance meter? if so, what are the values I should look for on the OT? The owner of the amp said he'd been powering on the amp many times with the 6L6 tube. When I looked of the 6L6, the key was broken off so no telling how many times it had been removed and replaced incorrectly. Before replacing the OT, I had a signal at pin 5 of the 6V6 and I knew the 6V6 was good. This was my justification for replacing the OT. Also, the customer told me the speaker was replaced with a known good speaker but I discovered the coil on the speaker was a dead short across the terminals. I think I'm going to put the original OT back in and see what happens.

I cleaned both input jacks with a 38 caliber wire brush and Isopropyl Alcohol. Thank you for the suggestion!

The chassis is VERY rusty (Texas Gulf Coast) and the grounds were broken on some connections. I repaired those broken grounds but wasn't sure about others. @Justin Thomas thanks for the reminder and I will check all connections again.

@Chuck_H what is the NFB lead? The amp was not factory stock. Someone replaced many of the electrolytic caps in the past so some things are out of place and newer than original.

I will post again with my findings.

CJ

[m1989jmp]

Thanks for the replies.

I have not tried swapping the leads on the OT mainly because the supposed drop-in replacement has the same color wires as the original OT. I will try this.

I did not test the original OT mainly because I don't know how to test it other than a resistance check. Can you test the OT with an inductance meter? if so, what are the values I should look for on the OT? The owner of the amp said he'd been powering on the amp many times with the 6L6 tube. When I looked of the 6L6, the key was broken off so no telling how many times it had been removed and replaced incorrectly. Before replacing the OT, I had a signal at pin 5 of the 6V6 and I knew the 6V6 was good. This was my justification for replacing the OT. Also, the customer told me the speaker was replaced with a known good speaker but I discovered the coil on the speaker was a dead short across the terminals. I think I'm going to put the original OT back in and see what happens.

I cleaned both input jacks with a 38 caliber wire brush and Isopropyl Alcohol. Thank you for the suggestion!

The chassis is VERY rusty (Texas Gulf Coast) and the grounds were broken on some connections. I repaired those broken grounds but wasn't sure about others. @Justin Thomas thanks for the reminder and I will check all connections again.

@Chuck_H what is the NFB lead? The amp was not factory stock. Someone replaced many of the electrolytic caps in the past so some things are out of place and newer than original.

I will post again with my findings.

CJ

Before dropping in old OT with new tubes/filter caps/etc. at least make a few basic impedance measurements across primary and secondary just to make sure it's not shorted.

Edit: some OT testing methods:

http://www.geofex.com/ampdbug/outtrans.htm

[m1989jmp]

I have to add that the old OT can be functional now, but stressed a lot due tube/speaker issues.

[Chuck H]

Negative feedback lead in red. For that matter you could use this layout to determine lead dress and grounding in general by comparing to what you have.

[glebert]

The fact that it does it only at the extreme positions of the volume pot make me wonder if someone could be wrong with that part.

[Chuck H]

The fact that it does it only at the extreme positions of the volume pot make me wonder if someone could be wrong with that part.

Well since the amp is reported to work fine at all points in between (and I assume this means "as it should" for a given volume setting.?.) I see this as a phase anomaly that only happens at the extreme ends of the volume adjustments. This could well be a grounding issue, either a fault or the actual ground locations, as Justin pegged in post #4.

[Dave H]

I have not tried swapping the leads on the OT mainly because the supposed drop-in replacement has the same color wires as the original OT.

You should really swap the OT leads but for a quick test try disconnecting the negative feedback wire at the board. It's the right hand end of the red wire on Chuck's marked up layout.

[galaxiex]

.......
I have not tried swapping the leads on the OT mainly because the supposed drop-in replacement has the same color wires as the original OT. ..................

CJ

I will also add to the suggestions to swap the leads.

I recently installed a "drop in replacement" Tx for a champ type circuit and the color of the leads matched the old Tx....

However... the colors were backwards on the new Tx because I had to reverse the leads to stop the oscillation at 1 and 10 on the volume pot.

PLEASE, swap the leads first before messing with it any more.

It's easy to do.... so just do it.

Just because the colors are the same, does NOT mean they are correct.

[The Dude]

Yes. I've even had OEM replacements that were backwards. It takes only a few minutes to check.

[Leo_Gnardo]

Yes. I've even had OEM replacements that were backwards. It takes only a few minutes to check.

Same here. What does Saint Enzo say... there's no excuse for NOT checking something, especially when it's easy as this.