1962 Vibrolux squealing

[sleepingAwake]

Hello guys and gals.

Obviously I have an oscillation issue. Happens when I turn on the vibrato.
I don't have a scope so I can't do the kind of isolation I'd like to.

All new tubes and yes it did it before. Besides replacing all electrolytics and tubes I reflowed every solder joint in the amp. There was around 10 cold joints so I said the heck with it and reflowed them all.
The tubes and caps were for maintenance. Now I'm moving on to troubleshooting.

The buzzing is from a fluorescent lamp about 3 feet overhead.

What do you think? Could it be DC sneaking through a cap?

[soundguruman]

Hello guys and gals.

Obviously I have an oscillation issue. Happens when I turn on the vibrato.
I don't have a scope so I can't do the kind of isolation I'd like to.

All new tubes and yes it did it before. Besides replacing all electrolytics and tubes I reflowed every solder joint in the amp. There was around 10 cold joints so I said the heck with it and reflowed them all.
The tubes and caps were for maintenance. Now I'm moving on to troubleshooting.

The buzzing is from a fluorescent lamp about 3 feet overhead.

What do you think? Could it be DC sneaking through a cap?

The switch(s) on the input jack(s) is not closing to ground.
You see the arrow on the schematic input jack?

each input jack must have a functioning switch, that's the arrow.
if the switch does not close
you will get oscillation, especially if the guitar is not plugged in.

you are giving the preamp an open input, with no load.
you are just associating it with tremolo because tremolo is making volume go up and down.

The switches on the input jacks have to close, when nothing is plugged in.

At V1A ideally, you would run the grid wire under the board, under the plate resistor.
It's an issue with the layout too...
Fender vibrolux_5e11_schem-1.pdf
Is that the right schematic?

I apologize in advance if the PDF does not open correctly.
you can twist the wires together from pin 1 and 2 of the first preamp tube.

this forms some added capacitance between plate and grid of the first preamp tube.
this can also help to stop oscillations.

plate and grid wires spaced closly together or twisted together, that's why in later amps, they run the grid wire under the board, under the plate resistor.

[sleepingAwake]

Actually the layout and schem is this.
http://www.ampwares.com/schematics/vibrolux_6g11.pdf

Does that change your diagnosis?

[Mick Bailey]

The first thing is to narrow down which stage is affected. Does the trem intensity control affect the squeal?

Also,

To eliminate the preamp, turn all the other controls to zero except the intensity control (while it's squealing). Does it still do it?

If so, remove the PI tube. How about now?

[Jazz P Bass]

I would recommend that you check Node A on the Tremelo 12AX7 for undue ripple (Vac).
You may have a bad ecoupling capacitor.

[Mick Bailey]

The last amp I had that did this was a Traynor YGM-3 (2 x EL84) and with the lights off I could see the output tubes red-plating in time with the squeal. That one was a combination of lead dress and new tubes.

[sleepingAwake]

The first thing is to narrow down which stage is affected. Does the trem intensity control affect the squeal?

Also,

To eliminate the preamp, turn all the other controls to zero except the intensity control (while it's squealing). Does it still do it?

If so, remove the PI tube. How about now?

The trem intensity does indeed affect the squeal. The only time the squeal occurs is when the trem is on and the intensity above 7 and speed above 5.
With all other controls turned to zero it still happens. I can pull P1 or P2 and it still occurs.

[sleepingAwake]

Lead dress seems ok but I will say that change the amount of noise it picks up from the environment effects the squeal.
In other words if I place my hand over the wiring anywhere around the preamp through power amp section it may increase the severity of the squeal.

All tubes are new and old tubes do the same thing.

[supawah]

bad astrons.....

Hello guys and gals.

Obviously I have an oscillation issue. Happens when I turn on the vibrato.
I don't have a scope so I can't do the kind of isolation I'd like to.

All new tubes and yes it did it before. Besides replacing all electrolytics and tubes I reflowed every solder joint in the amp. There was around 10 cold joints so I said the heck with it and reflowed them all.
The tubes and caps were for maintenance. Now I'm moving on to troubleshooting.

The buzzing is from a fluorescent lamp about 3 feet overhead.

What do you think? Could it be DC sneaking through a cap?

just throwing this out there, the last 6 amps I've had come in loaded with yellow astrons, both ampegs, and fender tweeds, the majority of them were leaking dc, I mean 95% of them were leaking, some as much as 12-20 volts. not totally shorted, but enough to throw the circuit out of balance. lift the ends of them and check them if you get really stuck.

[Chuck H]

I have a few ideas.

That amp is designed to work with a lower mains voltage than what comes out of the wall today. It was also designed to run the bias quite hot. The trem function lowers and RAISES the bias from a center point. You may be over dissipating the tubes on the raised swing of the trem function. The solution may be as simple as cooling off the bias a bit (increasing the -V on the power tube grids).

My other consideration is that you have the amp out of the cabinet right now. Has it always done this even with the amp in the cabinet? and does the cabinet have a shield in place for when the chassis is installed? I mention this because sometimes problems that occur with an exposed chassis are solved by simply shielding the open side. That might explain how you can change the nature of the problem by moving your hand over the open chassis.

Is this amp equipped with a properly grounded mains plug? Have you tested the coupling caps for leakage? It's a slim possibility, but if the chassis isn't at 0V and/or ther is DC leaking from coupling caps it could affect decoupling and cause momentary phase errors on the control grids.

It could be a combination of these things. Many designs only JUST work. Meaning that they were only JUST stable enough to pass muster as a new design and if a couple of things get hinky the amp acts funny. It's possible that cooling the bias a little and making sure the chassis has a top shield will stabilize the amp.

[Mick Bailey]

Given that the squeal still happens with the other controls turned right down, try removing the phase inverter tube. As Chuck H points out - your trem could be tipping the output tubes into excessive dissipation. Removing the PI will isolate the signal from the output, though you may have DC leakage from the PI caps throwing off your bias.

The Traynor I mentioned was fine until the tubes were changed for ones which pulled just a little more current, then the whole thing was upset. It must have been borderline to begin with.

[sleepingAwake]

I tried substituting the Astron caps in the trem and PI section with known working caps.
Still squealed.

I noticed that when I measure the voltage across the 1 ohm resistors I installed on the power tubes is was quite high. Equivalent to 73 v!!! Bias voltage is around 45v at the bias cap.

I tried something I've read about before but I didnt quite think it would fit the situation. I swapped the leads on the OT. Squeal is gone.
The OT was replaced at some point early on and maybe it was color coded wrong from the factory???

No more squeal but I'm not convinced its the "right" fix.

Chuck H raises some interesting points for sure.
If it was my amp I would probably replace all the caps but the guy whom I'm fixing it for likes original. For example a lot of the resistors have drifted up just a little over tolerance. I'd replace them if it was my amp but I know he likes original.

[Jazz P Bass]

That was my next post, but you beat me to it.

Reverse the OT leads.

If it is wired correctly, when lifting the feedback connection, the output waveform should increase in amplitude.

[Mick Bailey]

That was my next post, but you beat me to it.

Mine too!

The key bit of info was missing. Bit like going to the doctors with bright red urine and not telling him you've been eating beetroot

Nice you've got it fixed.

[Jazz P Bass]

Yup, you cannot always rely on the wire colors.