6G16 Vibroverb vibrato problem - red plating tubes!

[Josh_B]

I recently built a 6G16 Vibroverb. GREAT sounding amp!

BUT...when I turn the intensity up past "3", it starts to red-plate the power tubes - regardless of the starting bias. I can bias it at 20 ma and still get the same thing. I just don't get it.

I have a 10K bias pot with a 27K load resistor. With the footswitch defeating the Vibrato oscillation, the Intensity control can bring the negative bias control all the way down to -8 volts from -53.

I have checked and re-checked and re-checked the circuit for errors and the components for proper values, both visually, and with a meter.
Yesterday, I took the board out and checked all of the connections on the back. They were fine. I also checked the layout again and measured each component with a meter, and checked all of the grounds. I made sure there are no grounds except where they should be.

With the power tubes out and the vibrato switched off with the footswitch, I also checked through all of the voltages.

Bias tap = -45 volts
Bias board out to bias pot = -63 volts
Bias pot out to intensity control, left lug = -47 volts

V8 and V7 (6L6)
pin 3 = 477
pin 5 = -47
pin 6 = 475

V6 (phase inverter)
pin 1 = 270
pin 2 = 20
pin 3 = 29
pin 6 = 258
pin 7 = 20
pin 8 = 29

V5 (vibrato)
pin 1 = 225
pin 2 = 19
pin 3 = 2.3
pin 6 = 473
pin 7 = 225
pin 8 = 233

V4 (reverb recovery)
pin 1 = 204
pin 2 =
pin 3 = 1.7
pin 6 = 213
pin 7 =
pin 8 = 1.8

V3 (reverb send)
pin 1 = 465
pin 2 = 30
pin 3 = 7.5
pin 6 = 465
pin 7 = 30
pin 8 = 7.5

V2 (bright)
pin 1 = 205
pin 2 =
pin 3 = 1.7
pin 6 = 185
pin 7 =
pin 8 = 1.5

V6 (normal)
pin 1 = 205
pin 2 =
pin 3 = 1.7
pin 6 = 160
pin 7 =
pin 8 = 1.5

[tubeswell]

Can you post gut-shots of your build?

[Josh_B]

I went through tonight and cleaned up the layout and put shielded wire (grounded at input end only) on both grids of the first two tubes.
It's quiet and performs great, but there is no vibrato oscillation and I still have the red-plating of the tubes with the intensity above 3.

The pictures before I cleaned up the lead dress and shortened up a bunch of wires going to the tube sockets is here:
http://gallery.mac.com/joshbradshaw#100053

My "C" node is actually a yellow wire. I wasn't sure of the original color coding when I did it. Here are those voltages:
A - red wire - 474
B - brown wire - 472
C - yellow wire - 386
D - blue wire - 324
The A node is grounded to a power transformer bolt. The B, C, D, ones are grounded to the buss wire near the inputs.

[tubeswell]

Well I can't see your amp so I have no idea what wires you're talking about. If the tubes are red-plating, they are biased too hot and when the oscillator is cranked they probably have points in the oscillation where they are not getting enough negative voltage on their signal grid(s). Have you put the correct value resistors in the bias circuit, and did you measure them to check that they were not off as you were building it? Are the bias caps (correctly) reverse biased? etc? You need to start looking at what in the circuit is causing the insufficiency of negative voltage at the grid.

[Josh_B]

I have -53 volts at the junction of two 220K resistors for the power tube grids. I did measure the components to see that they are in spec and I have triple-checked the layout using Weber's and the original Fender one.

I also measure the components in the circuit with relation to ground. Everything jives.
Sometimes when I switch it on, it oscillates fine. Sometimes, it does nothing.

Since these pics were taken, I have taken the board out and checked connections on the back, cleaned up the layout, and run shielded grid wires behind the board.

[martin manning]

If I was in this quandary, here's what I'd try:

- unsolder the blue wire from the intensity pot's CW lug to disconnect it from the trem oscillator

- measure the resistance across the pot's element and from the open end to the wiper as the pot is turned to make sure that it is functioning properly

- power up (with the oscillator still disconnected and disabled) and measure the bias voltage as the intensity pot is turned. it shouldn't change much since the grid current is very small.

If that's all good then the problem is coming from the oscillator.

- connect a 'scope to the disconnected blue wire and find out what the peak-to-peak voltage is doing when you start the oscillator. I don't know what is expected here; maybe someone else can help with that, but I'd guess it shouldn't be more than the bias voltage. If you don't have a 'scope you can meter it with the speed turned down to at least get an idea.

MPM

PS: and check the resistance from the disconnected blue wire to ground with power still off (it should be infinite), and there should not ever be any DC on it with the power on.

[Josh_B]

Thank you!! I will post the results Sunday night!

[Steve Conner]

I think the problem has got to be in the .1uF cap that couples the trem signal into the intensity pot. This capacitor is here so that only the AC portion of the trem signal can affect the power tubes. It should block DC, so turning the intensity pot should have no effect on the bias voltage.

So, since you observed that the intensity pot does affect the bias, I say this cap has got to be leaky, shorted, or even missing altogether. What voltage rating did you use for the cap? It has about 300V DC across it in normal operation, and Lord knows how much if your trem oscillator is broken, maybe 500. So it should be a 600V cap.

But also try the things martin manning suggested, it's all good advice

[martin manning]

I think the problem has got to be in the .1uF cap that couples the trem signal into the intensity pot. This capacitor is here so that only the AC portion of the trem signal can affect the power tubes. It should block DC, so turning the intensity pot should have no effect on the bias voltage.

So, since you observed that the intensity pot does affect the bias, I say this cap has got to be leaky, shorted, or even missing altogether. What voltage rating did you use for the cap? It has about 300V DC across it in normal operation, and Lord knows how much if your trem oscillator is broken, maybe 500. So it should be a 600V cap.

But also try the things martin manning suggested, it's all good advice

...my thought too, or, that the intensity pot is bad, with the wiper making intermittent contact.

That 0.1u cap looks to be on the board in Josh's pics. It's the big (looks like it reads 600V) orange one right next to the four-in-a-row ceramic discs.

BTW, the recent Fender reissue Vibrolux Reverb schematic says the trem oscillator should produce 55V p-p at 8Hz with the speed and intensity controls at max. The trem circuit is not identical, but that should be close, given that it is doing the same job.

MPM

[Josh_B]

Well, you guys set me on the right track, and the problem is solved!!

I thank you a thousand times over.
The Intensity pot was/is defective. I replaced it and immediately had beautiful, glorious Vibrato and no red plating of the tubes whatsoever. They stay running cool, even with the Intensity cranked full up. There is no ticking or beating or anything - it sounds really great!
Unfortunately, the 250K pot I had lying around is of the wrong taper (Audio instead of Linear), but I will find the right one tomorrow and finish assembling this fine amp.

Steve, the cap I have in the position you describe is a 630 volt. I replaced it prior to trying out the pot. It never hurts to be sure!

Thank you all for your advice!

[martin manning]

Great, and thanks for the follow-up... it's always nice to hear how things turn out; even better when they turn out well!

MPM