Vibro Champ Rebuild - Plate voltage high

[jescher356]

Hi All,
A quick question from an enthusiast with not a lot of tech chops.

I have been restoring a silverface Vibro-Champ. Got it all together and it sounds good except:

1. Bass knob doesn't seem to affect tone
2. The amp Squeals when the tremolo intensity is turned lower than 4.

I checked voltages and found that the plate voltage on pin 6 on the 1st preamp tube is 300v when it should be 200v. All other supply voltages seem to be normal.

What's going on? Any thoughts would be appreciated...

[jescher356]

One more thing...

Pin 8 (cathode) is reading +4.5v when it should read +1.6v.

If anyone has any insight, please let me know.

[Tom Phillips]

High plate voltage AND high cathode voltage. Something is really goofy.
First suggestion is to check the actual values of the plate resistor and the cathode resistor on that stage.

[Tom Phillips]

Opps. Forgot to say try a different tube first and check the wiring in that area of course.

[jescher356]

Opps. Forgot to say try a different tube first and check the wiring in that area of course.

Tried a different tube, checked the resistors and re-checked wiring in that area. It all seems to check out fine. Any other thoughts?

[Bruce / Mission Amps]

Tried a different tube, checked the resistors and re-checked wiring in that area. It all seems to check out fine. Any other thoughts?

Have you tried lifting the cathode bypass capacitor from ground to see if it is leaking or shorted?
Also, sometimes one of the bare ground wires from the eyelet board to the brass plate (or wherever it grounded) becomes unsoldered on one end... usually the brass side. That will effectively, almost turn that stage off or at least be "biased" by a cathode bypass cap's DC charge, which does really bizarre things to the amp.

[jescher356]

Just went and checked the bare wire grounds to the brass plate and they are all good.

lifted the cathode bypass cap and the voltage on the plate and the cathode remain the same.

Both the first and second gain stage get their supply voltage from the same eyelet, then both have 100k plate load resistors. I don't understand why the first gain stage is correct at 200v and the second gain stage is high at 300v.

[Tom Phillips]

Interesting.

When you earlier said said you "checked" the values of the plate and cathode resistors did you actually measure the resistance with your Ohm Meter? Same with the grounds. They may look fine but it is good to measure them.

The more end-to-end test the better. i.e. measure the resistance from the top of the tune socket pin 8 to chassis ground. (Somewhere away from the spot where the cathode resistor ground wire is attached. Result should be ~1,547 Ohms. Note: I forgot to mention the 47 Ohm resistor in series with the cathode resistor. Measure that separatly.

Also measure from the top of the tune socket pin 6 to the eyelet where the tow 100k plate resistors join. Result should, of course, be ~100k Ohms. Do this with the tube removed and all the caps fully discharged.

Tom

[Tom Phillips]

Jescher,
I just edited my last post. Please note the corrections and the new comment "Note: I forgot to mention the 47 Ohm resistor in series with the cathode resistor."
Regards,
Tom

[Tom Phillips]

Another thing I'd have you do is temorarily disconnect the wire at the high side (right most terminal as viewed from the back) of the tremolo intensity POT so we can isolate that part of the circuit for now. Tape off the wire so it doesn't inadvertently touch anything.

[jescher356]

Yes I check with a VOM. But went back and re-check as you described... everything checked out correct.

I removed the right tremolo intensity lead. I turned the amp back on and now the squeal is immediate with volume knob less than 8, and at 8 there is a fair amount of buzz.

I've attached 3 photos that perhaps can shed light...

Thanks for the help!

[Tom Phillips]

Nice to have the pictures. I see that this was a complete rebuild. The lead dress looks nice.

My next guess is that the OT wiring was changed and you're power amp feedback is phased positive rather than negative. I don't see how that would affect the high voltage readings you are getting but there could be more than one thing going on. I'll look at the pictures again. Post more if you can including overall shot of the build with the controls at the top.

I think this is going to turn out to be 1 or 2 simple things.

[Tom Phillips]

OK. I can see from one of your photos that the OT wiring matches the Fender layout drawing. That is, the colors are hooked up as shown. It could still be a problem if the transformer internally different from the original spec. I've seen that before. An easy test is to temporarily disconnect the feedback by lifting the yellow wire that connects between the speaker jack and the 2,700 Ohm resistor on the eyelet board. (Either end will do. Whichever is easier.

It the amp gets louder with the wire disconnected then the feedback is negative and it was hooked up correctly. It's easy to tell when the amp is on the test bench hooked to a scope. A test by ear is a little harder but I think you will be able to hear the difference.

This is all just to rule out one potential issue. I'm suspicious because of the squealing you reported.

[Bruce / Mission Amps]

All these suggestions are good things to check and I'll add....

1. Are you sure the input jacks are wired right?
2. DC resistance: What do you find with the amp with the amp OFF.... and B+ grounded when you connected your DMM using OHMS scale to measure the DC resistance from lug 8 to ground?

If the combination of the 47 ohm and 1k5 resistor are not grounded correctly, with respect to chassis ground, your cathode voltage will be high and so will the plate voltage because the triode is not really turned on hard enough and not hard enough to strongly amplify the signal from the volume pot.

[jescher356]

Tom,
Sorry for the delay, I had some other business to attend to...

I just disconnected the feedback wire and it got quieter. The squeal is gone. So it sounds as though the transformer is wired backwards. I will rewire and re-measure voltage and get back with you. Sounds like we are making progress.