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Re: Help with '64 Vibroverb

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2/17/2000 7:13 PM

Re: Help with '64 Vibroverb

There's an old Heathkit signal tracer, which is basically an audio amplifier with a small speaker. It's a tube unit, so has an onboard (though tiny) output transformer. There are binding posts that allow you to substitue the output transformer (and speaker) into the amplifier circuit under test. This is really a neat feature, and has come in handy for me during radio servicing.

The idea here is, by the time you've serviced and built a few amps, you're bound to have some extra output transformer around the shop that will handle a pair of big tubes (6L6/EL34) which can be mounted on a board with long test leads. This can be utilized to verify gross defects in the output transformer of the amp under test. For those who do a large volume of repairs on tube amps, it might be wise to invest in a new general replacement (like the new sensor "bassman") trans just for this purpose. Even a pull from an otherwise unusable PA or Hifi amp would be good for this purpose. A 4-tube amp (Twin Rev., 100w Marshall) might need a larger trans to handle the heavier primary current. A 4-tube trans would work well for just about every amp you'd normally encounter, and be quite versatile if it had multiple secondary taps. So for those who could afford it, the replacement TR trans would be the ticket.

With this "test transformer", it wouldn't take too long to carefully unsolder the plate & CT leads from the amp under test, tack in the long test transformer leads, and connect to a known good speaker. (Also make sure about any negative feedback connection.) If the "test transformer" happened to be the type the amplifier actually needs as a replacement, it could be used for that purpose. Just pick up a new one on the next routine parts order.

2/17/2000 7:36 PM
Carlo	I obviously couldn't have (and didn't) say it better myself. As usual, Doc, nice solid thinking. Carlo

2/18/2000 4:22 AM

Found something strange

I had the chassis out again and noticed that without the reverb cables plugged in the reverb control acts like a strange gain control. As you turn up the reverb the amp volume and gain increases tremendously going into full distortion.

I have never seen another Fender amp do that. Also when the reverb control is turned up the ringing gets worse. It is not the reverb tubes.

Any thoughts? Should the reverb control act this way on this model with the reverb cables unconnected?

Thanks,
Tim C.

2/18/2000 5:11 PM
John Stokes	The reverb control should not work that way. You have some strange coupling going on in that amp. No wonder it rings.

2/18/2000 11:23 PM
Richie	Have you checked the little electrolitics.. sounds like the one is bad.. it makes the amp trebly and whistle..sometimes. also check the resistor values.. see if they are correct. Richie{~}==:::

2/19/2000 2:12 AM

I think John and Richie are on the right track, Tim.

Check the cathode bypass cap that is shared by the reverb recovery and mixer stages. If that cap isn't doing it's job, positive feedback could be coupled through the cathodes of those two stages. Bridge a known good cap across the suspect and see what happens.

Another possibility that comes to mind is that the reverb circuit may have been modified at some point to implement an "icecube" type distortion boost. Try pulling the reverb driver tube temporarily. If that stops the extra gain and distortion, this could be it.

--tboy

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