A tale of two Vibro Champs

[xtian]

My buddy the guitar dealer gave me two Vibro Champs to repair, a '64 black face and a '74 silver face.

The '64 had burned power tube cathode resistor (non-original 1K) AND blown open cathode cap. I put in a three-conductor power cord, did a full cap job, and replaced the burnt cathode components with a 1K 6.5W wirewound (was on hand!) and a 47u/50v cap. On power up, I'm seeing 55+ volts on the cathode, and it begins to smoke, so I shut it down.

Bad power tube? Why else would so much current be flowing thru the cathode?

[xtian]

With the power tube pulled, there is 470v at B+.

[Stevem]

What begins to smoke, the 5 watt resistor?
What fuse was in the amp when you got it?
Once that cathode cap burned up and shorted that fuse should have long since blown, with that much carnage I would take a guess that the OT is toast as is the output tube!
These amps bias in at no more than .048 amp unless its one of the blackface models where the B+ ends up being around 420 and then a 1k cathode resistor will be the place to start off.

I also really do not like the fact that the original can cap is still inplace and new caps are just tacked across it!

[ToneMerc]

I also really do not like the fact that the original can cap is still inplace and new caps are just tacked across it!

Well, it's looks like he unterminated the leads from the cap can. However, take a look at the power tube, it looks like the grid is tied to the cathode via a resistor or large SM 330pf capacitor maybe? Hopefully that's a cap.

TM

[cbass]

Yup i would just about guarantee that tube is toast.
I dont know what your wall voltage is out there. This amp would probably be a good candidate for a bucking transformer. They were already pushing the 6v6 at lower wall voltages . dropping the input voltage 6 or 12 volts will really help that poor little 6v6.
You can also drop the your b+ voltage with a big resistor since tese amps are single ended you won't get any sag.

[matt h]

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[xtian]

Thanks for the sanity check, guys. Wall AC is 120. Don't worry about my new caps, they're secure and safely wired. Yes, that's a 330p silver mica on the power tube socket. I'm going to set this one aside and recap the '74 next, and check the voltages.

[cbass]

Im not much into doing a bunch of stuff to vintage amps . this ones been bfed a little but not bfed to death. If you are really worried about the voltage an external bucking transformer is the only thing I'd consider. Dropping your input voltage dwn 10volts would probably bring your b+ down 30 or 40 volts easy.without any mods to vintage amp. If your customer doesn't want to run a bucking transformer tell them to stock up on 6v6s

[Stevem]

Is that a replacement pt in that, it looks so new, the original one was a 125P8?

[matt h]

(deleted)

[Firestorm]

1964 Champ (Vibro or otherwise) should have had around 360V B+. If that's the original PT, I bet it's fubar. You might want to check what VAC it's putting out to the rectifier. But now you've got 470V on a 450V cap and 55V on a 50V cap. None of that is good.

[pdf64]

An barely loaded B+ winding may be running probably 10% high, and the 5Y3 won't be dropping any volts.
The combination of those could cover the 80 to 100V excess on the B+.
With good tubes in place, all could be fine.
A good 5Y3 means one that was made in the USA.

[Stevem]

If the guy will pay for the work than its also a good idea to rewire the Heater string so one leg is not at ground!

[xtian]

These repairs are being paid by the seller, so he'll want cheapest.

[pdf64]

Are you running it via a light bulb limiter?
It may be a good idea to do so, until probs are resolved.