vibro champ issues...

johnny1000 · Post by **johnny1000** » Mon May 09, 2011 1:14 pm

I have a '65 BF Vibro Champ that needs a little help. It sounds alright, if a little bass light, at lower volumes, but when I dig in, especially louder, it just *folds*. It's like once you run out of headroom, instead of clipping it sounds like it's got a really poorly set compressor that sucks out the sound until it drops back down a bit and recovers.

What does this sound like it could be? Caps? Tube? Speaker?

joninc · Post by **joninc** » Mon May 09, 2011 9:32 pm

i am gonna go with tubes on this one.

but really i am not expert

The Scum · Post by **The Scum** » Mon May 09, 2011 10:35 pm

Tube or solid state rectifier?

johnny1000 · Post by **johnny1000** » Tue May 10, 2011 5:41 am

It's the stock 5Y3.

It doesn't sound like normal sag to me, and theoretically a single ended amp shouldn't have rectifier sag, right? (I know almost nothing about these things but what I read on the internets, so I probably know effectively less than nothing.)

The Scum · Post by **The Scum** » Tue May 10, 2011 7:49 am

Sag is a function of power supply output impedance, and not of amplifier topology. The first suspects would be the rectifier and filter caps. The rectifier is easier to experiment with because it's probably socketed.

If you do swap caps, don't forget to put a bleeder resistor across them before desoldering.

Are you equipped to safely put a scope or meter on B+, and see what happens when you hit it?

johnny1000 · Post by **johnny1000** » Tue May 10, 2011 10:11 am

Since I had to look up "bleeder resistor", I'm going to say that I'm probably not equipped to *safely* put a meter on B+.

I can follow a schematic, though. Assuming I did avoid the deadly electricities, I'm looking at the voltage coming off the 5Y3GT? (in the schematic it's 355v)
http://www.ampwares.com/schematics/cham ... _aa764.pdf

Thanks!

Scodiddly · Post by **Scodiddly** » Tue May 10, 2011 5:04 pm

How long does sound keeping coming out after you switch it off? Sometimes you'll see old amps where the filter caps have built up a lot of internal resistance, cutting into the effective capacity. Because the power goes in/out more slowly the amp might still produce sound for a long time (more than say 30 seconds) after being turned off.

johnny1000 · Post by **johnny1000** » Wed May 11, 2011 2:28 pm

Good question... I don't remember. I'm back in tomorrow and will have a look.

fossiltooth · Post by **fossiltooth** » Thu May 12, 2011 7:44 am

I had a similar issue with that kind of sag at high levels on my vibrochamp. When I replaced the 5Y3 rectifier with a new one it cleaned right up. Can't promise it'll work for you but it's worth a shot. Inexpensive part.

johnny1000 · Post by **johnny1000** » Sat May 14, 2011 6:22 pm

Alright, I spent some more time with it, and have a better description of the problem.

At low levels (say, below 4) it's fine. I cranked it up to about 6 or 7 and it sounded great for about 5 seconds, and then sounded like it had an asthma attack. Not so much sag, as totally running out of air and the notes disappearing into a fizzy hash, which continued even after I turned it down again. Turning it off and then down to 4 before turning it on seemed to resolve it. Turned up to 10, everything is instantly fizzy hash.

That doesn't sound like descriptions of a dodgy rectifier to me (but, again, I don't really know). Could it be something like cranky caps in the power supply that basically work with smaller current but fail at higher?

Dominick Costanzo · Post by **Dominick Costanzo** » Sun May 15, 2011 7:01 am

Seems like the PSU capacitors have given up the ghost.
Replace all electrolytic capacitors while you're in there.

Jim Williams · Post by **Jim Williams** » Sun May 15, 2011 9:34 am

You can get new multi section caps from Electro-Harmonix. I replaced mine with a a pair of dual 100 uf/500 volt jobs from JJ. On my VC, I also removed the 1000 ohm 2 watt plate resistor with a small Deluxe Reverb choke, that removed nearly all the hum these always have.

I also dumped the tube rectifier for fast recovery diodes and replaced the 6V6 with a 6CA7. The preamp was redone using the vibrato tube and a mid pot and master volume added. It does about 12 watts and screams. The clean setting don't break up so fast either. Use a 4x12 high efficiency cab and you can play with a loud drummer too.

fossiltooth · Post by **fossiltooth** » Sun May 15, 2011 9:46 am

Jim Williams wrote: I also dumped the tube rectifier for fast recovery diodes and replaced the 6V6 with a 6CA7. The preamp was redone using the vibrato tube and a mid pot and master volume added. It does about 12 watts and screams. The clean setting don't break up so fast either. Use a 4x12 high efficiency cab and you can play with a loud drummer too.

Jim, you do some brilliant mods, but that's exactly the opposite of what I use my Vibro Champ to do! What you're describing sounds like a killer amp in it's own right, but why retool a perfectly good champ to sound clean, loud, and hifi? On the other hand, if you build one of these from scratch or from some current production model, I'd be first in line to check it out! Anyway, that's just my outlook.

Jim Williams · Post by **Jim Williams** » Sun May 15, 2011 12:16 pm

fossiltooth wrote:
Jim Williams wrote: I also dumped the tube rectifier for fast recovery diodes and replaced the 6V6 with a 6CA7. The preamp was redone using the vibrato tube and a mid pot and master volume added. It does about 12 watts and screams. The clean setting don't break up so fast either. Use a 4x12 high efficiency cab and you can play with a loud drummer too.
Jim, you do some brilliant mods, but that's exactly the opposite of what I use my Vibro Champ to do! What you're describing sounds like a killer amp in it's own right, but why retool a perfectly good champ to sound clean, loud, and hifi? On the other hand, if you build one of these from scratch or from some current production model, I'd be first in line to check it out! Anyway, that's just my outlook.

I made those changes because I was not happy with the amp stock. It sounded perfectly small, tinny and weak. Not a problem here, all my old black face Fenders are rebuilt insides, because I can and I want to. I make the amp into what I want as a player, after all, that amp is an extension of the instrument. I have zero concern for their re-sale value as they are not for sale, ever.

The VC I have was modified to do clean and very dirty too. The preamp stage is now similar to a Marshall JMP model and I added the master volume for tone and loudness adjustments. I wanted a more versatile small amp and this Champ does just that. Not recommended for collectors and re-sellers, though. Highly recommended for players.

Scodiddly · Post by **Scodiddly** » Sun May 15, 2011 1:21 pm

Problem could be caps or rectifier. If the caps are old then I'd go ahead and replace them, it'll have to be done someday anyway and is probably overdue. Find somebody with a tube tester and test out the tubes.

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