Tune up advice for 65 VibroChamp

[clinkous]

Just got a Vibro Champ and would welcome any comments about what to replace as preventive maintenance.

PT 606540
OT 606513
Mallory Cap Can 235-6538x

Am I correct in reading these as 1965 parts?

Nothing looks ruptured or modified except for one new 20/450 electrolytic from one pin of the can cap to ground. Would this be a repair or an added capacitance mod?

The amp plays well and it doesn't sound weak when played next to my SF VibroChamp. Speaker is a Mojo and seems fine. Probably going to replace the paper electrolytics because of age alone but what about this can cap? Why is there an unused 20uF pin on the cap?

Anyway, I'm installing a grounded cord with a grommet setup for the strain relief and I was just fishing for anything you guys had to say.

Hope my attachment works.

Thanks,
Chris

[g1]

The added cap is probably a repair, it allows leaving the original can in there for looks.
What is the voltage rating of the unused 20uf can section? Those cans often had a low voltage section which could be used for the power tube cathode bypass. Which would make me wonder why they didn't use it for that.
Maybe someone knows of some disadvantage to that?

[Justin Thomas]

Which would make me wonder why they didn't use it for that.
Maybe someone knows of some disadvantage to that?

Because a whole 25/25 is cheaper than the 8" of wire needed to connect it? :P
I'd rather not have an extra cap tacked onto the can like that, but if the amp runs good and doesn't make noise, I just would leave it. Your reading of the dates is correct - all 1965 parts. It may be an "added capacitance mod," though, mightn't it? Isn't it in parallel with the existing can section, and therefore adding to it?

If you are the new owner, congrats!

Justin

[66 Kicks]

The can is still connected, so the added 20uf/450V cap is in parallel with it making it (probably) 40uf for the 6V6 plate. 40/20/20 cans were pretty typical in the '70's Champs and I think it may have been to reduce hum. Somebody may have done it on this amp for the same reason.

Is there is an unused 20uf section in the can or just an unused pin?

[Justin Thomas]

The can casing should be stamped with all the included sections. Usually they don't add a solder tab unless there's a cap in it... most likely it's a 25V cap, as g1 mentioned...

Justin

[66 Kicks]

The can casing should be stamped with all the included sections. Usually they don't add a solder tab unless there's a cap in it... most likely it's a 25V cap, as g1 mentioned...

Yeah, I agree. If it was another 20uf/450V, you would think the guy would have strapped that to the existing 20uf instead of adding a whole new cap.

[Tom Phillips]

I have seen many of these small amps where Fender installed a 4 section 20uF/450V can and only hooked up 3 of the sections. Who knows what they were thinking. A big opportunity to add an additional filter stage to lower the B+ and dramatically reduce the hum level. I guess the extra resistor was too much cost or trouble. My notes show that the extra filter stage reduces the B+ripple from ~1.45% stock to 0.05% while dropping the B+ ~13V. The resulting hum level was very much improved.

...If it was another 20uf/450V, you would think the guy would have strapped that to the existing 20uf instead of adding a whole new cap.

Maybe another person just following someone's layout diagram for a mod. I.e. someone who didn't really understand the circuit.

[Chuck H]

I vote that the extra cap is a repair. It was pretty common once upon a time for repair shops to parallel a good cap with an existing bad one that couldn't do the job by itself anymore. Faster and cheaper. I'll guess the original can wasn't doing it's job anymore and that's why the cap was tacked in.

I'd remove the added cap and replace the can with a 40/20/20/20 and just omit one section as Fender did or use a 20/20/20/20 and parallel two sections for the main filter. Or just leave the original can, but don't use it. Wire all the power section with new caps under the hood Someone is going to say something about rectifier tubes and capacitance now. Don't worry about it. 40uf on the main filter works fine and hums less.

[clinkous]

Thanks guys,

The can cap is a 20/20/20/20 all at 450V. Chuck H I had wondered about replacing the can and paralleling two sections. I'll try to find a 40/20/20 that fits. Otherwise just electrolytics and a cord?

Maybe someone can id the 12AX7s?

Chris

[Alex R]

yep cheap repair gets my vote too for the paralleled cap, seen it many times here in the UK. Not a good idea of course. Take it off and chuck it away.

Looks like it's definitely time to replace the filter caps. I tend to just grind off the old lugs and leave the old can cap in place, put in a tagstrip with some small modern radial caps on it along with the dropping resistors, and rewire up to that. Standard modern electro caps are definitely the way to go for reliability.

[Leo_Gnardo]

Looks like it's definitely time to replace the filter caps. I tend to just grind off the old lugs and leave the old can cap in place, put in a tagstrip with some small modern radial caps on it along with the dropping resistors, and rewire up to that. Standard modern electro caps are definitely the way to go for reliability.

I second that emotion & have found no harm in upping the first filter to 47 uF. Select 105C rated caps as they will be in a little oven with tubes & PT generating heat. I just put a bus wire from one PT bolt to the other & bridge the caps over the old cap, with + ends landing on the appropriate eyelets on the board. Doing it this way also gives you the option of placing the filter handling power for the preamp near the preamp circuit affording an extra degree of hum filtering.

For extra hum reduction you can add a pre-filter to the supply. The cheap way with a 20 or 22 uF cap and 100R. More effective and expensive and heavy, same cap plus choke - I use one intended for Twin Reverb power supply 90 mA 3 or 4 Henry. With this in place hum falls to an almost undetectable level, ideal for close-mic recording. You'll hardly know the amp is on, until you play it.

That 12AX7 is an Amperex "Bugle Boy", bit of a collectors item. If it's working well, leave it in and you'll have the premium tone others spend $$$ for. Of course, depending on what you put into it.

[Chuck H]

+1 to what Alex and Leo said.

The tube is "branded" Amperex. A lot of tubes in the day were made by someone else and rebranded. I don't know if Amperex ever rebranded other tubes. I don't think so. Someone cooler than me might even be able to tell you the era by the logo, paint, electrode build, etc.

I have made a couple of Champs very quiet by changing the 6.3V filament wiring to a twisted pair and referencing the false CT to the top of the power tube cathode. One was a VC. Wiring filaments is always a little bit of a PITA (Well, "I" don't like it much), but there's only three tubes on that winding.

[clinkous]

Thanks for all the advice and suggestions. I do not own this amp (unfortunately) so I believe my best bet is a CE replacement can with 40/20/20 and new board electrolytics along with a new power cord. The tubes seem fine but I didn't play it that much before I looked inside.

I do like the choke idea. I have one from a re-issue twin and may try this on my personal SF VC!

Once again I'm grateful for the information you all share. I'll do everyone a favor and NOT share my limited information! I'll try to keep the ears open and the mouth shut.

Chris

[Justin Thomas]

Look at the top of the tube - I'm pretty sure if it has four ridges in an X, it's real deal... I'd tell the owner it was old and worn out & needed to be replaced... I do occasionally have to tell owners I'm jealous of their tubes, though...

Justin