Silver face Fender Champ-needs work, lots of questions

[Dtmenges]

I acquired a silver face Champ a couple of years ago. There must have been an apartment cleanout because it was sitting beside the dumpster. The amp looks to be original inside but the speaker is missing. Actually there was a speaker laying in the cabinet but it is a Realistic brand so it goes in the junk box. I trained on tubes back in the 70s, worked at Crown on audio amps for several years, and have worked on tube ham radios so I am not totally clueless as to the work ahead of me. I would like to restore it to a good working amp and maybe make a couple of dollars for my effort. I do have a scope, signal generator, dummy load, Variac, and hiZ voltmeter. I haven't powered anything yet and don't intend to until I'm sure nothing else will smoke.

That said, here are the issues and questions.

What speaker?

The Tolex is in good shape, no rips. Grill cloth has a small snag.

Burned resistor--The cathode resistor for the 6V6 is cracked and toasted so the magic smoke is out of it. White plastic Malory cap next to it has a melted spot. Also the 1k 1W resistor in the power supply is open and looks burned.

Line cord--The line cord is a heavy 3-wire SJO type cord with an unpolarized 2-wire plug. The soldering inside of the amp looks good but something is doesn't feel right. Did this amp have a 2-wire cord? The death cap is still in place.

Power electrolytic-The schematic AA764 seems to match the amp except it calls for a 20,20,20 mF Malory cap. The amp has a 40,20,20. If the cap was replaced it was an excellent job of soldering. Is this a case of documentation not catching up with production changes?

Bass pot broken off and knob is missing.

I have tracked down some of the parts. Any suggestions and hints will be appreciated even if the suggestion is to sell it as a parts amp.

[The Dude]

Wow! Great find! I wouldn't necessarily toss the speaker. Try it first. Despite being a Realistic speaker, some of them sound pretty good. In fact, I have one running on my bench right now as a test speaker and it sounds better than a lot of others. Of course replace any obviously damaged/burnt parts first. I would definitely remove the death cap and install a grounded AC cord.

P.S. Welcome to the place!

[52 Bill]

Welcome to the place!

The later '70s Champs came with 3 wire grounded ac cords, adding one would be a good thing. I've seen the same 40/20/20 cap used in these. I think that it was just a later version change, not an aftermarket replacement. Does the cap have a date code?

The open 1K power supply resistor is fairly common as is the cooked 470 ohm 6V6 cathode resistor. The white Mallory cathode cap will sometimes short, causing the 6V6 to redplate. That could be the cause of the open 1K resistor.

The original speaker was probably a 8 inch CTS rated at 4 ohms. The Radio Shack one is probably an 8 ohm. You should find a 4 ohm replacement for the amp in order to get the most out of it.

[The Dude]

Good point, Bill! I hadn't thought about impedance.

[Leo_Gnardo]

What speaker?

If that Shack speaker doesn't suit your ear, your neighbors at Weber (Kokomo, Indiana) have an excellent 8" that's probably the closest tone match to the original, and even better it's the cheapest 8 at $33. There's lots of other 8's competing for your attention, some approach the $100 mark. Whether they really sound "better", well that's always in the ear of the listener isn't it?

https://www.tedweber.com/8sigf

If rebuilding to sell, probably best to stick to original circuit values. If you're going to keep it you may want to consider increasing the value of the cathode resistor (say 560, 620, 680 ohms) in a bid to limit the current passing thru output tube, also use a 50V or higher bypass cap. Also put some space between R & C so your new cap doesn't get roasted.

Champs deliver a disappointing low end response so don't be upset if you find power rolls off badly below 400 Hz. Lucky if you can get a watt out of it at 100 Hz. OTOH they're great amps for recording despite this flaw. And you can drive any speaker from its output. They sound surprisingly good running anything larger than that poxy little 8.

Nice find, your acquisition price is perfect!

[eschertron]

40/20/20 was what I found in the '76 Champ I picked up. I think you're right about undocumented factory changes.
I picked up a Weber speaker, too, so that I could drive the piss out of the amp and not tear up the original oxford. It sits, sealed, in a box with some moisture absorbent.
Have fun with the amp!

[Justin Thomas]

What Leo said. The weakest link in a Champ is the speaker. I have a 20W Eminence no-name in mine, I like it. But it's even better through my 1968 Fender Bassman 2x15"!

Justin

[g1]

I guess the only thing I have to add is you might want to verify the transformers before ordering many parts. If somebody tin-foiled the fuse while the 6V6 was smoking those resistors... fried PT or OT may have led to the dumpster.

[Chuck H]

What he said^^^^^^^^^^^^^^^^^^^^^^

Verify the transformers are functional and then fix it if they are. Otherwise it's probably more valuable sold "as is" and "needs work" and "untested". I'm not the sort of douche that would rook a buyer. I'm a different kind Anyway... If the transformers are good then it's worth buying an appropriate speaker and fixing. Otherwise it may not be. You can buy SF Champs in good working order pretty often on *bay for under $300

[The Dude]

On the other hand, it's unlikely both transformers would be bad and it's a lower power simple amp, so you could get either transformer at a relatively low cost. IMO, it would still be worth repairing.

[Chuck H]

The last time I replaced a small, inexpensive transformer it cost me as much for the shipping as the transformer!?!

So a minimum $45 and the time it takes for replacing, possibly retrofitting, diagnosing other repairs, almost certainly a 6V6, etc... Under $300

Can anyone say "lucky to break even"???

[The Dude]

Well, I guess it depends on the intent of the project. If you're doing it as a money maker to resell for profit, you're not going to get rich. I can agree with that. If you intend to keep it, have a little project, maybe learn something, have some extra time, end up with a cool little amp, etc.............

[Chuck H]

I was just working off Dtmenges post. I'd have snapped it up in a second. Regardless of the potential. My closet will attest to that

[The Dude]

I thought you were just being un-Dude.

[Dtmenges]

Welcome to the place!

The white Mallory cathode cap will sometimes short, causing the 6V6 to redplate. That could be the cause of the open 1K resistor.

We used those white Mallory caps at Crown back in the '70s. Techs made a point to keep their faces away from them on first power up. Even new they tended to go off like an M-80. But then again a Crown DC-300A had a bit more horsepower in the power supply.