Tweed Champ

[martin manning]

I don't know what the difference would be, but you might want to try measuring the OT turns ratio by putting a low voltage 1kHz sine into the primary with the right resistive load connected to the secondary. Then you'll know what the result is when it's measured the usual way.

[sunnydaze]

Thanks Martin & RP.

I went ahead and measured using a low voltage 1KHZ sine wave as Martin suggested. I couldn't get stable readings while the OT was in circuit, so I went I ahead and disconnected to measure in usual manner.

While disconnected I checked for continuity between primary and secondary = none, looks good. Primary resistance was 330 ohms, which is comparable to a brand new Weber Champ OT I remembered I had in my parts stash. Weber is 380 ohms on the primary.

I think the global negative feedback and secondary ground connection may have made the in circuit readings a little wonky. The secondary is grounded on the OT housing even though there is an unused factory hole and rubber grommet where the ground wire should be routed to the speaker jack.

Anyway, as you can see from the scope pic, measurements a bit closer to expected value when measured out of circuit in the traditional manner.

1.19 VRMS injected in the primary gets 25 MVRMS on the secondary. Indicated turns ratio is 47.6, which at 4 ohms is about 9,086. Again DMM measurements very similar to the scope. So fella's there you have it.

RP - the Mod speaker is OK. Conor said it was very inexpensive. It is a teeny tiny 8 inch speaker. I bigger, better speaker would most certainly help out I think.

Will put this one back together and call it a day.

On the other hand, now that I know I have a champ OT, might have to build one of these little buggers.

Thanks & best regards,
Mike

[sunnydaze]

The output is just too low. I hooked up the new Weber OT and it had about three times as much power. There is a street in Taipei where there are a bunch of tube hifi and electronics parts shops. Also there are several shops that recone speakers and several shops that build audio transformers. Turns out there are a fair number of Taiwanese that are into tube Hifi.

I've suggested that Conor take his OT to one of these hifi transformer places and ask them to repair, keeping it as close to original as possible.

Best regards,
Mike

[Colossal]

Mike and RP,

This has turned into a really cool thread! I have been long interested in demystifying the history and performance of these little amps. Nice work testing and documenting Mike.

[sunnydaze]

Thanks guys. Won't be seeing much more of this one for awhile. Conor picked it up this afternoon. The Weber OT is much louder than the original. Weber was 7k.

Don't give too much stock to the indicated impedance on the original - for sure had a problem.

One thing about this amp that I only noticed this afternoon - seems to have come from the factory with a 25 uf cap on the cathode of v1a. That's not on the schematic. The two 25 uf bypass caps that were in there sure looked original. Both had drifted way high. The one on the power tube was around 76 uf with a really high esr. The other was over 200 uf, esr looked ok.

Thanks & best regards
Mike

[martin manning]

Ya, that 25u bypass on the first stage is a known "thing."

[Colossal]

Ya, that 25u bypass on the first stage is a known "thing."

Martin,

I think this may have been discussed before, but I forgot the consensus if it was. The 5F1 schematic and layouts do not show V1a bypassed with 25uF, but IIRC, guys have found the cap on V1a in those models. So was this a documentation oversight and the cap was there all along, or maybe a transitional value that was added to later 5F1s?

I think RP has said bypassing V1a is a must, but again, I'm going off of TAG memory here. Long time ago, I came across a thread on another forum discussing a Champ derived amp which did not have cathode bypass caps on either V1a or b. The guy who had purchased it via a music store (builder unknown) said it was wonderfully dynamic.

[pops]

Gain stages without a bypass cap will have a flatter frequency response, less gain but i like what it does when not there.

[Colossal]

Gain stages without a bypass cap will have a flatter frequency response, less gain but i like what it does when not there.

Yep, more local feedback which is cool!

[rp]

My living room '80s thru '90s was friends' music stores, every Champ that came through, trade, consigned, repair, always got doted on and a steady workout from the regulars. No one plugged in 'cause they liked the clean jazzy sound, "ooo, this one sounds linear" didn't get said much, the appeal was the distilled rock & roll wildness at a comfortable volume that comes out of them, and they just went right up to 10: power on -> 10! Ancient Tweed, Haute Tweed, BF, SF, all are great, they all have a family sound. All but ancient ones I'm sure had the 25mf on V1a at least if they were screaming right. I wouldn't even think of leaving it off or putting it on a switch. Even backed down you still want that vim to the sound. I have a pull boost switching 1mf & 25mf and it's unnecessary, 25mf is all you need here - it's not like a Champ is going to be too woofy. If you want a tamer jazzy Champ for an apt. try the 5C1 with the stock grid leak input and a husky OT.

BTW Mike did you replace the OT and the bypass caps at the same time? If so that's your extra loudness right there and your old OT is fine. I'd double check that orig OT before getting it rewound, player tweed Champs are ~$1K now and sell right away, and with just a bit of effort on eBay that one can be made vintage stock. Also, 9KR isn't too far off from 8KR, and I could see the quality control not being as rigorous on these cheap little things. I doubt Triad or Schumacher would toss one reading even 20% high (assuming 7-8K was the spec - which it might just be it appears.)

[sunnydaze]

Cathodes were already bypassed, I just replaced the caps. The old OT was only putting out a little over 1 vrms into a 4 ohm load. The new one did more than three times that. All the testing was done with the new caps.

I doubt if Conor will do anything with the old OT for awhile.

Best regards
Mike

[martin manning]

So was this a documentation oversight and the cap was there all along, or maybe a transitional value that was added to later 5F1s?

Dave, I suspect the situation with the Champ's V1 Rk bypass is similar to that of the 5F6-A, where a few part values were changed in production after the schematic was drawn. If there was an updated version in use at the factory, it just never got into circulation.

[sunnydaze]

Hello RP,

Other than the low output, I couldn't measure anything that made me think the original OT was bad. Like you, I doubt if Leo or Triad would have thrown out the OT because the winding ratio was a bit high. If the thing had put out anywhere near a reasonable amount of power I would have been fine with the higher impedance. Leo and I share the same birthday and like Leo I tend to use what's on hand.

I've also got an old Epi EA 50 that sounds pretty good. Really basic circuit

http://www.gibson.com/Files/schematics/ ... emaker.pdf


Best regards
Mike

[martin manning]

Shorted secondary turns would cause both low output and a high turns ratio.

[sunnydaze]

Martin,

Right, a short on the primary side would cause a lower indicated turns ratio. Aside from the low output, still sounded pretty good, though. The new Weber OT has a 4 and 8 ohm tap for more options.

This was a fun little amp to work on.

Best regards,
Mike