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RE: WECo 91A/B--Additional points and some corrections

Posted by gilmorneau (A) on November 23, 2014 at 09:02:41

In Reply to: RE: WECo 91A/B--Long, but hopefully informative posted by maxhifi on November 22, 2014 at 12:46:51:

First, a couple corrections/edits to my post above.

I initially came up with a sensitivity of 0.25V but on re-thinking it, it's probably even more sensitive than that--around 0.05V input for full output. Still a 'back of a napkin' casual calculation, but I think it's closer to correct. Obviously, some sort of attenuation will be required for home use.

Also, I said the 'field supply' had a resistance of around 1000 ohms. What I meant, of course, was that the field winding on the speaker had about 1000 ohms (used as the choke for the amp's power supply filter).

Regarding the series connected capacitors on the original, WECo did this in order to reliably get the necessary voltage rating out of the electrolytic capacitors available at the time. There's no need to do it, and no advantage, if you can source capacitors with sufficient voltage ratings. OTOH, if you do decide to use series capacitors, it's essential to parallel the voltage dividing resistors across them to ensure that each cap gets equal voltage.

Regarding output transformers: As noted below, the original WECo 171A trans were wound on (what is now) an archaic type of lamination, namely a CIC type, which I understand is no longer available. There are those who salvage the cores from other WECo transformers and chokes in order to rewind them to something like 171A specification. There is (or was) at least one vendor in the USA who resells 171A on cores rewound in Japan. Note that the original 171A and its clones have very limited primary inductance, on the order of 10Hy. If you gap them for more DC in the primary, you get less inductance. If you gap them for more inductance, you get less DC in the primary before saturation. Nature is cruel.

The Peerless/Magnequest TFA-204 is the closest thing available new AFAIK. It is a good little transformer, wound on conventional EI lams, but requires adjustment to the original circuit values in order to perform optimally. The original 91 amps pulled 80mA through the transformer, the TFA 204 isn't happy with any more than 60mA.

I am unfamiliar with the Amtrans option mentioned by GSH, though I'm not surprised they're not mentioned in a 1935 catalog if they're really meant as model 91 replacement transformers--the model 91 wasn't introduced until 1936. Does anyone have a 1936 or later Amtrans catalog with showing the transformers GSH mentions? Please post.

I have to agree that any of the three 'original' types just outlined will be of limited utility for full-range, hi-fi use. Maybe adequate for reproducing an impression of 1930's sound (if you had the right speakers), but get a bigger, wider bandwidth transformer for highest satisfaction. I like the Partridge best, but they have become difficult to source.

Several people have mentioned a reference to 6J7 having lower distortion than a 6SJ7. Radiotron Designer's Handbook (4th ed.) page 510. It has probably been reprinted since, but there it is.

In his 'YAWE91' thread, Thorsten advocates for using POSITIVE feedback to boost the high frequencies a bit. WECo used frequency selective NEGATIVE
feedback to achieve the same end. Thorsten's is a viable (though tricky) approach, but is not authentic to the original model 91 ethos, which seems to be the OP's intent.

Almost hate to indulge them by mentioning it, but as I predicted in my point 5 above, the thread-jackers have apparently arrived. It would be funny if it weren't so tedious. In what sense is a suggestion to try a DC 2A3 amp helpful to someone explicitly inquiring about an authentic WECo model 91 clone? Sure, a DC 2A3 can make for a satisfying little amplifier, no doubt. But it's not the ONLY way, and it's not even the best. People who listen to other topologies are not ignorant--they may know things the DC 2A3 crowd doesn't. There's more than one way, for sure. But it needs to be its own discussion thread.

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