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Posts: 288
Location: Colorado
Joined: August 10, 2004
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The WECo model 91 amps are something I've learned a bit about over the (many) years I've been in audio, so hopefully I can be of help. I'll try to address your points/questions directly, but may wander off topic slightly to correct some misinformation that seems to resurface regularly.
BTW, people often refer to the 91-A and 91-B interchangeably, but there are differences. The 91-A was (not surprisingly) first, and had a power transformer suitable for use on 60Hz lines only, using power transformer number 352-A. The 91-B was introduced about 4 months later, and came with a larger power transformer, model 359-A, which allowed for operation on 50 or 60Hz lines. Other differences are more subtle.
Both the 91-A and 91-B came installed in a metal wall cabinet, with a speaker and field supply included. Together, the whole kit was called the 500A system, and was intended for wall mounting in the projection room of a theater. The 91 amps were never intended to be stand-alone amplifiers for hi-fi use. Neither were they solely relegated to duty as projection room monitors, as some on the web claim. The original WECo literature clearly shows them matched with and connected to the stage speaker of an auditorium.
1. Like you, I have never received what I believe to be a satisfactory explanation of this capacitor's purpose. I'm just a hobbyist, not an electrical engineer, though. Seems like feedback to me, and is probably meant to correct deficiencies in the original output transformer's limited FR, but this is only speculation. If one of the brilliant EE's out there could take the time to analyze this and give a definitive answer, I and the OP would be most grateful. BTW, if you look at the schematics for other WECo amps of the period (e.g. the 86C), you'll see the same arrangement.
2. Yes, a 6J7 or 6C6 would be suitable as a substitute for the originally spec'd 310A. Compare the data sheets and you'll see very similar performance, close enough that in later versions of the 91 amps, WECo themselves showed the 6C6 (an RCA developed tube!) in the schematics. Yes, contrary to what is widely reported on the internet, the 310A was originally specified in both the 91A and 91B variants (i.e. the 310A came first, the 6C6 later). The earliest original WECo Service Bulletin I have, dated June, 1936, shows the 310A. The earliest reference I could find to the 6C6 on original WECo literature is dated 1938. If you have original WECo literature dating the 6C6 to before June 1936, please share. The 6C6 came later, probably because of its ease of acquisition (if your movie theater was in rural America in the 1930's, your local hardware store probably had 6C6's, but you'd have to order 310A's from WECo. No 310A's meant no sound, no movie, and no money for you.) 310A had 10V filaments, 6-pin base. 6C6 has the same base, but 6.3V filaments (hence the voltage dropping resistor in the filament line of the 1938 schematic). 6J7 is a 6C6 with octal base. There are differences other than that, though. There are early mesh and punched plates, while most of the later tubes are solid. Each will have it's own sonic signature in an amp this simple.
3. In a home audio application, the input transformer is not necessary for gain. You may wish to use a 1:1 transformer for isolation, but that would be system dependent. You could also use a grid choke on the first 6C6, so it 'sees' the same impedance to ground as it would if using a transformer.
I have not calculated the gain of the amp minus the input transformer, but could probably make an educated guess. Total gain of the amp as spec'd by WECo is 92dB (the 91-B allows for reducing this to 82dB by strapping out one of the feedback resistors). I don't have the spec's in front of me, but a reasonable estimate for the gain of the input transformer is probably, say, 30dB? That leaves around 62dB for the amp circuitry (or 52dB with the feedback altered as noted). 52dB is still about a 400x voltage gain, if my numbers are correct, which means input sensitivity of, what, about 0.25V for full power out and is probably still more than you need, but not absurdly so. (All quick calculations, BTW--please correct me if I'm way off base). To reduce the sensitivity, you could further increase the feedback (more so than recommended by WECo).
4. The frequency correcting network was indeed intended to account for the HF limitations of optical soundtracks (it is explained in some detail in an article in International Projectionist magazine dated Nov. 1936). With a poorly focused soundtrack, it can judiciously extend FR a bit, but when combined with a source having a flat frequency response, it makes for a 6dB or so peak at around 7kHz. Not ideal. The circuit is not totally useless, however. A pentode driving a triode has an inherently limited HF response, and if the frequency correcting feedback network were tweaked to boost at, say, 20kHz, instead of 7kHz, it might be made to usefully extend the circuit's FR upward enough to satisfy modern 'hi-fi' standards. Worth experimenting with. Any EE's want to tackle how to do that? Post here.
5. Original field supply probably had around 1000 ohms +/- resistance (in order to get the voltages right). Could only guess at the Henry rating. The original power supply used relatively little capacitance (20uf before the choke, 10uF after) and probably had unacceptably high levels of hum for use in hi-fi. Unless you're going to be using the original power transformer, I'd suggest building a proper PSU using modern components. There are those who will propose all sorts of unorthodox designs for PSU's (I'm surprised they haven't thread-jacked yet), but a relatively conventional and well executed design was put forth by Thorsten in his 'YAWE91' thread a few years ago on the AA DIY forum. It would be a good starting point.
6. The coupling capacitors are indeed small and will limit FR somewhat. The feedback helps with this, and it was all likely designed together, as a system. Original WECo response curves show nothing below 50Hz or above 10kHz. You may find this is more satisfactory than you imagine. It's worth a try before you start to modifying things. It will depend a lot on your speakers.
7. Output transformer options are myriad. If you're going to be building the amp as originally spec'd, then I'd argue that there's no reason to get a super wide bandwidth, high power output transformer. If your plans include upgrades in the future that may widen the BW or increase the power, then all the usual suspects are suitable.
FWIW, I have built this circuit (with and without modifications) using Partridge, Tango, Magnequest, Electra-Print, and rewound original core WECo 171A transformers, all with good (though different) results.
Best,
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