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Since I'm a professional tube amp tech, people occasionally give me things they'd otherwise throw out. This mono amp apparently came from a 1957 Dumont phonograph console. Transformers have a 352 Essex EIA code. Based on the tube number sheet, which was saved, it seems to have had a remote control module that contained a single 6AV6 as a phono preamp stage and that plugged into the empty socket on the amp.
It's an extremely simple amp with 2 x 5Y3, a 12AX7 paraphase inverter, and 4 x 6V6 cathode biased in parallel push-pull. I found one other photo online of what seems to be the same amp, but that's about it. I thought the iron looked like it might be good...
Follow Ups:
After a fairly long electrolytic capacitor reforming (over 3 hours on my Heathkit C1 capacitor tester), I was able to test this amp out. It must not have been powered up in decades. It seems to have been intended for a 4 Ohm speaker load. The OPT measures ~4.3k into a 4 Ohm load, about right for parallel P-P 6V6s. Seems to sound OK.
With its stock 12AX7 PI/driver, I could barely get 15W out of it before clipping, leading me to suspect that its output may have been purposefully limited. I've seen a few amps from the 1950s that appear to have had their outputs limited, perhaps to protect inexpensive speakers. In this case, since it was used in a console phonograph, it might have been limited to avoid acoustic feedback. The bottleneck is the fact that the first 12AX7 stage has a cathode voltage of only 1.9V, so you can't drive it with a very large input signal. I was supplying the signal for it via a mixer capable of higher than line level output, so I swapped in a 12AT7 and was able to get 20W into 4 Ohms before clipping.
I've worked on a few Leslie 21H and 44W amps that run 6V6 outputs at around the same voltage and bias points, and I've bench-tested some of them at around 28 Watts. Those amps have a 6SN7 driver.
The 6V6 screens in this amp run at full B+ voltage, which I see more often in amps from the late 1940s/early 1950s. The aforementioned Leslie amps run the screens at ~40V less than the plates.
Hi. Dumont Laboratories was one of the early forces in television that did not survive. Alan B. Dumont - experimenter/innovator - was first in
in developing reliable cathode ray tubes. Do some searches. His work
lead to the first oscilloscopes. Our family's first TV had a Dumont
square 21 inch picture tube. This was unusual in 1952 when most kinescopes had round faces. He also while working in the nascent tube
industry figured out how to keep quality high and production greatly
increased. I think he was working for the DeForest company at the time.
He early on recognized the old mechanical scanning disc TV machinery
was wrong headed and must be eclipsed by techniques of electronic
scanning. Lots of history. Very smart man. Have fun reading about him.
Always known for quality.
W
So, Dumont was bought by Fairchild?
Also, was I've read elsewhere that Essex Wire was the parent company of Stancor. Is there more relevant history on the relationship between the two?
IIRC they also made early TV sets. I have several boxes of Dumont tubes, and while most seem to have been outsourced, some are a bit unusual may have been made in house
We had a DuMont silly scope, too, at my house when I was a lad. It wasn't a great one, though.
all the best,
mrh
From 1946-1956
I forgot :-P
silly me...
all the best,
mrh
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