Is there any proof the
right tubes are in it? Ailing orphans sometimes suffer idiot repair attempts.
The tube line-up is about as expected:
-2 OD3 - references
-3 6L6 - pass device
-1 6SH7 - voltage amp
-1 5Y3 - low-current rectifier
-1 6SL7GT - ??? volt-amp or trivial rectifier
-1 5Y4GY - high-current rectifier
5Y3 is apparently wired half-wave because it is a 200mA tube only supplying ~20mA, a half-wave transformer winding is cheaper and smaller, and while it would work with just one plate connected, you get two per bottle so they use both.
I'm not sure everybody understands the function of this box.
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When the polarity switch is negative
It isn't a "polarity switch". The box is dual-output, but just one meter to check both outputs. Yes, the outputs are different polarities, but there are two very different outputs.
To mess with tubes, particularly radios, you need three batteries.
The "A" battery heats the filaments. 6V was popular and you have this, working.
The "B" battery sucks electrons off the filament or cathode and is several hundred volts positive of the heater/cathode. This has to supply all the power the radio needs for gain and output.
In some designs (and much experimental work) you need a trivial amount of current at a voltage negative of the cathodes to bias the grid(s). This is obviously the "C" battery.
The B and C "batteries" are connected like the right side of this sketch:
We actually used batteries for some work. While a battery's voltage can't be "varied", they are really stacks of 1.5V cells so any voltage can be gotten if you bring a few taps out of the stack. A 3V/4.5V/6V/7.5V/9V C-battery was a common item.
But batteries can be expensive.
If all radios used the same power, we would just use transformer, rectifier, and capacitor to deliver the standard voltage.
But a repair shop gets a 45V beach-radio one day, a 300V Zenith DeLuxe Console the next day.
A tapped transformer would work....
But transformers sag, and simple cap-filters hum/buzz. Sometimes we have to test at a specific or buzz-free voltage.
So we start with a too-high voltage and waste the excess. Along the way we can reduce the buzz.
So this bench supply must start with a big ~500VDC supply for the "B battery" output, and a smaller negative supply for the "C battery" function.
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the output ranges 5V from 790 to 795volts
Measured where? Which of the two outputs? (It would be real good to have a closeup picture of the binding-posts.)
Let's go back to the power transformer and the DC we get from it. The big 5Y4 rectifier feeds a big cap. Is the negative end of that cap connected (directly or through small resistance) to the black terminal of the 0-350V output? What is the voltage on that cap?
Now let's find the raw negative power. Big clue would be a filter capacitor(s) with the positive end connected to common or a black binding post (very possibly through the 1-5 pins of a gas-tube). Do you get ~300V across that cap?
I'd half-bet that the 5Y3 supplies the negative voltage. However the logical way is to put the 5Y3 in the line to common, and take negative DC from the other end of that transformer winding. But there are several ways to do it.
That 6SL7 is a puzzle. This could be a high-class ultra-stable supply using the twin-triode as a diff-amp, and the 6SH7 for extra gain. But for best performance, the 6L6 pass-tubes need a floating screen supply. But we are out of rectifiers (unless they use solid-state for this). Hmmm.... extra volt-amp, missing rectifier.... a 6SL7 could be diode-strapped as a dual rectifier, and its voltage and current ratings are just about right for feeding several 6L6 screens. It seems silly not to use one of the dual-diodes, but most are too small or much too big. And 6SL7 was a VERY popular tube, always in-stock at competitive prices.
