The part of the schematic that I didn't post does indeed show 117 volts as
the expected input.

So one final question, and then I think I've got it. I've gone online and
found a few sources for what should be a compatible transformer. 270-0-270,
plus 5V and a 6.3V. I also looked up the current draw of a 5U4 and found
that I need at least 3A across the filmanent, so I'll be sure to buy one
that is appropriate. But about that 270V - do I need to take into account
any kind of voltage drop across the plate? Obviously the schematic is
showing resulting voltages, not transformer output voltages as I originally
though. So is 270 the right spec? The 5U4 has about a 50V drop across the
plate.

Jeff



"Carter-k8vt" wrote in message
. net...
Unrevealed Source wrote:
"Carter-k8vt" wrote in message
t...

The 268 volts you are seeing on each plate is 268 volts -AC- to ground;
to put it another way, you are looking at a transformer with a 268-0-268
volt secondary (or a transformer with a 536 volt *center-tapped*
secondary).

I've got that part - thanks for confirming.

Let me toss you a minor curve ball: If you were repairing this radio in
its era, you would indeed be looking for a replacement transformer with
approximately a 270-0-270 high voltage secondary winding; this is
because it was designed for 110 volts AC coming out of the socket on
your wall. Today, that voltage is 115 to 120 volts or even a bit higher
and using -today's- line voltage with a 270-0-270 winding would give you
a bit more than the 265 volts DC B+ that you are looking for. So, what I
am saying is that you could (should?) look for a transformer with a
220-0-220 or 250-0-250 high voltage winding; i.e. less (than 270-0-270)
is better.

The 265 volts you refer to is +265 volts -DC-, measured from the
filament (cathode) to ground. The voltage -across- the 5U4 filament
is 5 volts AC, the filament voltage of a 5U4.

So I need to apply 5V AC across the filament (pins 2 and 8). That's
consistent with what my understanding would have been.

Yes. That is why your replacement transformer needs a 5 volt secondary
winding (with enough current for a 5U4 filament).

And the 265V is generated by the tube, correct?

Yes, the tube changes (or 'rectifies') the 268 volts AC to 265 volts DC.

This would be a pulsed DC, would it not, coming directly off the tube?

Yes.

In circuit, it is constant DC due to the filter capacitors?

Yes (and also due to a filter choke if one is present (rest of the
schematic is cut off).

Do I understand this correctly?

Yes.

By the very nature of your question, it indicates that you are a
beginner -- which is OK; we were all beginners at one time. Just be
careful; these voltages, if not fatal, can at least be harmful.

Yes, relative to almost all of you, I am indeed a relative beginner. And
this is my first experience with a transformer/rectifier-based radio, so
I apologize if my questions are somewhat stupid.

Jeff

Nope, not stupid. Again, we were ALL beginners at one time. Asking
questions is how you learn...

"Unrevealed Source" wrote in message
...
The part of the schematic that I didn't post does indeed show 117 volts as
the expected input.

So one final question, and then I think I've got it. I've gone online and
found a few sources for what should be a compatible transformer.
270-0-270, plus 5V and a 6.3V. I also looked up the current draw of a 5U4
and found that I need at least 3A across the filmanent, so I'll be sure to
buy one that is appropriate. But about that 270V - do I need to take into
account any kind of voltage drop across the plate? Obviously the
schematic is showing resulting voltages, not transformer output voltages
as I originally though. So is 270 the right spec? The 5U4 has about a
50V drop across the plate.

That 50V drop is figured into the resulting DC output voltage, once
filtered. (DC filtered voltage = AC voltage (of one half of the transformer
at any peak) x 1.414)