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...

In article ,
Carter-k8vt wrote:

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.

How can we be sure it was designed for 110 volts AC coming out of the
socket on the wall? But that is really irrelevant because the schematic
says the voltages were measured with 117 volts AC coming out of the
socket on the wall, not 110 volts AC.

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 specifications listed with the schematic say the radio is designed
to operate with voltages coming out of the socket on the wall of 105-120
volts AC, which would seem to cover "today's" voltage of 120 volts.
Unless the OP has a voltage greater than 120 volts AC coming out of the
socket on his wall I would think he should go for a transformer to the
original spec. Also if his line voltage is high, going for a
transformer with a lower voltage high voltage winding is going to do
nothing to bring down the heater voltage to normal. If this is really
an an important issue, what he should do is look for a transformer with
a tapped primary, unfortunately these don't seem to be as common as they
once were when the transformers in some equipment had taps for low and
high line voltage as well as the nominal line voltage.


Regards,

John Byrns

--
Surf my web pages at, http://fmamradios.com/

In article ,
Carter-k8vt wrote:

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.

John Byrns wrote:

How can we be sure it was designed for 110 volts AC coming out of the
socket on the wall?

Well, I can't "be sure". See below.

But that is really irrelevant because the schematic says the voltages
were measured with 117 volts AC coming out of the socket on the wall,
not 110 volts AC.

I would humbly offer in my defense that I do NOT have the schematic for
this radio, nor did I try and find it on line. I just went by the
partial schematic sent by the original poster which
made NO mention of 117 volts.

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 specifications listed with the schematic say the radio is
designed to operate with voltages coming out of the socket on the
wall of 105-120 volts AC, which would seem to cover "today's" voltage
of 120 volts.

Again, I do not have the schematic so I cannot read what you are
quoting. However, I would respectfully suggest that the 117 volt figure
they quote is a NOMINAL number. Although I was probably either not yet
born (or was very young) when this radio was new, it is my best
understanding that, GENERALLY SPEAKING, line voltages were
(significantly?) lower back then and higher today, no matter what was
printed on the schematic. Also, please note that I stated "could
(should?) look for a transformer with a 220-0-220 or 250-0-250 high
voltage winding". So I -did- try and leave a little "wiggle room". :-)


Unless the OP has a voltage greater than 120 volts AC coming out of
the socket on his wall I would think he should go for a transformer
to the original spec. Also if his line voltage is high, going for a
transformer with a lower voltage high voltage winding is going to do
nothing to bring down the heater voltage to normal.

But isn't "half a loaf better than none"? Instead of having the B+ AND
filament voltage be high, you would at least be making the B+ lower.

Going from 110 volts primary to 120 volts will raise the B+ from 265 to
290 volts, while raising the filament voltage from 6.3 to 6.8+ volts.
Neither is a desirable situation, but I personally would rather have the
lower B+. Your mileage may vary.

If this is really an an important issue, what he should do is look
for a transformer with a tapped primary, unfortunately these don't
seem to be as common as they once were when the transformers in some
equipment had taps for low and high line voltage as well as the
nominal line voltage.

Agreed! Theoretically, a tapped primary would be the perfect solution;
-realistically-, that ain't gonna happen. Realistically, the choice is
between both high B+ AND high filament voltage versus just high filament
voltage. You pays yer money and you takes yer pick...

Regards and Happy New Year,
Carter K8VT

Again, I do not have the schematic so I cannot read what you are
quoting.

http://www.nostalgiaair.org/Resources/451/T0000451.htm

Look here. There are several to choose from. Perhaps there is a run or
version number to be had. Start with the first H104.

http://www.nostalgiaair.org/PagesByM...3/M0024073.pdf This page
identifies the different suffixed chassis. A tube change.



Paul P.

And perhaps some of these links may be helpful.
http://www.ppinyot.com/HomePage.htm

I put this together to help out other hobbiest. Nothing for sale on this
web site.

Paul
www.ppinyot.com

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)