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Faux center tap on transformer secondaries
Faux center tap on transformer secondaries
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Old 12th August 2019, 05:35 PM   #1
Cowtipper is offline Cowtipper  Canada
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Default Faux center tap on transformer secondaries

I picked up a junked Radio Specialty Mfg. FM Deviation Meter for $10 that still had most of the parts in it (including all the tubes!). I checked around for a schematic, but the only info I could find was that others were looking for the same schematic......so I tore it apart (a couple weeks of just desoldering.......).
I'd like to do something with the transformer but I'm not sure how to approach it.

I set the variac to 122vac (my nominal local potential) and did some no-load measurements on the secondaries and got:
825v CT
895v no CT (?!?)
91v no CT
65v no CT
7v CT (heaters)
5.6v (diode heaters)


I've been playing with 895v secondary on PSUD II and have pretty well come to the conclusion that there really isn't a practical use for 1270VDC (unless I maybe want to build a radio transmitter......).


Ideas welcomed, BTW.


I'd really like to somehow center-tap it so I can split pre+driver and output plate duties between the two high voltage secondaries.
I'm familiar with the technique of using equal resistors in series across the heater circuit to lift the heater voltage, but I'm pretty sure I read a long time ago that though it should only be used for heater circuits and (I think) NEVER used for plate voltages. I've searched the internet with every potential search term I could think of and can't find that reference.


So I guess my question would be: Why can't (or why shouldn't) a faux center-tap be used on the high voltage secondary (Safety? Explosion? Alternative dimensional travel?).
If it turns out that there actually is a way of doing it would it be connected to the center-tap of the heater circuit and/or chassis ground?


Or is this just wishful thinking and I'd be better off just not wasting my time trying to do something that isn't practical or impossible (and cheap).
One think I already thought of is using a 1:1 transformer with a center-tap but I'd rather just use the other high voltage secondary than go that route. It might limit what I can do, but that's half the fun......
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Old 12th August 2019, 06:18 PM   #2
JonSnell Electronic is offline JonSnell Electronic  United Kingdom
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You could build an audio amplifier using a pair of 813s and achieve an extremely high output power!
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Old 13th August 2019, 04:04 AM   #3
PRR is offline PRR  United States
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> Why can't (or why shouldn't) a faux center-tap be used on the high voltage secondary

You waste far more power in the resistors than you can get out of the scheme. ("CTing" a heater winding, we aren't trying to get power from the CT, just drain stray voltage.)

That "Meter" has a CRT 'scope. THAT'S why it needed 1,200VDC. At just a couple mA. There is NO use for this in audio, really in any non-CRT work. (Maybe a very heavy Geiger counter.) Certainly can't warm a '813.

The Deviation Meter is not much more than an FM radio. You can maybe power a preamp, but not more than a couple Watts of power stage. Even that would require an unlikely high impedance output transformer.
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Old 13th August 2019, 04:45 AM   #4
wiseoldtech is offline wiseoldtech  United States
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Quote:
Originally Posted by Cowtipper View Post

Or is this just wishful thinking and I'd be better off just not wasting my time trying to do something that isn't practical or impossible (and cheap).

Bingo, you nailed it.
In practical electronics design, what you're asking to do is simply not possible.
PRR's post sums it up nicely.
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Old 13th August 2019, 04:58 AM   #5
Cowtipper is offline Cowtipper  Canada
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Sure does. Shows what i know LOL.
I'll heat shrink and tape it out of the way and make do.
Thanks for clearing it up.
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Old Yesterday, 02:51 AM   #6
Ben Mah is online now Ben Mah  Canada
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You could do a choke input with bridge rectifier. VDC would be approximately 0.9VAC instead of 1.4VAC for capacitor input. A RC stage or two could lower the voltage some more.

Good for a SE transmitting tube amp.

Last edited by Ben Mah; Yesterday at 02:55 AM.
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Old Yesterday, 09:28 PM   #7
PRR is offline PRR  United States
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It is an *oscilloscope* winding. The design load is just a few mA.

Seriously not good for anything audio.

Pretend we did have a use for 800VDC at 2mA. What choke value is minimum to ensure choke-input action?

Currentmin. (in milliamps) = VIn (RMS) / L (in henries)

L (in henries) = VIn (RMS) / Currentmin. (in milliamps)

L = 895V / 2mA = 448 Henries

500H @ 2mA is not a common choke.
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Old Yesterday, 10:35 PM   #8
Ben Mah is online now Ben Mah  Canada
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Sorry, I missed the few mA part. My bad.
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