Power Transformer Specs

[mwelch55]

I need to understand transformer current rating specs a little better.

I have a transformer that is rated at 350V (175-0-175) at 60mA center tapped.  Does that mean that I get 60ma if I use a bridge rectifier across 350VAC?  That would give me about 350*1.4=490 volts DC after rectification and filtering.

If I use the center tap like most marshall amps do, that would give me 175*1.4=245 volts dc.  Would the current capabilities still be 60ma or would it double (since you have two 175 volt windings in parallel)?

Mike

[PRR]

Different winders spec the CT PT different.

Do you know what it came out of? What did it power? That would be an excellent guide.

[mwelch55]

Thank you for the reply.

This is an Edcor transformer (XPWR024-120).  I wanted to use it for a low power amp, but wanted to use the center tap so the DC voltage won't be so high.  If I use a bridge rectifier with 350VAC, it will produce a B+ that is too high for el84s or 12BH7/12AU7 output tubes.


Mike

[jjasilli]

That would give me about 350*1.4=490 volts DC after rectification and filtering.
Yes.

If I use the center tap like most marshall amps do, that would give me 175*1.4=245 volts dc.
I'm not aware of any amps that use only 1/2 the winding for HT. 

I have a transformer that is rated at 350V (175-0-175) at 60mA center tapped.
The maker is tersely saying this combination of things:
i) if your draw 60mA through the secondary, you will have 350VAC;
ii) this implies that if you draw less than 60mA, the secondary voltage will be higher; if you draw more than 60mA, the voltage will be lower;
iii) if you draw more than 60mA, don't blame the maker if the PT fails.


Does that mean that I get 60ma if I use a bridge rectifier across 350VAC?
No. Current is drawn from the power company, trough the wall socket, and through PT by the load presented to the PT by the amplifier.  E.g., if the load = -0- = short circuit, an infinite amount of current will be drawn.  Something will burn out, hopefully a fuse.  IOW, you the designer control the current draw by way of the load you present.

Would the current capabilities still be 60ma or would it double (since you have two 175 volt windings in parallel)?
Current handling capacity is determined by the gauge of the wiring used in the windings.  (In a coil, a wire's current handling gets more complicated, but that's the idea.)  If you were to use  ea side of the winding for different 175VAC power supplies (out of phase), ea side could handle 60mA. 

Hope that helps.

[tubenit]

Take a look at the "Data Cheat Sheet" for what the draw is for EL84 tubes.

With respect, Tubenit

[PRR]

I think that part from that company is sure to give 240V DC at 60mA.

Which is a single EL84/6V6, not a two-pack.

[jjasilli]

I think maybe I better see mwelch55's point.  His PT has relatively high voltage but relatively low current handling.  So how best to use it?  Hence PRR earlier suggestion to see what it was use spec'd for.

[jjasilli]

Further thought - since this is a standard Edcor PT, you might wish to ask them what power tubes and circuitry they think it is best suited for. 

[mwelch55]

Here is a schematic of what I was referring to.   

In the second example, would that be able to handle 2 push-pull EL84s or only single-ended?  Also, would the second one give you twice the current capabilities as the first (just trying to understand)?  I wouldn't think that the current capabilities would be the same for both circuits.  Would the Full-Wave Center Tap Grounded be able to deliver twice the current as the Bridge circuit?  Also, would the Full-Wave Center Tap Grounded circuit be closer to 60ma or twice that?

The second example is what I was referring to when I said "use the center tap like most Marshall amps do" (I'm not using only half of the winding).  Also, the second example is the one I am planning on using since 490v is too high for a low-powered amp.

I am trying to figure out the best way to use this transformer.

[shooter]

here's the math that I see 490vdc * .06 = ~ 29W available, loaded you'll be closer to 440 *.06 ~ 26W

Those should be good enough to do a SE KT88 running cool, more like bored 

maybe a modern 6L6 almost sweating.

245vdc once loaded becomes, from my experimenting, too low for anything besides a quiet noodling amp

[jjasilli]

EDIT
@ mwelch - Great, your schematic is helpful.  My 2 cents.


Note: For our purposes your R1 and the second cap are superflous (see: The Hammond Transformer Guide online - please consider this a mandatory reference piece). C1 should be followed by a shunt resisitor (B+ > Ground, in parallel with C1) of a value that would cause a 60mA AC current draw.  This R represents the load of the amp that you design.  This load is mostly caused by the power tubes, because the preamp tubes draw only a few mA.

Bridge Rectified.  Peak DC voltage is about 490; but average DC volatge is .9 X 350 = 315 VDC.  Current handling is .9 X 60mA = 54mA.  This might support a single EL84 or 6V6. A 500V JJ 6V6 would work.  This might barely support 2X small bottle tubes or 1 big bottle tube like a 6L6.  Fine if you like hotrodding for sag, and are willing to risk the PT.

Full wave.  The 245VDC you specify is the peak DCV = .71 X secondary AC voltage.  The average DCV is only .45X the secondary AC voltage = 158 VDC.  The voltage is low but do-able.  You retain the full compliment of 60mA.  A single 6L6 in Class A is within reason.

Again, Edcor may have a recommendation


EDIT
@ shooter:  245vdc once loaded becomes, from my experimenting, too low for anything besides a quiet noodling amp

254 IS the loaded (peak) DC voltage @ 60mA AC draw.  That said, you are still right.  The voltage is pretty low.

[mwelch55]

It sounds like it might be best suited to a single-ended amp.  I also thought about using it as a power transformer for a stand-alone spring reverb. 

If I made a stand-alone reverb using 12AT7 as the driver, I'd need to drop the 490v down to around 390-400 volts (like the reverb in the 65 Deluxe reverb).  Since the 12AT7 doesn't draw that much current, could I just drop the voltage with resistor?

I ordered that transformer about a year and a half ago by mistake and I am trying to make use of it.

Thank you for the replies.

Mike

[SILVERGUN]

I'd be tempted to go the lower voltage route and tweak for 6G15 reverb
http://el34world.com/charts/Schematics/files/fender/Fender_Reverb_6G15_schematic.pdf

[jjasilli]

I thought about using it for a preamp. Reverb unit is another alternative! Full wave @ 245VDC might work, but that's @60mA draw.  Preamp tubes will draw less than 10mA.  That might be closer to 300VDC.  What is the measured unloaded secondary AC voltage?

[shooter]

I used this Edcor in my current build;
XPWR105  360vac (180-0-180) @ 250mA
drawing ~100mA, VDC at plate of a kt88 440ish bridge rec, ~200uf filtering across a 400ohm dropper
So Mr.55's should land close to mine in SE config, If you're going for loudness, I don't see why you couldn't get ~10W audio pretty easy, killer speaker(s) and the beer-drinkers will swear it's LOUD 

I would try PSE JJ6V6's running 35mA each into the max B+ you can get, I'd do the math on 400 to 420vdc plate.   
That said, both SG and JJ have valid options

[jjasilli]

Shooter  you're drawing less than half the rated current.  mwelch could be drawing 50% to nearly 100% more than his rated current in some power tube scenarios.

[shooter]

100%

Edcor's numbers land pretty good at max I, I expected to do a PSE @ 200mA and I'm pretty confident I'd land pretty close to 400vdc plate, so maybe start the math at 390vdc plate for 70mA ~27W.  great speakers and a PSE 6V6 will get you pretty close to 5E loudness.  The filter caps need to be big n beefy, mate it to a EDCOR OT spec'd around 25W audio, you don't like, i'll pay for shipping the bad amp to my place