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 Post subject: Re: How do I go about finding a replacement power transforme
PostPosted: Feb Mon 05, 2018 8:33 pm 
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First, transformer ratings include a rating for each winding which implies that there is also a rating for the total VA. The ratings are set by temperature rise..be it the total or the local heating internally. While we'd like to assume that there is some margin, there are certainly examples where it was pretty thin.
If that one winding were run at 0.6 A instead of 0.3, everything might be just fine. But, if all the other windings are at the limit, and the input voltage is 10% high, then you are running out of margin.

In your case, you said you were not running the full load tha transformer was designed for. That, plus the lower input voltage, could put you in safe territory.

Try it...if the transformer does not get too hot, you may be OK.

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 Post subject: Re: How do I go about finding a replacement power transforme
PostPosted: Feb Mon 05, 2018 9:24 pm 
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egg wrote:
300195-1 (Magnavox Power Transformer) specifications.
Image
Service manuals for 93 series amplifiers can be found here, courtesy of Bill J... :)
viewtopic.php?p=2223559#p2223559

Greg.

Edit: At times this Magnavox transformer 300195-1 is shown in Sams with a...
600VCT rating of .130A


Edit: image updated.


If I add up all the VA out shouldn't it be equal or less than VA in?

If transformer 600v is 170ma as shown I get 144 VA or 1.25 amp for all secondaries.
If 130ma I get 1.04 amp.
Both are greater than specified 0.92 amp.

What am I doing wrong?


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 Post subject: Re: How do I go about finding a replacement power transforme
PostPosted: Feb Mon 05, 2018 9:37 pm 
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Here is what happened. The 600 volts is really 300 each side of CT so it's only 300 volts @ 170 ma.

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 Post subject: Re: How do I go about finding a replacement power transforme
PostPosted: Feb Mon 05, 2018 9:48 pm 
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Norm,
If we look at it that way then there would be two 300v primaries each at .170 amp, which is the same as one 600v, so no difference.

Right?


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 Post subject: Re: How do I go about finding a replacement power transforme
PostPosted: Feb Mon 05, 2018 9:54 pm 
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There is 300 volts going to each plate of the 5U4 tube. Out of this tube comes DC loaded to around 300 volts at 170 ma. That's where they get the 170 ma. That would be 51 watts rather than 102.

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 Post subject: Re: How do I go about finding a replacement power transforme
PostPosted: Feb Mon 05, 2018 10:11 pm 
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Is this really the convention? Wow! I am surprised.

If what you are saying is correct and the CT transformer power ratings are really only for half the stated voltage, I assume the reason is the manufacturer is expecting that at any time only half the winding is conducting. Which will be true as long as the transformer is actually used in a full wave rectifier circuit. If so, that seems like a dirty trick of overrating to me since it assumes a specific use case.

So if I try to use this transformer as a pure 600v AC supply (omit the CT) and draw .170a it will melt?

EDIT: What I mean by overrating is that, as specified, the secondary could handle .170 amps but only at a 50% duty cycle (every half cycle at 60Hz). THe wire itself cannot handle .170 amp AC continuously, which is different than all other winding specs.


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 Post subject: Re: How do I go about finding a replacement power transforme
PostPosted: Feb Mon 05, 2018 10:25 pm 
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rjp wrote:
So if I try to use this transformer as a pure 600v AC supply (omit the CT) and draw .170a it will melt?

It depends entirely on what the rectifier configuration is.

If you use a half-wave rectifier (a single diode), you can draw 170ma because it only flows half the time.

If you use a full-wave bridge you will overload the winding since current flows all the time.

- Leigh

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 Post subject: Re: How do I go about finding a replacement power transforme
PostPosted: Feb Mon 05, 2018 10:35 pm 
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Based on the spec sheet at Mouser for Hammond transformers it looks like my original understanding was correct. The input VA very nearly equals the sum of all the output VA for all the windings including the 600 volts at .172 amp in the example highlighted below.

This makes sense to me, as I believe the transformer spec should be a property of the transformer, not the circuitry that follows it.

But then Leigh is never wrong. So... I am getting ready to eat my words :) But for now I think I'm right.

I guess the question is, can that secondary wire carry .170 amps RMS AC continuously or not?

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 Post subject: Re: How do I go about finding a replacement power transforme
PostPosted: Feb Mon 05, 2018 10:48 pm 
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rjp wrote:
I guess the question is, can that secondary wire carry .170 amps RMS AC continuously or not?

The 272FX is indeed the proper replacement.

Your actual realized voltages in the radio will likely be slightly high for two reasons:
1 - Modern line voltage is higher than the nominal 115v for that transformer, and
2 - You're only loading the 6.3v winding with 4 amps (or less) v. the 5 amp spec.

BTW...
Transformers are always spec'd with ALL windings loaded at nominal current simultaneously.

- Leigh

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 Post subject: Re: How do I go about finding a replacement power transforme
PostPosted: Feb Mon 05, 2018 11:17 pm 
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I am less concerned with whether or not it is the correct replacement (although I thank you for confirming it as I may buy one).

But totally aside from that (just for educational purposes) I want to learn the correct transformer math formulas.

Can you help me here?

Let's just look at the HV secondary. The spec says 600 V at .172 A.

If I am following correctly, Leigh and Norm say this winding can supply .172 amps only at a 50% duty cycle.

First question. Is this in fact what you are asserting?

Second question. If this is true then why does the VA total in the example highlighted above seem to match the sum of all VA outputs at 100% duty cycle?


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 Post subject: Re: How do I go about finding a replacement power transforme
PostPosted: Feb Tue 06, 2018 12:31 am 
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I've done a little more reading online and it does appear that there is a special rating sometimes being used just for the high voltage secondaries in these power transformers.

One reference summed it up as follows: "The current rating is given for the two CT windings in "parallel", but the voltage rating is given for the two windings in series."

This seems to agree with what you guys are saying. The overall winding is 600 volts, but can only supply .170 amps if the two 300 volt legs are used "in parallel" (for example, to feed a full wave rectifier with 50% duty cycle on each leg).

This rating convention seems to only apply to the HV secondary. The filament windings, for example, may also be center tapped, but the current rating is on the full voltage.

This does not match the VA ratings used in the Hammond sheet shown above. It seems they used the .172 amp rating for the entire 600v winding (series).

So there seem to be two different rating schemes going on, which is why I am confused.

Why don't we have a standard for this?


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 Post subject: Re: How do I go about finding a replacement power transforme
PostPosted: Feb Tue 06, 2018 1:56 am 
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Going through the Hammond pdf catalog I found the following note on HV secondary rating.

Exactly as you guys said! Thanks for setting me straight on this peculiarity of transformer specification.

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 Post subject: Re: How do I go about finding a replacement power transforme
PostPosted: Feb Tue 06, 2018 10:52 am 
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I don't know if it is a universal convention, but it is very common to see the HV secondary rated in terms of the rectified DC current. I'll guess that one reason for this is that the DC is much easier to measure. Because any rectifier is a peak detector, the actual current waveform is very "spikey"-- not even close to being a sine wave. Since power dissapation is the basic concern, the DC current accurately characterizes what is happening.

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"Measure voltage, but THINK current." --anon.


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