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Old 19th November 2005, 09:05 PM   #1
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Default Using an old power transformer as a choke?

I have seen this referred to in some old texts, but have never done it.

I have a Hammond 273bx with a bad HV secondary, so I was considering using the primary coil as a breadboard choke rather than throwing it out. I have no idea of the inductance at tube amp DC currents, but I assume it would be high enough... at least over a dozen Henries, and the indulation is pretty rugged at 600v on the leads.

Any thoughts?
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Old 19th November 2005, 10:53 PM   #2
Gluca is offline Gluca  Italy
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A PSU choke you mean! I believe it would be a poor choice if you used it as an anode choke.

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Old 19th November 2005, 11:02 PM   #3
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Oh yeah... sorry, yes, I meant PSU choke as in a CLC filter. I imagine there might be some saturation problems at higher current, but I assume that in a preamp supply or small power amp it mght be cool.
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Old 20th November 2005, 05:25 PM   #4
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Default Power Tx as choke....

The Tx you plan to use has a faulty winding yes?

In what way is it faulty--If shorted turns, this could seriously affect its performance as a choke, effectively 'stealing' its inductance.

If the Tx is O/C secondary, then it should have some inductance that is usable.

How about trying a choke (ballast) from a florecent light fitting if your current requirements arnt too high..........
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Old 20th November 2005, 08:51 PM   #5
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Default Re: Power Tx as choke....

Quote:
Originally posted by Alastair E
The Tx you plan to use has a faulty winding yes?

In what way is it faulty--If shorted turns, this could seriously affect its performance as a choke, effectively 'stealing' its inductance.

If the Tx is O/C secondary, then it should have some inductance that is usable.

How about trying a choke (ballast) from a florecent light fitting if your current requirements arnt too high..........

The transformer is a Hammond 273BX that was in an amp that a buddy of mine decided to run without a speaker attatched to, so the high voltage secondary fried. I assume an internal short in the windings of the HV secondary is the fault. Impedance from each plate lead to the center tap is horribly unbalanced now--over 40 ohms on one side, and under 10 on the other. Where the lead to the the side with the lower impedance, just before it starts to join the coil, the plated magnet wire is almost burnt through as well. There is no short between primary and secondary, or from primary to core or any of the secondary taps to the core as far as I can tell.

I was planning to just snip off all the secondary leads and ignore them, and just run the 117v AC primary coil as a DC series PSU filter choke in a CLC section.
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Old 21st November 2005, 02:11 AM   #6
sasha is offline sasha  Canada
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Sorry buddy but I think You can't use this transformer because of
burnt/shorted turns in secondary. Primary will "see" those shorted
turns and it will be affected by it.
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Old 21st November 2005, 06:42 PM   #7
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Quote:
Originally posted by sasha
Sorry buddy but I think You can't use this transformer because of
burnt/shorted turns in secondary. Primary will "see" those shorted
turns and it will be affected by it.

Gotcha. I was not sure what the deal was when sending DC through it instead of AC. I thought maybe the secondary coil might just sit there and do nothing.
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Old 21st November 2005, 07:47 PM   #8
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"Gotcha. I was not sure what the deal was when sending DC through it instead of AC. I thought maybe the secondary coil might just sit there and do nothing."

The secondary would "just sit there and do nothing" if the current were purely DC. However, that is not the case when using the xfmr as a ripple choke. It will be passing the ripple current, and that will enable the shorted secondary to reduce the inductance, lead to more heating, and ultimately a progressive failure as more and more turns short out.
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Old 21st November 2005, 07:53 PM   #9
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Quote:
Originally posted by Miles Prower
"Gotcha. I was not sure what the deal was when sending DC through it instead of AC. I thought maybe the secondary coil might just sit there and do nothing."

The secondary would "just sit there and do nothing" if the current were purely DC. However, that is not the case when using the xfmr as a ripple choke. It will be passing the ripple current, and that will enable the shorted secondary to reduce the inductance, lead to more heating, and ultimately a progressive failure as more and more turns short out.

I am currently pounding my head against the workbench and going "DOH! DOH!" for not having thought of that... Thanx for the reality check.
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Old 22nd November 2005, 09:37 PM   #10
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How about cutting the cores and pulling out the burnt section, unless its interleaved or what have you? A little airgap will reduce inductance, but reduces saturation too.
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