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Re: Great posts and now to save my TFA2004s...

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In Reply to: Great posts and now to save my TFA2004s... posted by Garland on March 1, 2005 at 06:07:02:

If you do decide to use a Hammond power transformer I would suggest getting one that is of a higher current rating than you actually need as a hedge against noise and high voltage secondary failure. My experiences have been such that I do not use them in our products any more. My hunch is that the primary turns count tends to be so low that the transformers end up vibrating themselves into a failure mode as the HV secondary turns at the end of the winding slip and touch other layers. We have seen high voltage secondary failures of as many as 12 units in a batch of 50 in the past, and this week I am assisting someone who has experienced a high voltage secondary failure of a 300 series trans after it has been running (noisily) for a year or two, at a current below it's maximum current rating.
FWIW an MQ PGP 8.1 is more spendy, but they are extremely reliable. You might also want to look around for surplus iron like Thordarson, Stancor, or UTC.

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Were they used FW center tap or FW bridge? - VoltSecond 20:35:34 03/02/05 (0)
I collect failure mechanisms. Once you know what and why things fail, you can design that failure out.
1. Were they used full wave center tap (2 diodes) or full wave bridge (4 diode)?
A HAM I used to work with said never to use HV transformers bridge that come with a CT because the insulation can't take the higher voltage. He normally ran his transmitter B+ over 1000V.
2. I've noticed a higher failure rate in magnetics since the switch from fluorinated cleaners to aqueous cleaners. However, I doubt this style power transformer sees a cleaning step after winding.
3. The charts say 270V peak is the magic voltage for corona (electrical discharges), below 270V you really don't have to worry about it. However, I haven't seen problems with corona in transformer until I've exceeded 400V B+.

Play safe and play longer! Don't be an "OUCH!" casualty.
Unplug it, discharge it and measure it (twice) before you touch it.
. . .Oh!. . .Remember: Modifying things voids their warrantee.
Power Iron - Sector-7G 05:29:43 03/02/05 (0)
Get a full audio quality custom. Ias Doc said, salvave is a good place ot find a less expensive option, Triad and Stancor among others.
One way to get a few more primary turns in is to string the filament windings in series with the primary. Put it in both polarities and use the one which gives less voltage at the HV secondary.
If I can't find what I need in the scrap bin, I go to Heyboer TX in Michigan. They are cheaper than Hammond and I get to specify what I want. I penned a power TX for a linestage project which would do quite well for your porject( and others ) it has a 3 tap HV at 400, 325 and 250 with 3A of 5 and 6.3, 100 mA from any tap. I think they went for $66 or so. L-C from the 325 taps ( 650 CT for use with vacuum diodes ) ought to get very close to your OP.
regards,
Douglas
Snake oil is just that...rendered snake

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