The AM Forum

The plate transformer in my GPT-750 is toast.  2800-0-2800 @ .8 amps.  I have an old buzzard Thordarson 62P84 5160V (2580-0-2580) CT, 1500 VA, out of an old PS that Vortex Joe gave me.  I figure it's rated at half an amp. 

Given the Thordarson's voltage rating is given at 220V primary vice the 240V I have at my line (giving just about the right voltage at the secondary), any guesses as to how long before I would fry it in the GPT-750?  Are these old xformers generally under-rated?  This is kinda my first experience messing with ancient HV xformers, any opinions welcome  ;D

Hi John,

I have not had to fire up an old high voltage transformer, but will put my $0.02 worth in.  The transfomers back then were designed and built more conservatively than today.  I don't feel concerned about the 220/240 difference but it is a factor.

I am most concerned about firing it up full voltage after sitting around for many years not being used, unless it has always been in a low humidity enviroment.

Seems like it should be baked for a few days first.  Maybe load the secondary with load resistance that would develop current somewhat below the current rating with less than 120 volts in from a variac, and walk up to this value over a number of days, with the transformer and resistors in a non-flammable box to form a sort of oven.  Maybe say 200 Watts of load with 50 vac in to start.  Fuse the feed to the primary.  You get the resistor heating and the internal heating of the core and windings.  What say?

The lowest class of transformer temperature ratings is 105 degree C maximum.  You don't want to get to this of course.

Seems like there was a thread about 6 months ago on re-varnishing and baking?

From my  experience with old iron..........Don't be scared to use the 220v iron on 240v.....It is a good idea to dry out old transformers before using them...I set them behind the woodstove for a couple weeks..don't let them get too crispy.....remember ...shaken not stirred,....Baked not fried.

Thanks, guys, I'm not so concerned about running it on 240 as I am exceeding its current capacity.  I'd hate to burn it up.

73 John

Jn,

Don't forget the original trans was for 24x7 operation. Your replacement trans should be fine for ham usage, even with old buzzard transmission usage...

John,
Put a AC amp meter in series with the primary and run it up with a variac.
Secondary unloaded.
You will see a slow rise in resting mag current as the voltage is iscreased. at some point the current will take off. You want to stay below that voltage because you are approaching saturation. A 220 /240 volt primary should work up to about 250 to 260 volts before the current takes off.
Sounds like a job for Gary the transformer man.

Only 1 way ta find out Johnnie !!!
Keep that red bottle handy though.

Just dont run the GPT at full strap. CHT was the best Thor had, I think it would be ok if you dropped the load on it by about 10%. My HB rig had a M77 mod xfmr in it and lawd knows that I gave that tranny hell when I got down and did the james brown.

I have a collection of dead plate iron rolled down the hill behind the shack and into the woods. It's a "Ham" thing. If it doesn't get hot your not running enough snot.
:)

i get away with radio homicide using some of Dad's old plate iron from the 1930's. i have a big Kenyon 3500 CT @ 500mil job and have  full wave bridge on it, drawing up to an amp for years with no sweat. with a variac that supply runs his whole station.  my "220" reads 242V here. 

Cool!  Thanks for the poop, guys.  Getting the plate tranny outta that beast is gonna be a chore and a half.  I think I'll have to remove the whole back of the cabinet to get to the back 4 bolts holding the tranny down.

I always liked Kenyon iron. They used to run full page ads in 1930's QST showing their entire manufacturing process. They applied hot varnish at several steps baked into whatever they were trying to seal. At one point they used a vacuum chamber set to -15 to pull in the varnish to every part of the unit. I never did like their connection terminals though - but most Kenyon iron is cheap even today.

I have Kenyon iron in the 4x1 rig. Big cube runs cool as a cucumber at 500w out. It has stood the test of time. Like timmy, I anit crazy about the terminals either.

                                                 The Slab bacon

Teh audio phools and such dont like kenyon. Too plain looking. lotsa amer's too. They just dont have the "look". I would blast and apply HD black crackle to them if I ever built another HB rig. Maybe have the mounting flanges chromed. You gotta admit that would be the shit, Frank.  8)