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SB 220 Plate Transformer

F

FlatlandBusa

Well-Known Member
Nov 20, 2022
230
422
73
South Central Nebraska
I recently picked up a very nice well built but completely stock SB 220. Before ordering the upgrades I disconnected the high voltage leads from the plate transformer and turned the amp on to make sure the transformer wasn't junk. I got approximately 980 volts on both leads. Fast forward to today, with a new Harbach metering/rectifier board, new filtering capacitors, and a soft key kit installed, I started it on a variac and got good plate voltage, took the variac out of line, turned it on and got a bang and a tripped breaker on the back of the amp. Unhooking the plate transformer high voltage leads, resetting the breaker, the amplifier turns on and has 980 volts on one lead and 60 volts on the other :mad: . Looks like the plate transformer decided to die from the first inrush of the capacitor bank...

Is the heavy duty Peter Dahl plate transformer worth it for this amp? Cutting the case isn't a problem for me. Or is it best to stay with the stock replacement plate transformer? I'm not looking for every last watt out of the amplifier since I have bigger amps to use when I feel like it.

Thank you in advance!
 
Last edited:

The usual failure mode for that transformer is for the high voltage winding to arc or short to the chassis. This tends to put a big surge through R3, the grid-current shunt resistor. And if the meter switch is set to "Grid" when the transformer fails, the coil in the right-hand meter gets evaporated. Heathkit saw fit to leave out any diodes to suppress surges and protect the meter. Peter Dahl used to sell both a stock version that would fit okay, and the beefier c-core design that's too tall for the RF shield. That may be the only one that Hammond will sell you. Last one I bought involved a flaming hoop or two. Hammond doesn't sell direct. The transformer gets ordered through Digi-Key. Prepayment was required and the lead time was several weeks. Being effectively a custom order, there are no returns.

And if someone has a cheaper or easier-to-buy option, I wouldn't mind hearing about it.

73
 
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