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RE: How much current capability to replace a blown power trans?

Posted by Alex Kitic (A) on October 27, 2014 at 11:59:25

In Reply to: How much current capability to replace a blown power trans? posted by DAK on October 27, 2014 at 11:43:25:

For the heaters, it's rather easy. Assuming that the preamp uses 2x 6SN7 and the heaters are AC, the 6.3V winding needs to be capable of at least 1.2A (2x 6SN7 heaters in parallel).

The same is true of the winding for the rectifier: if the rectifier is meant to be 5Y3GT, than the winding needs to supply 5V 2A.

As for the HT windig, which is probably CT - it depends on the voltage current it is required (meant) to supply, and the type of power supply.

Usually 1x 6SN7 (both triodes) per channel requires between 6 and 15mA, thus both channels would require between 12 and 30mA. If the HT secondary is CT for full wave rectification, and the power supply is cap first (like C-L-C), the AC current of the HT secondary needs to match the DC current of the B+.

You can add some current capability to the HT secondary, but it is not strictly necessary. Most probably it would end up being capable of 50mA and not 30mA because of standard wire thickness... and some would jump in ecstasy at the "oversized" power transformer.

If we reassume, 6.3V 1.2A (would be probably 1.5A in practice) = 9.45VA
and 5V 2A = 10VA, plus let's say 300V-0-300V 50mA = 30VA, which in the end sums to 50VA. That's most probably a fair measure for your preamp ;)
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