Monday, October 10, 2011

Yes, You Can and the Champion 600

Part of the Mark II Champion 600 to 5C1 modification program was intended to lower the price of the modifications substantially by reusing the original Ningbo Chaobo power and output transformers.
That presented a smallish problem because the power transformer was without a center tap or the more typical 5v rectifier winding.

The first objective was to wire up the filament circuit and see if the 6v filament winding had enough stamina to survive. This was done by comparing the unloaded and loaded voltages. Unloaded the winding produced 6.7 volts ac. Installing three tubes-a 6X5, a 6V6GT, and a 6SJ7-and powering up produced a voltage drop of only .3 volts, and it stabilized at 6.4v which was serviceable.

The second objective was to see if there was a way to use the transformer with the rectifier tube. A bit of looking around and chatting to people led me to a drawing of a hybrid power supply on diyAudio drawn by A. Ciuffoli, which is sometimes called a hybrid Graetz bridge.

It consists of tying the plates of the rectifier tube to ground by means of the diodes shown. I used the 1N5408 which is a bit heavier duty than the omnipresent 1N4007 diode, and then I doubled them up for good measure. Connecting pin 8 of the 6X5 tube to the load produced a good solid 360v of DC right where it is needed.

With all tubes installed and a load connected, I'm getting 330v-316v-273v on the B+ rail which is plenty for this job. By comparison, the original solid state rectifier setup in this amp produced 360v according to my notes, which puts this right on the money. According to the Sylvania technical manual, the voltage drop for this tube should be about 22v, which means I'm close enough for guv'mint work.

So. The proof of concept is good, much money will be saved, and you heard it here first.

Thanks to Mr. Ciuffoli, wherever he is, and the folks over at diyAudio who think hard about such matters.

ERRATA: I got the diode numbers screwed up here and on the schematic so just to be on the safe side use the omnipresent 1N4007 for the legs from the HV to ground and double them up as I did.

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