power supply question

[John]

Hi fellas, I picked up a Heathkit sig generator at a hamfest this past weekend. I was expecting to work on it, and sure enough it's got short in it somewhere past the rectifier section. I'm going to first pull the tubes before I dig deeper, but meantime, I'm curious about the initial caps and big diodes coming off the PT. Why did they do it that way exactly? I'm sure it has to do with the fact we're making a sine wave and not amplifying audio, but I don't know *why*.

Thanks in advance!

[eleventeen]

It's a totally conventional voltage doubler. Note the stacked filter caps and the to-and-fro diodes.

A very, very common "mode" of low-cost transformer from that era was a no-CT, 125 volt secondary, with enough filament power (maybe 1.2-2 A) to run (at maximum) half a dozen little tubes. All the mfrs made them. Usable for anything small-signal, maybe up to a 3W record player amp w/a SE 6AQ5 output tube. You get the isolation, you get fil power for a few little tubes, and you get to use cheap diodes for your rectifier. No 6X4, no tube socket. If you look at 5-year earlier versions of this same piece in the Heathkit line, I would bet you'd see a 6X4 7-pin rectifier tube.

[John]

Thanks for the reply, makes perfect sense now.

Thanks!