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zobsky 3rd July 2007 07:41 PM

Schematic Question
I'm curious (and maybe dumb too).

What's the pupose of
1. The 2K resistor in series with the OPT
2. Supplying high voltage to the output stage at 2 points (A and B). Looking at the power supply, at first glance A and B seem to be de-coupled via a cap. However, tracing the high voltage through A, it loops through the OPT and returns to the power supply via B. Is this an application of an OPT as a filter choke?

Tom Bavis 3rd July 2007 08:32 PM

It's a hum-canceling circuit, cheaper than a choke... the 2K resistor mirrors the plate impedance of the 6BQ5, resulting in cancellation of the 120 Hz power supply ripple on the primary. A screen resistor and filter cap would be needed anyway, so the only cost is for the extra turns on the primary.

Since the plate impedance of the 6BQ5 is about 40K, would think the tap is about 22%? (square root of 2K / 40K). Maybe less, since any hum that gets to the screen would be amplified out of phase and cancel too...

zobsky 3rd July 2007 09:29 PM

Neat, Thanks .

One of my old SE 6BQ5 consoles used the same scheme and I was wondering what the rationale was.

As an aside, Looking at the Sams Photofact a few years ago, I'm pretty sure the OPT primary impedance was less than 5K (closer to 3 K, if I'm not mistaken). I've always wondered why. Would one want touse a lower impedance OPT when using the OPT this way?

I still think I'd use a choke nowadays.

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