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RE: GM20 questions

Posted by cpotl (A) on June 8, 2010 at 11:16:15

In Reply to: RE: GM20 questions posted by Ralph on June 8, 2010 at 08:45:07:

No, I don't have one; I was thinking about constructing an OTL along those lines, which is why I was wondering about what exactly the component is.

What you and Legendre are suggesting certainly sounds right, I guess. I hadn't thought of the point you were making, that actually it makes sense to send the 6C33C grid bias voltage a bit more negative as the amp warms up, to compensate for drifting. I guess I could always start just with a fixed resistor to get the bias range on the pot about right, and see how it changes as the tubes warm up. If it seemed appropriate, I could then experiment with thermistors.

In fact, now you mention it, it's true in the two 6C33C OTL amps that I have already constructed (designs by Hans Beijner and Tim Mellow), that the current drawn by the 6C33C's tends to creep up slowly to its final value, over the course of an hour or two after switching on. I've been leaving the bias set so that the current is a bit on the low side at start-up, so that it ends up about right after they warm up. I reckoned that is probably OK, since I don't tend to play them at anywhere near maximum power output, and so the rather low initial current doesn't seem to cause any problems. But the thermistor idea could allow the current to be kept more or less optimal from startup onwards, I guess, if the temperature coefficient worked out just right.


May I ask aslo what you make of the 5O KOhm offset pot, with the resistors indicated as 52R2 across each of the arms of the pot in that schematic? Assuming 52R2 means 52.2 Ohms, that seems awfully low when bridging a 50 KOhm pot, doesn't it? Would you think 52.2 KOhms might be more likely?

Chris

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