In Reply to: RE: GM20 questions posted by Ralph on June 7, 2010 at 13:08:17:
Thanks Ralph. Yes, I know NTC thermistors have their uses (like hefty ones on the power inlet to reduce the current surge when switching on an amp), but it just doesn't seem very plausible in this particular location. I found the GM20 schematic at
http://cid-a332bc3aefc89f6b.skydrive.live.com/self.aspx/Public/GM20%20schematic.pdf
and looking at it, it seems to me that the component labelled MV1 connecting to the bias adjustment pot. ought probably instead to be something like a 130 KOhm resistor, in order to get about -30V bias on the 6C33C grids at midrange of the bias pot. It seems odd, somehow, having a thermistor there, I would have thought. Surely stability is what is wanted, not temperature dependence? Or if it's meant to reduce its resistance (to about 130 KOhms?) as it heats up due to current flowing, then that means the grid bias would start out less negative, meaning the 6C33C's would be drawing even heftier anode current than normal, at startup. Something doesn't seem right here?
Chris
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Follow Ups
- RE: GM20 questions - cpotl 14:15:21 06/07/10 (10)
- RE: GM20 questions - Legendre 09:35:14 06/08/10 (0)
- RE: GM20 questions - Ralph 20:50:27 06/07/10 (8)
- RE: GM20 questions - cpotl 22:11:06 06/07/10 (7)
- RE: GM20 questions - Ralph 08:45:07 06/08/10 (6)
- RE: GM20 questions - cpotl 11:16:15 06/08/10 (5)
- RE: GM20 questions - llwhtt 14:45:01 06/08/10 (3)
- RE: GM20 questions - Legendre 18:22:48 06/08/10 (2)
- RE: GM20 questions - Lew 07:57:17 06/09/10 (1)
- RE: GM20 questions - Ralph 08:52:21 06/09/10 (0)
- RE: GM20 questions - Ralph 11:32:56 06/08/10 (0)