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Old 22nd February 2014, 03:02 AM   #11
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The low plate voltages of this circuit is exasperating me! A B+ of 108VDC and plate current of 108/470K= 0.11 mA is barely even on the plate graph for a 12AT6.... How does this tube even work?? Has anyone written anything about this realm of tube application?
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Old 22nd February 2014, 09:42 AM   #12
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Quote:
Originally Posted by LATUBEGEEK View Post
The low plate voltages of this circuit is exasperating me! A B+ of 108VDC and plate current of 108/470K= 0.11 mA is barely even on the plate graph for a 12AT6.... How does this tube even work?? Has anyone written anything about this realm of tube application?
See the other thread, you need to change the B+ voltage drop resistor to a much smaller value (plus a bypass capacitor say 20uF) , and add a plate resistor.
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Old 22nd February 2014, 03:11 PM   #13
DF96 is offline DF96  England
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To run at low voltages you need low mu valves. 12AT6 has mu=70, far too high.
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Old 22nd February 2014, 07:11 PM   #14
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The 12AT6 was in the intercom originally.....I'd hook you up with the original designer, but he's probably dead (RIP)....I added a 12AU7 (low mu), but have a 12BH7A, with a Mu of 16.5... may try it instead. Don't have many other 7pin tubes (other than a matched pair of 6V6 clone 6AQ5A's that I'm dying to push-pull for 10 watts, but that's a future project!).
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Old 23rd February 2014, 04:58 AM   #15
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I think DF96 was just mentioning IF you want to run at low voltage, but there is really no reason to run any of the tubes at low voltage, when you are converting it the intercom to a guitar amp.
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Old 23rd February 2014, 12:58 PM   #16
kuroguy is offline kuroguy  United States
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Go to your local hamfest. 7 pin tubes are a buck a peice all day long.
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Old 23rd February 2014, 05:04 PM   #17
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Thanks all for the info and suggestions. I'm just wanting to understand how tubes work at low voltages (there are plenty of vintage stuff that had them). Am finding out not that desireable...., but is still good to explore and learn.
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