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Old 20th September 2007, 12:54 PM   #1
engels is offline engels  Israel
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Wink New design: 12V Pushpull tube amplifier

I was fiddling with spacecharge tubes for a while and I'm very exited about them. Now I've finally found a suitable small OPT with CT primary (not made for tubes but should withstand this). So, I'm planning a pushpull guitar amp with 12V power transformer.

The problems are - of course - the PI and the power stage. The preamp is not a question, it already worked in SE design.

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From my experience all the 12V tubes work (sound) better with grid-leak bias, that's why the output stage looks so stupid and I'm shure I'm missing something here.


12AJ6 - hi-mu triode
12FM6 - medium-mu triode
12DU7 - output tetrode
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Old 20th September 2007, 01:13 PM   #2
EC8010 is offline EC8010  United Kingdom
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There's no way you will get 15V from a 7815 supplied from a 12V transformer. You need at least 15VAC and a bigger reservoir capacitor.

Don't space charge output valves draw grid current? If so, your concertina will be in trouble.
The loudspeaker: The only commercial Hi-Fi item where a disproportionate part of the budget isn't spent on the box. And the one where it would make a difference...
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Old 20th September 2007, 01:18 PM   #3
engels is offline engels  Israel
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Unhappy oops!

sorry, I forgot the coupling caps between the PI and the power tubes...
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Old 20th September 2007, 03:02 PM   #4
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I agree that a higher-output phase inverter is needed - I'd probably use a pair of 6GM8 dual triodes and a pair of 12K5s - since I have 'em. G1 is usually at 12V and G2 used as control grid - I wonder about that 12DU7 data sheet...

At 15V, some cathode bias may be necessary on the outputs - the 12K5 has a more complete data sheet, and says there is -2V bias (from rectified signal) with 2.2 meg grid resistor, and draws 40 mA with no signal.

Sounds like a fun project - I'd run it from a 12V gel-cell battery, of course!
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