807/6au6 pure pentode mode amplifier - diyAudio
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Old 20th November 2011, 03:32 AM   #1
ryuji is offline ryuji  United States
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Default 807/6au6 pure pentode mode amplifier

I had to take a vacation from tube amplifier fiddling after my life got too busy. Im back at it. I drew this up some time ago, but i just now got to wire it up in a prototype form. Best sounding circuit I've yet to listen to. Compared to a triode driver/preamp this tube is far more precise/articulate without losing the part that makes tube amplifiers awesome

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Old 20th November 2011, 03:35 AM   #2
ryuji is offline ryuji  United States
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Following up my sharing with a question. Anybody put together a 6au6 line level amplifier or have an idea of how to go about designing one? I want to build a tube buffer for my subwoofer channel that is partially for balancing the volume with the main amp. is it as simple of copying my circuit as seen above and using a voltage divider on the other end to attenuate it down to line level voltage?
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Old 20th November 2011, 08:47 AM   #3
dgta is offline dgta  United States
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No, a buffer has unity gain and is typically a cathode follower. You can use a tube stage with lots of gain and then attenuate it, but it's a very poor way of making a buffer.
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Old 20th November 2011, 03:18 PM   #4
DF96 is online now DF96  England
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The unbypassed 807 screen grid resistor will give it some of the characteristics of UL, so not pure pentode mode. Whether better or worse I cannot say, but certainly different. By the way, the 807 is not a pentode but a beam (kinkless) tetrode.
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Old 20th November 2011, 05:42 PM   #5
ryuji is offline ryuji  United States
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i was actually considering bypassing it, i dont have parts on hand to do that tho, need to order some. using up my stock of 10 uF caps on building the gain stage

maybe that feedback is desirable tho. ill have to carefully compare them. my quick and dirty test used a 10k resistor in there(as thats what is installed at this moment)

and yea, i guess your right on it being a beam tetrode.

i was actually told in the past that as my 300v supply is regulated that the bypass on the 807 screen wasnt needed. i was going to compare but you agreeing with my original thoughts changes my mind back on that decision

going to rip apart my amp and rebuild it as described in the schematic this coming week, ill report my findings with experimentation and actual b+ voltage

Last edited by ryuji; 20th November 2011 at 05:49 PM.
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Old 20th November 2011, 09:35 PM   #6
DF96 is online now DF96  England
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A screen grid is both an input and an output. If you are lucky, the signal voltage developed across the g2 resistor will move the valve more towards triode-mode operation. If you are unlucky, you will get extra harmonics and IM as the signal, in effect, goes through the valve twice.
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Old 20th November 2011, 09:44 PM   #7
ryuji is offline ryuji  United States
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what are your thoughts on directly connecting g2 to my 300v regulated supply, this gets around the whole problem? or is it better practice to use a bypassed resistor
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Old 20th November 2011, 09:51 PM   #8
DF96 is online now DF96  England
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A small resistor might help maintain stability, by acting as a stopper. Apart from that, a fixed DC voltage will enable true 'pentode' mode operation.
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Old 20th November 2011, 09:53 PM   #9
ryuji is offline ryuji  United States
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so in other words you like the idea of ~2.5k resistor there with an 10uf or so bypass like i assembled the 6au6

resistor i guess in theory helps prevent oscillations

I should try it out as each... who knows, like you said you get varying results and i might in fact enjoy one of them.

anything else i should experiment with? perhaps bypassing the cathode resistor on the driver?
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Old 20th November 2011, 09:55 PM   #10
DF96 is online now DF96  England
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2.5k is not small, in this context. If you want 'pentode' sound then either bypass the resistor, or put a 100-200 ohms to the supply.
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