6MD8 triple triode utilizing separate bias - diyAudio
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Old 21st April 2017, 08:24 AM   #1
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Default 6MD8 triple triode utilizing separate bias

Hello All,

I have several of these and wondering if one can ground the mutual cathode and set each triode with different negative grid bias so that I can use one section as preamp and the other two as push pull output.

I think that it can be done as long as the current in all the sections is the same.

What do you think?

Thanks.

Glass_painter
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Old 21st April 2017, 09:56 AM   #2
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As far as I understand you can ground the cathode, and also bias each triode to different anode current with separate fixed bias voltage, if required.
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Old 21st April 2017, 10:51 AM   #3
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Quote:
Originally Posted by artosalo View Post
As far as I understand you can ground the cathode, and also bias each triode to different anode current with separate fixed bias voltage, if required.
Correct.
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Old 21st April 2017, 11:52 AM   #4
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Hi,

Thank you. I will try building it and see how it sounds.

BR

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Old 21st April 2017, 12:15 PM   #5
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You can also use a series of resistors between cathode and GND, and get the correct tap for each unit, returning the grid leak to the right point.
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Old 21st April 2017, 01:23 PM   #6
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Several years ago there was a Hundred Buck Amp Challenge. It was a competition to make a guitar amp for less than $100 in parts cost. I explored several designs early on and one of them used two tubes and a mosfet.

The output stage used a 6MD8. I simply used three or four (don't remember) generic red LED's in series from the cathode to ground. These developed about 10 volts? (again, I don't remember exactly) of cathode bias. Each tube section had a resistor from it's grid to ground so all the bias came from the cathode voltage. One section was intended as the reverb driver so it was a stand alone SE amp stage. It was tested, but never used. The other two sections formed a push pull power amp section which delivered a little over 1 watt cleanly, almost when driven well into clipping. The phase inverter was an LND150 mosfet in split load configuration. The OPT's were small Antek toroids. This part of the amp worked perfectly.

The preamp section used a 6MN8 biased with a single red LED. Each triode section was configured as a voltage amplifier stage. I tried several different configurations but the perf board implementation was always plagued with oscillation, and eventually the entire design went into the box of dead projects to be robbed for parts. I believe much of it still exists (I stole the transformers), and I may try to work on it again some day.
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