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6MD8 triple triode utilizing separate bias
6MD8 triple triode utilizing separate bias
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Old 21st April 2017, 08:24 AM   #1
Glass_painter is offline Glass_painter  Israel
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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
artosalo is offline artosalo  Finland
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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
JonSnell Electronic is offline JonSnell Electronic  United Kingdom
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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
Glass_painter is offline Glass_painter  Israel
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Hi,

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

BR

Glass_painter
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Old 21st April 2017, 12:15 PM   #5
Osvaldo de Banfield is offline Osvaldo de Banfield  Argentina
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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
Tubelab_com is offline Tubelab_com  United States
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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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Old 26th June 2017, 06:01 AM   #7
Glass_painter is offline Glass_painter  Israel
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Default ecll800 power amp conversion

Hello all,

The ecll800 is a two pentode triode tube utilizing mutual cathode and internal connections so it can serve as a single tube small power amp.

My question is can I use the same topology to build an amp utilizing 6MD8 tubes which are triple triodes with mutual cathode.

Logically, I do not see why not.

Your comments are welcome.

Glass painter
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Old 26th June 2017, 06:38 AM   #8
jazbo8 is offline jazbo8
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6MD8 triple triode utilizing separate bias
Please do not start multiple threads on the same subject which is against the forum rules. Your threads have been merged together.
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Old 26th June 2017, 10:23 AM   #9
DF96 is offline DF96  England
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The triode in ECLL800 has unusually low mu so it can be used as a signal inverter. You would need to use feedback instead to achieve the low gain in the first triode.
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Old 26th June 2017, 11:39 AM   #10
Glass_painter is offline Glass_painter  Israel
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Hi,

The triodes in the 6MD8 are low u units similar to 12bh7 so I think it would be OK.

I am concerned about the topology of the ecll800 pp circuit do you think it will work with the 6MD8?

Thanks for the input.

Glass_painter
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