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6GM8 Operating Point
6GM8 Operating Point
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Old 20th November 2004, 08:37 PM   #1
ALBQ is offline ALBQ  United States
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Join Date: Dec 2003
Location: Albuquerque
Default 6GM8 Operating Point

Playing around with a C4S'ed Bottlehead Foreplay clone using 6GM8 low voltage dual triodes on a PCB. Thinking will use either a 24V or 30V B+ (max plate voltage rating). Anyone have any thoughts on a recommended operating point for this tube?
Thank you -ALBQ
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Old 22nd November 2004, 06:46 AM   #2
Morgan L is offline Morgan L  Sweden
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Join Date: Jan 2004
Location: Umea, Sweden

I've used the ECC86 in a CDP output at 3 mA / 15 Va-k. Works fine, maybe a bit on the smooth side sound-wise (which is usually a good thing).

I would try 3 mA / 25Va-k. So, in a Foreplay circuit, set both CCSs to 3 mA, use a 330 ohm cathode resistor (unbypassed) on the input tube's cathode, and use 50V B+.
Check that plate voltage at the input tube is ca 25V (20-30V) after warmup. Grid stoppers - say 1k on input and 10k on cathode follower - might be a good idea.
Ground the internal shield (pin 9).
If you use AC heating, make sure to balance the feed with 100-ohm resistors. I would also raise the heater to 10-15Vdc, with cap bypass to ground.

Good luck,
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Old 23rd November 2004, 02:45 AM   #3
ALBQ is offline ALBQ  United States
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Join Date: Dec 2003
Location: Albuquerque
Default 6GM8 Operating Points

Thank you. Will give it a try
Cheers -ALBQ
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