6GM8 Operating Point

ALBQ · 20th November 2004, 08:37 PM

All,
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

Morgan L · 22nd November 2004, 06:46 AM

Hello,

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,

ALBQ · 23rd November 2004, 02:45 AM

Morgan,
Thank you. Will give it a try
Cheers -ALBQ

23rd November 2004, 02:45 AM	#3
ALBQ diyAudio Member Join Date: Dec 2003 Location: Albuquerque	6GM8 Operating Points Morgan, Thank you. Will give it a try Cheers -ALBQ

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22nd November 2004, 06:46 AM	#2
Morgan L diyAudio Member Join Date: Jan 2004 Location: Umea, Sweden	Hello, 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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