Substituting 6N2P tube in LMNC Safety Valve VCA

Ayab · Joined: Oct 27, 2015 Posts: 173 Location: London, UK

Got a couple of 6N2P tubes on eBay as a substitute for 12AX7/EEC83 to build LMNC Safety Valve Distorting VCA. But then I discovered the 6N2P has a different pin out to 12AX7/EEC83 (look before you leap, I know....).

The safety valve VCA stripboard design is attached. And also the schematic (which is difficult to read the numbers from and the schematic heater wiring differs from the stripboard, showing pin5 to ground and pin? to the 27r):

Also diagram of the pin out difference (from diy audio.com):

I’m planning to try using two 9v batteries in virtual ground configuration to test but am not really sure where to start with altering the connections for the tube.

I anyone understands tubes and can advise it would be great not to fry these lovely tubes and have a working distorting vca!

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Ayab · Joined: Oct 27, 2015 Posts: 173 Location: London, UK

Sam from Look Mum No Hands said it looks like it should be a direct replacement as pin 9 is not used. He is a great. Very Happy

Will confirm when have built it

piedwagtail · Posted: Sat Apr 11, 2020 9:06 am Post subject:

The 6N2 is a 6.3V tube. You can't use 9V DC unless you use a 9 ohm 3W resistor (likely two 4.7ohms) to drop the voltage down at 300mA .Valves will burn through your batteries quickly, a dedicated 6V transformer supplying the filaments with AC would be more suitable; either ground as shown in the picture or use a humbucker pot in parallel. If you must have DC, then rectify and use a LDO 6V regulator.
Whichever way, it's not a drop in replacement; he uses the filaments in series ie. 12V when 9V isn't going to damage the filament. The 12xx7s can run at 6.3V with pins 4 and 5 joined for parallel operation.

Ayab · Joined: Oct 27, 2015 Posts: 173 Location: London, UK

Thanks very much for the info piedwagtail

The circuit I am intending to use is for +12/-12V

So I need some hefty resistors! How would I wire them up?

Ayab · Joined: Oct 27, 2015 Posts: 173 Location: London, UK

HOW TO ADAPT 6N2P to 12AX7

This describes a plug and socket solution where;
12AX7 = plug
6n2p = socket

12AX7 pins 1,2,3,4,6,7 and 8 go to 6n2p pins 1,2,3,4,6,7 and 8.
12AX7 pin 9 goes to 6n2p pin 5.
6n2p pin 9 links to 6n2p pin 8. (Cathode K2)

As the wise piedwagtail says the 6N2P is 6.3V so it cannot be used as a replacement for a 12AX7 when any voltage above that is being used.

I will see how it goes adapting the safety valve circuit to 6V