12A_7/Russian 6N2PEV Heater Wiring-Switching Between Both

[Nichesound]

Today I am building a Fender AB763 circuit with some preamp options. One option will use an EF86 tube on one channel, another option is to be able to use two different types of tubes on the same socket, but with different heater wiring configurations.

I want to be able to use a 12AX7 tube with the typical heater wiring connections, and still be able to use a Russian equivalent tube using the heater wiring connections for that type of tube. The Russian 6P2N (-EV) tube has similar characteristics of the 12AX7 tube, the differences being the 12AX7 uses a parallel wiring layout, where the Russian 6N2P tube uses a serial wiring layout for the 6.3VAC heater circuit.

I will be using the 6P2N-EV tube, as this is the more robust tube, made for their armed service for harder service applications, similar to the 7025 tube in the USA.

To solve this problem for the differing heater wiring arrangements, I came up with a layout using a 3pole 2throw mini switch, wired in the configuration shown in the picture enclosed.

I haven't tried the application yet, as the amp is about 80% complete, but am looking forward to trying some of these 6P2N-EV tubes I have purchased recently, to see how they compare to the American brands.

Chime in please with any comments on this idea. Thanks.

Johnne in Seattle

[Richie]

there use to be a diagram on the internet of a switch to use between the 2 tubes.

The russian type were used in the "red bear" amps. the tubes in a red bear amp i worked on once, seemed fine.
Probably like most tubes you'll just have to try some and see.

[VacuumVoodoo]

Here you go, DPDT ON-OFF-ON switch, use the OFF position while changing tubes, then flip to correct position.

[img:640:480]http://i26.photobucket.com/albums/c149/ ... erw_sw.gif[/img]

[Nichesound]

"Someone I know has done some extensive experimentation and based on this, always ground pin 9 of a 6n2p as static charge can build up on the internal shield and cause the valve to act faulty until grounded, don't worry as much for a 6n2p-ev"

This is from another post referring to Pin 9 on the socket. I figure on more pole will take care of that duty, so why not.

[VacuumVoodoo]

Be aware that 6N2P and 6N2P-EV are structurally different tubes, at least Cold War era NOS. Later on they began using same commercial version. Myself, I use only NOS EV version as it exhibits much lower noise and microphony levels than the commercial bottle.
Grounding internal shield pin 9 is a good idea when the tube services the input stages, further down the signal path it's not absolutely necessary.

[bancika]

get the EV version if possible, they look totally different, pins, electrodes, everything seems higher quality in EV.

btw, their heaters are hard-wired in parallel, not series. In 12a*7 you have the freedom to choose.

[VacuumVoodoo]

Reheating this thread. Here's a switching arrangement that grounds pin 9 on 6N2P (or ECC88 if that's your pleasure). Marked in red.

[Merlinb]

I use this: