Wiring for 6DN7

[tvr2500m]

I'm building my second S.E.X. 3.0. First one is stock, this one with some minor tweaks. When I built my first one, I grabbed a bunch of NOS 6FJ7s and also a bunch of 6DN7s. I have a healthy stash of each. I'm especially glad I did this, and when I did this, for the 6FJ7.

Overall, the S.E.X. is my favorite headphone amplifier of the many headphone amplifiers that I have. It sounds superb with every one of the also many headphones I have and will drive all of them; it does an excellent job with the Hifiman HE-6SE V2. Plus, it has that Bottlehead user aesthetic and user experience thing that I very much enjoy with this amp and the Crack.

I would like to use the 6DN7 for this build. In planning for this build, reviewing the very good assembly instructions (though no one did kit manuals as well as Dynaco), looking at the socket wiring (because I have to map the 6FJ7 Compactron to the 6DN7 Octal, running and connecting everything correctly), with the Compactron socket, all the unused 6FJ7 pinout socket tabs are bent out of the way except for pin 8, which gets used to connect the 220 ohm 1/4W resistor between pins 8 and 10.

From the schematic, this is a grid stopper resistor. This would be pin 4 for the 6DN7. Do I just need to get one end to ground from pin 4 (like the other grid stoppers are wired to the other grids and the terminal strips)?

Thanks. Again.

[Paul Birkeland]

Grid stoppers are a good idea in general, but I wouldn't be too concerned if you just left the grid stopper off in this particular application.  Do note that the non-socket side of this stopper resistor does not go to ground.

[tvr2500m]

Thanks for your reply. I did notice that one side of the resistor doesn't go to ground. Pin 8 isn't connected to anything. Copy that, I could leave out that resistor. What is it doing/how is it working that one side of the resistor is connected to an unconnected pin?  If I might want to use a grid stopper resistor here, how might I do it?

[Paul Birkeland]

Thanks for your reply. I did notice that one side of the resistor doesn't go to ground. Pin 8 isn't connected to anything. Copy that, I could leave out that resistor. What is it doing/how is it working that one side of the resistor is connected to an unconnected pin?  If I might want to use a grid stopper resistor here, how might I do it?

That pin on the socket isn't connected to anything internal to the tube, so we use it as a terminal. It's working because a wire leaves pin 10 and goes to the output of the volume pot.

Grid stoppers mitigate against unwanted oscillations and radio frequency contamination, though the risk of running into this seems to increase rather substantially with certain tube attributes.  I would definitely rather have the one present that goes to pin 3 if at all possible. 

[tvr2500m]

Got it. The grid stopper to pin 3 of the 6FJ7/pin 1 of the 6DN7 is no problemo. My plan was/is to just map the pinouts from one tube to the other

I see that connection to the potentiometer in the schematic. Could the other side of the pin 10 resistor just be connected to a terminal strip?

[Paul Birkeland]

Yes, if you have an empty terminal, you can land the resistor there.

[Thermioniclife]

Try this.

[tvr2500m]

Thanks. That's what I'm trying to do - octal/6DN7 change, upgraded potentiometer.

Does that grid stopper resistor have to be part of the volume control circuit, or can one end just go to ground like some of the other resistors do?

[Paul Birkeland]

Does that grid stopper resistor have to be part of the volume control circuit, or can one end just go to ground like some of the other resistors do?

Again, you cannot tie one end of a grid stopper to ground.  This will not allow any signal to get into the tube. You can leave the grid stopper off the first stage if that's helpful for your layout.

[tvr2500m]

Big doh! on my part. That's the signal input to the grid. Got it.