6J4 Experiment!

[DualTriode]

Hello,
I just sat down to sketch up a circuit to prototype for fun. The project will be a two stage SET amplifier. There will most likely be several iterations. The output tube will come later.
Not that it is the best tube or worst I have a shoe box full of 8532’s (6J4). So this time the driver will be a pair of 6J4’s. The plan is to use two per side in a series configuration much like a SRPP but the output will be tied to the plate of the bottom tube as opposed to the cathode of the top tube.
As I sit to sketch I notice that pins #1, #5 and pin #6 of the tube tie to the grid. Here are a couple of questions.
Does anyone know this tube? Do pins #1, #5 and #6 need to be used? There is a similar thing with the 12B4 both grid pins need to be used to avoid oscillation I read. There are a lot of Darling amplifiers made with this tube and I have not read about any issues.
Any stories to share?
DT
All just for fun!

[Miles Prower]

Originally Posted by DualTriode

Does anyone know this tube? Do pins #1, #5 and #6 need to be used?

The 6J4 is a UHF triode designed for grounded grid, small signal, RF amplification. It includes all those extra grid connections since these were meant to be connected to the ground plane to minimize impedance, so that it wouldn't oscillate.

For normal audio use, this isn't a consideration (you probably won't be using it as a GG amp anyway either).

[DualTriode]

Hello,
So I will tie all 3 grid pins together with no grid stoppers included, that is what was done with the Darling.
Any 8532/6J4 stories to share?
The first power triode will be the 6BQ6GTB, another crate full to work with.
DT
All just for fun!

[tubelab.com]

Any 8532/6J4 stories to share?

I have about 1000 of them, all used. They are in various conditions from dead to new. I tested a few several years ago and decided that they were worth saving. I found a few references that said that they made good input stages. My notes said "lower distortion than a 12AX7". That doesn't tell me much, and I was testing tubes quickly to decide which ones to keep.

I have over 100 6BQ6's, so let us know what you cook up. I'm thinking P-P though. The 6BQ6 works OK in SE. I have melted a few in the Simple SE, but they absolutely rock in P-P. Note the pinout is compatible to the popular audio tubes, just wire the plate cap to pin 3. They like screen drive too as long as you are reasonable with the power requests. Thermonuclear meltdown occurs at 110 watts output on 550 volts B+.

[drj759]

I tried 8532's in my Darling build a few years back. It was my first tube project. The 8532's I had sounded shrill and are the most microphonic tubes I have used to date. I abandoned the 8532's and moved on to the Hitachi 6GK5 with much better results. Shrill sound from 8532's may have been from high frequency instability. The 8532's were so microphonic that I didn't bother to scope for instability. You might want to plan to use grid stoppers with your 8532/6J4’s.

[DualTriode]

Hello,
See the attached sketch below; This is the starting point of the 6J4 experiment.

The series input stage is lifted from Volume 18 of the 1946 MIT series post WWII. The top tube acts as a resistor, as a tube it is nonlinear in the opposite direction of the bottom tube. Gain is equal to ½ the mu.
drj759
I will try the grid stoppers if the shrill happens thank you.
DT
All Just for Fun!

[Eli Duttman]

That 6J4 seems more than a bit like the 5842 and 6C45П. Tubes of that nature can oscillate if you look at them cross eyed. Put large value Carbon comp. resistors on all grid socket connections and use the junction point of the multiple stoppers as the I/P location. Ferrite beads on the heater wires close to the socket are also a good idea.

[DualTriode]

Direct-Coupled Amplifiers.pdf
Hello,
If you have an interest or comments I have attached a pdf scan of the page from the 1946 book that shows the first stage of the amplifier.
DT
All just for fun!

[Eli Duttman]

Sure looks like SRPP to me.

[SY]

But it's not. Look at where you're sampling the signal. Output impedance is higher, and when the cathode resistors are matched, the linearity is better. However, it needs to be buffered to realize the linearity; Broskie uses a White cathode follower with a hum cancellation gimmick, Landee shows a cathode coupled amp.