6j6 twin triode running at low voltage (18V)

[tca]

I've found a 6J6 tube in my drawers. It is a cheap twin (common cathode) triode (http://www.r-type.org/pdfs/6j6-1.pdf)

Here's de schematic I've been playing with (for each triode):



Works nicely with a nice compression. Some xosc pics:





With the two triodes in series one gets:



The two triodes in series reacts nicely with the attack but with high input signals the triode cuts of and it takes about a few seconds to recover (have to attenuate the signal before the second triode). Needs some more testing but so far so good.

BTW, the heater is taken from the 18V with a 47 Ohm power resistor in series.

Comments/improvements?
Cheers.

[duck_arse]

I bought some 6j6's for 1$ each, some many years ago. they are still laying around, waiting for some 7 pin sockets to magically appear, and something to build them into. excellent!

[amptramp]

If you are running off 18 volts, you can get a 19J6 with a 0.15 A heater that may relieve your power supply load.  It is rated for a nominal 18.9 volts so it should work perfectly.  The 19J6 was developed for TV antenna amplifiers that were mounted on the mast right at the antenna.  They usually had a transformer behind the TV sending 18 volts up the line and there was either a DC supply or a transformer in the unit to take 18 volts and convert it to 110 VAC which was rectified and used as the B+ supply.  This could all be done carrier current with the signal coming down the power line so you could add an amplifier without adding anything to the normal coax cable run.

[tca]

^ The 19J6 is a a bit more expensive and harder to find in europe. Is there a european equivalent? Thanks.

P.S.
Schem updated (see first post).

[DDD]

"... with high input signals the triode cuts of and it takes about a few seconds to recover ..." - maybe it's time to reduce C2 capacitance down to 1uF or even to 0.1uF?

[amptramp]

Here's de schematic I've been playing with (for each triode):




The two triodes in series reacts nicely with the attack but with high input signals the triode cuts of and it takes about a few seconds to recover (have to attenuate the signal before the second triode). Needs some more testing but so far so good.

You are getting "gulp distortion" or "blocking" when the signal disappears.  When the signal goes positive, the cathode follows it in the positive direction but when the signal goes negative, the cathode stops conducting and the cathode voltage falls at a rate set by the R-C time constant on the cathode.  With 330 ohms and 10 µF the corresponding corner frequency is 48.3 Hz.  It takes a while for the cathode voltage to decline to the point where it starts conducting again and this is the gap in operation.

I would use a fixed bias on the cathode if you intend to operate both tube halves as a two-stage cascaded amplifier.  The fixed bias could come from a LED or other diode string in the cathode path totalling 2.5 volts.  If you use a resistor without the capacitance, you will get a multivibrator that oscillates - not what you want.

[tca]

Thanks for the replies. Decreasing C1 (4.7u) and C2 (1u, or 220n, 15n)  partially solves this issue. Reducing C2 greatly controls the blocking distortion.

The second stage has a 100k at the plate, although a 33k would do, but it gets out more distortion with a higher value which I like.

Removing C2 it oscillates as amptramp said.

Also both grids have some DC, about -0.35V  (grid-leak) with one 1M to ground.

[PRR]

> grids have some DC, about -0.35V

They really shouldn't. <0.1V is more right.

However if this very high-gain/bandwidth tube is oscillating above 50MHz, it just might self-rectify a third-Volt on the grids. What happens with 100pFd plate to cathode? (Might be better OR worse; parasitic oscillations can be paradoxical.)

However VHF oscillations may be part of "the tone". OTOH, they make the effect hard to reproduce.

Or you may just have a wheezy 6J6 with mucho grid current.

[tca]

Measured with a decent device grid DC voltage is -.26V.

> What happens with 100pFd plate to cathode?
The same reading.

> However VHF oscillations may be part of "the tone". OTOH, they make the effect hard to reproduce.
+1I'll post the final schematic in a couple of days.

Thanks.

[tca]

Some fresh news on this:

[rankot]

You may try with this one: https://robrobinette.com/SixShooter_Build.htm

[tca]

Here's the final version. I've used a cellphone charger (5.3V 0.5A) instead of the 18V power source. Works nicely.





Cheers.

[briandress]

sound clips?

[tca]

sound clips?

https://soundcloud.com/tca-4/testting-a-6j6-tube-booster-runing-at-53v

[rankot]

Tiago, if I want to use this booster for bass guitar, what shall I change to avoid distortion? 

[tca]

> if I want to use this booster for bass guitar, what shall I change to avoid distortion?

Increase the output caps to 1u and try the following:  use a 100k on the 2nd triode xor use only the 1st triode.

Cheers.

P.S.
This circuit does not have much headroom.

[bool]

... or try both triodes in parallel (i.e. single-stage).

[tca]

^ that would work too.

[rankot]

I have built this with modifications shown on picture:



Added Drive pot, so I can adjust amplification of the second stage, and switches to allow testing with constant cathode current source, as well as 9V/27V B+. Still no success with 9V B+, but 27V works excellent! I have also added tone stack. I want to test it a little bit more, expect PCB files as soon as I return from summer vacation 

[tca]

Excellent!