Suppresor Grid used for Feedback?

[dshortt9]

Do you have an EL34, EF86, SV83?

[smoking-amp]

"Do you have an EL34, EF86, SV83? "

No, sorry. I have mainly TV tubes, nothing much audio except 6L6, 12AT7, 6CG7/6FQ7, 6SN7, 6BQ5/29GK6/EL84, 12AX7, 12AU7, 5687, 6DJ8.

Some of those should have good data sheets available with g2 plots. Not likely to be much g3 effect with non "dual control" tubes. I tried some horiz. output tubes, and g3 does flat nothing. 29GK6/6BQ5/EL84 shows just a little subtle change around the knees with +30V on g3.

[Wavebourn]

Originally Posted by smoking-amp

You have mail.

Hi Don;
I'm shipping you now 6BA6, GU-50, 12L6, and some exotic tubes. No need to return, enjoy them after torturing!

[smoking-amp]

RE: Wavebourn

Got the tubes today, arrived in good condition.

1st tube up for torture: 5763
Axis: 5 mA/ div vertical and 50V /div horiz.
1st plot is with g2 at 140V and g3 at 0V, 1V steps on g1
2nd plot is with g2 at 140V and g3 at -250V, 1V steps on g1
3rd plot is triode mode, 1.9V steps on g1

Seems that the g3 is not effective enough in this tube since -250 V on g3 fixes it up. Seems to be a bit of osc. near the knees too when g3 is 0. Changing g3 voltage in triode mode had almost no effect.

Step size here is rough yet, I still have to do some calibration work on the curve tracer, plus I added a cont. variable control to the step amplifier gain.
I will also have to increase the power supply voltage for the step generator amplifier before I can test the GU-50 fully, I think. Only have around 60V max available at the moment. The step amplifier is already modded for up to 250V.

[smoking-amp]

Next tube up: 6BA6W/5749

Plot1: 2mA/div vert. , 50V/div horiz.
100V on g2, 0V on g3, 1.8V steps on g1

Plot2: 1 mA/div vert. , 50V/div horiz.
0V on g1, 0 V on g3, 5.5 V steps on g2

Plot3: 2 mA/div vert., 50V/div horiz.
1 V on g1, 0 V on g3, 5.5 V steps on g2

Plot4: 2 mA/div vert., 50 V/div horiz.
2 V on g1, 0 V on g3, 5.5 V steps on g2

Plot5: triode config. 2 mA/div horiz., 20 V/div horiz.
2.75 V steps on g1

g3 voltage does have noticeable effects on the knee type plots, +2V squares them up a little tiny bit, over +2 V or minus V rounds them off.

Plot 4 is showing some hum effects from the Tracer power supply I think, comes and goes, need to do some work on the power supplies. Every other trace is from leading or falling 60Hz power sweep, so supply hum crowds every two traces together a little. With 9 traces setup, this alternates between sweeps as to which sets of traces crowd together some.

I wonder if the strange waves in the g2 drive plots are caused by the g1 weird spacing affecting g2 gm locally.

[smoking-amp]

Back to the 5763 to try g2 drive on it. No funny waves in the curves here.
5 mA / div vertical, 50 v / div horiz. 5.5 V steps on g2, +2V on g1, 0 V on g3

g3 volts best at 0V here.

[dshortt9]

Interesting stuff. With three grids to play with curves can be altered to make "poor" tubes look good and vicey versy.

[smoking-amp]

next up, 12L6

Plot 1, pentode: 10 mA/div vert., 50 V/div horiz.
g2 at 95V, g1 1.4V per step

Plot 2, triode: 10 mA/div vert., 50 V/div. horiz.
5.5 V per step on g1

Plot3, g2 drive: 10 mA/div vert., 50 V/div. horiz.
5.5V per step on g2, g1 at +4V

Nice triode.

[Wavebourn]

Thank you Don!

Still, 12L6GT is the winner: very nice triode indeed!

[smoking-amp]

"With three grids to play with curves can be altered to make "poor" tubes look good and vicey versy. "

Some other tricks:

12L6 with resistor in g2 circuit. All plots 10 mA/div vert. and 50 V/div horiz., 1.4 v per step on g1, 105V g2 supply

Plot1: no g2 resistor
Plot2: 1.3K Ohm g2 resistor
Plot3: 10K Ohm g2 resistor