Triode plate resistance == internal negative feedback?

[smoking-amp]

"Some valves don't do what we expect. The 6J6 is basically two planar triodes back to back, so it ought to be linear but it isn't. "

Looks similar to the 12AT7 curves. (see the Mu curves at end of data)
I don't have one handy to look at, but I would guess it has the grid 1 very close to the cathode, and maybe a somewhat coarser pitch to the grid wire windup. Both are rated for mixer use. I would guess they are both using grid wire "island effect" to get the non-linearity.

http://tubedata.milbert.com/sheets/093/6/6J6.pdf
http://tubedata.milbert.com/sheets/093/1/12AT7.pdf

[DF96]

My guess is a coarse grid pitch, and perhaps quite thick grid wires so that it can cope with a bit of grid current in Class C.

[Wavebourn]

Originally Posted by DF96

My guess is a coarse grid pitch, and perhaps quite thick grid wires so that it can cope with a bit of grid current in Class C.

Very possible: in computers they were used in logical elements. Multi-vibrators and triggers work with positive voltages on grids.

[ruffrecords]

Originally Posted by smoking-amp

"Some valves don't do what we expect. The 6J6 is basically two planar triodes back to back, so it ought to be linear but it isn't. "

There appears to be a strong correlation between plate tallness and linearity (for a given tube) - taller plates have better linearity. I am not familiar with the constructional details of the 6J6. How tall are is plates compared to the 12AT7?

Cheers

Ian

[Wavebourn]

Originally Posted by ruffrecords

There appears to be a strong correlation between plate tallness and linearity (for a given tube) - taller plates have better linearity. I am not familiar with the constructional details of the 6J6. How tall are is plates compared to the 12AT7?

It is true when grids completely surround cathodes: edge effects affect linearity. Planar tubes have 4 edges, so taller electrodes do not mean higher area covered by uniform field. Planar tubes usually have denser grids, but grids of 6J6 and 12AT7 seems to be rare.

[Bigun]

Originally Posted by DF96

Those embarrassed by the idea of having hidden feedback in their amplifiers could always look at it another way:
- an ideal triode is a linear voltage amplifier with a non-linear output impedance (so for best results have a very high load impedance)
- an ideal pentode is a non-linear transconductance amplifier with a very high output impedance (so for best results either use very small signals, or arrange in push-pull with external NFB to reduce the output impedance)

Nicely said - and you could write <Solid State device> where you have written 'pentode'

[AJT]

Originally Posted by sawreyrw

A triode can be modeled as a voltage controlled current source (gm*Vgc) in parallel with the plate resistance (Rp). If the cathode resistance is zero or it is bypasses, then there is no feebdack through Rp. Rp will be in parallel with the anode resistor. If there is a cathode resistor, then Rp could be thought of as providing negative feedback. However, Rp varies with operating point so the feedback in nonlinear for large variation in plate voltage.

what about Rk? isn't it that there is supposedly a cathode resisistor equal to 1/gm at operating current of the stage?

[DF96]

You can either talk about gm or Rk - using both at the same time is double counting.

The post you quoted is a bit confused. There is, of course, "no feedback through Rp" - but Rp is itself the consequence of feedback. Feedback creates Rp.

[DF96]

Typical 6J6 anodes appear to be about 10mm tall, but the active part of the cathode is a bit shorter at about 8mm. There could be some end-effect, but as all electrodes continue past the active cathode surface I would expect this to be small. The grid pitch is not much smaller than the grid-cathode spacing (maybe 2:1 ratio?), so island effect will be fairly strong.

[ruffrecords]

Originally Posted by DF96

Typical 6J6 anodes appear to be about 10mm tall, but the active part of the cathode is a bit shorter at about 8mm.

That is similar to the 12AT7. The 6SN7 and its derivatives (6CG7/6FQ7) have an anode height of about 24mm and much better linearity. I tested about 50 6CG7/6FQ7 types some with 24mm plates and some with 18mm plates and the shorter plate types were always less linear.

This also appears to apply to the 12AX7. The EH12AX7LPS (a long plate version of the 12AX7) is a lot more linear than the standard height plate types. Despite its detractors, the 12AX7 is actually intrinsically very linear, even more so than the 6SN7 in the test I did.

Cheers

Ian