# Calculate triode strapped characteristics from pentode data?

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#### mnturner

Hey guys, I'm thinking about building a headamp using subminiature jan6418's strapped as triodes, which I've seen done before. I was thinking about using the triode to drive a MOSFET source follower. without knowing the Rp of the tube, I have no idea if it will be capable of driving the gate capacitance of the FET though.
The datasheets for this tube only give pentode characteristics, and none for the triode strapped configuration. Is there a way to calculate/estimate things like the mu and Rp of a triode strapped pentode from the pentode data?

#### jechentau

ah here on the bottom seem triode curves http://www.shinjo.info/frank/sheets/127/6/6418.pdf .
seems like a nice tube.
the screen grid voltage controls the current at a with a certain (more or less) fraction of the control grid sensitivity, in volts giving a certain change in current. the ratio is the effective mu; µg1/g2. one can roughly trace it out of the penthode curves. (since the g2does a certain magnitude of current, for the voltage it has indicated on graphs)
this tube has a quite low plate current so can only drive not too big mosfets fast enough.
the rp seems to be around 16~17k.
if one looks at a suitable point to trace out a easily countable amount, at a fixed g1 voltage (say -1 here) with 20V Uplate say 240 uA, with 30V 800uA, that is 560 uA for 10 V, 17.8K (oops i was off ;p)
the grid controls about that amount (560 uA) with about 1V variation. one may thus guess the µ is around 10.

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#### mnturner

Thanks jechentau, that's exactly what I was looking for.
I've read good things about kits using this tube and I think it could be a nice sounding little project. Gives me something to use the 6418's I have lying around in.

#### mnturner

Just got some VN10KLS power mosfet and a 19v switcher and proto'd it up in an hour, doesn't sound bad at all for a first run. I think an LM317 regulator to get rid of the hum from the switcher would have it sounding pretty damn good.
only one question, this FET has a max. input capacitance of 60pf, the output Z of the triode stage should be approx 18k, rough calculation says the HF rolloff should have the -3dB out at about 150kHz, just looking at the output on the scope while sweeping a sin wave through it, i seem to see the highs beginning to drop off at about 16k.
Any ideas?

#### mnturner

Oh, and Plate load is 150k.

#### jechentau

i have not calced it through but it may be that the possible current swing of the tube is just not large enough for the fet for sufficient speed. what does the scope pic look like when the rolloff is reached?
does the sine go triangular?

#### mnturner

I'll have to retest it, the test conditions were far from optimal. It was slapped together on one of those solderless test boards so there would have been plenty of stray capacitance and inductance. For the test I used a triangle wave from a crappy little handmade signal generator and it was tested with a speaker load, not a resistive load. Now that I think about it, it was most likely just the frequency response of the speaker causing it. I'll set it up properly and retest, then get back to you.
Thanks for the interest though.

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