D3a in triode mode

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RhythMick
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#31 Re: D3a in triode mode

Post by RhythMick »

Nick wrote: Thu Jul 01, 2021 2:30 pm Thats a lot of step up for a 400uv cart. I would have thought 10:1 would be better. Also remember all things being equal 20kHz is 20db Up on the 1kHz signal (not that there is that much signal at 20kHz, but pops and clicks are to be considered.
And now fully reading Nick's post, always a good idea :-)

OK I see what you are saying about the 20kHz end. So if I want to leave some headroom with a D3a grid bias of 1.5V lets say maximum 1V on g1. I wonder how the cartridge is specced, is that nominal output at a given frequency ? 1kHz ?
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Nick
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#32 Re: D3a in triode mode

Post by Nick »

Noise should be fine. I used it as a pentode with 40db gain and it was almost quiet enough to use without a step up.
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RhythMick
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#33 Re: D3a in triode mode

Post by RhythMick »

Nick wrote: Thu Jul 01, 2021 11:25 pm Noise should be fine. I used it as a pentode with 40db gain and it was almost quiet enough to use without a step up.
impressive thanks
RhythMick
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#34 Re: D3a in triode mode

Post by RhythMick »

OK set up and measuring/recording properly.

First config is :
PSU is 350v mains TX -> Bridge Rectifier -> LCLC : output from LCLC under load measured and recorded
D3a triode wired (g2-A, g3-k)
47k g1-GND
Anode load is 2K + LL1667-20mA (200H)
Bias : cathode 78R + 1000uF bypass
Output load = 100K

Vak = 159V
Vg1 = 1.65V
Ik = 21mA
100mV (pk) square wave input -> 8.5Vpk
Gain = 85 (!)
Capture.PNG
Capture.PNG (29.96 KiB) Viewed 3161 times
Red = Sq Wv In
Blue = Out
RhythMick
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#35 Re: D3a in triode mode

Post by RhythMick »

RhythMick wrote: Fri Jul 02, 2021 2:36 pm OK set up and measuring/recording properly.

First config is :
PSU is 350v mains TX -> Bridge Rectifier -> LCLC : output from LCLC under load measured and recorded
D3a triode wired (g2-A, g3-k)
47k g1-GND
Anode load is 2K + LL1667-20mA (200H)
Bias : cathode 78R + 1000uF bypass
Output load = 100K

Vak = 159V
Vg1 = 1.65V
Ik = 21mA
100mV (pk) square wave input -> 8.5Vpk
Gain = 85 (!)

Capture.PNG
Red = Sq Wv In
Blue = Out
Interestingly removing the cathode bypass cap reduces the gain but only by a touch, down to 82. No change to the shape of the output wave.
RhythMick
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#36 Re: D3a in triode mode

Post by RhythMick »

OK been away for a week but back now. Repeated the above with the same measurements. Triode mode, cathode bias resistor unbypassed, into 100K load and I get a gain of just over 80.

Before I proceeded I want to experiment with wiring the D3a as a pentode and I've not done that before. If I understand it correctly it's just a matter of inserting a resistor between g2 and b+ with the objective of reducing the voltage on g2 - is that correct ?

I tried that using 1K and then 8K1 resistors.

R (b+ to g2) = 1K, Ik = 20.5mA, Ia = 15.9mA, Ig2 = 4.7mA, Va (GND) = 161V, Vg2 (GND) = 155V

R (b+ to g2) = 8K1, Ik = 18.7mA, Ia = 14.5mA, Ig2 = 4.1mA, Va (GND) = 173V, Vg2 (GND) = 139V

So it does look like the larger resistor drops the g2 voltage but the gain shows no change where I expected a big increase in pentode mode.

Have I misunderstood pentode wiring ?
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Nick
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#37 Re: D3a in triode mode

Post by Nick »

You need to decouple g2 to ground or the cathode with a cap so the voltage stays constant.
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RhythMick
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#38 Re: D3a in triode mode

Post by RhythMick »

Nick wrote: Sun Jul 11, 2021 3:18 pm You need to decouple g2 to ground or the cathode with a cap so the voltage stays constant.
Ok thanks Nick. What value cap is normal?
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#39 Re: D3a in triode mode

Post by Nick »

Whatever gives you -3db with the g2 resistance in parallel with the load resistance.
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#40 Re: D3a in triode mode

Post by IslandPink »

I would try at least 470µF with that cathode resistor , maybe 1000µF , for which of course I have a Black Gate NX ... :)
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RhythMick
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#41 Re: D3a in triode mode

Post by RhythMick »

Nick wrote: Sun Jul 11, 2021 6:07 pm Whatever gives you -3db with the g2 resistance in parallel with the load resistance.
Thanks Nick. My humble apologies for asking without doing my own research. Boooo

"The Valve Wizard -Small Signal Pentode" http://www.valvewizard.co.uk/pentode.html
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Nick
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#42 Re: D3a in triode mode

Post by Nick »

You can also use a gas regulator. The goal is to fix the voltage on g2 so it doesn't vary with signal. Note that the g2 current with vary with signal so a simple resistive dropper wont work. Up to some point the lower the impedance of the g2 supply the better. I use a mosfet source follower in my d3a phono to set the voltage. Of course this is a sensitive point WRT noise, so a low impedance low noise is needed.
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RhythMick
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#43 Re: D3a in triode mode

Post by RhythMick »

Cap applied between G2 and cathode. Gain is now 215.

Thanks.
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#44 Re: D3a in triode mode

Post by RhythMick »

Made some time to continue the experiments with the D3a today. Having got triode and pentode mode working I wanted to try ultralinear, suggested by Jac of JacMusic.

Choke load, LL1667-20mA (200H). The two coils of the LL1667 are connected in series (correct polarity observed), anode connected to output from the choke, g2 connected to the mid-point. 47K grid load, 100K output load.

g2 measures at 5mA, Ia = 17mA, Ik = 22mA, Vak = 157V.

Interestingly though, the square wave input at the grid becomes horribly distorted at the output. Looks like overshoot and ringing ?
Capture.PNG
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IslandPink
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#45 Re: D3a in triode mode

Post by IslandPink »

What's the frequency ?
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