D3a in triode mode

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RhythMick
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#1 D3a in triode mode

Post by RhythMick »

I'm wiring up a D3a in triode mode, planning to scope the output with a 1khz square wave on g1, play with the operating point.

G3 to cathode, tick.
G2 to anode ... Hmmm. Won't that mean the screen sees a larger current than designed for?

I'm planning led cathode bias around 2.2v to begin with and around 150v on the plate. I'd just like to hear confirmation that connecting g2 to anode is correct before I apply power and fry an expensive valve.
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IslandPink
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#2 Re: D3a in triode mode

Post by IslandPink »

I can't remember without checking what the max G2 dissipation is, but I have been running triode D3a's in my phono for years with 15mA current and about 120V on anode and G2, if that's any help.
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RhythMick
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#3 Re: D3a in triode mode

Post by RhythMick »

IslandPink wrote: Sat Jun 26, 2021 10:02 pm I can't remember without checking what the max G2 dissipation is, but I have been running triode D3a's in my phono for years with 15mA current and about 120V on anode and G2, if that's any help.
Thanks that helps.

With G2 connected direct to anode (no resistor) how does the current split between anode and G2 ?
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#4 Re: D3a in triode mode

Post by RhythMick »

Screenshot_20210626-232751.png
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Max G2 1W
But
Max anode + G2 (triode) 4.5W
RhythMick
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#5 Re: D3a in triode mode

Post by RhythMick »

OK a day spent setting up a test rig for the D3a wired triode. I have 10R sense resistors connected to cathode, anode and g2 (so I can easily measure current through each). I'm using one of Andrew's DC boards to put 6.3V across the filaments. I have a PSU circuit consisting of mains tx, SS bridge rectifier, LCLC with a bleed across the first cap. Choke load into anode. Cathode 10R connects to 47R making total cathode load of 57R (unbypassed). G1 has a 47K to ground, but for now G1 is grounded. The 10R resistors to anode and g2 are connected and B+ from the anode choke connected to there (so anode and g2 are connected, but with the 10R sense resistors to each). G3 is connected directly to Cathode. Circuit diagram below (using spice, but I'm not simulating, just drawing, and the Pentode model doesn't show the separate filament connections please ignore that).

I have a variac before the mains TX so I can gradually increase voltage while measuring the currents. The TX feeding the DC board is independent of the variac.

I'm aiming for an operating point (to begin with) of 160Vak, Cathode at 1.5v, grid grounded, bias -1.5V, 24mA current through cathode.

I'm wary of too much current going through g2 and I'm not familiar with using pentodes - hence the cautious approach. max dissipation on g2 is 1W which (with 160Vak) means no more than 6.25mA through g2 (so 0.0625V measured across the 10R).

Meters in place across the 10R resistors. 6.3V on the heaters (pins 4/5), Variac gradually increased. Way before reaching mains voltage g2 is showing 13mA, anode 3mA, cathode 16mA. Powered off after a few seconds to avoid burning out the valve.

I've checked the connections and all seems well. No shorts, everything wired to the right pins.

Heaters pins 4 & 5
Cathode pins 1 & 3
g1 pin 2
g2 pin 9
g3 pin 8
Anode pin 7

Any input welcomed, as I say I've not used anything other than diodes and triodes before. All research indicates that D3a in triode mode should have g3 connected to cathode, g2 direct to anode.
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IslandPink
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#6 Re: D3a in triode mode

Post by IslandPink »

I'm surprised to see G2 conducting much more, given that it's got less surface area than the anode.
I'd be inclined put a proper anode resistor ( 5 to 10k ) above the connection from G2, and see if this changes anything.
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RhythMick
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#7 Re: D3a in triode mode

Post by RhythMick »

IslandPink wrote: Sun Jun 27, 2021 7:36 pm I'm surprised to see G2 conducting much more, given that it's got less surface area than the anode.
I'd be inclined put a proper anode resistor ( 5 to 10k ) above the connection from G2, and see if this changes anything.
Thanks yes I'll try that. Of course that will put it into pentode mode right? I'll start again by getting it working properly in pentode mode.
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#8 Re: D3a in triode mode

Post by RhythMick »

I wonder if gradually increasing the voltage actually causes a problem because the larger but more distant anode isn't sufficiently attractive but the screen grid is close enough to pull the current. Hmmm.
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#9 Re: D3a in triode mode

Post by IslandPink »

RhythMick wrote: Sun Jun 27, 2021 7:41 pm Thanks yes I'll try that. Of course that will put it into pentode mode right? I'll start again by getting it working properly in pentode mode.
No, not unless you have a separate connection with its own resistor, connecting G2 to the B+ supply.
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Nick
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#10 Re: D3a in triode mode

Post by Nick »

Yep, if you look at the g2 current curves

https://frank.pocnet.net/sheets/128/d/D3a.pdf

They are higher until the anode voltage gets past 100v or so. I would continue the experiment, checking the current and actual dissipation at each anode voltage rather than setting a limit to g2 current based on the 160v point. I think you will find that after a bit Ia will start to increase and Ig2 will start to fall. And of course as the total current increases the Vg1 will become more -ve reducing current as the cathode voltage increases.
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#11 Re: D3a in triode mode

Post by IslandPink »

Which page number is that Nick ? - I'm struggling to find it.
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RhythMick
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#12 Re: D3a in triode mode

Post by RhythMick »

Nick wrote: Sun Jun 27, 2021 8:36 pm Yep, if you look at the g2 current curves

https://frank.pocnet.net/sheets/128/d/D3a.pdf

They are higher until the anode voltage gets past 100v or so. I would continue the experiment, checking the current and actual dissipation at each anode voltage rather than setting a limit to g2 current based on the 160v point. I think you will find that after a bit Ia will start to increase and Ig2 will start to fall. And of course as the total current increases the Vg1 will become more -ve reducing current as the cathode voltage increases.
Thanks Nick that makes sense, though I couldn't see which were the G2 curves. I'll have another look later on a bigger screen. And with my glasses.
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#13 Re: D3a in triode mode

Post by Nick »

IslandPink wrote: Sun Jun 27, 2021 8:42 pm Which page number is that Nick ? - I'm struggling to find it.
page 90, its not exactly what we need, but I believe the dotted lines are Ig2, though that is with a fixed voltage on it.
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RhythMick
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#14 Re: D3a in triode mode

Post by RhythMick »

I'll keep the triode connections and cathode bias as I currently have them. I'll work through a range of anode voltages and record anode, g2 and cathode currents and share results.

My objectives for now are to get familiar with the operation of the D3a as triode by observation. I want to see a signal going through, measure gain, compare bypassed Vs unbypassed and different bias settings etc.
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#15 Re: D3a in triode mode

Post by RhythMick »

124v across Vak, 22mA G2, 6mA across anode.

Except I then realised I'd labelled the top of the board wrong. Sorry.
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