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Wiring the D3a as triode - What is best?

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Bas Horneman

Bas Horneman

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#1
Wiring the D3A as triode - What is best?

I wired the d3a as triode, in order to use it as driver for my SE6c41c.

I did it the following way. Pin 8 (g3) to pin1 (cathode). Pin 9 (g2) with 1k to pin 7 (anode).

But I saw DHTROB use it differently (g3 to anode) and (g2 to cathode)

Anyone have experience with the d3a or any other pentode re-wired as triode?

(ps forgot to draw the grid leak resistor + I drew pin 8 to cathode as an internal wire (which it is not))


An externally hosted image should be here but it was not working when we last tested it.
 
#3
Why not try it both ways and let us know what you find.
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Mmm I should.... should I not 🙂 ...only the amp weighs a ton (3channels) 3 power transformers 3 output transformers 9 chokes and 3 filament transformers !!! (And now 2 anode chokes as well)

As far as I know, your connections are correct.
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Ok...if no-one can convince me otherwise...I'll just leave it the way I've done them.
 
#4
But I saw DHTROB use it differently (g3 to anode) and (g2 to cathode)
Click to expand...

With 0V on the screen, you probably wouldn't be getting much current out of it at all.

I have seen schematics with the suppressor to the anode, or the cathode (with valves that have a separate suppressor pin) in triode mode, but all have the screen tied to the anode. IIRC, I've read there may or may not be some difference on where the suppressor is tied, something to do with interelectrode capacitances or something - not particularly significant.

Is this the triode-strapped D3A schematic you're referring to?

http://www.dhtrob.com/schemas/afbeeldingen/phonique_d3a.GIF

If so, look carefully. The screen is tied to the anode.

EDIT: BTW, DHTRob seems to have messed up the connection of the 0D3s, connecting 200V across them with no means of current limiting will likely end in disaster :bigeyes:
 
#5
Is this the triode-strapped D3A schematic you're referring to
Click to expand...
Yes..that's the one.

The screen is tied to the anode.
Click to expand...

That's right. Only I have the screen connected to the cathode, hence my question which is better 🙂

[edit] just found this in the Telefunken datasheet
An externally hosted image should be here but it was not working when we last tested it.
 
#8
I'm sorry, I still don't follow.

The screen (g2) is connected to the anode. You can't have the screen connected to the cathode as well without shorting the valve.

EDIT: Oh, I get it now. We were referring to different things - I meant screen grid...
 
#10
Nah, we're both right 😀

The screen grid is connected to the anode, but the shielding screen is connected to the cathode (as it's connected to the same pin as the suppressor)

Damn my vague terminology :bawling:

Hoping I understand this now, the question seems to be where is the best place to tie the suppressor (g3) and the shielding screen...

Well, in triode mode, I don't think it matters. So we get back to what Frank Berry said ages ago, which is try both and see what happens 😎
 
#14
Don't tie the suppressor grid to the cathode: tie it to ground. The suppressor grid likes to be at the same DC potential of the control grid. Some people like to connect them (ex. in EL34) to the bias supply if it is a fixed bias output stage (and the bias is extremely clean). They think the tube sounds better with a suppressor much more negative than ground. I don't know... but maybe there's a max voltage allowed between anode and suppressor...
 
#17
I will bypass it soon..but the funny thing is that I had the idea that the bass was better with the D3A than with the 6N1P?!

The datasheet gives 1k9 as Ri triode connected. (Or are they assuming a bypassed cathode...and does it go up that much unbypassed?) wow 😱

An externally hosted image should be here but it was not working when we last tested it.


Anyway...chucking the 1,9k Ri figure in the Rp box in the spreadsheet give 4Hz at -1dB...enough for me
 
#19
With a mu of 77, the D3a will multiply any unbypassed cathode resistor by a factor of 78 (1 + mu), and add it to ra. That's a big increase...

I shall be most interested to hear what people think about playing with g3 voltages. I wouldn't have thought such a coarse grid would have much of an effect with such a small voltage, but nothing beats experiment.
 
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