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Old 16th April 2016, 12:47 PM   #1
adrieo is offline adrieo  Netherlands
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Smile Please comment on D3a tube testing

Hello Diy members,

I need some help in understanding the way to check the D3a valves and judging their quality.

I tested some NOS D3a tubes triode mode.
My setup is: separate 6.3 v supply
Variable HT supply: set at 160 V
mA meter in Kathode - Anode loop.
(Separate bais supply 0-30 V - with 0.1 volt steps.)

I tested at 160 V in the range of - 1.5V to - 2.0 V negative bias supply.

I get for 4 tubes at:
-1.65V 21.5 mA
-1.65V 22 mA
and
-1.85V 21.5 mA
-1.85V 24 mA

with a change in the negative bias per 1 volt the change in mA is between 30 and 40 mA. So transconductance seems within factory spec.

Is it normal that you need to bias these valves individually to get the same current through them.
And does it tell something about its quality? (Lower neg.bias=lower quality?)

According to the datasheet: Testing with a kathode resistor of 470 ohm and a positive bias of + 10 volts and a HT of 160 V should give 24mA.
I tried this, but maybe I did something wrong as the current went through the roof.

Thanks!
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Old 16th April 2016, 01:07 PM   #2
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Hi!

Quote:
Originally Posted by adrieo View Post
Is it normal that you need to bias these valves individually to get the same current through them.
Yes


Quote:
Originally Posted by adrieo View Post
And does it tell something about its quality? (Lower neg.bias=lower quality?)
No, as long it is within the allowed range this is ok. High transconductance tubes tend to vary a lot in terms of current at a given neg. grid voltage.

Quote:
Originally Posted by adrieo View Post
According to the datasheet: Testing with a kathode resistor of 470 ohm and a positive bias of + 10 volts and a HT of 160 V should give 24mA.
I tried this, but maybe I did something wrong as the current went through the roof.
Yes the current goes through the roof if you apply positive grid voltage you probably fried that tube. This is meant to be used with a larger value cathode resistor which will set the cathode at a higher positive voltage than the +10V. This results in an overall negative grid voltage. The purpose of this is to take care of the large current variations between tubes which you noticed.

BR

Thomas
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Old 16th April 2016, 01:56 PM   #3
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At 24ma, a 470r resistor should develop about 11,24 volt, so 10 at the grid should not send values through the roof. Maybe oscillation?
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Old 16th April 2016, 03:08 PM   #4
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Or maybe the cathode resistor was not really there or shorted?
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Old 16th April 2016, 05:48 PM   #5
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or the OP misinterpreted the datasheet and put +10V between cathode and grid instead of between power supply ground and grid ...
that's why the datasheet uses Uba, Ubg1 ...instead of Ua, Ug1, the "b" to indicate that numbers are referenced to power supply, not cathode ...
so 24mA(plate)+3mA(screen)=27mA(triode connected) across 470ohm put cathode at +12.5v relative to gnd and +10v relative to gnd actually means the grid is at -2.5v relative to cathode for this test
(and plate at 160-12.5=147.5v by the way)
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Old 16th April 2016, 06:06 PM   #6
adrieo is offline adrieo  Netherlands
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Thank you all.
Especially VinylSaviour for his detailed explanation.
I will check if I made a mistake with the 470 ohm resistor. But my main question has been answered. I will bias every D3a tube individually and also put stoppers against possible oscillating.
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Old 16th April 2016, 10:44 PM   #7
kevinkr is offline kevinkr  United States
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Yes stoppers are mandatory with the D3A. Make sure you have all you are going to need, they've gotten a bit hard to find lately. (Your measurements are consistent with what I find in my batch of 30 or so.)
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Old 16th April 2016, 11:13 PM   #8
adrieo is offline adrieo  Netherlands
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A seller here (NL) asks for Siemens D3A goldpin pentode ( 7721 ) €15,25. Not really extreme for those who spend xxx money in their search for their ultimate sound and of course the enjoyment in building.
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Old 16th April 2016, 11:23 PM   #9
kevinkr is offline kevinkr  United States
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I paid about €7 - €8 for most of mine, I was an early adopter.. lol
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Old 16th April 2016, 11:42 PM   #10
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D3a featured first stage in one of SY's phono preamps.
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