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Valve Itch phono
Valve Itch phono
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Old 18th April 2015, 11:04 PM   #2051
merlin el mago is offline merlin el mago
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What's the expected bias, the voltage across the diode?
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Old 19th April 2015, 12:38 AM   #2052
Salas is offline Salas  Greece
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Some voltage across the diode, yes. Since the current there is low it won't be full 0.7V that is why we may use IR LED or two diodes if the tube sample don't go very little mV across open input.
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Old 19th April 2015, 06:33 AM   #2053
stajo is offline stajo  Sweden
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I think a well choosen IR is by far more linear then 1N4148 at those currents. A bypass, like a low noice 100k from top anode to diode anode might get the current up to linear levels, and maybe to levels where grid current stops with the strong bias level of one diode.

Edit. Maybe not on second thought. It looks hopelessly nonlinear up to some 50 mA. http://www.nxp.com/documents/data_sh...148_1N4448.pdf

Would a low noice 1k-2k be worse noisewise then the unlinear effect of the diode to the sound?

Last edited by stajo; 19th April 2015 at 06:44 AM.
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Old 19th April 2015, 01:56 PM   #2054
Salas is offline Salas  Greece
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V1.2 depicts an IR for those reasons you mention, if not having one and the diode is not working well but using a resistor, better bypass it with a large value electrolytic 1000uF 6.3V to filter its noise
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Old 19th April 2015, 02:11 PM   #2055
stajo is offline stajo  Sweden
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Yes. Depending on where the grid current starts its a call between harder biasing for more signal current with a resistor or rectifyer diode and less bias current and presumably lower noice with an IR diod.
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Old 19th April 2015, 02:58 PM   #2056
Salas is offline Salas  Greece
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This tube we use has good GM to small Ik ratio presenting low hiss plus very quiet mechanically, and it got slowly recognized for phono input use these days, obscure if not used at all for that back when used in the Itch, so its worth solving for Ig carefully by sample basis. They generally differ by sample when it comes to grid current negative bias threshold, but its relatively low always.
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Old 19th April 2015, 03:10 PM   #2057
stajo is offline stajo  Sweden
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I have some boxes, and one of them is OTK-marked (military tested). Will measure them up.
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Old 19th April 2015, 06:51 PM   #2058
stajo is offline stajo  Sweden
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I guess Merlins problem might be just grid current. A SUT should be able to take more DC then a PUP-coil.
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Old 19th April 2015, 08:30 PM   #2059
stajo is offline stajo  Sweden
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I made an attempt to measure grid-anode and grid-cathode capacistance after running a few 6N2P's in the AVO. Since a difference of 1 pF can matter quite alot, especially concerning that Cmiller= C(1+Af), important for the second stage and riaa comp values it was interesting to see that Cga and Cgk measurments differed quite a bit on how I held the crocs. If I added a socket on the tube it increased by nearly double. Then I tested just with the crocs distance and got a difference of like 2 pF just by holding them near or far away from each other. A lession for when doing interconnection cabling, yes indeed. Croc/cabling distance is far more important then tube sample differences concerning grid capacistance.

Measures made with DER EE DE-5000
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Old 19th April 2015, 08:43 PM   #2060
Salas is offline Salas  Greece
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The method used for determining Miller is comparing -3dB upper limit in a common cathode circuit between two different source resistances (virtually series to the grid) then calculating back from the measured Av. It takes gen & scope.

With the LCR you could roughly say a type has more parasitic than another when steady for wires, but to be sure of the absolute figure is quite tricky.
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