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Old 24th March 2013, 02:53 PM   #1161
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Quote:
Originally Posted by Salas View Post
Some samples of 6N2PEV may show more or less grid current. Its specified as low grid current tube but still has. Grid current manifests closely to 0Vgk like we use here.
With MC and SUT the cartridge will not see it but some SUT may care about 1uA or so on its secondary losing performance. Most will not care.
With MM cartridges connected directly there can be a problem with performance though, like neskor had yesterday.
There are two solutions if that happens. Either capacitor couple the input or raise the cathode voltage a little so Vk>0.
An ordinary diode like 1N4007 can do that while keeping the circuit near enough to its prior operating point. An LED would change it too much. At 1mA+ bias used the diode is not at its best resistance slope but the signal is very low and its distortion is expected to be low. A low current & low junction capacitance diode like 1N4148 is an even better part for the purpose.

To sum up: When you see DC input offset with some 6N2PEV, lift the input tube's cathode with a diode. Measure again, it should be much lower than what you had.
Match tubes for lowest final reading if you have many. Your MM cart should do OK if you end up in the mV offset region. If with MC, each SUT model may care or not for grid current, so listen with that diode lift and without. If the performance does not change its better for the diode not to be there avoiding its little additional noise and THD.
Notice that you should measure for lowest DC on input without the cartridge or SUT connected. After achieving few mV only, when connecting an MM cart should show DC offset next to nothing across the input.
Thanks Salas
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Old 24th March 2013, 02:53 PM   #1162
neskor is offline neskor  Serbia
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I'm pleased with the sound after this diode tweak.
Previously I had around 0.4 VDC on grid.
After diode tweak I have 0,025 VDC on input grid. I had to search from big bunch of tubes to find ones with the smallest grid current.
With cartridge connected I have 0,001 VDC on input grid.
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Old 24th March 2013, 02:57 PM   #1163
Salas is offline Salas  Greece
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So you can save your MKP capacitors for better use.
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Old 24th March 2013, 03:02 PM   #1164
neskor is offline neskor  Serbia
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What is a recommended, and also max and min value of interstage and output capacitors?
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Old 24th March 2013, 03:08 PM   #1165
Salas is offline Salas  Greece
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100nF max inter-stage capacitor, 47nF min. For output depends on what impedance you drive. Say no bigger than 3.5uF when driving low kOhm vol pot.
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Old 24th March 2013, 03:10 PM   #1166
neskor is offline neskor  Serbia
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Ok, thanks!
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Old 24th March 2013, 03:14 PM   #1167
Salas is offline Salas  Greece
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47nF inter-stage and 1uF output when driving 50K typical pot example response with useful sub bass roll off.
Attached Images
File Type: png Example_FR.png (9.6 KB, 107 views)
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Old 24th March 2013, 03:50 PM   #1168
Salas is offline Salas  Greece
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Quote:
Originally Posted by merlin el mago View Post
Thanks Salas
Grid current leakage was mentioned as early as in post#5.
I added the diode trick link there also.
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Old 24th March 2013, 03:54 PM   #1169
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I guess I don't have the issue because sound amazing
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Old 24th March 2013, 03:58 PM   #1170
neskor is offline neskor  Serbia
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Probably because you are using SUT
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