Valve Itch phono

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Joined 2009
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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:)
 
diyAudio Chief Moderator
Joined 2002
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47nF inter-stage and 1uF output when driving 50K typical pot example response with useful sub bass roll off.
 

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