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Old 23rd March 2013, 10:16 PM   #1151
Salas is offline Salas  Greece
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Bit more current running on the 6SN7 than in others but not too much. That bottom 6n2p-ev -0.4V on grid is not OK to meet the generator. Is it so with the cartridge connected? Do you have a couple of 2.2uF capacitors MKP or bit bigger value to AC couple the cartridge and see if the sound changes? Also is the 15nF Riaa capacitor checked for value?
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Old 23rd March 2013, 10:28 PM   #1152
neskor is offline neskor  Serbia
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That -0,4V is without catridge, just PSU connected nothing else.
Just tried with 3,5uF to Ac couple the catridge and sound is much better.
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Old 23rd March 2013, 10:34 PM   #1153
Salas is offline Salas  Greece
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Leave the cartridge capacitors on. Is there enough treble now?
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Old 23rd March 2013, 10:38 PM   #1154
neskor is offline neskor  Serbia
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yes
should I put capacitors inside preamp, directly on tube grid?
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Old 23rd March 2013, 10:41 PM   #1155
Salas is offline Salas  Greece
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Yes and use good ones. Is there sudden change of sound when you power on/off like before? Any photo of the spare parts simple build?
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Old 23rd March 2013, 10:45 PM   #1156
neskor is offline neskor  Serbia
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no sound change now
no pictures made, one power supply is from previous preamp boxed and other is just two capacitors, one resistor and gretz
do you know what causes that -0,4V on grid?
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Old 23rd March 2013, 10:49 PM   #1157
Salas is offline Salas  Greece
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Grid current probably. AC couple with the 47K still on grid side when you will move the caps inside the box. What TT and cartridge on use? Listen to some records and tell us if all is OK for smaller sound details too. What was the previous preamp?
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Old 23rd March 2013, 10:55 PM   #1158
neskor is offline neskor  Serbia
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Righ now AT 95E, other catridge Denon DL103 with Cinemag SUT.
Previous preamp is some DIY LCR with EF80 Srpp
Turntable right now is old Dual idler drive.
Sound is now soooooo much better.
Thanks Salas for this midnight buffet ))
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Old 23rd March 2013, 10:57 PM   #1159
Salas is offline Salas  Greece
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No worries, enjoy.
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Old 24th March 2013, 02:45 PM   #1160
Salas is offline Salas  Greece
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Default Grid current situations

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.
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