Interesting what I read today:
"The μ-follower proved to be an excellent test bed for determining the irreducible distortion of a valve.....Surprisingly, given its good reputation, the 76 did not measure particularly well...One triode was significantly better than the others...this valve is the 6SN7, 12SN7, 7N7, 14N7 or 6J5."
VALVE AMPLIFIERS FOURTH EDITION - MORGAN JONES (page 618).
So there we are, it wasn't a bad choice for Itch's 2nd μ stage.
Today I build Valve Itch phono with spare parts, just for fun.
But unfortunately sound is not really good, like covered with blanket.
Strange thing happens when I switch off power supply, sound suddenly become clear and good. Same thing happen when I switch on power supply, first couple of second sound is clear but after that sound again become dull.
Tried different power supply but same thing
Any clue?
But unfortunately sound is not really good, like covered with blanket.
Strange thing happens when I switch off power supply, sound suddenly become clear and good. Same thing happen when I switch on power supply, first couple of second sound is clear but after that sound again become dull.
Tried different power supply but same thing
Any clue?
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?
Grid current situations
Some samples of 6N2PEV may show more or less grid current. Its specified as a 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 their 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.
Select 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.
Some samples of 6N2PEV may show more or less grid current. Its specified as a 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 their 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.
Select 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.
