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Old 24th February 2013, 07:00 PM   #1141
Salas is online now Salas  Greece
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Noise.
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Old 24th February 2013, 07:14 PM   #1142
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Perfect.

So if I get it correctly, grounding the lower tube cathode, bias the grid at 0 volts so the triode is fully open and working at max current. You dose the current with the upper ccs so there is no overdrive even if the signal goes positive.
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Old 22nd March 2013, 01:10 PM   #1143
Salas is online now Salas  Greece
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Quote:
Originally Posted by cygnus x1 View Post
I was never a big fan of 6SN7 (but I have not heard a 1578melz)
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.
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Old 23rd March 2013, 08:53 PM   #1144
neskor is offline neskor  Serbia
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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?
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Old 23rd March 2013, 09:22 PM   #1145
Salas is online now Salas  Greece
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Not normal. What types of PSU? Can you scope the rail during those on/off cycles?
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Old 23rd March 2013, 09:26 PM   #1146
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Unfortunately I don't have oscilloscope.
PSU is just plain, build in 5 min. gretz, capacitors and resistor, no Salas HV
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Old 23rd March 2013, 09:34 PM   #1147
Salas is online now Salas  Greece
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Can you measure the DC voltages of all tubes from plate to ground after 5 minutes you turn it on? So we see how it biases first. Do you use some SUT and MC cartridge? Having any hum?
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Old 23rd March 2013, 09:43 PM   #1148
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No hum at all
Right now no MC or SUT, just playing ordinary MM catridge.
All voltages are good, checked several times.
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Old 23rd March 2013, 09:51 PM   #1149
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Maybe you got some DC offset on the input? What tube you use on input?
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Old 23rd March 2013, 10:06 PM   #1150
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input tube is 6n2p-ev
checked again, so voltages are:
from top 6n2p, anode is on 270V, cathode 187V, anode bottom 6n2p is 77V, grid on - 0.4V.
from top 6sn7, anode is on 310V, cathode 185V, anode of bottom 6sn7 is 78V and cathode is 1,8V
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