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I have recently built a Vox AC15 clone and I am having some problems with the EF86 preamp stage.I did a circuit pop test and confirmed hearing the proper disturbance when checking the plates and grids.Until I got to the EF86! I am getting no guitar signal,but I do hear hiss from the speaker which gets louder as I turn up the volume.My voltage coming of of pin three from the EZ81 is 343. phase inverter pin voltages are pin 1:244 pin 2:40,pin 3:61, pin 4:3.15, pin 5: 3.15, pin 6:239, pin 7:34 pin 8:53, pin 9: 3.14 but when I measure voltage at pin 1 of the EF86 I am supposed to be getting around 95 volts.Instead I am getting 35 and on pin 3 I am getting .585 volts. and pin 6 I am supposed to be getting around 95 volts but I am getting Zero volts here. If I measure volts at the plate resistor with the EF86 removed I get a reading of 343v.Also with the tube removed the resistance of the plate resistor is 219Kohms. When I put the tube in its socket and measure resistance I get a measurement of INF and Zero volts.Does this point towards a possible bad tube?I have double checked all my soldering connections and lead dress and cant figure out were to go next,short of ordering another tube!!Once I do get a signal what would be the best way to tweak the pre-amp circuit and bring down some of the voltages.I seem tho think that they are maybe to high. maybe this is whats wrong with circuit?
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A schematic would help figure out just what you are talking about.
Either the socket (circuit) is wired incorrect or the tube is bad.
That is my take.
343Vdc through a 22K R & then through a a 220K R, plug the tube in and it drops to zero volts.
That is severe to say the least. -
Thanks for your help, I am a newbie with trying to link schematics.I will do some more research on this and hopefully figure out how to scan and post my schematic.I can tell you it is a 1959 Vox Ac15 shematic if that helps. Yes it is severe!! I see no arcing in the tube,or anything obvious for that matter.I have checked and double checked the board and under for shorts etc.I would think that that would show up even if the tube was not installed.I wish I had another pentode to try,but this is my first ef86 build and all I have are 12ax7s. If there is that much of a voltage drop I would guess that could mean a direct short in the tube? I have searched the forum all day,learned some ways to tailor the ef86 voltage diff. between the plate and screen. I just cant think of any procedure of things to check like a flow chart of sorts to logically come to a conclusion.Comment
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After, double-checking the circuit I think that the ef86 tube is bad and dropping all the voltage.Got the new tube a few days ago.Its a jj ef806s,Amp works great!!! nice and clean up to about noon on the volume.Great sounding tube!! not microphonic at all,lots of chime and super loud.I did however put a 300 ohm resistor to ground and the center tap.Now I am getting 309volts at the filter caps.Amp has JJ ef806,GT ecc83,2x nos Sylvania El84,and a JJ ez81.This amp just gets better everytime I play it,There should be no problem using it live because it starts to break up nicely at high volume!!Comment
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Glad to hear that you got it fixed.Comment
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