I'm working on an old Philips EL6425 and would like some advice. This is my first PP amp and I haven't come across phase inverters before , and this one seems out of the ordinary. It is an LTP (I think) , but there is a negative voltage on the cathode resistor R27 (marked as -3 voltage ), around -40v . Also the second grid on pin2 is grounded without an AC coupling capacitor , and there is a diode and resistor connected to the input which is explained in the manual by "GR3 in the grid circuit of B5 improves the RC time of coupling capacitor C18 and leak resistor R26 so that when Grid 7 of B5 becomes too far negative this charge can leak away faster through GR3"
The diode GR3 has a negative voltage (marked -2) on it , which is about -3 volts.
I want to put a Master Volume control before the Phase Inverter , so my questions are ...can I do so while still leaving the existing PI as it is without interfering with the function of G3 ? Should I replace the PI altogether ? I'm trying to keep the amp fairly original where I can , but maybe it doesn't make a great deal of difference here , and having the MV in this position is probably more important to me . Any help much appreciated.
I want to put a Master Volume control before the Phase Inverter , so my questions are ...can I do so while still leaving the existing PI as it is without interfering with the function of G3 ? Should I replace the PI altogether ? I'm trying to keep the amp fairly original where I can , but maybe it doesn't make a great deal of difference here , and having the MV in this position is probably more important to me . Any help much appreciated.
That "phase splitter" is about the most common of the various circuits, along with the cathodyne single tube section circuit.
It's just a differential amplifier, with the signal applied at the first grid, and with the second grid at AC ground.
The cathodes go to a resistor, and then to a negative supply, approximating a current source.
A volume control would have to go at the input of the first EF86 tube. The volume control should be 250k or more.
It's just a differential amplifier, with the signal applied at the first grid, and with the second grid at AC ground.
The cathodes go to a resistor, and then to a negative supply, approximating a current source.
A volume control would have to go at the input of the first EF86 tube. The volume control should be 250k or more.
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Thanks for the speedy reply , Merlin's description of LTP says "The decoupling capacitor for the second grid (Cg2) must decouple all frequencies of interest." Is one not needed in this instance?
The capacitor isn't needed in this version, since the first grid is at DC ground potential. So the second tube grid can be
connected directly to 0VDC ground, without a decoupling network, and both grids will still be at exactly the same DC potential,
as is necessary. Then the decoupling capacitor is not needed.
This circuit is possible because the cathodes connect to a resistor that goes to a negative DC voltage.
The cathodes will then automatically bias the tubes at a suitable positive DC voltage for the circuit.
connected directly to 0VDC ground, without a decoupling network, and both grids will still be at exactly the same DC potential,
as is necessary. Then the decoupling capacitor is not needed.
This circuit is possible because the cathodes connect to a resistor that goes to a negative DC voltage.
The cathodes will then automatically bias the tubes at a suitable positive DC voltage for the circuit.
That has a ton of gain turning 8.5mV into 50v . Turning B5 into a concertina would lower the gain by 15x . This would improve S/N and make 150mV full power input signat.