The schematic as posted has a mistake. The bottom 680K resistor doesn't go to the plate of the triode - it goes to the output ie the cathode of the triode.
Shoog
Shoog
This preamp has been running for about two years now. Still sounds great and will be the keeper for me. The only thing left to do would be go differential.
As for R14, its simply taking the load off the Mosfet and its absolute value is not at all critical. I would shoot at 40K.
I need to open it all up and do a full trace to check the values, but won't be doing that any time soon. The use of the Pentode CCS means that the whole circuit is extremely forgiving and will adjust to just about any operating conditions.
Shoog
As for R14, its simply taking the load off the Mosfet and its absolute value is not at all critical. I would shoot at 40K.
I need to open it all up and do a full trace to check the values, but won't be doing that any time soon. The use of the Pentode CCS means that the whole circuit is extremely forgiving and will adjust to just about any operating conditions.
Shoog
I would be inclined to increase R4 a bit as I seem to remember that the pentode takes 200V of the stack which is not necessary. Still it has been operating like that for two years and has not been an issue.
Shoog
Shoog
I definitely going to try this one.
B+ is about 400 volt ?
Does it sound better than the ecc88 FVP5 clone you made in the past ?
B+ is about 400 volt ?
Does it sound better than the ecc88 FVP5 clone you made in the past ?
I am running both at the moment. The sound is very similar and it would be quite hard to choose between them. I think the ECF80 version has more punch and authority in the bass region and so on balance I think it has the edge.
The voltage is indeed about 400V, but I would say it would be happy at anything from 300-400V without any circuit adjustment - possible it might need very slight tweaking to the value of R4 to ensure that the Pentode has enough volts to work with. Overall the ECC88 version would have much more sensitivity to component values, and the balance of the preamp changed quite significantly with all adjustments. The ECF80 version self adjusts with minimal change in tone.
I had it up against a friends Aikido and it was a dead heat.
Shoog
The voltage is indeed about 400V, but I would say it would be happy at anything from 300-400V without any circuit adjustment - possible it might need very slight tweaking to the value of R4 to ensure that the Pentode has enough volts to work with. Overall the ECC88 version would have much more sensitivity to component values, and the balance of the preamp changed quite significantly with all adjustments. The ECF80 version self adjusts with minimal change in tone.
I had it up against a friends Aikido and it was a dead heat.
Shoog
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hi, i bought this 9AU7's for a dollar each RCA cleartops and Tung-sols at Rogalski's.....i run them cascaded ala PV12 at B+ of 300volts and currents of 5mA plate.....sounds good so far....
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