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In Reply to: RE: Do i need to modify this driver circuit for PSE KT88 posted by DAK on August 27, 2014 at 15:30:38
Hi DAK,
The line stage circuit you linked to, is very similar to Mr. Ciuffoli's input/drive circuit. The octal tubes you want to use are roughly similar to the 9-pin minis used in the EL34 PSE amp. So yes, my previous mod suggestions still stand, you just might have to adjust the 6SN7's cathode R a bit to fine tune the tube's bias point, but that's about it (perhaps less than 1K up or down adjustment in resistance). Also note that in the EL34 PSE amp the finals' grid leaks are at 270K and not 470K. Of course he's using fixed bias on the output stage, but even if you go cathode bias I'd still keep this same value for the grid leaks with a .22uf coupling cap. Hope this helps.
Follow Ups:
That is a relief! I am feel a bit more comfortable pursuing this knowing that a more experienced diy'er has seen nothing bad happening with using the circuit with minor changes.
I had a question concerning the B+ for the 5814 plates in Andreas' circuit. In it he has full B+ of 430v sitting on the 5814 plates. In my implementation using 6sn7 I would also follow that ? cheers, Dak
If you were to do my initially suggested bias point, you would use a dropping resistor with a decoupling cap to ground to get your voltage at the plate load resistor of the input stage to about 330V. This additional PSU rail will offer better filtering of the 120Hz ripple.
If you want to go unfiltered and use all of the available B+, then you will need to use a larger plate load, such that the higher voltage drop across it, will get you back to the original ~125V at the plate of the 6SL7. From a quick look, it appears that a 270K resistor might get you very close to that target. I will have to take a better look though. This larger resistor will have the advantage of less distortion and better gain at the expense of more hum. You could also split the difference and use a plate load midway these two values (say 220K) and still use an additional psu filter stage for better isolation. I will have to work out that new bias point though.
Had another look, you could use your octals with the same resistors as used in the EL34 PSE, by making two revisions. Change the cathode resistor of the input tube to 1.5K and set the B+ voltage feeding both the input and driver tubes to 380V. This will bias the 6SL7 at around -1.5V, with just over 1mA of current going through it and about 150V at the plate. With the 22K at its cathode, the 6SN7 will be biased at around -7V with plenty of room for swing at ~40V RMS. Use PSUD II to figure out the dropping resistor to set your your B1+ to 380V.
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