source amplifier, as a gate-source-drain amplifier or GSD  for short. The advantage to such a system is that, like the periodic table of elements, the holes are readily apparent, which prompts us to fill in the blanks.

impedance of about 10 k and a mu of about 8 when used in this way.

Simon B.

    Thanks for the information. Battery tubes are definitely worth looking into. There a whole group of vacuum tubes designed for use in car radios that use the car's battery to supply both the heater voltage and the plate voltage! The 12DS7 and the 12DZ6 are capable of putting out a huge amount of current with only 10 volts on their plates.
    The reason I avoided these tubes was to keep the tube sourcing easy. Which was something of a mistake, as special tasks require special tubes.

Subject: 6N3P preamp,
do you have some advice?
   Thanks for a great webzine and great software! Recently I bought your Tube CAD program from Antique Electronic Supply and I have been playing with it for a while. From a sale I acquired several tubes from the former USSR including the 6N3P and I would like to design a simple line-level preamp with these. I was thinking of a constant current ground cathode amplifier and I have come up with the following part values (see schematic on next page). Two prototypes which I have made seem to perform well (when tested with sine and square waves and looking at the output on a scope). Even a 100 kHz square wave (4 Vpp output) looks good. But I have not yet performed listening tests as the dedicated PSU is not finished. My 6N3P tubes seem to have tight tolerances. They show almost identical measurements and they aren't microphonic. Maybe they were meant for the military. Current draw of the preamp is about 4 mA/channel. Measured voltage gain 25.3 (channel 1) and 25.8 (channel 2).
     Do you have any suggestions or advice? Should I bypass the electrolytic cap with polystyrenes, for example? Forgive me for bothering you, I was just wondering if I had

Subject: portable headphone amplifier
    Thank you for an excellent publication, which is building into a powerful resource for all those who are interested in valves (which of course is
what we call them over here!).
    Your article on a 'portable' headphone amplifier was interesting, particularly as you decided not to use battery valves.  There are of course many battery valves available at cheap prices, and they have the advantage of low LT consumption (25 mA at 1.4 V for the DX96 series) and equally low HT consumption (a few mA at 90 V). The DL92/94 would make an excellent battery headphone amplifier, and the HT consumption would be very low if they were used in push pull biasing largely into class B.
     But even in SE class A the maximum consumption would be 10 mA.   I have recently made up a battery pack consisting of a single D cell and 10 PP3 batteries for this purpose.   You can get cheap alkaline PP3s from Walgreen if you live in the USA.   Forget Energizers etc they are far too expensive.
    There is also the DL33/35 octal output valves which look excellent and give good performance. All battery pentodes can be strapped as triodes and make good AF amplifiers. The DF91 (1T4) has an anode

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