Is a top quality class A operating amp possible with an UL84 electron tube

At your risk. Even 40V can be very dangerous. Follow all known safety precautions.


A top quality input transformer could be used for great sound passive stepping up of the signal voltage without the need of a extra preamp electron tube. The coupling capacitor is important as the grid wants to be undisturbed to assume it's DC potential formexamolemif used at space potential.


A few 12V batteries may be needed to get to 48V volts. Always use a suitable fuse in series with a battery for safety.


So 48V DC would power the heaters at pristine low noise quality.


The same 48V may  power the anode through an output transformer.


It may be a good experiment to see if the grid can be left open or floating or at space potential. If this fails then what high pull up resitor can be used from grid to cathode, cathode being to ground. If this fails then perhaps using a cathode resistor to reduce anode current by increasing negative bias. It is advised to bring up anode voltage slowly and observe anode current. Note: In most cases even with pull up bias the grid is more negative than cathode due to the fact that the cathode is loosing electrons by thermionic emmision to the electron cloud. Perhaps a good way to know what is happening is to monitor the anode current. Or connecting a pH meter or an electrometer to grid so that its potential wrt cathode is its bias can be measured by not being disturbed by the measuring device itself.


The schematic of this power amp may be a variation of the Pleiades V6 front end mic booster front end amp. Do not connect a high Megohm resitor from anode to grid unless you wish to play with much lower plate voltage such as 12V or 6V with a PL84, EL84 or other electron tubes.


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It should give a world class, state of the art sound.


It should be relatively simple to make.


We should be using electron tubes to reproduce nice music. Not electron tubes using us to reproduce music.


Further reading: Open grid vs pull up (Pleiades bias) or pull down bias - euroelectron


For even simpler and pristine sounding class A power amps (electrons flowing all the time) the Pleiades 2N3053 may be tried.


More further reading:


Electronic Valves, Fundamentals of Radio Valve Technique Deketh - Philips


Applied Electronics - T. S. Gray - MIT