I would advise choosing voltage to match the availability of 400V and 450V capacitors. If you go higher you will need capacitors in serial with balancing resistors - not a problem but bear it in mind. Poly caps are easy to get at 630V and 1kV but silver mica are 250V etc.
400V electros are common because they are used in switching power supplies.
Thank you; I have a good supply of 300V caps and I often put them in series and use 220K resistors to force them to behave! I wonder if it would be feasible to use constant current feed with these tubes.
I have always wondered at the reason for this distinction-is it simply that the screen tends to draw less current when it is connected directly to the anode?
no - its because in pentode mode the anode voltage can swing BELOW the stabilised screen voltage at which point the screen effectively becomes the anode and dies a horrible death...
I see, but does the larger screen resistor not mitigate against this somewhat or is this factored in when specifying the maximum screen voltage?
Grungeman91,
Assuming a stereoblock HIFI amp, build with PP 7591s. Unlike all the the other types mentioned, the 7591 is easy to drive. Small signal circuitry that works with "12" W. types, such as the EL84 and 6V6, also works with the 7591. Legendary brands, like Fisher and Scott, took advantage of that fact. You can too. A UL mode PP pair will yield an honest 30 W. The 7591 is in production.
The 7591 data sheet is here.
Assuming a stereoblock HIFI amp, build with PP 7591s. Unlike all the the other types mentioned, the 7591 is easy to drive. Small signal circuitry that works with "12" W. types, such as the EL84 and 6V6, also works with the 7591. Legendary brands, like Fisher and Scott, took advantage of that fact. You can too. A UL mode PP pair will yield an honest 30 W. The 7591 is in production.
The 7591 data sheet is here.
The larger is resistance of screen grid supply, the higher are distortions.
By the way, Russian curves for GU-50 don't tell the whole story... They look as if were cut intentionally, the tube does not look optimally designed and built!
How come?!
In reality, it can work much better when abused. If to imagine the rest of curves that had been cut off by scissors, a tube is very nice for class A2 amp!
Also, it is nice as well for class B amp: if to connect all grids together it is a class B push-pull triode, one pair with 1200V on anode and zero volt grid can give 180W of output power. However, about 30K P-P transformer is needed, but for speech purposes it is not a problem: 300Hz-3400Hz would be more than enough to amplify military and police commands...
LS-50 was designed by Telefunken to sustain "normal" abuses by German police and army, that did not hesitate to abuse anything, nor anybody.
http://www.scottbecker.net/tube/sheets/043/l/LS50.pdf
By the way, Russian curves for GU-50 don't tell the whole story... They look as if were cut intentionally, the tube does not look optimally designed and built!
How come?!
In reality, it can work much better when abused. If to imagine the rest of curves that had been cut off by scissors, a tube is very nice for class A2 amp!
Also, it is nice as well for class B amp: if to connect all grids together it is a class B push-pull triode, one pair with 1200V on anode and zero volt grid can give 180W of output power. However, about 30K P-P transformer is needed, but for speech purposes it is not a problem: 300Hz-3400Hz would be more than enough to amplify military and police commands...
LS-50 was designed by Telefunken to sustain "normal" abuses by German police and army, that did not hesitate to abuse anything, nor anybody.
http://www.scottbecker.net/tube/sheets/043/l/LS50.pdf
Thank you Eli. I'm building a hi-fi audio amp for this project. I'm new to the subject in general. I have been reading the radio designers handbook and valve amplifiers by morgan jones, so I am very much in the learning phases. Your suggestion has been the best yet.
Interesting and individual perspective as usual! I have never even considered building an A2 amp but it would be a way of bringing back from the dead some inefficient,but reasonable quality speakers that are lying around. I suppose then that GU-50 would be as good as any for this whilst keeping the voltages not too frightening.
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