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Old 17th April 2011, 05:47 PM   #1
r2k is offline r2k  Estonia
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Default first experience with retro electronics

mkay so i have no prior experience with vacuum tube electronics. but i do have a fairly decent grasp of modern electronics(make my living with it).some time ago i got my hands on a boxfull of old vacuum tubes. so i figured, lets make something out of it - some arcane knowledge never hurts. so i asked a bit from old time engineers, read around the internet, looked at bunch of schematics and experimented a bit so now i think i have enough to put together a simple mono amp. i selected from box two el36 tubes for output and two el82 tubes for input. salvaged together some more materials and hey ho i have a makings of a tube amp

picture
schematic

the principle is pretty simple. differential pair(2xEL82) driven by current mirror for input stage. capacitive coupling to output stage(2xEL26) the two out of phase output tubes are biased with regulated voltage insead of cathode resistors. transformer is used to match the output to 8ohm speaker and feedback from the speaker leads is used to control the gain. the power supply circuitry is omitted from schematic but the anode supply, biasing voltage and filament supply will all be regulated. filament supply will be periodically switched with an hbridge. should help with tube longlivety(i think i'll use long period 555 circuit to drive the hbridge)

what you see on the pic is still in very early stages, there will be sockets and proper anode clamps, one tranformer will be replaced for a more powerful one(EL36 filaments alone take 1.25A)

so.. from those who actually know a thing or two about retro electronics. thoughts ideas improvements fixes about the design. pitfalls to avoid reccommendations to keep in mind. comments on the work ive done so far etc. lets hear it
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Old 17th April 2011, 07:33 PM   #2
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There's no need to switch the polarity of the heater-volts--wont make 'em last any longer...

PL/EL36 valves are Indirectly heated, therefore do not suffer from the voltage variation along cathodes, as directly heated types do...

Indirectly Heated are also known as 'Uni-Potential'....

Indirectly heated valves will run out of emission long before the heater expires through old age!
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Old 17th April 2011, 07:36 PM   #3
r2k is offline r2k  Estonia
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ah, that simplifies the design a bit then. just that i heard that heaters last longer when the polarity is switched. guess the guy forgot to mething that this is only relevant for directly heated tubes
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