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EL509 OTL for high impedance speaker array
EL509 OTL for high impedance speaker array
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Old 5th December 2017, 08:44 AM   #1
Mikael Abdellah is offline Mikael Abdellah  Sweden
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Default EL509 OTL for high impedance speaker array

I think I would like to try and build a OTL amp based on EL509, but I would like to run a single tube and construct a large array of high impedance speakers to achieve a suitable impedance for the output tube without using parallel tubes. What would be a good impedance for a EL509 or similar tube? I was thinking just above 3kOhm. How do I calculate and decide working parameters? Desired frequency range is roughly 250Hz-16kHz so this is no attempt to build the ultimate OTL amplifier capable of driving everything, I just want to satisfy my curiosity and build another tube amp
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Old 5th December 2017, 03:32 PM   #2
artosalo is offline artosalo  Finland
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As a Circlotron some 500 to 1k would be fine.
I have built - not an OTL - but Circlotron(s) with 1k load and two JJ EL509.
With 480 V +Ub the output is 100 W with less than 1% THD and no GNFB.
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Old 6th December 2017, 07:33 AM   #3
Mikael Abdellah is offline Mikael Abdellah  Sweden
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Thank you artosalo. I think I will keep it simple and just use a single tube. The circlotron looked a bit more advanced than I was aiming for. Also, I don't need that much power. I would be happy with a bit over 10W which shouldn't be a problem with a EL509.
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Old 6th December 2017, 09:23 AM   #4
tubetvr is online now tubetvr
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I did my first OTL test with the circuit described by Philips for EL86, I didn't use EL 86 but some other tubes that I had including PL509, (0.3A series heater version of EL509). The result was so good that I continued to develop OTLs. This schematic is simple and use 2 tubes but it is not 100% balanced and give a lot of 2nd order distortion but sound is quite good!

Click the image to open in full size.

Components need to be adjusted for other tubes than EL86
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