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Old 22nd November 2006, 05:54 PM   #1
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Default Anyone tried this method for OTL?

I came upon an interesting web page last night while surfing. Just curious if anyone has tried this guys configuration.

http://members.aol.com/sbench102/inverted.html
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Old 22nd November 2006, 08:43 PM   #2
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Old 23rd November 2006, 12:35 AM   #3
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I experimented with this idea back when it was written. As explained in the article the drive voltage requirement is huge. The drive voltage requirement is the output voltage MULTIPLIED by the Mu of the output tube. Think voltage regulator triodes with a low Mu like the previously mentioned 6080/6AS7 (I have a few hundred myself) or 6336. There is a lot of capacitance when driving a bunch of tubes in parallel.

This idea has merit, and I did get some decent sound out of a SE OTL (maybe 1/2 watt) using a couple of 6AS7 tubes and a 2 henry 2 amp choke. I also fried a couple of tubes. There was no sign of imminent doom, no glowing grid wires, the current just kept dropping, and the sound got distorted, and then faded out. The (used) tubes came from a big box full that I found in my warehouse, and may have been defective. I made a note to experiment further, but never found the time. That was several years ago.
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Old 23rd November 2006, 11:46 AM   #4
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I was briefly playing with this about 30 years ago when I found an article in an old electronic magazine "Radio & Television" describing this way of connecting triodes.

My experience, (as I recall it) was that it works as a method for giving low output impedance but it is difficult o use this method for giving substantial output power, (due to limitation in the current a grid can carry). In addition we have the problem with low gain which requires a very special driver.

IMHO the generic problem with OTL amplifiers is not to reach low values of output impedance but instead to design an amplifier that gives high enough current to give substantial output power.

To give 25W in 8 ohm the amplifier must be able to source a peak current of 2.5A, that would take quite many grids using the inverted mode even when using large power tubes but can relatively easy be accomplished by just 2 6C33C tubes in a more "normal" OTL circuit.

Regards Hans
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