Tube DIY Asylum

Posted by kevinkr (R) on June 30, 2008 at 13:52:35

In Reply to: RE: The 300b is one of the most linear amplifying devices ever invented posted by Caucasian Blackplate on June 29, 2008 at 20:38:00

I've done this with 6SN7/7N7 where an MQ EXO-001 choke provided the inductance (100H, 900 ohm DCR) This ends up working very much like a mu-follower without the additional caps and resistors being required. Even more inductance would not hurt as long as the dcr can be kept in a convenient range to derive the required bias for the upper tube.

Gain was slightly under 26dB with this configuration, about as good as it gets with a 6SN7.

While I do not have the numbers available both linearity and available swing on a 400V supply were quite good. Used this to drive a 2A3 direct coupled. Swung 100Vpp with no difficulty at all.

Comments on linearity of the 300B, testing at 5% THD was a convention at the time as this was generally considered the onset level of objectionable distortion. (How things change) My 300B amplifier has about 5% at 10Wrms, but at 1W it is well into respectable solid state territory at ~0.15 % or less depending on OP and tube brand. The JJs I use are quite good, but date from 2001 - current ones may differ significantly.

My old 300B PP amplifier was 25Wrms at 1% thd, predominantly 3rd harmonic.

Edits: 06/30/08

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