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Re: Build ideas for Allen Wright PP-1C Push Pull Power Amp

Posted by Ray Moth (A) on April 16, 2007 at 02:38:16

In Reply to: Re: Build ideas for Allen Wright PP-1C Push Pull Power Amp posted by Burney on April 15, 2007 at 23:56:12:

[RANT] The 12AX7 is a VERY poor choice for ANY purpose WHATSOEVER in a Hi-Fi amp. [/RANT]
There, that feels better! :)

In this particular instance, the input stage comprises two cascodes, connected as a long-tail pair splitter. Cascodes work best with high gm triodes and the 12AX7 does not meet that criterion. Not many tubes will do a good job here, for that matter, because you also need a tube that will operate well at quite a low plate-cathode voltage. The 6DJ8 (family) is one of the few that are suitable (not a coincidence - it was designed for RF cascode operation).

A pentode might do very well (I'd love to try it!), since a cascode can be considered a way of making a pentode out of two triodes. The cascode doesn't suffer from partition noise like a pentode does, but it poses the problem of having to share B+ between top and bottom tubes and usually doesn't have quite as much gain as a pentode.

One problem with using pentodes could be conflict between the current the pentodes want to draw, under influence of their screen grids, and the current for which the CCS is set up. A way around this could be to experiment first with a shared cathode resistor, find out what cathode current you get, then set up a CCS drawing the same current to replace the resistor.

Obviously a pentode solution makes some things easier, like not having to share B+ between two tubes in series, and not having to worry about heater-cathode voltage limits for the upper tube of a cascode. Allen's design and choice of tubes solves both of these problems comprehensively.

A big cathode bypass cap is something Allen found to work well. Personally, I'd prefer to avoid it by using fixed bias, as you suggest.

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