Posted by PakProtector [ 64.12.116.134 ] on February 28, 2006 at 16:38:59:
In Reply to: Anyone use 6S45P with 12B4A concertina for PP 7591A or 6L6? posted by Norris Wilson on February 28, 2006 at 13:20:00:
Hey-Hey!!!,
The split load PI, or concertina circuit is a bit wasteful of B+ with respect to its output capability. The absolute max swing possible is 25% of B+, and in practice it is more like 20% of B+, per phase, peak to peak.
For a Class A amp, with its required lower B+, this can be an issue. The 7591 does make the best of this, requiring more like 10% of Class A B+ input to its grid. Even less if a lower g2/full pentode circuit with a large load a-a is employed( large numeric that is ).
Since my current choice of circuits requires a slightly different set of performances from its components, the split load doesn't get much playing time. Low plate and cathode loads would make it a bit less sensitive to load imbalance. Since my linestage 12B4 seems quite happy with 10k plate loads, 5k + 5K ought to be quite a good operating point. With enough B+ to make 30 mA of idle, it's too far from Class A for my current preferences.
I suspect that plate-to-grid NFB will be hard to implement around the finals with a concertina PI if you wish to keep things balanced.
Anyway, that's my take on it, remember that your amps are for your ears, so building to somebody else's opinion may not be the thing to do. There are reasons to do it, and there are reasons not to...I do hope to listen to Scott's amp, so that I can hear what he's been talking about.
cheers,
Douglas
[ GroupBuild Forum ] [ Help ]