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Thx for the schematic ... a few suggestions for you ...

Posted by Naz (A) on March 12, 2007 at 16:35:34

In Reply to: "...do away with a complete gain stage...". The extra jack coupled to the 2nd... posted by jeffreybehr on March 12, 2007 at 10:30:28:

First, it's not all that straight forward to eliminate the first stage because the second is referenced to neg 133V instead of ground. The easiest option here is to IT couple from a new input to the second stage but the downside is that you need very good iron and a place to put the trannies. I may be in the minority here but it wouldn’t be my first preference.

Opt 2 is to Ref stage 2 to ground but then you'll be marginal on linear swing to drive the 845. This can be easiest fixed by increasing the B+ to stage 2 (not a lot of headroom here though with your supply, maybe 20V) in tandem with a favourite of mine, changing the plate R of the stage from a 1K to an inexpensive choke measuring around the same resistance. If you have difficulty finding one even a Hammond 154E will do but because it measures 1.666K you'd best change the cathode R to 1K68 as well.

There is a third option which is a much bigger job but can yield great gains. You could convert the driver to a White CF which has slightly less than unity gain and keep the first stage. Advantages are extremely low OP impedance to drive the 845 and the ability to drive the grid positive yielding around 40W for greater headroom. You'll also eliminate one coupling cap ... always a good thing. The downside is having to increase the neg supply voltage to the driver to at least -360V. You'd also need to reduce the B+ to say 120V or so. Well worth the trouble IMHO.

Cheers,
Naz

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