I went to the site mentioned a few days ago with all the amp pictures,
http://www.angenent.org/W7IUV/amp-pix.htm and noted that most of these put
the vacuum variables horizontal with the tuning shafts straight out the
front panel. With all the talk about stray inductance and grounding, I'd
like to ask why they all do this?
I saw an amp by an anonymous fellow that seemed like a much better design.
This baby used four 8877's in parallel grounded grid and two vacuum
variables mounted vertically to the same chassis plate, right next to the
tubes. The plate inductor jumped across the two standup vacuum caps. The
tuning shafts went into the bottom chassis, through right angle drives and
out to the front panel.
The key advantage of this approach is excellent ground return paths with the
tradeoff being two right angle drives. This eliminated any seams in the
ground returns.
In general, it appears that amp builders don't worry much about seams. In
looking over the pix, I saw tubes mounted on a plate that then mounted on
the chassis with a few screws or caps mounted on the front panel with the
front panel vaguely connected to the chassis, etc, etc.
I could imagine all kinds of problems with feedback caused by loose screws
or corrosion buildup in the multiple connections between the grounded end of
the caps and the grid of the tube. Just bump the case, jiggle the feeble
connections through the chassis, the circulating currents find other routes
around the chassis (like the input circuits) and _bang_.
BTW, the amp cruised at 4 KV and 4A. The meter attached to a 10 KW Bird
element slid smoothly up to the right and went tink on the pin. Very
impressive, very clean design.
jeff
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