> I can always experiment with grid bias settings, anyhow.
** In AB1, one can not adjust the grid bias to the point where grid
current flows -- i. e., the grid must not be allowed to go positive
yes, that is in fact part of the definition of AB1 (180<x<360 conduction angle
but no grid current).
** My Eimac spec sheet says 600V max for Class AB1. In AB1 one needs
to use high screen potential in order to utilize the full emission
ok, about what grid bias would be required at 600v screen, 3kv anode?
** I have a paper copy of curves for E-screen = 350v. With 0V peak
on the grid, the anode draws c. 0.11a-peak, so for an AB1 4-125A with
350V on the screen, the max average anode-I would be c. 38mA - which is
hardly good news. Since I-anode is a function of E-screen ^1.5. with
600V on the screen, the peak I-anode would be (600/350)^1.5 x 0.11a =
225mA-peak which is still only about 75mA average.
however, with 3kv anode, the average per-tube anode current has to be kept
below 41 ma to avoid exceeding the 125 watt plate dissipation rating...
.Even 600V on the screen would
allow using only c. 1/3 of the max rated emission potential of 225mA
agreed, but this amp appears to be dissipation limited, not emission limited,
since I will be using it in AM (100% duty cycle).
>Another way to go would be to use the 4-125As in g-g.
probably requires too much driving power (I'm assuming I only have 10w carrier
to drive with).
> figure at rated 500 watts dissipation (125 per tube), at 33%
> efficiency worst case would give me 250 watts carrier in Class A,
> which should be plenty (about +14 db)...
>
> And with no signal, or pure carrier, a class A amp will still
> dissipate the same 500 watts - so no heat-related tuneup problems
> unless I want to go above 250 watts out, right?
** The characteristic curves suggest otherwise -- at least for AB1.
please explain further?
** pushing ratings is like unto pushing greenbacks into the crapper.
agreed... 4-125A's are cheap used, but they aren't THAT cheap.
-Charles
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