[Amps] Grounded B+ amplifier
r at somis.org
Fri Feb 27 03:40:22 EST 2004
>Years ago I recall reading about an amplifier where the B+ supply was
>grounded and the cathode was "hot" with the B- supply. I'm wondering if
>that might have some advantages for high power amplifiers.
>Specifically, the pi-network output could be simplified by eliminating
>the plate choke. Here's how the output would look:
** Such a circuit is high-pass, although it does eliminate any RFC in
the anode circuit. For a low-pass configuration, a RFC would be needed
to keep the anode at DC-ground potential.
>Believe it or not, this is a pi-network output circuit, just redrawn.
>The same component values would be used as with a conventional circuit.
>I see some drawbacks:
>1. The filament transformer would have to be insulated for the full
>plate supply voltage.
>2. Likewise, the grid current meter would be at the full plate supply
>and would have to be carefully insulated from the chassis. Metal case
>meters would not be a good idea.
** Insulating the entire meter and metering circuit would probably be
needed, even with plastic meters.
>In fact, everything in the
>cathode/grid circuit would have to be well insulated, such as the zener.
** At this power level, my guess is that there is no cathode zener
because the amplifier would probably be AB1 grid-driven -- i.e., 8973, et
>3. Switching between standby and operate modes would require a
>well-insulated relay, perhaps a vacuum type or reed relay.
** Switching could also be done electronically with an optoisolator -
such as in:
>4. The load capacitor would have it's frame grounded as always, but the
>tune cap would have to be insulated on both sides.
** Good point. And the Tune-C bypass would be required to carry the
full circulating current in the tan. For an 8973 using a Q of 10, the
bypass would need to be able carry c. 700A-rms. This is probably why the
low-pass configuration tank is typically used on elevated-cathode
>And there may be others, but eliminating the plate choke with it's nasty
>resonances would be a big plus.
** Indeed, however, this problem can be addressed by bandswitching two
HV-RFCs - or even by parking the resonances in unused spaces of the
>While it appears at first glance that
>the coupling capacitor is gone, it's function is actually shifted to the
>B- connection at the cathode.
** Good point. However, the cathode must be well bypassed to circuit
common in a cold-cathode grounded-screen configuration. Thus, there are
tradeoffs with both the hot-cathode and the cold-cathode circuits.
>Comments are welcome.
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