Bill Turner wrote:
On Sun, 7 Dec 2003 07:24:04 -0500, "John Walton"
wrote:
or an LM317HVT with PASS transistor
The original question was about a zener diode, ie a shunt regulator. I
wasn't aware that a 317 plus a transistor could be configured to emulate
a zener... can it?
The TL431 certainly can.
_________________________________________________ ________
If you're going to use such things, you must protect them against
inadvertent arcs. One good arc and they will be history.
Normally an arc from the B+ supply of a grounded-grid triode is no
threat to the cathode bias device. Arcs inside the tube will ground out
to the grid, and external arcs to the chassis; and then the current
flows back to B-minus. The cathode and the bias device are not part of
that current path, so the bias device is not at risk.
The exception is if an internal arc is severe enough to burn right
though the grid (I've seen that in a mesh-grid tube) or if the current
is high enough to drive the local grid potential positive. Then some of
the arc current - perhaps tens of amps, peak - will flow down through
the cathode. In such extreme cases, no bias device is likely to
survive... at least, not without some additional protection.
That's why, whatever bias device you use, an 80-cent varistor connected
from cathode to chassis is a very good investment.
A big, fat
zener on the other hand, is relatively immune to such abuse.
"Relatively" in this context is hard to judge. In most cases, the bias
device survives because it wasn't actually under threat - see above. If
a device does fail, it's only guesswork to say how a different bias
device would have fared in the same amplifier.
A more reliable picture emerges from a larger number of amplifiers.
There are more than 350 Triode Boards out in the field, with tubes
ranging from the 2C39 to the 3CXP5000, and they all use basically the
same TL431/TIP147 circuit protected by a varistor. Between them, these
boards have seen a lot of arcs, and there's no indication that the bias
circuit is notably fragile. Certainly there are occasional failures -
but in those cases it seems likely that a big, fat zener wouldn't have
survived either.
--
73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB)
Editor, 'The VHF/UHF DX Book'
http://www.ifwtech.co.uk/g3sek