Ulf Tjerneld wrote:
>Hi all,
>
>Thank you for all great replies to my question!
>
>I understand now it is enough with only a couple of ohms for the B- to
>chassis resistor. However, contrary to this I came across a couple of
>articles written by G3SEK in Radcom about HV PSU design (Jan-Mar
>2009). Excellent articles that really explains a lot to me. But, in
>his example he actually uses a 100 ohm from B- to chassis. The
>resistor is also in parallel with 3 big diodes. He explains that they
>serve a a return path to ground for surge currents and that the 100
>ohm resistor is there to act as a backup for the diodes. But he does
>not mention anything about how the metering is accomplished in this
>case; with a 100 ohm you would have maybe 25-35 volts over the
>resistor, so a meter would have to be shunted. And when shunted we
>would bring down the resistance to a low value again which in its turn
>would make it unnecessary with such a high value in the first place.
>At least according to my logic!
>
>Anybody likes to comment on that?
My pleasure :-)
Metering circuits were outside the scope of the Radcom series, which was
only about HV power supplies. My approach to metering is fully explained
in the Triode Board User Manual:
http://www.ifwtech.co.uk/g3sek/boards/triode/triode-manual.pdf
The aim is that even very large current surges shall not cause any
hazard to either the operator or damage to the amplifier. Large diodes
divert large current surges away from the anode and grid current meters,
and the voltage levels are carefully chosen so that even a huge current
surge shouldn't make the meter needles slam violently against the stop.
The resistor from chassis to B-minus is intended *only* as a backup for
the safety diodes. It only comes into play if the diodes have failed
open-circuit (unlikely) or - much more likely - if the diodes are not
connected at all, eg if the HV supply is energized as a separate module
for testing. Then the resistor forms the last line of defence to prevent
the B-minus rail from floating far away from ground.
This resistor needs to be there, but it's only ever intended as a
backup; its value doesn't need to be a low enough to allow the HV supply
to deliver normal current. There's also another problem about making the
value too low: it creates a sneak current path directly from chassis to
B-minus, bypassing the meters. This wouldn't be noticed in a
grounded-cathode tetrode amplifier, but with a grounded-grid triode the
sneak current path will make the grid current meter read slightly
negative when the tube is drawing zero-signal anode current. To avoid
that problem, the backup resistor can safely be increased to 100-1000
ohms.
In the Triode Board schematic I suggested 1000 ohms (for reasons too
detailed to go into here). The Radcom article wasn't assuming anything
at all about metering circuits, so I made the value 100 ohms and backed
that up with a longer string of diodes.
--
73 from Ian GM3SEK
http://www.ifwtech.co.uk/g3sek
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