Hi Ian
What is "safe" practice in a grounded screen tetrode amplifier where the
requirement is that the power supply negative rail is left "floating" ?
It always makes me nervous poking around a grounded screen amplifier when some
parts of the circuit is below chasis potential and the negative of the power
supply is left floating.
73
Craig
VK3HE
--- On Sat, 11/7/09, Ian White GM3SEK <gm3sek@ifwtech.co.uk> wrote:
> From: Ian White GM3SEK <gm3sek@ifwtech.co.uk>
> Subject: Re: [Amps] PSU design question
> To: amps@contesting.com
> Date: Saturday, November 7, 2009, 12:20 PM
> 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
> _______________________________________________
> Amps mailing list
> Amps@contesting.com
> http://lists.contesting.com/mailman/listinfo/amps
>
_______________________________________________
Amps mailing list
Amps@contesting.com
http://lists.contesting.com/mailman/listinfo/amps
|