Re: [Amps] 3.5 kV 2A REGULATED Power Supply: Schematic ?

["Ian White, G3SEK" <G3SEK@ifwtech.co.uk>]

R. Measures wrote:

> **  Ian -- For AB2, yes, but for AB1, control grid V-stability matters
> not since there is no grid current.

Yes it does matter, because you cannot guarantee that grid current will 
always be exactly zero.

Even if you use speech processing or ALC, the transient response of 
filters and ALC systems means that occasional spikes of excess drive 
cannot be totally eliminated. This situation is real life, and the grid 
bias supply must be able to handle it.

Also, some tetrodes show significant reverse grid current at lower drive 
levels. If the bias supply can't handle this situation too, there will 
be an unwanted shift in operating point.

A grid bias supply whose output voltage changes with even a small trace 
of grid current will allow - correction, will *cause* - serious IMD.



> > To achieve such good voltage regulation, you need a transformer with
> > very low winding resistances. Voltage doubling is not a good idea for
> > high-current supplies, because it *always* has worse regulation than a
> > full-wave bridge unless the winding resistances are extremely - no, make
> > that extraordinarily - low.
>
> **  Transformer secondary-winding resistance is inherently low with a FWD
> becaise only half as many secondary turns are required for the same
> output potential -- which means fewer layers of paper insulation are
> needed for the secondary.  Less paper means that more space is available
> for copper.  The result is a transformer that provides the same potential
> as a FWB configuration transformer but is more efficient because it has
> less copper loss.  Also, the FWD configuration has the benefit of ripple
> cancellation since, as one half of the filter is charging, the other half
> is discharging in the opposite direction.

That isn't really how it works, for several reasons.

1. The situation you describe is only true for the relatively short time 
while capacitors are being charged. All the rest of the time, the caps 
are discharging.

2. In the doubler, the voltage across half the capacitor stack is going 
down while the other one goes up. In the bridge (or biphase with a CT 
secondary) the whole capacitor stack gets charged.

3. Even in a so-called "full wave " doubler, each half of the capacitor 
stack is only charged on alternate cycles. With a 60Hz supply, each half 
is discharging for almost a whole cycle (16.7ms) before it receives 
another boost. In a full wave bridge or biphase circuit the whole 
capacitor stack is recharged every half-cycle (8.35ms).

A good situation to compare the two configurations is where you have a 
transformer with two identical secondaries (or two identical 
transformers) and you use the same two capacitors connected in series. 
You have the option to connect both secondaries in parallel and 
voltage-double, or both in series and use a bridge. In that situation, 
the bridge *always* gives better regulation.

The other side of the argument, as Rich points out, is that a 
transformer for bridge use requires more insulation and is generally 
more expensive.

I won't deny that voltage doublers are good value, and can be made to 
give adequate performance - especially in high-V / low I applications. 
But let's not kid ourselves that the voltage regulation is better than 
bridge or biphase. In any fair comparison, it's always worse.


Later:
> > 25 - 30mA will often be OK, but it won't prevent runaway in all possible
> > cases. Some tubes - or pairs of tubes - will generate larger negative
> > screen currents than that.
>
> ** Wow.   Ian must be uing some humungous tetrodes.  Perhaps these are
> the ones that have a  chain hoist loop on top because handles simply
> wouldn't do?
Quite the opposite - it's the small tetrodes like the 4CX250B/R, 4CX350A 
and 4CX400A that seem to be the worst. Also, some are notably worse than 
others, especially after they have already suffered some overheating of 
the screen.

For all of those tubes, Svetlana recommend a screen current sinking 
capability of 15mA per tube, so 25mA might not be enough to guarantee to 
keep a pair of tubes out of runaway. Other manufacturers are less 
specific, but that recommendation is probably a good design value for 
other makes also.

With larger tubes, negative screen current seems to become less 
important, relative to the normal positive current.



--
73 from Ian G3SEK         'In Practice' columnist for RadCom (RSGB)
                           Editor, 'The VHF/UHF DX Book'
http://www.ifwtech.co.uk/g3sek
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