>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.
** I adjust the grid potential so that there is no grid current under
max drive. There can be grid current only if a more powerful driver is
used.
>
>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.
** Which is why I adjust the grid potential for zero grid-I with a pulse
which produces maximal output from the driver.
>
>Also, some tetrodes show significant reverse grid current at lower drive
>levels.
** Now there's a new one.
>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.
>
** Which is why I adjust for zero grid-I with max pep drive.
>
>
>>>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).
>
** Sure, it's not perfect cancellation, but it does cancel. The FWD
anode PS at:
http://www.somis.org/pb.ps.gif
uses 20, 300uF, 450V capacitors. The net C is 15uF @ 9000V. Many
builders considered this inadequate during construction, however, no
ripple could be heard on the air and the amplifier produced 1200V-pk into
50-ohms.
>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.
>
** However, a same-core transformer that was designed for FWD service
would have less secondary R than both windings in parallel in the above
example.
>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.
>
** Cu costs more than paper.
>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.
>
** Your comparison was apples vs. oranges.
>
>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.
>
** I was talking about tetrodes with 1000 - 2000 V screens. For 250 -
800 V screens, the zener-string screen shunt regulator fed through a high
resistance from the anode supply is as foolproof as it gets.
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