>R. Measures wrote:
>>1. Do not skimp on anode and screen V unless you are a QRP fan. If I
>>was going to build an amplifier with a 4cx3500A, I would start with a
>>least 6000V under load on the anode and 1200V on the screen .
>>Otherwise, you might not be able to utilize the full emissive capability.
>>2. Neutralize (see http://www.somis.org/D.a.05.GIF).
>>3. Under no circumstances tune the grid with a capacitor.
>>4. Except for >100MHz, consider using the 8171/4cx10,000D. It does
>>not require a noisy high-pressure blower; it costs less, it can make
>>more P; a low IMD version is available; plus it also has handles.
>>5. For any tetrode, to prevent a potentially fatal, runaway condition
>>of reverse screen current, bleed a constant 25 - 30 mA from the screen
>>supply.
>
>Several people, including Rich, have pointed out that screen and control
>grid voltage stability is much more important than anode voltage
>stability for tetrodes.
** Ian -- For AB2, yes, but for AB1, control grid V-stability matters
not since there is no grid current.
>
>However, anode voltage stability cannot be completely ignored, because
>maximum RF drive and anode current will coincide with minimum anode
>voltage. So right at the moment when you want maximum undistorted RF
>output, the tube has the least capability to deliver it.
>
>If the B+ sags too much, the result can be a large reduction in
>available RF output at low distortion... or a large increase in IMD if
>you insist on being greedy.
>
>This applies especially to tubes that need high current but at low
>voltage. The GU84b is a typical example - say 2.0kV at 1.2A - and of
>course two of any tube is two times harder on the power supply.
>
>As a rule of thumb, 10% voltage regulation is a good figure to aim for -
>in other words, with full drive the B+ sags to 90% of its zero-drive
>value. You can live with 15%, but worry if the voltage sags as much as
>20%. Don't worry about the voltage rising (within reason) when the amp
>is on standby.
>
>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. The FWB configuration doesn't
do this However, a FWB is the only game in town for a resonant-choke
filter power supply, which has advantages for RTTY, FM, AØ, and AM modes.
>
>A very common amateur solution is to increase the anode voltage and run
>the tube in more of a high voltage / low current mode. Most tubes will
>handle this quite comfortably. For example, the GU84b seems to run
>happily at 2.8-3.0kV instead of the rated 2.0kV, and that is much better
>for the B+ supply.
>
>Triodes are even more sensitive to anode voltage (because they don't
>have a constant-voltage screen grid) so the same problems and the same
>solutions apply - and even more strongly.
** Good point, Ian. Pentodes even more so.
>
>However, I don't see much role for precision regulated B+ supplies in
>amateur amps, except maybe to allow you to use an existing transformer
>that would deliver too high a voltage for the tube(s) you want to use;
>or perhaps for portable use where the mains regulation is very poor.
>
>
>Just one final point about tetrodes...
>Rich wrote:
>>5. For any tetrode, to prevent a potentially fatal, runaway condition
>>of reverse screen current, bleed a constant 25 - 30 mA from the screen
>>supply.
>
>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?
>The important thing is to bleed *more*
>current than *whatever* maximum screen current the tubes can generate,
>at the worst possible combination of tuning/loading/drive.
>
>The worst case will be at intermediate levels of RF drive, so runaway is
>most likely to happen while you're tuning up or testing a new amp. If
>you can get safely through the critical period when you and the new amp
>are breaking each other in, then you'll be home free. So do bleed *lots*
>of standing current through the screen regulator of a new amp (you may
>be able to reduce it later, when you've found out how the amp behaves)
>and also use current trips, VDRs and everything else you can think of to
>protect the tube(s).
>
>
>
>
>--
>73 from Ian G3SEK
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