<<** Ian -- For AB2, yes, but for AB1, control grid V-stability matters not
since there is no grid current. >>
Look. It does not matter if the tube draws grid current or not. If there is
any signal impressed on the grid voltage via any means, it can lead to
distortion. It?s a simple concept. Regulation of the bias voltage is one
way, then, to ensure that it does not happen. The better regulated the bias
supply is (e.g. the lower the Z of the supply) the less likely this is to
happen. Here is a test: will the amplifier's unit impulse or step response
will be different with poorly regulated supplies?
Sincerely,
Dr. William J. Schmidt, II K9HZ
Trustee of the North American QRO - Central Division Club - K9ZC
"Collector of Edison Wind-up Phonographs... Do you have one for me?"
Email: bill@wjschmidt.com
Alternate Email: wmschmidt@charter.net
WebPage: www.wjschmidt.com
-----Original Message-----
From: amps-bounces@contesting.com [mailto:amps-bounces@contesting.com] On Behalf
Of R.Measures
Sent: Monday, December 29, 2003 8:42 AM
To: Ian White, G3SEK; AMPS
Subject: Re: [Amps] 3.5 kV 2A REGULATED Power Supply: Schematic ?
>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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