Manfred, you can be sure that tube life is adversely affected by frequent
on/off cycles of the filament. One of the current tube manufacturers (Burle
I think) has a paper on his website that suggests that one on/off cycle of
the filament power is roughly equivalent to reducing tube life by 60 hours.
If you expect say 18000 hours, the 300 on/off cycles pretty well uses that
up.
This was for a larger transmitting tube so perhaps the 60 hour figure is
not accurate for a 3-500Z or similar but the principle applies.
I use a TH347 tube in a 1296MHz homebrew cavity amplifier. This tube is
rated for 34A filament current and the manufacturer specifies a maximum turn
on current of 2X or 68A for the first AC cycle. Considering that the cold
filament resistance can be as low as 10% of the hot resistance you can see
the need for a current limiting circuit.
After loosing a couple of tubes due to open filament caused by frequent
on/off filament cycles I became very interested in how to protect my tubes
during turn on. Now the filament is turned on with surge limiters in the
primary switched in three steps to limit the current to about 35A maximum. It
takes about two minutes to reach the full 6vac operating voltage.
So, it is no longer "instant on" but the tube is fully protected.
I don't know how to apply this to a smaller tube like a 3-500Z but I see no
way that the principle in invalid.
73,
Gerald K5GW
In a message dated 1/29/2015 8:58:57 A.M. Pacific Standard Time,
manfred@ludens.cl writes:
Well, I got suggestions for three different tubes. All of them being
directly
heated glass bottles, which makes a lot of sense given my requirements.
I don't think I will ever actually build that amp, but I wanted to see
what
could be done, in the line of a reasonably cheap and efficient amplifier
using a
conventional tube-type RF section, combined with a modern switching power
supply.
In calculating the efficiency of amplifiers, I think we need to consider
_all_
power taken from the supply line. That includes filament power, and in teh
case
of tetrodes, screen power.
Instead I think we do not need to include the drive power in the formula,
despite the fact that GG amps feed most of it to the output, while
grid-driven
amps either need very little drive power, or burnit up in a dummy load. In
fact
most ham amplifiers are built to be driven by standard 100W radios, set up
to
100W or near that, so this area will be similar for all amps.
There are three power drains to consider: At full output, in TX at no
output,
and during RX. In order to get lowest possible overall power drain from a
simple, conventional tube amplifier, I set the requirements for class AB2,
with
low idling current, and instant-on filaments that can be shut down during
RX, at
least for slow ragchew-type communication. Shutting down filaments means
that
also the fans can be shut down, putting the amp into near-zero power drain
mode.
So, let's see:
A pair of 3-500Z bottles indeed seems like a pretty optimal choice, being
able
to run in a simple, zero-bias GG circuit. Although it looks like to really
operate at legal limit and low idling current, it would need some bias -
but
that's easy enough to do. Running at a tad above 3000V, 120mA bias, 800mA
max at
PEP, it delivers 1500W PEP out at 61.7% plate efficiency. Considering 146W
filament power, the overall efficiency is 58.2%. Total power input of
2578W,
plate dissipation of 932W. Cooling requires a good air stream from fans,
but no
noisy blower. The output matching is reasonably easy, and the drive
requirements
are 92W over 57 ohm, allowing a radio with higher Q output to drive the
cathodes
directly, while a typical radio would have an easier job driving it if
simple
resonant circuits are used at the cathode.
At idle during TX it consumes 511W, and during RX it's down to 146W.
To the above figures we have to add the fan power (maybe 20 watts), and
power
supply losses. On the other hand, if we shut down the filaments during RX,
and
after a minute shut down the fans, power drain is nearly at zero.
Cost for those tubes ranges from about $340 for the cheapest ones, to $570
for
ones with better reputation. I don't know what the sockets cost.
Now let's see what happens with a pair of 4-400: Judging from the data
sheet,
class AB2 operation at 1500W would require roughly 3230V, 700mA at the
plates,
500V 39mA at the screens, and 146W for the filaments. That means 61.8%
overall
efficiency, slightly better than the 3-500Z. The price for that is the
screen
supply. Plate dissipation is 761W.
Grid bias would be -83V, which means that a 100W radio can drive the grids
directly, no impedance transformation needed, just a dummy load at the
grids,
which also has a stabilizing effect. So we have simpler drive than with
the 3-500Z.
During TX idling, plate current is 150mA and screen current is zero.
Including
the filaments, that is 630W idling power. Worse than with the 3-500Z, but
this
can be tweaked, probably sacrificing some IMD performance.
Power drain during RX is the same as with the 3-500Z.
Cost for these tubes, Taylor brand, is $478. No idea about socket prices.
The third submission I got (well, actually it was the first!) was a pair
of the
GU81 pentodes. These are really huge bottles, gorgeously beautiful, and
very
inexpensive compared to the other two! From an emotional point of view, I
would
say, go with them and build a window into the front of the amp, so that
the
tubes can be seen all the time!
Performance data for class AB2 operation is not as easily available for
these
tubes. I worked it out from the data given in the sheets, but I'm not very
sure
of it all. These huge tubes are frequency-challenged by their high
capacitances, so that at 30MHz they need to work at reduced voltage, not
much
more than 2kV.
It seems that to get 1500W output, the parameters would be roughly 2200V,
1.1A
at the plates, 600V at a whopping 400mA at the screens (can that be
possible?),
and 277W for the filaments! That would mean an overall efficiency of only
51%.
On the 10 meter band, the plate tank Q would need to be higher than 20,
because
of the huge capacitance! And the grid requires a rather high drive
voltage, so
that bandswitched PI tanks would be needed at the grids, or maybe a very
well
made broadband transformer, followed by a dummy load.
It seems to be a far less than optimal choice, from the technical point of
view.
But the sheer beauty of these tubes, and their rock bottom price, around
60
dollars for a pair, are attractive... It would be a fun project, but not
really
a technically competent amplifier, it seems.
Comparing these three tube options to my cheap MOSFET amplifier project,
which
also seems to be pretty much shelved: I'm getting an efficiency of roughly
55%.
Of course there is no filament power, and the low efficiency is due mostly
to
the poor saturation characteristics of the MOSFETs, given by the
voltage-dependent internal capacitances. So, even with the power lost in
the
filaments, good tubes are more efficient. If we shut off the filaments
during
RX, tubes win this efficiency contest, although only by a small margin. On
the
other hand, my MOSFETs cost only about $70 for the full set, and allow
making a
no-tune amplifier. Then again, they are easier to kill than tubes, if an
antenna
connector comes loose or such.
It's somewhat of a tie.
One more question, maybe a bit stupid: How fast is the heating of 3-500Z
and
similar directly heated tubes? Are there specs available? Is it reasonable
to
switch off the filaments during RX, or would that mean cutting off the
first few
words of every transmission? Would it damage the tubes to switch the
filaments
on and off very often?
Manfred
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