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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