[Amps] What tube?
Carl
km1h at jeremy.qozzy.com
Thu Jan 29 18:51:52 EST 2015
If you use care in selecting the filament transformer it can be self current
limiting. The one
in the SB-220 is a good example. Many HB and commercial amps start off with
a "robust"
transformer capable of quite a bit more current than the tube requires that
is often overvoltage
to add to the problem.
When you get to real high current tubes a Variac or controlled step start is
needed.
Carl
KM1H
--------------------------------------------------
From: "Gerald Williamson via Amps" <amps at contesting.com>
Sent: Thursday, January 29, 2015 12:36 PM
To: <manfred at ludens.cl>; <amps at contesting.com>
Subject: Re: [Amps] What tube?
> 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 at 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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> http://ludens.cl
> ========================
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