[Amps] What tube?

Thu Jan 29 12:36:35 EST 2015

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

========================
Visit my hobby  homepage!
http://ludens.cl
========================
_______________________________________________
Amps  mailing  list
Amps at contesting.com
http://lists.contesting.com/mailman/listinfo/amps