[Amps] Any new tube research?

[Roger (sub1)]

On 8/19/2011 11:26 PM, jeff millar wrote:
> On 08/19/2011 07:05 PM, Roger (sub1) wrote:
>> I really don't see wide adoption of SS legal limit amps until the
>> devices can be made more rugged, can be cooled more easily, and become
>> available at much lower prices.  Prices are based on tooling set up
>> costs, number of units produced per batch, amount of silicon in the die
>> compared to the base wafer size, and yield efficiency.  At present they
>> are just too fragile and the protective circuitry (which is more
>> complicated than the amp) really pushes the prices far too high for wide
>> spread adoption.
> I used to think the same thing...but not any more.
>
> The Freescale MRFE6VP61 can withstand 60:1 VSWR for 10 msec, which is plenty of

Try as I might I've not been able to find a data sheet on the MRFE6VP61.

  When it gets to 5 or 10 seconds then I'd be comfortable. I do not like 
having to depend on fast acting protective circuits. You are still stuck 
with the sensitivity to over voltage spikes which must be suppressed one 
way or another. So the reliability of your device becomes the 
reliability of the protective circuit.

> time for a simple controller to shut it down.  That part costs $271 for about
> 1000W out...a lot cheaper than a 3cx800.  Transistors will get cheaper. Tubes
> will get more expensive.

That part I agree with, but I'd be looking at at least 4 of them in PP, 
parallel for a comfortable delta T at 1500 out and they still wouldn't 
lend themselves well to running the legal limit in class A.

>The commonly used reference design for the amplifier uses a large block of
>copper to spread heat into a large Aluminum finned heat sink.  But water cooling
>makes better sense.

Water cooling is almost as easy to implement (with the proper 
precautions) with high powered tube amps.
With tubes and SS You also have to be careful about the coolant temp and 
humidity.  I've worked with water cooling from both perspectives. 
Condensation can ruin a solid state amp almost as quickly as a voltage 
spike.  For the best efficiency the water needs to be a cold as possible 
without causing condensation to maximize the delta T. The higher the 
delta T the better the cooling. The same is true in computers. Actually 
the liquid cooling systems used in computers work very well with solid 
state amps.
>    Mount the transistor to about a 1/2 inch square water pipe
> to minimizes the thermal path from transistor to water and use the water to
> spread the heat rather than a lot of heavy expensive machined metal.
You do need a certain amount of metal in there above the minimum 
required for heat transfer, or thermal inertia to take care of cooling 
failures.  It really doesn't take much water to remove a KW of 
dissipation which is capable of well more than the legal limit. 
Unfortunately cooling systems can run the gauntlet from very simple to 
elaborate. The simplest is to just throttle tap water through the heat 
sink but from a conversation approach is not very efficient when it 
comes to resources.  It, at minimum needs a solenoid valve to turn the 
water on and off to eliminate the "memory" factor.

These figures may sound like a lot but 1500 watts for an hour is 
1,290,631 calories.  To hold equilibrium that is 1,290,631 grams of 
water per hour. One gallon of water = 3,785 grams so that is roughly 341 
gallons per hour or 5 gallons per minute BUT it takes one calorie to 
raise one gram of water 1 degree C. If we let it raise the water 5 
degrees we've just dropped the water requirement to about 1 gallon per 
minute. (and increased the cooling efficiency)  If we are willing to 
raise the water 10 degrees (which is a typical delta T) we are now down 
to about a pint per minute (give or take a bit) and these figures are 
for continuous power.  For tubes you can average it out due to the mass 
of the cooling elements. SS devices have only a tiny fraction of the 
thermal mass which makes it desirable to have more copper than the 
minimum necessary for the proper operation when the system is working. 
IOW I'd want a copper block roughly a 1/4" thick by 2" on a side for 
thermal mass.  For the average system this would not add much if 
anything to the over all physical dimensions but would certainly add to 
the reliability.

BTW when you figure 20% duty cycle for SSB AND the typical 10 minutes 
out of 30 transmit time the required water becomes miniscule.

The best "to me" are the refrigerated systems  that cool a reservoir 
down to just above the condensation point.  These systems work well 
summer or winter as they put the excess heat outside (or inside if you 
wish).  But being mechanical and requiring circulating pumps in addition 
to the refrigeration system  are more complicated and more prone to 
failure, which means at least a method of shutting down the amp if the 
cooling system fails. IOW coolant temp and flow.

The main thing I like about water cooling for high powered amps...IT's 
SILENT!  No blower noise.  OTOH you still need a blower on the tube 
seals where with SS cooling the drain cools everything.
> Transistors dump heat at ground potential while tube dump heat at high DC and RF
> potentials.  That makes cooling a LOT easier.
Depends on what you are used to.  If you have been working with several 
hundred KW that was water cooled, then water cooling a legal limit amp 
is not all that difficult. Also almost any external anode tube can be 
easily converted to water cooling. OTOH it is true that water cooling a 
tube with 3 or 4 KV on it takes a bit more plumbing than a grounded heat 
sink and there are safety considerations.

OTOH you are running a far higher delta T which means a more efficient 
cooling system. The downside for HV water cooling is the necessity for 
using either distilled water or deionized water which are both highly 
corrosive where metal and particularly copper and brass are concerned. 
With the grounded heat sink you can use plain old tap water although 
when recirculating it you do have to treat it as if it were a swimming 
pool. IOW Algae and bacteria can be serious problems.

My choice of water/liquid cooling would be a 10 gallon PVC tank using 
either direct flow to the amps or a heat exchanger in the tank made of 
copper tubing.  The water would be cooled by a modified air conditioner 
negating the need for running any of the coolant outside which here in 
the frozen North presents a number of hazards.  Each amp would have flow 
and temperature monitoring. Loss of pressure would shut down the 
circulating pumps and close safety valves to the reservoir. The reason 
for the heat exchange would be that little coolant would be lost even in 
case of a piping failure.
> The cooling system becomes a transistor mounted on a short length of square pipe
> that has some hoses leading to a bucket for thermal mass or maybe a fan cooled
> heat exchanger.

My cooling systems would be some what different than that arrangement.
I want the heat dumped outside in the warmer weather and inside during 
cold weather, but without affecting the humidity in the house or shop.

 >I'm designing a transistor control board that uses a FET to switch the 
50V drain supply. The micro-controller cuts off >the amp in about 2 msec 
on any fault conditions. The FET doesn't need protection from drive 
without supply >voltage. Homebrew amps can use a 3000W supply available 
on ebay for $25. Commercial manufacturers will have >to pay real money 
for a switcher, but you get power factor correction, light weight, and 
huge volume discounts >because lots of supplies use the same components.

Agreed

 >The trend from 12V to 24V to 50V supply has greatly simplified >output 
network design because the device output >impedance goes up by 4 for 
every doubling of supply voltage. >Total amplifier size will shrink and 
become separate >subsystems, especially with water cooling.

Agreed, although the same concept can be used with tube amps where RF, 
Control, and PS are separate modules.
I have one like that and am building several others following the same 
approach.   Unfortunately I've not found a switcher that would give me 
around 3KV at roughly 6 or 7 KVA which is required for the legal limit 
out when running class A.

As I stated previously the RF circuits are relatively simple in SS amps, 
it's the control and protective circuitry as well as cooling that is 
both complex and expensive.

 > Amplifier pallet >smaller than a paperback. Power supply smaller than 
a shoe box. Water cooling smaller than a >bucket. Control box >just 
large enough for a meter and display. Multiple amplifiers can share the 
subsystems.

Having worked in Industry with many types of controls, I do not like the 
idea of amps sharing subsystems such as control and protective circuits. 
The same is true for switching power supplies powering multiple amps at 
the same time. You increase the odds of a total system failure when 
taking this approach. I have no problem with the PS being switched 
between amps and that works well with SS amps as there is virtually no 
warm up time although the control circuitry may take a couple of seconds 
to become fully operational depending on its configuration.

I would end up with an amp about twice the size of a paperback, a PS 
about the size of two or 3 shoe boxes and a control board about 8 X 10 
inches. Even with new components the control board should run less than 
$50 USD so sharing them would save little and add to the likely hood of 
failure.

That would all end up in one enclosure which would be about the same 
size as today's SS legal limit amps.
 >

 >Parallel power supplies for >more capacity.

That is one of the main advantages of low voltage.

 > Routing 50V around the shack makes more sense than routing 4 kV.

routing a high current buss around the shack scares me almost as much as 
routing a HV buss around the shack and it's more of a fire hazard.  It 
can be done but takes a lot of precautions if properly done.

 >Multiple amps can share the >same water cooling bucket.
 > Multiple amps can share the same controller. jeff, wa1hco _

Even with one amp I'd want at least a 5 gallon reservoir with temp 
control on it. I'd probably aim for 50F but even that could be too low 
on a high humidity day. Yes, multiple amps could share the same 
reservoir but I'd want separate circulating systems for each. A single 
system could be used but then it'd need flow control and monitoring at 
each amp.  Flow monitoring is not as simple as pressure monitoring. You 
measure a pressure differential across an orifice and the system will 
shut down with either insufficient or excessive differential. Exhaust 
temp needs to be measured right at the amp as well.

No way would I use a controller on more than one amp. They are cheap and 
I'd prefer the reliability of one for each amp.

73

Roger (K8RI)
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