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From: john larkin <JL@gct.com>
Newsgroups: sci.electronics.design
Subject: Re: dumping a lot of heat
Date: Sun, 08 Dec 2024 07:47:25 -0800
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On Sun, 8 Dec 2024 14:15:54 -0000 (UTC), Phil Hobbs
<pcdhSpamMeSenseless@electrooptical.net> wrote:

>Liz Tuddenham <liz@poppyrecords.invalid.invalid> wrote:
>> john larkin <JL@gct.com> wrote:
>> 
>>> On Sat, 07 Dec 2024 11:49:14 -0500, legg <legg@nospam.magma.ca> wrote:
>>> 
>>>> On Thu, 5 Dec 2024 22:03:41 +0000, liz@poppyrecords.invalid.invalid
>>>> (Liz Tuddenham) wrote:
>>>> 
>>>>> legg <legg@nospam.magma.ca> wrote:
>>>>> 
>>>>>> On Thu, 5 Dec 2024 08:55:32 +0000, liz@poppyrecords.invalid.invalid
>>>>>> (Liz Tuddenham) wrote:
>>>>>> 
>>>>>>> john larkin <jl@glen--canyon.com> wrote:
>>>>>>> 
>>>>>>>> I'm thinking about building a biggish rackmount dummy load box. It
>>>>>>>> would simulate series resistance and inductance. Part of the problem
>>>>>>>> is that it will need to dump a lot of heat.
>>>>>>>> 
>>>>>>>> We are using copper CPU coolers on PC boards, which are great up to a
>>>>>>>> couple of hundred watts, but I'd like to do a kilowatt or two.
>>>>>>>> 
>>>>>>>> https://highlandtechnology.com/Product/P945
>>>>>>>> 
>>>>>>>> It would take a heap of expensive extruded heat sinks and fans to get
>>>>>>>> rid of a kilowatt. At 1 K/W, a pretty good heat sink, that's 1000 degC
>>>>>>>> temp rise.
>>>>>>>> 
>>>>>>>> A small hair dryer can dump a kilowatt. So some sort of red-hot
>>>>>>>> nichrome coils and a vicious fan might work.
>>>>>>>> 
>>>>>>>> I'd prefer to not use water.
>>>>>>>> 
>>>>>>>> I wonder if there is some sort of runs-red-hot power resistor.
>>>>>>> 
>>>>>>> If you are using elements at near red heat, remember you need to keep
>>>>>>> the radiant heat away from the outer walls of the cabinet.  Reflectors
>>>>>>> just throw the problem elsewhere and eventually will tarnish, the best
>>>>>>> system is several spaced blackened steel baffle plates with vertical air
>>>>>>> passages between them (visual black is not always IR black).
>>>>>>> 
>>>>>>> If you need a rapidly-controllable load, valves can dissipate energy at
>>>>>>> a much higher temperature than transistors, so they might be worth
>>>>>>> considering.
>>>>>> 
>>>>>> https://ve3ute.ca/2000a.html
>>>>> 
>>>>> Most of my valve designs err on the safe side.  Putting electrolytic
>>>>> capacitors where they can get hot is just plain stupid.  I have seen
>>>>> loads of shoddy radio/audio designs where the cathode resistor was
>>>>> strapped along the side of the bypass capacitor or the main smoother was
>>>>> stood up right next to the rectifier or the output valve.
>>>> 
>>>> By standard 'safe' design, tubes have a pretty limited life.
>>>> 
>>>> By 1956, the heater was no longer the weakest element in 
>>>> the life equation for these parts - glass electrolysis was.
>>>> 
>>>> Electrolytic caps and their use has always been an issue.
>>>> Cuffing the tubes not only enforces distance to other 
>>>> components, but reduces radiant effects in the viscinity.
>>>> 
>>>> Win, win.
>>>> 
>>>> RL
>>> 
>>> Tubes were awful. Still are.
>> 
>> The techniques for designing with them are quite different from
>> transistors and ICs, you have to think a different way; they aren't just
>> poor transistors, they have a different lifestyle altogether.  They also
>> have some advantages over semiconductors:
>> 
>> 1)  Withstanding short term overloads without damage.
>> 
>> 2)  Dissipating energy in a smaller space at higher temperatures.
>> 
>> 3)  Easier to make with few exotic materials or processes.
>> 
>> 4)  EMP-proof and radiation-proof in the event of nuclear war.
>> 
>> Those properties aren't needed most of the time but when they are,
>> valves are a lot easier to design with than transistors if you are
>> conversant and comfortable with the technology.  There are still very
>> few single transistors that can out-perform a humble EF91 from the
>> 1940s.
>> 
>> 
>
><nearly dies laughing>

Yup. EF91 is known as "Not the valve that won the war."

Pentodes are *noisy*, at least 10 times more than some 3 cent jfet.

There are a couple of high-voltage tubes that were, until recently,
worth at least considering. But multi-KV fets are more sensible these
days.

I used to use the 1B3 HV rectifier diode as an amp, with the filament
voltage modulating the conductivity. The downsides were the bandwidth
and the X-ray hazard.

One could make an optocoupled tube half-bridge, or full-bridge, with
maybe a 30 KV supply. I guess you'd use batteries for filament power.