I made a side-thread for this since it isn't directly related:
This discussion of "super semis" makes me wonder if anyone is still
engaged in vacuum tube research these days. Could any of the basic tube
wear points be improved on with modern technology? Could filaments be
made to emit twice as long without degradation?
I just looked at the CPI Eimac site and I can't see where they're
motivated or have the resources to innovate at the basic level. They
list "Amateur Service" as a product category, which can't bode well for
volume sales... Assuming MOSFET technology passes tubes by at high power
levels, will they just go quietly into the night?
I hope not - there's no second chances with current solid state devices.
Generally speaking, they do not degrade gracefully... ;-)
73,
Jim, N7CXI
On 8/13/2011 10:53 AM, Roger (sub1) wrote:
> On 8/13/2011 8:53 AM, Ron Youvan wrote:
>> Dr. David Kirkby wrote:
>>
> <snip>
>>> I don't know how close we are to the theoretical limits of what is
>>> possible, but
>>> perhaps not too far.
>> I've never heard of this, but I see no theoretical limit problem, we
>> can tolerate any output
>> impedance if we are allowed to use a transformer/s.
> There are physical limitations, but I'm embarrassed to say that although
> I used to teach semiconductor circuit design It's been so long that I do
> not remember any formulas. The limitation being talked about sounds like
> it applies to bipolar transistors rather than today's and would apply to
> a "given" transistor where the gain is inversely proportional to
> frequency and dissipation goes up dramatically near the upper end of the
> operating range.
>
> Like tubes frequency limitations are due to transit times, however the
> transistor has junction temperature, gate temp, gate to source and drain
> capacity in addition to temp limitations.
>
> Like David I see no reason for a theoretical limit. The main limitation
> in today's RF Power transistors is getting rid of the heat fast enough.
> They've made great strides, but the thermal mass of the internal
> components is so small that even with the best of heat sinking
> techniques the internal delta T can easily become the power limiting
> factor well below the device's actual power rating.
>
> Those same small pieces that limit heat dissipation are what give the
> transistor the ability to go to such high frequencies. The relatively
> large size of tubes limits their upper frequency due to transient times,
> although in some systems the tube itself becomes part of the resonant
> circuit or even the actual circuit. (I phrased that poorly but I think
> the microwave guys will understand what I was trying to say). For many
> tubes the internal lead length limits their usefulness to HF or very low
> VHF.
>
> 73
>
> Roger (K8RI)
>
>
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