[Amps] Tubes vs. Solid State (was) Expert Amps 2K-FA: Any Opinions ?

[Jim Barber]

On 4/29/2012 9:20 PM, Roger (K8RI) wrote:
> On 4/29/2012 11:02 PM, Jim Brown wrote:
>> On 4/29/2012 5:24 PM, Roger (K8RI) wrote:
>>>> Most of the tube designs have been significantly "over engineered".
>>>>>    That is, the tubes have excess plate dissipation for the power output
>>>>>    and grids that will take a significant beating.
>>> Which is a plus for tubes.
>> Naw, it's a plus for conservative design.
>>
>> One thing that seems to be missed in this discussion about home brewing
>> amps was clearly pointed in a comment in this thread -- the key to solid
>> state amp design is THERMAL design and protection.  Those are very
>> different skills than most amp designers of our generation learned, so
>> at least in part, it's an "old dog new tricks" problem, not a limitation
>> of the technology.
>
> It helps to have a computer systems design background<:-))  Although
> many systems are running right at edge of the device limits.
> Even with the best heat transfer using the latest heat transfer
> compounds  such as Arctic silver which has a thermal resistance of less
> than 0.010 watt per square inch you are then limited by the internal
> thermal resistance which may be more than the compound between the
> device and heat spreader.
> Arctic Silver 5 has a thermal conductance of 350,000 W m^2, and a
> thermal resistance of 0.0045W per square inch. Achieving these figures
> takes mastering the technique of spreading the compound and then
> thermally cycling the required times.  Often the mating surfaces have to
> be polished, or lapped flat enough that when clean surfaces are placed
> together you can not pull them apart with a straight pull.  The
> installation requires working the compound down to a thickness of 0.001"
> to 003".  The copper heat spreaders between the devices and heat sinks
> are usually 1/4 to 1/2" thick.
>
> Many devices rated at a KW are only good for a couple hundred watts on
> SSB and there's none of this long tune up, key down.
> The protective devices or rather circuitry is usually far more complex
> than the amplifier circuitry.  Also as the devices are usually very
> broad band you use low pass filters band switched in the output as even
> the smallest harmonic output from the exciter gets amplified just as
> much as the fundamental. There is also such a thing as a device with too
> much gain.  which can lead to instability when operating on the lower
> end of the frequency range.  There is also protection from input on the
> wrong band as these are no where near as forgiving as a tube amp driven
> for a short time on the wrong band.  It only takes one over voltage
> spike to take out a semiconductor although they are improving.
>
> The protective circuits when taken one at a time are quite simple, but
> taken as a whole can appear quite intimidating.  OTOH setting them up
> with the proper values and time constants can be a real chore.
> Fortunately that can normally be done without the expensive and fragile
> devices in the circuit.
>
> 73
>
> Roger (K8RI)
>
>
>>
>> 73, Jim K9YC

And then there's the question of who works on all that complexity, and 
supports it in the long term?

Amateur amplifier suppliers come and go. I think it's fair to say *most* 
amateurs aren't qualified or even motivated to service complex 
solid-state amps themselves. At least some are motivated enough to work 
on, modify and tinker with vintage tube amplifiers. If that were not the 
case, I don't believe this list would exist.

73,
Jim N7CXI