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Re: [Amps] Tube amps are not a thing of the past

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Subject: Re: [Amps] Tube amps are not a thing of the past
From: "Paul Christensen" <>
Date: Sun, 4 Jun 2017 14:06:16 -0400
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">Another disadvantage of LDMOS is the input side of the LDMOS has a “Glass 
Jaw”. If you over drive them it can be a quick death, while a tube will take an 
occasional over drive."

Current SS amp protection circuity is very good and getting better all the 
time.  In the case of the SPE 1K-FA - a design that's already a decade old, I 
cannot force it to fail with direct output shorts, nor pulsing the input with 
high exciter input power of 150W.  The amp uses a 7-point summing circuit that 
quickly biases off the MOSFETs.  Concerning VT amps, it doesn’t take much abuse 
to destroy the control/screen grids of many hi-mu types.  At this point of 
LDMOS development, I don’t think we can say that today's SS amps are any less 
reliable than their VT counterparts in their respective power class.  We cannot 
ignore the accelerating use of SS in broadcast RF, medical, commercial, and 
military applications.

>"Amplifier Life and longevity is strongly on the side of the tube amplifier. 
>If you want it to last 25 to 40 Years."

Possibly true of the older VT amps that don’t use obsolete semiconductors and 
proprietary firmware code.  The B26 uses a RaspberryPi for control and is thus 
easy to hack, manipulate, and clone.  My SPE and newer Alpha amps all use 
proprietary firmware.  If the final round of support on those amps dries up, I 
think we can forget about 25-40 year longevity when we're cannibalizing old 
amps for spare parts.  Of course, LDMOS devices face the same problem with 
obsolescence.  Only time will tell if newer, better devices will be made with 
the same component footprint and operating parameters.  

I think the contesting argument is a good one.  But concerns in a contest 
environment may be more dependent on the quality of other components like those 
used in ultra-fast RF switching, cooling, and the power supply.  And that’s a 
shared concern across any type of SS or VT amplifier.  Assume for a moment that 
a SS amp fails during a contest.  Do we say it failed because it’s a SS amp, or 
because some other component failure that has no relationship with active RF 

The remote site that N4CC and myself share on the FL/GA state line uses two 
desk-crushing Alpha amps: a 9500 and 8406.  Our plan is to replace both with 
one Elecraft KPA1500.  Will the KPA1500 be more prone to problems?  We honestly 
don’t know.  However, based on what's been made public about the LDMOS design 
and PIN diode T/R design, I'm comfortable with the change -- especially since 
the root of the design can be seen in the time-proven KPA500.   Ultimately, it 
may be the wrong decision, but for every new technology that comes along, 
someone needs to take that risk and for us, the risk is acceptable. 

Paul, W9AC

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