[1000mp] RE: AMPS

[Tom Rauch]

> How does Eimac tube nomenclature work?  Does the 4CX or 3CX indicate
> the type and the following digits the size?  Do all 3CX tubes have
> thoriated tungsten filaments and hence are more robust?

The first number indicates number of operational elements, 2 would be 
a diode, 3 a triode, four a tetrode, etc.

The first letter in the letter group indicates envelope style. A "C" 
means ceramic, nothing means glass.

The next letter is anode style, X is external and nothing is internal 
anode.

The long number group is the approximate dissipation. Some tubes 
dissipate more than the number indicates...for example a 5000 cooler 
actually is rated at 6000 watts in some tubes...but it is reasonably 
close.

The last letter indicates special features like flying leads (F) or 
grid flange with flying leads (H) or coaxial ring (A), or even an 
internal change in the tube. 

The last number is relative class of mu, 7 being best for grounded 
grid and/or linear service while 1 is best in grid-driven class C. 

So a 3CX1500A7 (8877) is a triode ceramic insulated external anode 
1500 watts or so dissipation with the highest mu.

None of this has anything to do with the tube being directly or 
indirectly heated.     
 
Many amps, for some reason, do not protect the grids of MOx cathode 
tubes with fast electronic systems. I would avoid them. MOx cathode 
grids are gold plated to prevent contamination of the cathode, and 
the gold can be stripped in a matter of seconds. The grids are also 
subject t accumulated damage over time, as kinetic energy of the 
electrons striking the grid can dislodge gold without actually 
overheating the overall structure.

I've seen IC-775DSP radios eat up 3CX800A7 grids because the radio 
has 250 watt transients on the leading edges of key closure. In the 
ETO or Ameritron, the transient will knock the amp off line telling 
you something is wrong. Without a fast system, it will slowly munch 
away at the tubes without alerting you something bad is happening.
            
> One thing I consider very important is good CW QSK.  What I've heard
> is that vacuum relays are quite fast enough and robust (if a little
> noisy) whereas PIN diode switching, although faster (and silent) is
> not worth it's relative frailty. What has been the experience with
> these and with the Ameritron PIN diode QSK accessory?

PIN diodes are damaged by hot switching, just as relays and band 
switches are damaged by hot switching of relays or antenna transfer 
systems.

With a relay, you can open the input before opening the output relay 
so the high current line does not switch hot. With PIN diodes, that 
is usually cost prohibitive because of component requirements. That 
does give some advantage to relays since the diode fails immediately 
and the relay slowly accumulates damage over time. I use ALL vacuum 
relays in my own gear because I never know what radio I am going to 
use.
   
Other than that, they are both good systems.

Some radios give PIN diodes a bad name. I remember measuring one 
Kenwood model that actually held the carrier on for several 
milliseconds AFTER the output control line to the amplifier told the 
amp to receive! Switching like that can damage almost anything. 
It can even ruin a bandswitch or cause a tuning capacitor to fail, 
because all the kilowatts that used to go out the antenna have no 
place to go when the antenna relay or diode transfers open while full 
power is applied.

Most Yaesu's do NOT seem to have either excessive leading edge 
transients or hot switch problems. The FT1000MP overshoots a 
bit, but nothing like many other rigs I've seen. 

The early 775DSP is the record holder, and my AL1500 was useless with 
that radio because the 250 watts or so of transient into a 
50 watt drive PA just drove the protection system nuts. When I would 
load the amp heavy enough to not trip on the transient, the pulse 
power into the antenna was over 5kW causing my KW Matchbox tuners to 
arc and fail!! ICOM told me most radios do that...and the solution 
was to just not use an amp...but I think they corrected that problem 
later.

My next amp will be 100% solid state. Mostly because I can do it for 
about the same cost as an autotune tube amp, and it will be many 
times more reliable than a servo driven tuning system.73, Tom W8JI
W8JI@contesting.com