[AMPS] QSLing

[Rich Measures]

>For those who are picky,  Which Amp that uses the 811 tube is best (do not
>include the Collins in your answer)
>
>Thanks in advance,
>Tom
IMO, a reasonable 811A amp. design would use:
1.  At least 6, 811As in parallel.  (I-anode=1.2A, RL = c.700 ohms) 
2.  Under 3mS T/R switching.  
3.  The Collins Radio method of increasing the grid's resonant freq.  
(ground the grid through 220pF paralleled with 30 ohm, 2w.).
4.  Adjjustable, Q= c. 2, PI tuned inputs with at least 2 adjustable 
elements.  
5.  Glitch protection to limit HV fault current to 200 peak amperes.  Use 
200a pk glitch diodes as needed,  
6.  Resistance-wire Ls vhf suppressors to decrease VHF voltage gain.  
7.  Step-start all transformer primaries.  
8.  Not more than 1700v anode supply, with transformer secondary LV tap 
for CW/RTTY.
9.  Rheostat-adj. in the filament transformer primary, and a filament-V 
position on the multimeter.  


NOTES:  
--1.  Some people believe that adding tubes increases anode C which 
reduces ability to perform at 10m.  Does this belief wash? 
  When 6, 811A is used, the required size of C1 is 6 times what is needed 
when 1, 811A is used.  In other words, as more tubes are paralleled, the 
output RL decreases---and as RL decreases, C1 proportionally increases.  
The bottom-line is that it does NOT matter how many tubes are used 
PROVIDED that the tank is designed to transform the existing RL to 50 
ohms.  For example, with 6, 811As (RL= c.700 ohms) at 29mHz, at a Q of 
10, total C1 needs to be around 80pF (X = -70 ohms).  This means that the 
6pF x 8 = 48pF of 811A anode C is only going to provide about half of the 
needed C to be able to tune the tank down to 28MHz.  .  
--2.  Why use 6, 811A tubes?  Because today's 150w transceivers will 
drive 6, and will overdrive 4. Half a dozen tubes may raise some 
eyebrows, however, the filament power of 6, 811As is equivalent to a pair 
of 3-500Zs.  For those who have a 200w radio, 8, 811As are indicated.  
--3.  If keyed-down with a steady carrier, using a 1700v supply, the 65w 
anode dissipation rating of the 811A is *Grossly* exceeded.  Using a 
tuning pulser might be a good idea-- except possibly for those who have a 
free, unlimited supply of 811As.  .
--4.  In multi-tube amplifiers, positioning the Ls in one VHF suppressor 
so that it couples to another Ls, is an invitation to regeneration.   
--5.   If the amplifier is to be used on 120v electric-mains, consider 
using a FWB rectifier and a resonant-choke filter  to reduce the peak-I 
demand on the mains by about 85%.  

If you find a manufactured version, Tom, I'll be surprized.  If you are 
interested in homebrewing, constructing one might be fun.  Considering 
the current price of 8877s, 3CX800A7s, 3-500Zs and 811As, such an 
amplifier makes economic sense to me.  
Rich---



R. L. Measures, 805-386-3734, AG6K   


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