>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
--
FAQ on WWW: http://www.contesting.com/ampfaq.html
Submissions: amps@contesting.com
Administrative requests: amps-REQUEST@contesting.com
Problems: owner-amps@contesting.com
Search: http://www.contesting.com/km9p/search.htm
|