>
>On Thu, 09 Oct 97 19:28:44 PDT "james fisher" <k4gf@worldnet.att.net>
>writes:
>>Oct, 1997, from K4GF: My winter project this year will be to
>>up-date and rebuild an old amp using a pair of 811-A tubes. Of
>>several ways to limit the available 100 watts of drive I would
>>like to simply install an attenuator of 3 to 4 db in the input.
>>
>>Hamcalc will tell me the values of resistance,
>>...but I am concerned
>>about the dissipation and any special features of the resistors.
>>
>>I am guessing now, but the peak dissipation would be some 60
>>watts or so and the average one-fifth of that.
For ssb, yes.
- The most rugged film resistors I have tested are made by Matsushita.
Digikey carries the 3w size. These resistors will dissipate 12w
continuously in Free Air for one hour, provided that the maximum voltage
rating of 500v is not exceeded. The 100 ohm 3w unit we use in our
suppressor retrofit kits measures c. 11nH (0.011uH) of inductance. At
12w, the unit glows dull red in a darkened room. After 1 hour, the
resistance typically decreases roughly 10%. However, if two such
resistors are paralleled, they can not handle 12w each unless a
substantial gap separates them due to mutual radiative heating.
>>Anyone who can
>>help would save me lots of R & D time. Thanks...Jim Fisher
>
My appoach would be cut and try. The goal is 400mA of anode current with
100w of drive. However, one might do well to keep in mind that the
wattmeter is not going to read accurately when an SWR is present. . //
For starters, I would connect 100 ohms in series with the drive coupling
capacitor at the cathodes, and connect 150 ohms from the input side of
this capacitor to chassis-ground. By juggling these two resistances, it
should be possible to: bring the load on the tuned inputs into line
with the design value - - and reduce the drive on each 811A cathode to
the needed 20 or so watts.
- [note: I would do all testing with a tuning-pulser, or with 50-wpm
dits from a keyer because the 811A's anode-dissipation ability is
somewhat less than robust. ]
>Pi and T pads values are also in every ARRL Handbook.
...snip...
>An alternative is to replace the 811A's with 572B's. A pair of those can
>easily handle 100W.
The 572B has the same emissive capability as the 811A because both use a
6.3v, 4a thoriated-tungsten filament/cathode. IMO, anything over 200mA
on either bottle key-down is not very linear.
Rich---
R. L. Measures, 805-386-3734, AG6K
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