>
>Good evening;
>
>I am in the process of repairing a 76CA for a friend. It appears that the
>tubes are weak. Rathert than paying top $$ for 3 - 3cx400's, I remember seeing
>someone mention a web site regarding a convversion to 2- 3cx800's. Does anyone
>know the URL for the site ?
>
>Also, other than the heater voltage going to 13.5v, should the input and
>output circuits be compatible, or do I need to modify the tank or input ?
>
The tank RL of 2, 3cx800s is abt the same as 3, 3cx400s. The drive
requirement is c. 45w instead of c. 80w. . Mo' drive is not mo' betta.
>I thought I saw someone mention using a voltage doubler rather than adding a
>13.5v transformer. Is this possible ?
? Yes, but a pair of mucho-uF 10v electrolytics would be needed, as well
as an adj. regulator to set the heater potential to a bit less than
13.5v. It might be easier to add a small 6.5v transformer in series with
the existing >6v heater winding. // note: The recommended heater
potential range is c. 12.9v to 14.1v. If higher heater potentials are
used, barium evaporates from the cathode and lands on the gold-plated
grid, causing primary grid-emission, which ain't good news. / I have
heard from some owners that
*Commander* 3cx800a7 amplifiers use heater potentials of c. 15v.
>
>Lastly, in order to lower the resonant freq of the plate choke, do I need to
>add turns or remove turns ? Any rough idea as far as the turns/Mhz ratio ?
>(10/12m issue)
? ... removing turns from a hv-rfc is a lot less risky than trying to
add turns. In any event, to avoid choke-fires, check with a dipper
before "firing up" to make sure there are no resonaces within approx. 5%
of an operating freq. Check the rfc wired normally.in the amplifier.
- If you do the mod. yourself, there is a way to compensate for the fact
that the 76CA has no tuned inputs. E-mail me if you want to discuss it.
I would prefer talking on the phone on my dime rather than
finger-talking'.
>
- later, Eric
Rich...
R. L. Measures, 805-386-3734, AG6K, www.vcnet.com/measures
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