re: The pair of 813s plate modulated by triode connected 813s as posted on the AM Window site...
http://www.amwindow.org/tech/htm/813/813.htmThere was an error made when the schematic was drawn up. The RF final screen circuit needs a slight change. The rig will still work fine as-is, but this will make it right.
The output of the screen power supply should go directly in series with the 100 mA screen meter, then in series with the screen 15 H modulation choke, then a .001 capacitor bypassed to ground at the following 2.5 mH RF choke, then in series with the 2.5 mH RF choke, to a .001 disc bypassed to ground at the 813 tube's pin and the circuit ends at this tube screen pin. (The 2.5 mH choke and two .001s should be located as close to the tube screen pin as possible. The other tube also gets a .001 uF bypassed to ground directly at the screen pin)
As an additional note: The high fidelity audio transformer which drives the push-pull triode connected 813s is a standard backwards connected tube amplifier output transformer. 8 ohms to 2K or anything like this will work fine. This can be found in most any 20 watt or larger older audio amplifier. The UTC Linear Standard Series version is also an excellent choice if found.
If we ever change the schematic again, we should add this correction. Otherwise, please include this note when recommending the rig to anyone.
Thanks.
Tom, K1JJ
