2
The initial schematic circulating for these amplifiers was hand drawn on a drafting table by pencil around 1990!
This instruction manual and the 811 kits are the result of direct feedback from servicing several dozens of
amplifiers. Read these instructions thoroughly before you start working on your amp. Make sure you understand
every step to avoid errors or omissions.
AL811 Stability
Tubes with long thin grid leads and large internal structures sometimes have significant anode-to-cathode
feedthrough. This can destabilize amplifiers at upper HF. 811 and 572 tubes are particularly bad about this.
The three-tube 811 model is not neutralized. As with two unneutralized 572B tubes, three 811A tubes are
marginally stable at upper HF. The AL811 should have a 200-ohm cathode swamping resistor, or even lower
swamping resistance. Kit PN 811R200K has one chassis mount RF-rated 25W resistor, one 0.01uF coupling
capacitor, and necessary hardware. In addition to standard hand tools, you will need a 0.113 to 0.125” (1/8”)
drill to add one or two holes for 4-40 machine screw clearance.
AL811H
The four-tube model is neutralized. Four 811 tubes, regardless of layout, become unstable someplace above 15
meters and behave poorly on ten and fifteen meters when not neutralized. Any swamping resistance is optional;
it just makes neutralization less critical and closer matches drive level to 100W exciters. With bias changes the
swamping is not required.
NOTE: I find many amplifiers that have NOT been properly neutralized from the factory. This is not from a variation
in tubes. The only time tubes might require neutralization is when 572B’s are swapped in. To equipment operators,
poor neutralization is most easily noticed by a fairly large 10-meter input SWR variation as the amplifier is tuned.
A second neutralization indicator is exciter power varying as the amplifier is tuned past 10M grid current peak.
A third and by far least dependable identification method is observed by plate current dip not coinciding with
maximum output power or maximum grid current.
Neutralization
I neutralize 811’s by activating the relay system with an external low voltage dc supply of about 14-18 volts
positive applied to where meter lamp positives connect. The antenna relays are activated by grounding the
RLY jack. Feed an antenna analyzer into the RF input or output jacks, and listen with a radio on the other port.
With the amplifier on 10 or 15 meters adjust the analyzer and receiver to the same frequency. Adjust the
PLATE and LOAD for maximum signal. Now you can safely vary flapper plate positioning for minimum signal
level. The best flapper setting is the compromise between 10 and 15M nulls, with 10M having priority.
Caution: Do not neutralize before grounding grids directly to chassis at sockets! The amplifier must be
functioning normally with grids properly grounded to neutralize.
Arc Major Concerns
To Next Page
Loading...
Need help?
General