[YC-156]

Hi again Oskar,

That looks very promising. I do have a 'few' suggestions, as you probably figured.

• Loose the shield between the VFO and the 807 socket below chassis. The RF potential is the same on either side of it and it serves no purpose.
• The second shield should have a few pieces of angle stock added, so that you can ground it to the base of the chassis at a few points between the front and the rear panel.
• Turn 807 socket 90 degress counterclockwise as seen from below the chassis. This will make the G1 pin point toward 'From VFO'.
• Move tie point for the grid blocking cap more in line between 'From VFO' and the 807 G1 pin. Aim to keep connections short.
• Now you can connect Z1 straight from the G1 pin and to the grid block cap on the tie point. Other end of grid block cap still goes to 'From VFO'.
• The 807 G2 decoupling cap must be located right at the valve socket, from the pin straight to ground. Just like the cathode decoupling cap.
• Turn tie point, which carries G2 voltage dropper resistor, 90 degrees and move it closer to the point where the B+ connection passes through the remaining shield. This limits the opportunity for RF to be picked up or radiated from the 'cold' end of the resistor.
• Move the 'cold' G2 decoupling cap up on the shield wall, decoupling right at the hole. Ground return must be boled to aluminium shield wall right at the hole. In other words forget putting any of the caps on the G2 resistor tie points.
• Move R4 and its decoupling cap closer to Z1, which should now be pointing toward the right. The decoupling should return as closely to the hole for the valve socket as possible.
• Make positively sure that you have an excellent connection between the top of the aluminium chassis and the copper foil on the PCB. Add more screws and locking washers around the valve socket if you feel like it.
• There is no need to route the ground return of the PTT signal back to the PCB. Just bolt it to the chassis at the front panel.
• The PTT connection from the 807 cathode pin (now pointing back toward the rear panel) should be routed up, right and down low along the shield toward the front panel mounted PTT switch. That way it will cross neither the 'hot' G1 nor G2 circuitry.
• Add decoupling for PTT signal at the ext PTT socket right at the front panel. Bolt ground return straight to the chassis at that point using as short connections as possible.
• Decide on one side of the 6,3V heater circuit, which should be routed through the chassis instead of through wires.
• Connect one heater pin of the 807, the one closest to G2 would be my suggestion, straight to the PCB copper foil.
• The other remaining heater pin should also be decoupled at the 807 socket. The wire will be routed back to the rear connection as it is now. Yes, that means two decoupling caps for that short piece of wire.
• Resist the temptation to let the DC/heater power carrying wires hang up in the air. It is better to let them hug the chassis base or the shield. Lower RF field for them to pick up/radiate down low.
• Wires intentionally carrying RF, like the G1 connection, should be spaced well away fromt he chassis, but this is less critical than the previous point.
• Drop one of the 6,3V connections at the back, as you are now routing that part of the circuit through the chassis.
• The wire to the grid current meter going topside (up into the nice and 'hot' RF environment up there ) must be decoupled right at the point where the wire passes through the chassis plate. Considering adding a high inductance/low current capacity moulded choke into the wire at that point as well, thus further enhancing isolation.
• Depending on what the layout inside the VFO box looks like, then you probably need an internal shield inside that one as well. This should isolate the anode of the 6AG7 from the grid/cathode oscillator circuit and will hopefully prevent frequency deviations as you tune the 807 grid circuit. That means a shield from copper foil or a similar material across the valve socket.
• Swap the location of the B+ connection and the RF output coax connector on the rear panel, possibly moving the B+ more to the right. It is OK to route the B+ wire a bit 'back' topside along the large RF choke for this to fit. Better than having the wires cross below deck.
• Use a short piece of coax to connect the output loading variable cap to the output connector. Ground shield both at cap and output coax connector. This means less RF pickup by the B+ wires.
• Decouple the B+ connection right where it passes through the chassis plate to the compartment below *in addition* to the existing decoupling cap at the connector on the rear panel. You know the drill by now.
• Contemplate that in addition to providing electrical safety a wire mesh shield cage of the large 807 anode components topside would also largely prevent radiation of intense harmonic energy directly from the 807 anode structure. A class C stage generates intense amounts of hard harmonics, and those can severely disturb TV and radio reception nearby if allowed to spread.
  
  One reason why amateurs today have abandoned class C amps whenever possible are the difficulties in efficiently reducing the radiation of the strong harmonic energy RF class C amps generate at low VHF frequencies. An open frame transmitter like the one you are suggesting is downright illegal from an interference point of view, and the lack of proper RF shielding must be considered a fundamental design flaw. I would 'F'ail your design for that reason alone, since all electronic equipment designed today must by law ensure that they are 'nice neighbours' toward their surroundings.

Think about it and we can discuss all of this in the days to come.

...and keep up the good work.

Frank N.