UK Vintage Radio Repair and Restoration Discussion Forum - View Single Post - Homebrew transmitter project (Frank vs. Oskar :) )

YC-156 · 27th Mar 2006, 6:30 am

A few additional points, which I forgot the first time around:

All the coils should preferably be spaced at least their own radius from other components and any metal plate. That goes for the ends of them as well.
You should add an electrostatic shield to the 807 around its base, on the top of the chassis. It should be cylindrical and go from the chassis plate up to the point of the valve envelope, which is where the anode starts. Judging solely from a drawing of an 807 it looks like the dimensions should be something like 5cm in diameter and 5 cm in height. It shouldn't be a tight fit. Leave 5 mm of clearance at the widest point of the valve envelope.

This shield should largely prevent pick-up of 'output' RF directly via the connection wires inside the vavle envelope, as they go from the valve base and up to the grid/cathode structure. It won't affect cooling of the valve either, as all the heat is generated above the point, where the shield stops.
You would be well adviced in moving the grid tuning cap back from the front panel a bit and isolating its frame from the ground. Use an isolated shaft extender. This will make adding neutralisation of the 807 stage at a later date much easier, if you ever need it.
I don't recall if we have discussed this, but since you have moved the TX to the 160m band, you *could* replace the VFO with a crystal oscillator. A 1.8432MHz crystal is easy to source at most electronic suppliers, and you wouldn't need the large shield box topside. Just a bit of local shielding around the 6AG7 socket, which could then be moved down onto the main chassis plate. Let me know if you want more details on this.
If you want to keep the VFO, then, if you want to be able to change and predictably set the frequency, you almost certainly need some form of mechanical reduction drive on the shaft for the VFO variable capacitor. Your layout doesn't seem to provide room for one, so maybe you have already mentioned this before.

Best regards

Frank N.

Edit: By convention the 'flow' of a transmitter circuit mostly go left-to-right, but it doesn't really matter, of course.

27th Mar 2006, 6:30 am	#66
YC-156 Retired Dormant Member Join Date: May 2005 Location: Aarhus, Denmark Posts: 281	Re: Homebrew transmitter project (Frank vs. Oskar :) ) A few additional points, which I forgot the first time around: All the coils should preferably be spaced at least their own radius from other components and any metal plate. That goes for the ends of them as well. You should add an electrostatic shield to the 807 around its base, on the top of the chassis. It should be cylindrical and go from the chassis plate up to the point of the valve envelope, which is where the anode starts. Judging solely from a drawing of an 807 it looks like the dimensions should be something like 5cm in diameter and 5 cm in height. It shouldn't be a tight fit. Leave 5 mm of clearance at the widest point of the valve envelope. This shield should largely prevent pick-up of 'output' RF directly via the connection wires inside the vavle envelope, as they go from the valve base and up to the grid/cathode structure. It won't affect cooling of the valve either, as all the heat is generated above the point, where the shield stops. You would be well adviced in moving the grid tuning cap back from the front panel a bit and isolating its frame from the ground. Use an isolated shaft extender. This will make adding neutralisation of the 807 stage at a later date much easier, if you ever need it. I don't recall if we have discussed this, but since you have moved the TX to the 160m band, you could replace the VFO with a crystal oscillator. A 1.8432MHz crystal is easy to source at most electronic suppliers, and you wouldn't need the large shield box topside. Just a bit of local shielding around the 6AG7 socket, which could then be moved down onto the main chassis plate. Let me know if you want more details on this. If you want to keep the VFO, then, if you want to be able to change and predictably set the frequency, you almost certainly need some form of mechanical reduction drive on the shaft for the VFO variable capacitor. Your layout doesn't seem to provide room for one, so maybe you have already mentioned this before. Best regards Frank N. Edit: By convention the 'flow' of a transmitter circuit mostly go left-to-right, but it doesn't really matter, of course. Last edited by YC-156; 27th Mar 2006 at 6:36 am.