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Using chassis-mounted sockets with the kits
Hi Guys

Hobbyists often ask if they can use a chassis-mounted tube socket with the kits, to which the answer is 'yes', but with caveats.

The PCB layout incorporates the tube sockets and this controls the physical form of the connections to the sockets. All the parasitic capacitances and inductances are then controlled and stability is assured for the assembly. There is a further benefit that unit-to-unit consistency is assured even though most hobbyists will only build one of a thing.

If you really must use chassis-mounted sockets but want to useĀ  London Power PCB project as the basis, then the grid-stop resistors should be mounted right at the socket pins and a wire link installed in those positions on the PCB. The wiring to each triode or pentode should just be #22 stranded wire, properly twisted. For example, each triode in a dual-triode needs three wires. You can solder the wires to the PCB end, say, then measure out more wire than the distance to the socket side to be wired; then cut the wires; then twist them starting at the anchored end and allow ALL 3 free ends to rotate as you twist; only then do you begin terminating at the socket. Of course, you can reverse this order and begin at the socket remembering to install the grid-stop between the grid wire and the socket pin. Each triode then has its twisted-triple.

Heater wires should be twisted unto themselves. London Power PCBs that carry dual-triodes accommodate both 6V and 12V wiring. 12V wiring is preferred for 12V-capable tubes, as TUT4 explains, and in that case you do not have to bring out the pin-9 wire. The PCB keeps the heater wiring away from the triodes but now you are changing the effective layout and need to do so with care.

DO NOT use multi-conductor shielded cable to the tube socket. The cable capacitance will kill the frequency response and you'll be replacing it with twisted hook-up wire, so save a step Smile

The same guidance applies to octal tubes, pentodes and tetrodes. With the latter, especially with transformer connections, the transformer connections can still go to the PCB unless it is neater to have them go directly to the tube socket. Remember that the screen-stop should be tied to the screen-pin at the socket. The PCB achieves this, but now you have to place the resistor on the socket in the traditional manner. Where 300V wire is okay for everything else, the plate and possibly the screen wires for a power pentode - tetrode should be rated higher, at least 1kV.

Have fun

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