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The coil is made of 316 copper tubing which is connected directly to one of the tubes filament terminals, and an insulated 12 wire passing through the interior of the coils tubing is connected to the other.

The filament voltage is then applied to the cold end of the coil, as well as its interior wire, as shown in the schematic drawing below.

The LC circuit is shunted by a 2.5K ohm resistor to reduce its Q, and tunes from 7 mHz to 30 mHz, thus eliminating the need for an input band-switch.

For this reason it is necessary that C3 have off-center rotor plates for ease of tuning the higher frequencies.

With the input tap position on L2 set properly, a near-perfect 50 ohm input impedance will be realized on all bands.

The maximum output power of a single 813 would normally be around 300w PEP below 21 mHz.

With approximately 50w input, this amplifier produces 600w PEP output nominally, and around 500w PEP output at 29 mHz.

Its intermodulation and harmonic distortion products are well below 40 dB down from its maximum power output, and its tendency toward parasitic oscillations is so low that a parasitic plate choke is unnecessary.

To operate the 813 at these output power levels, a sufficient volume of air moving over the tubes glass envelope is required to dissipate the increased heat.

In this case, the tube is mounted horizontally, about above the case bottom and about 1 from the rear of the case, directly in front of a 3 exhaust fan that is mounted on the rear panel which draws cooling air around the envelope, from the air holes in the top and bottom of the case (see photo of case below).

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A low value 10w resistor can be placed in series with the Fans AC supply to slightly reduce the fan speed, thus reduce noise.

A 3kv 350ma continuous duty rated power supply is required for the 813 to achieve its maximum output power.

A schematic for a suitable HV power supply is shown at the bottom of this page.

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Counting from the C1 end, the following taps are placed at: 2 turn for 10m, 2 turns for 12m, 4 turns for 15m, 5 turns for 17m, 7 turns for 20m, and the full coil for 40m.

For this, the current rating of T-3 must be increased to 1.5 amps.

I would also like a complete list of parts if that is possible.

Licensed Amateur Radio operator in 1996 as IW5EDI, active member of ARI Firenze and ARRL.

Class 1970, married with two childrens, love experimenting and antenna home-brewing.

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IT System Engineer, recently started having fun with morse code and Raspberry Pi.

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