[Amps] Tank tweaking
Sun, 22 Sep 2002 10:42:29 -0400
>A few months ago I posted a query about crummy efficiency on my homebrew
>5CX1500 AB1 amp. I have cured most of the original problems by re-doing
>the input circuit and tweaking the output tank coils. Oh, 'tweaking'
>means creating a lot of scrap copper.
>It works well on 10 through 80. Efficiency on the order of 60%, the
>'real world' tune and load pF are very close to the calculated values.
>I'm using a spreadsheet I cooked up from the W5FD formulas.
>But... I have been chasing my tail on 160M. The calculated C1 / L / C2
>values at 1.9 MHz are 298pF / 25.47 uH / 1708 pF. Actual values are 276
>pF / 25.53 uH / 600 pF. Efficiency is off to 48%.
>The tank coils are 3/16" copper tubing for 10/15/20 and a hunk of coil
>stock for 40/80/160. Plate blocking may be a bit small @ 800 pF - would
>that have such a huge effect on loading? The plate choke is an RFParts
>RFC-3 (Ameritron) 225 uH & I don't feel any undo heating. Plate bypass
>is a pair of 500 pF @ 30KV caps & I've tried adding an additional 1000 pF
>6KV disk ceramic with no effect.
>The input is across a 200 ohm grid resistor with a 4:1 balun. It is
>untuned on 160 and 80, tuned on 10 thru 40.
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