I'm no guru's Bill but I was going to suggest a larger value plate blocker.
At 1.9MHz, 800pF has a reactance of 105 Ohms. If you were to replace it
with 2000pF it would fall to just 42 Ohms.
73, Alek.
VK6APK
At 12:45 PM 22/09/2002 -0400, Bill wrote:
>I left off some info, target Q is 12. Have also made some progress.
>
>I stuck in an additional 1000 pF on the plate blocking cap (from 800 pF to
>1800 pF) & the results are more reasonable. Am now able to get 1500W with
>reasonable efficiency, but the actual load C value is still low (was abt
>600 pF, now 1000 pF, should be closer to 1700 pF).
>
>The tune C value is right in line with the calculations (calc'd at 298 pF,
>actual 285 pF).
>
>Now I need to find a cap or two that will survive on 10M.
>~~~~~~~~~~~~~~~~
>Bill Coleman N2BC
>http://home.stny.rr.com/n2bc
>----- Original Message -----
>From: Bill Coleman
>To: AMPS
>Sent: Sunday, September 22, 2002 9:51 AM
>Subject: Tank tweaking
>
>
>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.
>
>Ideas?
>
>Thanks! 73, Bill N2BC
>
>~~~~~~~~~~~~~~~~
>Bill Coleman N2BC
>http://home.stny.rr.com/n2bc
>
>
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