>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.
>
** However, the DC blocker cap dissipates virtually no power. More Xc
ohms results in a slightly different setting of the Tune & Load caps, but
no change in P out.
>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
>>
>>
>>--- StripMime Report -- processed MIME parts ---
>>multipart/alternative
>> text/plain (text body -- kept)
>> text/html
>>---
>>_______________________________________________
>>Amps mailing list
>>Amps@contesting.com
>>http://lists.contesting.com/mailman/listinfo/amps
>
>_______________________________________________
>Amps mailing list
>Amps@contesting.com
>http://lists.contesting.com/mailman/listinfo/amps
>
- R. L. Measures, a.k.a. Rich..., 805.386.3734,AG6K,
www.vcnet.com/measures.
end
|