[Amps] Fw: Tank tweaking

[Gary Schafer]

Hi Bill,

Are you sure that your watt meter reads ok on 160 compared to the higher bands?

The other possibility is that the coil stock for the 160 coil has too small wire. The lack of skin effect gets to be more important as the frequency drops. The RF penetrates the wire deeper.

73
Gary  K4FMX


Bill Coleman wrote:

> Thanks for all the ideas.  I've had a few requests for a couple bits of data...
>
> On 160 the target Q is 12, Plate load is 3200 ohms.
>
> I have tried changing the plate blocking cap from 800 pF (it was 4 200 pF HT57's) to 1000 pf and 2000 pF using some HH58's. This appears to have helped, but it's still way down on efficiency (<50%) compared to any of the other bands (even 10M - 60%).
>
> I tried changing the physical layout of the coil stock relative to the 10/15/20 meter coils. Thinking (?) there was some wierd cancelling/coupling.  No change.
>
> Next experiment will be to tune the input circuit on 160.  I didn't bother tuning either 160 or 80.
>
> Right now the only band that is troublesome is 160.
>
> Thanks all.   73, Bill  N2BC
> ~~~~~~~~~~~~~~~~
> 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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