Hi Gary,
The grid current is almost the same at each point. Within the variation
I would see in each itternation of a tune process.
And the efficiency number, that was not discounting the feed-through
power. The numbers below are adjusted for the feed through and you can
see they are more in the range of what would be more typical.
73/jeff/ac0c
garyschafer@comcast.net wrote:
>
> Hi Jeff,
>
>
>
> You didn't tell us how much grid current there was in each part of the
> band for each test. You also seem to have the amp loaded heavier where
> the efficiency is lower, you are running more power there. That can
> make a difference.
>
> Try loading it to the same power input and see what it does.
>
>
>
> The plate choke can also make a difference. There will be more
> circulating current in the choke at the lower frequencies so more loss
> in the choke. I did not do any numbers so off hand I don't know how
> much difference there.
>
>
>
> The 70 some % efficiency that you quoted in an earlier email is
> unrealistic for a linear amplifier also. If you were seeing that much
> then the amp was no longer operating in a linear mode. You were
> overdriving it.
>
>
>
> Another possibility is an error in the wattmeter from one end of the
> band to the other.
>
>
>
> 73
>
> Gary K4FMX
>
>
>
>
> ----- Original Message -----
> From: "Jeff Blaine" <keepwalking188@yahoo.com>
> To: amps@contesting.com
> Sent: Wednesday, March 11, 2009 5:12:03 PM GMT -05:00 US/Canada Eastern
> Subject: Re: [Amps] Odd efficiency shift on 80m band
>
> Thanks to the several guys who have asked a few additional questions.
>
> Please find the answers here:
>
> 1. Actual data taken just now (indicated SWR on the rig side 1.15-1.25
> range):
>
> 4.00 mhz 73w drive Po=985w Ip=0.62A B+=2420v Pin=1500w
> 60.8%
> 3.75 mhz 71w drive Po=985w Ip=0.645A B+=2400v Pin=1548w
> 59.0%
> 3.50 mhz 73w drive Po=1000w Ip=0.710A B+=2375v Pin=1686w
> 55.0%
>
> 2. Results taken driving into a HK dummy load. 1.05 approx SWR typical.
>
> 3. Adjustments to coax length, both on the input and output, have
> minimal effect on the SWR but none on the efficiency readings.
>
> 4. Measurements taken with the Telesync LP100a on both sides.
>
> 5. With respect to the "things tried" list, they have made some minor
> difference in the absolute level - but the "slope" of the change remains
> constant. Meaning, if (for example) one thing bumps the efficiency up
> 1%, the change is 1% on both ends of the band. The approx 6-percentage
> points drop from the top to the bottom of the band remains consistent.
> Unfortunately!
>
> 73/jeff/ac0c
>
>
>
> Jeff Blaine wrote:
> > Gentlemen,
> >
> > 2xGI7B HB amp. Shows a funny efficiency shift on the 80m band and I
> was
> > hoping that some of the more experienced hands on the board may have
> > seen something like this before....
> >
> > On the top of the band (only), the calculated efficiency
> > [(Po-Pdrive)/Pin] varies quite a lot on the 80m band..
> >
> > 4.00 mhz 61%
> > 3.75 mhz 57%
> > 3.50 mhz 55%
> >
> > On all other bands, the efficiency is essentially flat over the range.
> > I can understand a significant variation on say 10m, but on the low
> > band, the efficiency of the amp should be more linear across the
> band as
> > stray L/C are a much smaller % of the lumped values.
> >
> > Things tried:
> >
> > 1. Replaced filament choke (much more L)
> >
> > 2. Increased coupling capacitance (2x C)
> >
> > 3. Varied the Q of the input matching network (Q=1.5-3.0)
> >
> > 4. Varied the L of the tank (+/- 15%)
> >
> > Things not tried:
> >
> > 1. Increase plate choke L (it's a RF parts/MFJ stock unit of 225uH with
> > no resonances in the ham bands). This is the stock choke for all
> > Ameritron amps. Approximate Xl=5K ohms @ 3.5Mhz vs an estimated 1800
> > ohms plate load Z. Seems like it should be plenty...
> >
> > None of these things made any difference. Sure would appreciate any
> > suggestions on where to look next...
> >
> > 73/jeff/ac0c
> >
> >
> > _______________________________________________
> > Amps mailing list
> > Amps@contesting.com
> > http://lists.contesting.com/mailman/listinfo/amps
> >
> >
>
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