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
>
>
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>
>
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