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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