[Amps] Odd efficiency shift on 80m band

[garyschafer at comcast.net]

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 at yahoo.com> 
To: amps at 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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