FYI, for my 80m/160m duoband amp (2500V, three GU-74Bs), below the "main"
200uH plate choke, I'm using a supplementary 62 uH choke wound on a ferrite
core and bypassed at the cold end with a Sprague 0.01uF/6000V poly
capacitor. Both the cap and choke are below deck, out of the RF field. I
measured the self-resonant frequency of the capacitor to be 5.7 MHZ, so it
wouldn't be suitable for bypassing above 80m.
73,
Jim W8ZR
> -----Original Message-----
> From: Amps [mailto:amps-bounces@contesting.com] On Behalf Of peter
chadwick
> Sent: Friday, October 18, 2013 3:01 AM
> To: audioguy@q.com; Fuqua Bill L; Amps@contesting.com
> Subject: Re: [Amps] Alpha 76A - 120hz hum on 160m only
>
> I'm guessing a bit here.....but the numbers should be 'ballpark'.
>
> The 'cold' end of the plate choke is bypassed with maybe a total of around
5000pF to
> ground. That's about -j17 ohms. The plate choke is say 250 microhenries or
+j 2800 ohms.
> The HV is around say 2500 volts so the plate swing is around 1500 volts
rms. This gives
> around 500mA of RF through the choke, and so the 'cold' end of the choke
will be at about
> 8.5 volts of RF. If you haven't got another choke and bypass as you work
back to the PSU,
> you're impressing that back on the rectifiers.
>
> The situation would be worse on 160, of course. something like a small 50
or 50 microhenry
> choke (even wound on a bit of lossy ferrite) and another bypass - 0.005 or
even 0.01
> microfarads should help. So would a 50 ohms glitch resistor with a bypass
on the PSU side
>
> Of course, you may already have a choke between PSU and plate choke, in
which case this
> is horse feathers...
>
> 73
>
> Peter G3RZP
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