[Amps] Amps Digest, Vol 122, Issue 28

[Carl]

Where possible run wide copper strap for all RF, even the input and grid 
grounding.
Using modern procedures on ancient tubes Ive been able to eliminate 
neutralization and parasitics on several grid driven amps thru at least 20M.

Carl
KM1H




----- Original Message ----- 
From: "Mike Waters" <mikewate at gmail.com>
To: <amps at contesting.com>
Sent: Monday, February 25, 2013 6:26 PM
Subject: Re: [Amps] Amps Digest, Vol 122, Issue 28


> Well said, Bill! That's exactly what I had in mind.
>
> 73, Mike
> www.w0btu.com
>
> ---------- Forwarded message ----------
>
>> From: Bill Turner <dezrat1242 at yahoo.com>
>> To: Amps <amps at contesting.com>
>> Cc:
>> Date: Fri, 22 Feb 2013 17:50:02 -0800
>> Subject: Re: [Amps] Why always one parasitic suppressor per tube?
>> ORIGINAL MESSAGE:
>> On Fri, 22 Feb 2013 11:18:14 -0600, Mike wrote:
>>
>> >Earlier handbooks, etc, show plenty of amps with different tubes with no
>> >parasitic suppressor at all, and that's what I'm going to try.
>>
>> REPLY:
>> Remember your oscillator basics:  RF energy has to be fed back to the 
>> input
>> circuit in the correct phase to sustain amplification. IMO, the best way 
>> to
>> suppress VHF oscillation is to make the input circuit a very low 
>> impedance
>> at the frequency of the VHF parasitic tank in the plate circuit. If the
>> impedance is low enough, it will swamp out enough RF energy so that the
>> tube(s) will not oscillate.
>>
>> Probably the most important rule in keeping VHF impedance low i to keep 
>> all
>> wiring in the input circuit as short as possible. One of the worst things
>> you can do is to run coax from the back of the chassis where the tube is 
>> up
>> to a band switch on the front panel. Sections of coax like this act as a
>> transmission line transformer and can exhibit very high impedance at VHF
>> frequencies, just what you don't want. Instead, mount the input 
>> bandswitch
>> right next to the tube socket. Yes, I know this means having two separate
>> band switches but it goes a long way toward stabilizing your amp.
>>
>> I have built several amps with this in mind and NO parasitic suppressor 
>> in
>> the place circuit. All were unconditionally stable, even with the antenna
>> disconnected.
>>
>> Bill, W6WRT
>>
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