ORIGINAL MESSAGE:
On Mon, 24 Aug 2009 12:44:13 -0400, "Carl" <km1h@jeremy.mv.com> wrote:
>
>Then give us an example of a parasitic free 3-500Z amp....if you can.
REPLY:
Ok, although I am not a fan of 3-500Z's and have only used one once. I converted
a Heath SB-1000 to six meters, removed the parasitic suppressor and replaced the
input circuit with a high-Q pi network and it was and is perfectly stable.
I believe the stability in this amp comes mainly from the high Q of the input
circuit, together with very short leads in that area. Ideally, you want the
input circuit right at the tube socket, not off somewhere else in the chassis.
The net result is very low impedance at the VHF parasitic frequency, too low to
sustain VHF oscillation.
I believe designers make a mistake when using an input circuit of relatively low
Q at the operating frequency. That allows the VHF impedance to creep up into the
range of potential instability. I have built several amps with the input circuit
having a Q of five or more and they are all stable without any parasitic
suppressor at all, although only one used a 3-500Z. The others used either an
8877, a 4CX1000A or a 4CX1500B. All of those are physically small tubes which
lend themselves to short leads in the anode circuit. Short is good because it
raises the VHF parasitic frequency and thus lowers the gain at that frequency
and also places it further away from the operating frequency.
I can't guarantee that approach would be effective in all cases but it certainly
is worth a try. Eliminating that pesky parasitic suppressor in the anode circuit
is a definite plus.
73, Bill W6WRT
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