Bill Turner wrote:
>
> ORIGINAL MESSAGE:
>
> At 10:19 AM 1/2/2006, Steve Thompson wrote:
>
>> I can't help but think that a low inductance Xc=5-10 ohms from cathode
>> to ground would make a big difference to the calculation. Cutting the
>> loop gain by 7-10dB is a good recipe for better stability.
>>
>> Steve
>
>
>
> ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>
> I wonder about this too. In my amp, I have a low-Q parallel resonant
> tank circuit on each band from cathode to ground. Q of about 2-3. I
> would think this would greatly reduce the tendency to oscillate at VHf
> compared to an untuned input.
It will depend on the impedance presented to the cathode at VHF. If the
capacitance to ground is on the end of a wire and/or bandswitch, it's
not going to provide a low Z to ground except where it's series resonant
with the wiring. Up to 30MHz the usual pi input matching could include
(say) 200pF fixed at the tube socket, and allowed for in the switched
sections. Make this 4x50pF distributed round the cathode pins, and the
effect is likely to be better.
The key point is that there's nothing magic about parasitic oscillations
- the rules are still the same, loop gain >1 and phase=0. Prevent this
and your amplifier is stable. The low internal inductance in most V/UHF
ceramic tubes means that you have more options.
Steve
_______________________________________________
Amps mailing list
Amps@contesting.com
http://lists.contesting.com/mailman/listinfo/amps
|