Mike asks:
>Without access to a spectrum analyzer, how does one determine what
>frequency your new amp has any parasitic oscilations occuring on, if
>any?
To some extent, does it matter?
In the 'good old days', the recommendation was to reduce the bias and the HV
so the tube was dissipating about 2/3rds of rated plate dissipation. Using a
dummy load on the output (and I'd figure on the input too) rotate the plate
tune and load caps, looking for a flicker of plate current (and/or grid
current), especially at the ends of the capacitor range. The real problem is
going to be with a tube rated for zero (or very little) grid current, such
as a 4CX1000, a good rip roaring parasite could over dissipate the grid.
This is where the gold balls that Rich has found come in, I suspect.
Once the parasite is established, you can use a frequency counter or an
absorption wavemeter to measure the frequency. I still think you want to get
the input power as low as possible, so reducing plate supply volts on a
triode is good start. On a tetrode, dropping the screen supply rather than
plate supply is probably the best approach, plus keeping a careful eye on
the meters. As the taming procedure progresses, you keep gently pushing up
towards full supply volts.
What I am not sure of is what knowledge of the parasite frequency gives you
if you don't have access to other test gear to start measuring reverse gain
at that frequency.
73
Peter G3RZP
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