[R-390] R390A RF deck advice
Charles Steinmetz
csteinmetz at yandex.com
Sun Jan 11 02:38:32 EST 2015
Nelson wrote:
>some online research about the parasitic suppressor
>resistors suggests that (at least in audio circuits, and I don't see
>why that wouldn't apply
>to HF circuits) if the resistor values are too high they can roll
>off the gain on the higher
>frequencies where good gain might be most needed. In view of that
>and after all the
>other work done, it would be a waste not to finish the job, and
>replace the over-value
>suppressor resistors.
Correct theory, but missing some particulars. The "grid stoppers" in
HF amplifiers are designed to prevent oscillations well into the VHF
region (hundreds of MHz). They could increase by 400% or more and
still not impair the in-band performance of the stages.
>I have no idea whether carbon comp resistors get quieter with age.
As a grid stopper, the only noise of concern is the Johnson (thermal)
noise, which does not change over time. If there were current
through the resistor, its "excess noise" would come into play, and
that may change over time. But in the case of grid stoppers, only
the vanishingly low grid current flows through the resistors, so the
excess noise is completely insignificant.
There is no reason to change grid suppressor resistors in a 390 that
have increased by 30%, 44%, or even 200%.
Best regards,
Charles
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