Tom asks:
>Exactly how much change do you see in a tube. Let's take an 8877 for
>example? What parameters change so drastically?
Now, I don't know with an 8877 - never used one. New RCA 6146B show a
variation in plate current for fixed plate, bias and screen voltages from
4mA to 27mA for new tubes, with a standard deviation of about 11mA.
4CX250B's show similarly high variations. Then there's the variation in
interelectrode capacity. I looked into the variation side when I did a
marine tx with 6146B's in the 70's to decide whether it was better (ie
cheaper) to use individual bias pots, or select tubes. We went for the
pots...................
>Two more questions. How did you know the amps had parasitics?
>What lead running near the glass?
'Cos with a 50 ohm input 'load' and a 50 ohm output load, the plate current
varied as you varied the tuning, as would the grid current. Some tuning
points, just standing plate current and no grid current. Others, lots of
both. A neon lamp on a stick offered up to the tank circuit had that sickly
bluey - yellow - orange sort of glow that goes with VHF energy, and at the
tank capacitor, it didn't glow as brightly - lots of variation as you move
it around the leads. That, to me, (and, I hope, to you) is adequate evidence
of VHF parasitics. LF parasitics caused by chokes resonating in plate and
grid usually give a very deep red colour in a neon on a stick, and the
intensity doesn't vary much as you move it along the leads.
The lead running near the glass was the bypassed HV lead as I remember it.
Trouble is that memory fades, and it was about 25 years ago, but to this
day, I can picture that amp and the glow of the neon.
One tx I did I tried ferrite beads on the 6146 anodes, with no success. The
best answer for those tubes that I've found is a 68 ohm grid stopper, a 47
ohm screen stopper and a 'conventional' plate suppressor with about 33 ohms.
With that approach, I put one tx into production without ever having
parasitics to sort out at all.
But I still wouldn't go for an HF amp without suppressors without a lot of
evaluation and development - and probably not even then. Even if I don't
agree with Rich about bending filaments with parasitics!!!
73
Peter G3RZP
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