>> From: wrt@eskimo.com (Bill Turner)
To: <amps@contesting.com>
>> Date: Wed, 27 Aug 97 00:37:58 +0000
>
>> A question about possible VHF parasitic oscillations in a VHF
>> amplifier:
>>
>> Using a grounded-grid 8877 on 50 MHz, with the tube output capacitance
>> serving as the only tank capacitance, is there a need for a parasitic
>> suppressor?
>
>There usually isn't even a need at HF with that tube Bill, if the
>grid is properly grounded. ...snip...
I have a collection of kaput, gold-sputter damaged 8877s, which were
removed from amplifiers that use Eimac's grid collet to properly ground
the grid.
>Tubes that use a cone type grid
>connection and a short grid length, when grounded directly to the
>chassis, are most often unconditionally stable.
> That's because the grid design maintains a very low impedance all the
>way up to the frequency where the tube has little or no gain.
The next time anyone has an 8877 amplifier apart, try measuring the grid
resonance with a dipmeter. The 8877 is rated for full input in amplifier
and oscillator service to 250MHz, but it reportedly has considerable gain
above that frequency. (Joel Paladino, n6amg)
>If you look at tubes, you'll see this is true. Tubes like the 811A
>and 572 with long thin leads are very unstable, so is the 3CX1200A
>and D7 (the Z7 doesn't even need a suppressor,
Eimac does not say this.
... ...
>Tubes like the 8877, 3CX3000, ... ...
Henry has apparently had plenty of VHF parasite grief with their
3CX3000A7 g-g linear amplifier.
>
>Other tubes, like the 3CX800 and 3-500Z are in between.
The 3CX800A7 is virtually identical to the construction of an 8877.
>
... ... ...
>...... If you do have parasitics, you
>should easily be able to correct them.
The next time you run into an AL-1500 owner, ask him how many 8877s he
went through during the first year. Owners who experience parasitics in
the AL-1500 'should easily be able to correct them' by adding a VHF
parasitic-oscillation suppressor---like the AL-1500 should have had in
the first place---like all other 8877 amplifiers have.
- IMO, the reason why the AL-1500 comes with no VHF suppressor has
something to do with a river in Egypt. -
Rich---
R. L. Measures, 805-386-3734, AG6K
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