On Jul 15, 2005, at 2:30 AM, Geert Jan de Groot wrote:
>
> For restauration work on a SB220 I'm looking for specs of
> suitable replacement caps for the grid bypass.
> In the SB220, each of the three grid-pins is connected by a
> 200p mylar cap to ground.
We use 2000v-rated disc ceramics in our parasitic-suppressor retrofit
kits. However, if you wish to incorporate grid-fuse resistors to
protect against grid shorts in the SB-220, the amount of total grid
bypass per socket needs to be roughly tripled or the grid fuse
resistors will blow on 10m.
>
> mylar caps are not easy to obtain these days.
> I have no idea what kind of volts, amps or other specs these caps
> need to be?
Hello, Jan -- The combined bypasses on each tube need to be able to
carry a total of 1.8A-rms at 28MHz. We have found that 4, 470pF,
2000v caps in parallel will do this job.
>
> I'm a bit surprized by the circuit anyway - can someone please explain
> why the grid of a G-G amplifier isn't connected to ground, but
> connected by a 1mh choke and bypassed by 3 200p C's instead?
This was seemingly Bill Orr's idea.
Some have said that his intent was to introduce RF negative feedback,
but there is no evidence that it did -- even on 80m. Since fuses are
cheaper than tubes, I like the idea of RF-grounding the grid through
multiple capacitors and using a 3AG fuse or a fusing resistor to stop
DC grid current during a glitch.
- Others have theorized that Orr's intent was to improve the VHF
stability of the 3-500Z by raising the grid's resonant freq. This
cancels out some of the grid's internal XL with external XC. The 600pF
Orr used raised the grid resonance from c. 88MHz to about 89MHz, but
the stability changed little because the SB-220's parasite is near
110MHz.
>
> 73,
>
> Geert Jan PE1HZG
>
>
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>
Richard L. Measures, AG6K, 805.386.3734. www.somis.org
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