Manfred,
Maybe my nomenclature of plate bypass capacitor is incorrect. It is
capacitor connected to the DC feed end of the plate choke. I have never
seen a capacitor of that large value used for that application. In my
experience, they are usually a 0.001 uF.
My desire to replace the original paper capacitor is this; the SBE-33 is
a compact unit and in the physical layout, the capacitor is mounted in
very close proximity to one of the PL-500's with the axis of the
capacitor parallel to the axis of the tube. I am sure it gets rather
hot in operation. I wanted to replace it with a capacitor that would be
smaller in physical size and could be moved further away from the tube.
0.1 uF 1KV or higher rated ceramic capacitors are not easy to come by
these days.
As far as the cathode bypass capacitor goes, space and heat is not a
problem there and I could easily replace the original paper capacitor
with a new film capacitor.
David
KC2JD/4
Manfred Mornhinweg wrote:
> Hi David,
>
>> the plate bypass capacitor is a 0.1 uF paper capacitor.
>
> Plate BYPASS capacitor? Is that circuit a cathode follower? Otherwise
> there shouldn't be a plate bypass capacitor! Perhaps you mean a plate
> COUPLING capacitor?
>
>> There is also a 0.1 uF
>> paper capacitor as a cathode bypass on the pair of PL-500's. Is there
>> any reason why I shouldn't replace these with 0.001 or 0.01 ceramic
>> disks?
>
> Most certainly yes. A 0.1uF capacitor has a reactance of one half ohm at
> 80 meters. A 0.001uF cap has 50 ohms. When the cathode impedance is just
> a few tens of ohms, which is probably the case with those PL-500's, a
> 0.1uF is a good bypass, while 0.001uF isn't a bypass at all!
>
> If the plate cap you mean is a coupling cap, then a much smaller value
> might work fine there, because the plate impedance is pretty high. But
> not at the cathode!
>
> The main technical reason to use a 0.1uF cap as plate coupling cap,
> might be to achieve low frequency stability. Although not so common with
> tubes as with transistors, leaving the device unterminated on a low
> frequency can easily lead to self-oscillations. So, while the 0.001uF
> cap will surely work well enough coupling RF, it might make the PL-500s
> self-oscillate at audio, depending on the rest of the circuit!! But it's
> not very likely.
>
> Anyway, if you are RESTORING the radio, try to keep as close to the
> original design as possible, even if just for the sake of authenticity!
> There are some high quality 0.1uF 1000V polypropylene caps out there,
> which even look good in old equipment!
>
> Manfred.
>
> ========================
> Visit my hobby homepage!
> http://ludens.cl
> ========================
> _______________________________________________
> Amps mailing list
> Amps@contesting.com
> http://lists.contesting.com/mailman/listinfo/amps
>
>
_______________________________________________
Amps mailing list
Amps@contesting.com
http://lists.contesting.com/mailman/listinfo/amps
|