On Thursday 08 October 2009 07:43:29 am Roger Parsons wrote:
> Whilst the explanation below makes perfect sense, I have 1kW and 500W ex
> commercial FM (100 MHz) broadcast transmitters made by CCI - the former
> uses an 8877 and the latter a 3CX800A7. Neither of them had filament chokes
> and I presume they worked OK! (The 1kW has been modified to form the basis
> of my 160m remote station and the 500W awaits a project.)
> 73 Roger
We recently finished redesign of several RF amps, all using the 3CPX800A7.
The original amps all had one filament lead tied directly to the chassis
without any filament choke. The other lead used a simple choke. We are
working up a couple of designs with the 8877.
According to the data sheets for both of these tubes, the filament leads
require RF isolation, however, the original (working) design didn't adhere to
this particular data sheet recommendation. I called CPI (Eimac) for
clarification. According to CPI, both filament leads should be at high
impedance for the intended frequency of operation (ie filament choke in both
In the redesign, we used a bifilar choke, wound on a 3" long, 1" diameter,
ferrite rod, thereby isolating both filament leads.
Using this bifilar choke, in the dual-3CPX800A7 amp, with 37W drive, we are
now getting 1500W output on 6m. Anode voltage is 2900V. The original design
(with one filament tied to ground) was definitely not this easy to drive...
>From a conceptual standpoint it makes sense that RF drive power will
(capacitively) couple from the cathode into the filament. Making the
filament a high-impedance to RF would therefore be the prudent thing to do...
>From experience, the amp works without the choke in both filament leads, but
certainly works better with high RF impedance in both filament leads.
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