> ----------
> From: Ian White[SMTP:G3SEK@ifwtech.com]
> Reply To: Ian White
> Sent: 20 December 2001 10:22
> To: Peter Chadwick
> Subject: Re: [AMPS] Bypassing Screen of 4cx800a for 50 MHz
>
> Peter - please can you forward this to AMPS. My messages are currently
> being blocked by contesting.com's paranoid Spamcop robot!
>
>
> 2 wrote:
> >>If I were going to build a single Tetrode amp on 6m or 2m I'd consider
> >>running the screen at deck - RF simplicity at (slight) HT complexity
> (ie.
> >>need for a B- lead).
> >>
> >// A chassis-grounded/earthed screen solves the screen's bypass-C
> >resonance problem, however, an anode (B+) to ground arc can briefly
> >drive the HV negative lead to 1000s of volts. Unless precautions are
> >taken, the arc may cause damage to components that are connected to the
>
> >HV negative
>
> Even for grounded cathode, you still need a separate B-minus lead
> (unless you want to do the anode current metering up there at B+
> potential) but that isn't normally a problem because B-minus is clamped
> very close to chassis ground by protection diodes.
>
> Rich is correct that the advantages in RF performance of grounded-screen
>
> are balanced by disadvantages on the DC supply side. Basically you have
> to float all the anode, G1 and G2 power supplies, which must be commoned
>
> at the cathode - not the chassis. The *only* chassis ground for the
> entire power supply must be through the screen current meter.
>
> As Rich says, all the components on the cathode/G1 side have to be
> insulated to withstand a large negative voltage in the event of an arc
> or other current surge - including the anode and G1 meters and all the
> bypass caps. A varistor between cathode and screen (chassis) will limit
> the negative voltage surge, but it has to be rated to handle the full
> surge current... which is why you still need a limiting resistor in the
> B+, to limit the maximum current that the Varistor has to handle.
>
> Another problem with grounded-screen is that all your RF bypass
> capacitors are grounded to the chassis, not the cathode, and when you
> switch to TX there is a surge of current through the screen connection
> to charge these bypass caps. This makes screen-overcurrent protection
> more difficult... still possible, but more difficult.
>
> > In a typical filament-type tube amplifier, such as 2, 3-500Zs, such an
>
> >arc usually will not breakdown the filament-transformer secondary
> >insulation, but it will often damage the grid-current meter shunt
> >and/or the grid current meter. In a typical heater-type tube amplifier,
>
> >provided that one side of the heater is grounded, a HV+ arc to chassis
> >can cause the cathode to briefly rise to several kV negative - which
> >causes an arc between the cathode and the grounded heater (typical
> >cathode-heater rating: 150v-max). The secondary arc can burn the
> >heater wire in two, thereby destroying the tube.
>
> Also very true. If the screen is at chassis ground, the cathode will be
> at -350V or more. Unless you tie the heater to the cathode, the
> heater/cathode voltage rating will be exceeded even in normal
> operation... and with an arc it will be even worse.
>
> --
> 73 from Ian G3SEK Editor, 'The VHF/UHF DX Book'
> 'In Practice' columnist for RadCom (RSGB)
> http://www.ifwtech.com/g3sek
>
> --
> 73 from Ian G3SEK
>
--
FAQ on WWW: http://www.contesting.com/FAQ/amps
Submissions: amps@contesting.com
Administrative requests: amps-REQUEST@contesting.com
Problems: owner-amps@contesting.com
|