Re: {Collins} Isolation Transformer



The original Collins plate transformer provides both plate and screen
voltages for the 4cx1000A. The screen voltage winding provides approximately
265 VAC @350mA CCS. After rectification and filtering, the screen supply
places a negative voltage about -230VDC on the cathode of the tube.  The
screen grid pins on the tube are grounded.  

The reason for this unusual design is to simplify regulation of the screen
supply. In fact, the screen voltage only drops by about 25V under full load,
with nothing but the supply's simple choke input filter.  Another advantage
is that the screen voltage adds to the high voltage, raising the effective
plate voltage by the amount of the screen voltage. The disadvantage of the
design is that the screen supply must handle the full cathode current of the
amplifier, which is about 550-750mA, rather than the screen grid current,
which is only about 25 mA.

There are several modifications for bumping the screen voltage of the 30S1
by about 100V (to 330V), which is highly recommended.  Peter Dahl makes a
replacement plate transformer (now sold by Harbach Electronics) which has
the higher screen voltage winding.  A cheaper, if less elegant, method is to
put the secondary of a 115V isolation transformer in series with the screen
voltage winding of the existing plate transformer. This is literally a five
minute modification, with the transformer resting on the auxiliary shelf in
the 30S1 power supply compartment. No additonal filtering or rectification
is required. However, to do this modification, the isolation transformer
should be able to handle roughly an ampere of current, the reason, again,
being because the screen supply must carry the full cathode current of the
amplifier. 

With the higher screen voltage, the 30S1 really comes alive. Although the
drive requirement increases from 60W to 100W, the power output climbs from
1000W (at the postive screen current threshold) to 1500W.  The plate current
(at the screen threshold) jumps from 560mA to 880mA. In SSB or CW service,
the power supply and cooling of the amplifier easily handle the power
increase.

73,

Jim Garland W8ZR

> -----Original Message-----
> From: collins-bounces@xxxxxxxxxxxxx [mailto:collins-bounces@xxxxxxxxxxxxx]
> On Behalf Of Colin Lamb
> Sent: Thursday, January 29, 2009 9:34 AM
> To: Clint Hasty; collins
> Subject: Re: {Collins} Isolation Transformer
> 
> All Electronics sells a 40 vac ct @ 250 ma transformer for $2.25.  I do
> not remember the voltage you need, but you can either bridge rectify that
> to ge 60 vdc or double to get 120 vdc, which can be added to the existing
> screen voltage.
> 
> 73,  Colin  K7FM
> _______________________________________________







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