{Collins} [Fwd: Re: 30S-1 Screen voltage questions]

>From the original paper Eimac data sheets (both the 4CX1000A and the 4CX1500B have same spec here): Max screen voltage is 400 VDC. 411VDC is kinda high. I'd add a small transformer to buck it back down to 325VDC in the SSB position. Kind of a reversed screen mod. hi. Like two wrongs making it right. 

Russ WQ3X

> From: "James Thorusen" <entropycat@xxxxxxxxxxx>
> Date: 2006/12/06 Wed AM 09:07:58 EST
> To: <jperone@xxxxxxxxxxxxxxx>
> CC: "AAACollins Mail List" <collins@xxxxxxxxxxxxx>
> Subject: {Collins} Re: 30S-1 Screen voltage questions
> Greetings to John and the list.
>    In response to your question below:
> <snip>
> "   Let me explain what I have found...
> The screen voltage at pins 10 & 11 on T201 measure 288VAC in CW mode &
> 400VAC in SSB mode.
> This results in 323VDC / CW & 411VDC / SSB on the screen of the final. It
> has a 4CX1000A installed.
> I think it must have a Peter Dahl transformer with these voltages, but I can
> find no markings on the transformer?
> Will these voltages hurt the 4CX1000A or should I be looking for a 4CX1500B
> to install?   "
>    According to my Eimac power grid tube catalog, typical operating voltages
> for the 4CX1000A in class AB1 linear amplifier service are 3 Kv plate
> voltage, 325 V screen,  plate amps 0.88, output power 1630 watts.   The
> maximum ratings for the tube specify only screen dissipation, which is 12
> watts.   Using your SSB screen voltage of 411 volts, this would be 29 mA of
> screen current.   It would be correspondingly more for the CW screen
> voltage; about 37 mA.
>    Screen dissipation is usually the limiting factor due to heating of the
> screen grid;  the voltage is not usually of much concern except to operate
> the tube in the appropriate region of linearity.   Usually, you will exceed
> screen dissipation long before you experience flash-over as the voltage goes
> up.
>    Incidentally, on another thread,  Jan Sterneckert, N5YPK states something
> to the effect that the Collins book on the 30S-1 states that the screen
> current should be "never more than +/- 25 MA".   That would be in keeping
> with my calculations above, but my 30S-1 book (4th edition, 1 December,
> 1959) states in a table of  expected values about screen current:
> "Disregard.   Use only for maintenance purposes."   It would be interesting
> to see whose book is newer.
>    In the meanwhile, I personally do not like to see negative screen
> current.   It can be zero, or positive, but negative screen current means
> screen emission.   The screen cannot emit electrons unless it is running
> hot, and this is a condition best avoided, in my humble opinion.
> Regards,
> Jim T.
> entropycat@xxxxxxxxxxx

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