I received the following email from Isaac Díaz <fs12742@autovia.com>
about 4CX250s. It appears to me that he has a problem with instability,
which I have pointed out, and that the amplified needs to be
neutrilised. However, some here might have some more comments. If so,
please send them directly to Isaac Díaz <fs12742@autovia.com>, as he
does not read this group.
David Kirkby, G8WRB.
> Isaac Díaz wrote:
>
> Dear Ph.D. Kirkby,
>
>
> My name is Isaac and I am a student of marine radioelectronics at
> La Laguna University (Tenerife, Canary Is., Spain), the career needed
> to get the professional degree as Radio Officer of the spanish
> merchant navy. Also I am an amateur radio.
>
> At the moment I am finishing my fifth year (the last one course),
> where I have to write little thesis as a final proyect, this is why I
> am restoring RF power amplifier from the 70s, which was a part of a
> marine transmitter. It uses four 4CX250B in parallel, maximum
> power with modulation A1 is 1000W, 1400W with J3E.
>
> During the restoring process I have been working with non new
> Eimac valves, just to check the amplifier, and the power supply I had
> to design, however, I ordered four new boxed valves (to England), they
> will be the valves to use in the amplifier after all things seem to be
> working properly. The valves I received were built by STC Components
> in England, and the problem is I cannot drive a RF level to the
> amplifier to get over 400W because anode current grows suddenly
> so high, as a short circuirt. An engineer I know told me problem is
> due to internal impedance of the valves, he told me to change resistor
> in each cathode (2.5ohms), but I couldn´t get good results. I
> checked there is not problem if three new valves are used to work
> with old one valve, problem appears only when are used the four STC
> valves.
>
> What do you think? What may I do?
>
> Many thanks
>
> Regards Isaac Diaz
>
>
> Isaac Díaz wrote:
>
> Dear Mr. Kirkby,
>
> Yes, please, copy my email to "amps".
>
> I wanted to adjust neutralization as showed in the handbook of
> this amplifier, no easy to do, that means I should tune on 22MHz
> (I don´t have a RF exciter for marine bands so I have to tune on
> 21.5MHz, 15m.ham radio band), then rotate load control one turn
> anticlockwise, retune, rotate tune control equally clockwise and
> anticlockwise a check that anode current doesn´t change, if this
> current was different then adjust neutralization trimmer, the problem
> I cannot tune on that frequency properly. The amplifer have this
> marine ranges and bands:
> 1.6-2.5MHz, 2.5-3.8MHz, 4/6MHz, 8MHz, 12/16MHz and 22/25MHz. On 3.5MHz
> ham radio band (2.5-3.8MHz is where I can tune the amplifier, however,
> tuning a frequecy of 7MHz on 4/6MHz isn´t easy at all, quite difficult
> tuning on 14MHz in bands 12/16MHz and 21MHz in 22/25MHz.
>
> Today I destroyed (I think this was the tent time) the series
> voltage regulator I build for screen grids (probably a internal arc
> between anode and screen grids) while I was trying to tune on 14.2MHz.
> Tuning is quite difficult, high anode current appears suddenly, or the
> "tunemeter" changes quickly for a slight movement in tune control, I
> hope there isn´t adapting problem with load (50ohms dummy load).
>
> Well, I will try to adjust neutralization as you said, applying a RF
> source to control grids without DC.
>
> Thanks.
>
> Regards Isaac
--
Dr. David Kirkby Ph.D,
email: drkirkby@ntlworld.com
former email address: davek@medphys.ucl.ac.uk
web page: http://www.david-kirkby.co.uk
Amateur radio callsign: G8WRB
--
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