PYE 151 High Power VHF Transmitter

[John-js46]

Can anyone provide me with or direct me to any information on PYE Tx 151 10D/CA5751 units which are high power (200 - 300W) VHF Transmitters used by National Air Traffic Control on 115 - 138 MHz. (Mine were on 121.495 MHz). I have not been able to find any information on this equipment and I wondered if anyone has any knowledge of these units as I would like to convert them into a 2m linear.

John G3WTO

[Herald1360]

Presumably these would be AM transmitters and as such unlikely to be linear. Could be an interesting rebiassing exercise if nothing else!

[timewave]

John

The kit I used to work on at Greenford in the 70's was the T55AM. The receivers were located at Hillingdon.
http://www.qsl.net/gm8aob/PYE%20T55%...0MODULATOR.htm

RF unit and seperate high power mod unit.
Contact cooled tetrode in the final I think.
You need insulated trimming tools for tuning the PA through the heat sink. The usual Beryllium warnings apply. Big chunk of material between the final valve and the heat sink!!
Lost a lot of brain cells since my days with the CAA
Might have a manual for the T55AM in the depths of the garage. Let me know if you have the same beast and I will check.

John G4IJD

[John-js46]

Hi John,

It sounds like it. I am told that the valve is the contact cooled equivalent to a 4CX250B. Certainly your description of tuning the grid and anode of the PA via the heat sink apply.
Thanks for going to all the trouble I much appreciate the help.
73
John G3WTO

[timewave]

John

I have located the manual so will get to work scanning it. Send me a PM with your email address and I will get it off to you this evening if the scanning goes well.

John G4IJD

[Peter C]

Hi All,

In my somewhat limited experience many of the high power AM transmitters modulated a low power stage and this was followed by a linear amp output stage so might be easier to convert.

By the way I have always wanted one of these, I have a few of the contact cooled valves.

Regards

Peter G8BZR

[Sean Williams]

From memory, these are high level plate and screen modulated units.

I converted one to a 4M (70 Mhz amplifier) many years ago, the thermal link is very fragile, and not at all happy with high power output, I wouldnt expect to see much more than 150w of RF to be reliable.

The main issue is that of getting rid of the heat, while the heatsink seems massive, the thermal link is not as efficient as the anode cooler of the 4CX250B - a single valve amp of this nature will not stay clean much above the 200W level.

A nice looking bit of kit, but not really the greatest thing for a decent, clean and reliable linear amplifier.

Good luck!
Sean

[timewave]

Here's how the modulation is carried out........plate and screen as Sean stated.

John G4IJD

[Peter C]

I stand corrected, always good to learn something. I guess that these use a beryllium oxide heatsink insulator and it is quite delicate and needs careful treatment if handled.

Peter G8BZR

[Sparky67]

Some police forces used the T55AM prior to the mid-80s - used a CC1 or CCS1 RFPA valve if I remember correctly?

[G6Tanuki]

The beryllium-oxide thermal conduction modules are quite safe provided you don't start drilling/sanding/machining them: I've used plenty of such in the past in wideband amplifiers.

IME the worst thing about the CCS1-type valves is the whole issue of how you handle expansion and contraction/shock-loading issues when the anode is bolted up hard to a few kilograms of heatsink for good heat-dissipation and the valve-base is also mounted solid to the chassis/grid-cavity box to get a good low impedance to earth. Making it survive a few thousand thermal cycles and 100 10G shocks was not easy.

[Sean Williams]

This was the problem I experienced, when trying to push the output up from the T55, as a linear amp - the expansion cracked the thermal link - not helpful!