valve modification - air siliguri AIR SILIGURI

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by Abhijit Sengupta Sharma (EA) & Uttam Bhattacharjee (Tech)

As per previous record the HMB 140 transmitter at AIR Siliguri runs with single Modulator Valve since long back. The only Valve BEL 40000 CX which is used in the Modulator section also pass its stipulated running life as per VSD report. For previous certain days its gives following problems as per record in the day to day transmission logging
The put on/ put off condition of transmitter is not at all smooth. Certain frequent tripping was observed during few days with huge tripping sound from the transmitter, suspecting short circuit of HT.
It gives intermittent shorting problem between screen grid and plate (i.e. HT)
Intermittent modulation failure.
Intermittent distorted modulation with the noise generated at the receiver due to spurious frequency.
Last of all on 20th August 2014 at 14:27 hrs it shows permanent shorting between HT and Screen grid / control grid.
There are two ways to get the alternative AM process for this transmitter
The station can run with low level modulation using PWM with MOSFET arrangement as pre-driver to PA.   
Modify the Modulator section with the Valve BEL 100000 CX which is actually used in PA section of this particular transmitter & eventually also a Pentode Valve. This particular Valve is also available as spare of the other transmitter (Transmitter - I) which is awaiting for dismantling with respect to installation of DRM(200KW) transmitter as proposed for this station
To resolve the problem quickly and to reduce the breakdown time it is decided to go with the 2nd option. But there are following problems to achieve this process
The BEL 100000CX vacuum tube valve is quiet heavier (about 38 Kgs) than the CQK 25 / BEL 40000 CX vacuum tube. Therefore the holding structure should be robust enough accordingly. 
The BEL 100000 CX valve is dimensionally bigger than the CQK 25 / BEL 40000 CX vacuum tube. Therefore to fit it in the final Modulator cubicle is a rigorous and time consuming job.
This particular Valve takes about 200 amps current at the filament instead of 180 amp taken by the modulator valve filament. Therefore filament transformer should also arrange to fit in the particular cubicle which also is quiet bigger in dimension (12”X14”X10”) than existing one.
If the portion of the holding structure is laying outside the cubicle it may cause RF shock hazards.
The connectivity of screen grid supply, control grid supply should be given from the existing biasing circuit.
The water flow pipe which is ¾ “ in diameter for this vacuum tube should fit with ½” existing water flow pipe without giving any sort of problem for quantity of water flow as the water jacket cavity of BEL 100000 CX is more than the existing vacuum tube. So that ‘WF’ indication (water flow safety interlock) may arise.    
The air flow pipe also fit to the proper persists as the previous agreement is a rigid ‘T’ section for supply of Air to both modulator valve, it should be modify with the flexible air pipe to fit with valve base.
TUBE CQK 50-2 
Frequency                     26 MHz
Anode voltage               11 kV
Anode current              10.5 A
Screen grid voltage      800 V
Screen grid current      1.4 A
Control grid voltage    -650 V
Control grid current     0.7 A
Filament voltage          12.6 V
Filament current          335 A
Anode voltage                       15 kV
Anode dissipation              100 kW
Control grid dissipation      0.7 kW
Screen grid dissipation        2.2 kW
Diameter                                 230 mm
Length                                      595 mm
Mass                                          38 kg

To drill the above problem we have done the following job
First observe the dimension of modulator cubicle as follows.
To make the space available remove the two numbers of filament transformer of Modulator valve from the outside.
Use ½ “/ ¾ “expander with ¾ “to 3” Nipple and a rubber ‘O’ ring for water pipe connection to the valve.
Remove the one valve and valve base from modulator cubicle to fit the structure and necessary connections.
Put the structure inside the cubicle with the valve base at the top and the filament transformer placed at the base of the structure.     
Connect the filament the screen gird and control gird connection to the valve base by using proper guage of cable with the filament verification and the modulator cathode current measurement cable connection. After that the HT connection has been made from one point.
Fit the flow pipe at the bottom of valve by a flexible pipe take from the Txr 1 and fit directly to the LP blower out let after remaining the pipe connection for existing set up.

After connecting all the necessary connection following observation has been made
First check the water and air leakage from the respective joints.
Check the 200 AMP filaments current with Tongue tester.
The screen gird voltage used at PA section for this particular valve is 800V but for the modulator section it is 1070V therefore necessary tap changing of screen gird supply transformer is made to get about 900V at screen gird. As per specification of the BEL 100000CX use the screen gird can take up to 1200 VDC, also the plate voltage up to 15KV.

After all the above observation the transmitter is put on in normal mode up to HT. As there is no abnormality in the meter reading and also physical checking the transmitter PA screen and modulator screen are given and take transmitter on AIR at about 01.00 hrs for testing. Modulations of 20% run for 15 min then give 50% of modulation for 20 units and then give about 80% of modulation for about 1½ hrs for testing found no abnormality. Finally the transmitter runs for morning transmission of 21st August 2014 on right time at 05.53 hrs.
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