Conversion of the Pye 200W UHF/SHF Transponders for 1296Mhz
First Published in DUBUS 2/96
As the TV repeater stations are being converted to solid state, a number of these cavities have appeared on the surplus market in Europe. Constructed by Pye (Phillips) as a TV transponder PA the YD1336 coaxial triode, which is similar to the Thompson TH308 series of tubes produces a linear output of 220W and a gain of 16dB at 470-860Mhz. The valve operates in class AB with a constant voltage bias supply applied to the isolated grid, the cathode being grounded to DC.
All of the assembly is constructed of thick walled silver plated brass and is designed for 24 hour operation in a install and forget environment. The input cathode circuit, is continuously tuned, as is the input coupling, which is a via a sliding arrangement onto the cathode assembly. In its original state the input lines are 3/4 lambda and the output cavity, along with the second output cavity are 1/4 lambda long.
There are three variants of the cavity,
- LF version, 470-660MHz
- HF Version, channel 660-780MHz
- HF Version, channel 780-860Mhz
The only difference is the inclusion of a shorting strip to stop spurious resonances in the cathode of the two HF versions. But the highest frequency version does have low inductance fingerstock. There is a difference in the neutralising between the different variants, this is achieved by milling four of the grid fingers in the higher frequency models.
There is very little mechanical work required to convert the cavity to 1296Mhz, where it will function as a 3/4 wavelength anode cavity and a 5/4 cathode line.
- Remove the output coupling cavity and coupling arrangement completely.
- Disassemble the PA cavity, no need to touch the cathode assembly.
- Increase the available tuning range of the anode cavity by removing the tufnel spacers from the cavitytuning posts.
- Invert the anode tuning ring to increase the tuning range.
- Fabricate a new output coupling probe alla OE9PMJ’s * TH308 cavity design.
- Tap the cavity where the original output coupling was to match the new output coupling arrangement.
- The original design uses a VERY LARGE blower which SUCKS air through the cavity. This is attached via a 24 inch length of 4 inch air hose.
- Note there is no need for a ‘anode cavity’ fine tune, it does not suffer from drift.
In its original service the grid was decoupled to RF and DC bias supplied to it. However when operating at 1296Mhz it was found that using the original bias supply the PA was liable to flash-overs at high power levels.
This has since been rectified by wiring the original grid wire, via a suitable meter, directly to earth. Bias is then supplied via a transistor arrangement in the cathode.
In its original condition the heaters were operated at DC, balanced to ground by a variable resistor mounted on the cathode assembly.
YD 13336 – Typical Operating Conditions at 1296Mhz
- Heaters 6.0 to 6.3V AC or DC at 4.8 to 5.8A (6V recommended)
- HT 3000V 0.5A
- Zero Anode Current 200mA
- Grid Current 0.5mA
- Cathode Heating Time 180 seconds
- Output 500W
- Drive 30W
As with the vast majority of TV transponder triodes, the most important factor is the low grid dissipation i.e. practically none. I have seen 700W from the cavity, but it is somewhat thrashing the tube. However, it will run at 400-500W key down all day with no thermal drift, typical drive requirement being 30-40W.
Though the YD1336 is designed to operate in any position, I am sure that mounting the cavity vertical, as originally installed, will reduce the possibility of grid sag when operating under adverse conditions.
YD 1336 – Manufactures Maximum Conditions
- Anode Volts 3500V
- Anode dissipation 1800W
- Cathode Current 0.7A
- Grid Current 5mA
* 1296 Mhz PA using TH308 by OE9PMJ – DUBUS 4/85
The dimensions to set the cavities tuning sliders for resonance on 1296Mhz