Audio intro:
26/9/24:
Look at this beautifully made homebrew transmitter. Firstly, I have to make it clear that I didn’t build this transmitter. Whoever built it made a really professional job of it. I’m impressed! The transmitter covers the 160 metre amateur band as well as 80 metres AM and CW.
It uses a 5B/254M in the PA (B8B base), also known as a miniature 807. The modulator uses two EL84 valves in push pull with a Collins 7R3 modulation transformer. This takes me back to the days when people built their own gear. Parts were easily obtained back then, transformers, valves, variable capacitors, tank coil formers… Happy days!
I think this must have been built in the 1960s or 70s so the capacitors will need replacing. There’s no power supply, so that will be the first job. 600 volts for the PA. 250 volts for the rest of the valves and 6.3 volts for the heaters.
Just look at the wiring, beautifully laced and routed. As with most of my projects lately, I don’t have a circuit diagram. With an eight-way jones plug on the rear of the chassis, I will be spending some time tracing wiring. It shouldn’t be too bad as it’s a fairly straightforward design.
I intend to use this tx on top band in conjunction with my National HRO receiver. It will sit nicely on top of the HRO. I’ll also use it on the 80 metre band AM frequency. Plus, I’m looking forward to a few CW contacts. There will be videos of my progress. Im sure you wouldn’t expect anything less!
27/9/24:
Thoughts on the power supply. The transformer below is ideal. The secondary windings are: two 6.3 volts at 4 amps, one 175 volts at 100mA, one 342-0-342 volts at 238mA, one 400-362-0-362-400 volts at 200mA. I should be able to do something with that lot!
Finding a suitable chassis or cabinet to house the PSU is almost impossible. Long gone are the days of freely-available aluminium chassis or boxes of various sizes. I will keep searching on-line and, hopefully, find something suitable.
The PA valve:
The PA valve is a 5B/254M in the PA (B8G base), also known as a miniature 807. Pin-outs 5B/254M.
| Vh | Ah | Va | Vs | Vg | mAa | mAs | P out | Frequency |
| 6.3 | 0.9 | 600 | 250 | -45 | 100 | 8 | 40W | 60MHz |
Other valves:
TheVFO appears to be an EF80, not marked. VFO HT stabiliser OA2. The driver appears to be a 6CH6, not marked. The PA valve is a 5B/254M.
Mic amp ECC83. Phase splitter ECC82. Push-pull output EL84.
Collins 7R3 transformer connections:
Push-pull EL84 valves on the primary with centre tap for 250 volts HT. Secondary is 600 volts HT and PA anode. But… there’s a problem: Pins 2 and 5 are joined, allowing one common HT supply. This can’t be 600 volts, which the PA anode needs, because the EL84s would als0 have 600 volts HT. Why are pins 2 and 5 joined? The max anode volts for an EL84 is 250. Is the PA valve also running on 250 volts?
