A 75 watt power amplifier for 70MHz

by Geoff Pike GI0GDP

This amplifier is intended to be simple in design, easy to build and as cheap as possible. Typically, 90% of the cost of a solid-state amplifier is in the main PA device used, so rather than specify a particular device, this project should be able to accomodate any of a number of different devices, allowing the constructor a choice.
The original used an SD1487, but I have tried a variety of HF devices with very similar results, so if you have one of the MRF454/455/458 types designed for HF service, then you should try these. The essence of this design is the fact that HF power transistors are still useable for low VHF, albeit with reduced gain, typically 7dB (x5), but this is an advantage from a stability point of view.
Another possible source for these is devices is from old CB amplifiers. Using a SD1487, the results are typically 15 watts in and 75 out.

Construction

As this was a club project, an etched PCB was not used: the board is simply "dry-etched" using a sharp modelling knife to cut the copper cladding, and peeling the waste off using a 25 watt soldering iron.
Either a single or double-sided 4” x 3” (120 x 75mm) copper board may be used.

As the picture shows, a broad track the same width as the transistor leads is cut running from the left side to the right hand side of the board. Also a +ve rail is cut in a similar fashion across the top of the board.
An isolated pad is made for the junction of L4 and L5 ,C1 and C2 by sticking down a small offcut of PCB.
A hole is cut in the centre of the board to allow the flange of the PA transistor to pass through and contact the heatsink.

There is no particular order to assemble the components, however note the following:
Diode D2 should be in good thermal contact with the PA transistor - use some thermally-conductive grease to help;
The silver-mica capacitors used for C8 and C9 would be expensive so I have substituted ceramic 1206 SMD types as an experiment and so far seem OK;
Be certain that the mica washer and bush used for mounting TR2 is not shorted out;
Remember that TR1 should be the last thing to be soldered after the board has been bolted into its case/heatsink (I used a cooling fan from an Athlon processor chip for low-cost cooling;
The relays are controlled from the PTT line the driving source, however it may be necessary to make a carrier-controlled switch if DC control is not available.

The marked-up photo should be enough to illustrate the construction without a long-winded explanation. There is also a larger photo of a completed unit in the photos section of this website.

Testing

First, to set up the bias control, disconnect L3 and L5 from the PA device, and check that the insulation on TR2 is good.
Apply 13.8V via a 10-ohm 10-watt resistor: this will prevent most disasters in case of shorts or other faults! Check the voltage at L3, and set this to 0.65V with RV1. Check that the total supply current is about 50mA.
If all is OK then switch off and connect L3 to TR1 base and L5 to its collector and thenre-apply the 13.8V through the 10-ohm resistor. The current should be about 100mA at this time, but should be adjusted with RV1 as needed.
Finally, apply a warm finger to D1 or D2 and check that the collector current falls.

Now for the RF side of things. First pre-set the trimmers thus:

C6	20%
C7	50%
C10	65%
C11	20%

Next energise the relays and apply 2-5 watts of RF drive initially.
Check that there is RF output with a suitable power meter and dummy-load.
If this is OK, then remove the 10-ohm resistor and repeat the above, peaking the trimmers C6 and C7 for maximum RF out (but don’t move C10 and C11).
Next apply 10 watts and set C11 for a collector current of 6.5A and then adjust C10 for maximum RF output.
Now readjust C6 and C7. At 50 watts the collector current will be ~6.5A and at 75 watts ~8A.
Take care when adjusting C11 as this can have an adverse effect on the collector current of TR1 (if you play with C11 and C10 they are a bit like the "tune" and "load" in the pi-network of a valve PA).

Without any harmonic filter, the 2nd harmonic is only -35dBc on full output, and the 3rd and 5th-order intermodulation products are between -30 to -40dB at 50W PEP on a two-tone test (spaced at 900Hz).
Without performing your own two-tone tests, a set of CW input/output power measurements will allow the gain-compression of the PA to be measured, so you can get an idea of its linearity for SSB transmissions.

Circuit description

If you want a full description, then e-mail me and I can send you all the info which I used to design this amplifier.

You may notice a few small differences between the picture of the amp and the schematic diagram, but both variations will work.

Parts List