I've been fiddling some more with my BITX:
and more specifically with the PSU that's built into it. I was using a 9-0-9 transformer, taking the 18V into the BITX. Once rectified and smoothed this was generating a basic DC supply of about 24V. Now, the 24V is a good value to feed directly to the IRF510 PA to get a good output, however, when I passed the 24V into the LM317 to regulate the value down to 12-13V the regulator was running really hot.
Thinking about the maths; then we have 24v in - 13v out = 11 * 0.8 (800mA) = 8.8 watts power dispassion! That's loads for a little LM317 to cope with even with the heatsync I've given it. I've been looking around for some time to try and find a suitable multi-voltage transformer such that I can have separate AC windings for the 13V regulated board supply and the PA. I found this on e-bay:
It was only the transformer I found on ebay, but I've built myself a power pack with a cabled 11.5V, +21V and 0V to the BITX. I'm about to drill the BITX case to take a 4-pin DIN socket and plug to act as the power pack DC connector so later today I'll be BITXing...
I'll keep you posted, but should be nice and cool!