"I noticed that the output power seemed a little low, so I biased the PA as per instructions. With the bias set to 100mA with PTT down and no audio, when I yell as loudly as I can into the mic I only pull about 840mA, as measured with a Fluke 117. Normal voice level gets 500mA or less. Increasing the bias does not increase output, it stays right around 840mA. The stock mic does exactly the same. Input voltage is 13.8 from a 25 amp supply, so no voltage sag. It does increase slightly at the low end of the band and decrease on the high and as one would expect. Any ideas or is this normal?"
Jon, every IRF510 is different, Yours is the second instance I have heard of a BITX with a shoutout of about 800 ma.
It is probably normal. If you want to get the "normal" 1000 ma. response, you will probably have to change to another IRF510.
But these devices are so variable that there is no guarantee the new one will do what you want! If it works, you are
probably best advised to leave it alone. Certainly do not exceed 4-4.5v on the gate (even that is iffy for some devices).
"Another question, I do see mention of increasing the PA voltage but have not found a maximum recommended VIN."
You can easily go to 24v. But (and it is a big but!) you must increase the size of the heatsink about 4-8 times. The
No. 1 cause of failure in these devices is heat. They have very poor heat transfer characteristics and a very small
die area. All the current that passes through the device goes through this die. If it is too much, it can cause instantaneous
failure. Remember to isolate the heatsink from ground as the tab is "hot". And provide the best transfer you can between
the IRF510 and the heatsink. Vital to success. (I am sure you have already noticed that). Probably if you are contemplating
48v or so, it can be done -- but you are best advised to change the IRF510 for something heavier, like a IRF520 or 530.
Note however, that the input and output circuits will have to be reworked as these devices have considerably
different input capacitances. Read KB1GMX and NA5N for nitty-gritty on these MOSFETS. As far as maximum voltage,
it should be about 2/3-3/4 of the rated voltage Pk to PK. on the input (AC) side. You would use a corresponding DC voltage to cover
that, without exceeding the data sheet values. For the IRF510, the maximum DC voltage drain to source is 70v. But
the almost universal recommendation is to not exceed 24v DC. Note that 2x voltage = 4x power. And you are not confined to
the IRF series. There are many other excellent MOSFET power devices.
Hope this helps.
john
AD5YE
