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Good questions.. I have considered it actually, but to add a relay would require a whole new case and heatsink. There's not much room in the KL300p.. The resistor gets hot to the touch, but nothing outside it's spec or even close to it. You would have to hold your finger on it for 15-20 seconds or more for it to burn you. My IR thermometer died not long ago or I'd be able to tell you exactly how hot it gets, but in testing I've left the amp powered up for days without any trouble whatsoever.

EDIT: I got to thinking, and there are some very small relays around.. it might be feasible to add one for the bias circuit, but given that the circuit can run indefinitely without issue I don't know if I will or not.

Glad to help Eagle. As I looked at it some more I think you should add one more of those .01 caps directly across the bias diode. This is to keep RF off the diode. Because of the polarity of the diode, if RF is present across it's junction, only the positive half of the cycle will be shunted to ground through the forward biased diode. The negative cycle will remain and effectively reduce the forward bias. One more cap will prevent it and keep clean DC at the transistors base.

Mack points out another good one. That bias should definitely be switched on in TX only. It's wasting power and keeping the transistors heated between keys. The 10 watt resistor won't get hot because it's only dissipating about a watt and a half.

I hear ya about the available space and if you could fit a 20 watter in there that would drop that heat quite a bit and a relay would bring that heat down to basically nothing as the circuit would only be charged in tx but as you've said it's pretty much a no go due to space.

Cool project anyway.

Thanks again Shockwave and mack! it's cool that there's some real knowledge here! I'm figuring this all out bit by bit, but so far I've honestly been pretty surprised by how straightforward much of it is. I've been having a lot of fun with it.

Regarding the relay, it does waste a small amount of power but the transistors don't get warm at all just idling, at least not that I can tell. I don't really keep the amp on when I'm not using it, so the wasted power is kept as minimal as I can keep it without adding complexity to the mod.

You could fit another relay in there just like the stock ones. Mounted on it's side with double sided tape. The keying transistor will probably handle the current of having a second relay coil in parallel with it. The more professional way to switch a positive voltage source is to use your friend the PNP transistor. Connect the emitter to the +12 volts from the power switch. Connect the collector to the side of the 100 ohm resistor you want to switch the 12 volts into.

The base lead should be connected through a resistor approximately 1000 ohms (or more) 1/4 watt. The other side of this resistor goes to the side of the keying relay coil that goes from 12 volts unkeyed to 0.6 volts when the amp is keyed. As long as the voltage between the base and the emitter remain close to the same positive potential, the connection between the emitter and collector appear as an open. Once the base is brought to a potential that is lower then the emitter by .6 volts or more, the transistor turns on and looks like a short between the collector and emitter.

PS: It's late and I hope I explained that right.

Very nice solution! So what you're saying if I understand correctly is that I can use the signal from one of the existing relays to trigger a PNP transistor, thereby supplying the bias voltage to the circuit? No third relay required?

Also, wouldn't the transistor need to be somewhat heavy duty, like something in a to-220 package with higher current handling capabilities?

Using ohms law I came up with about 120 milliamps of collector current and 84 milliwatts dissipation on the PNP transistor. That indicates even the smallest PNP from Radio Shack should work fine here. Nothing wrong with using a bigger one here. Just be sure if it's a TO220 case that you insulate the metal tab. It need not be attached to a heat sink since it won't get hot. Heat shrink tubing works good for insulating here.

This transistor will be activated by the amplifiers original keying transistor. The transistor is connected to the keying relay coil mounted on the board. This relay coil will have a diode soldered directly across it. You can feed the 1000 ohm resistor directly from the anode side (without stripe) of this diode. Check with your volt meter first to be sure this point changes from about 12 volts with the amp on but not keyed, to around 0.6 volts keyed.

I'll have a poke around with my DMM later after work.. Many thanks for your input! I've known about using transistors in this manner for a long time, I've used something similar in a few of my other projects (non radio-related) but for whatever reason it didn't even occur to me that the same would work here. This has been a productive discussion so far..