[Amps] EB104

[Steve Thompson]

tjjapha at earthlink.net wrote:
> Amps experts:
> 
> Even after speaking with Roger at Communications Concepts, I can't
> visualize the placement of the thermistor on or under the pc board (QST
> 6/06 600 watt solid state amp).  Can someone who has built this one
> describe the thermistor placement?  By the way, I've completed the bias
> circuit on the pc board and simulated the thermistor using both a 10k
> resistor and a 2.4k resistor, and the bias voltage drops by just 0.2 volts
> as the resistor is changed from 10k to 2.4k (as the thermistor will
> change).  Will a drop of that amount be enough to cut output significantly
> in the event of high temperatures?

It's a few years since I worked with one of these. I used a small bead 
thermistor and you just hang it off the bottom of the pcb on wires so it 
sits between the pcb and the heatsink. There's no heatflow involved so 
it isn't necessary to drill a hole in the heatsink for it or use thermal 
grease. It it has bare connections then put a bit of sleeving over it. 
Position it over towards the transistors a bit - say under T1 and it 
will do its job fine.

It's there to tweak the bias voltage and keep the idling current steady 
when the heatsink warms up, not as a protection device and a 0.2V shift 
is right sort of change for this. In my experience, it (in fact, the 
whole complex bias regulator) is overkill and not necessary.

Steve