Dang... just fired up my first channel of the JLH 20w...
Well, my original plan was a small board for each amp, containing the psu and the circuit. I built one (finished it yesterday) and proceeded to do a test run on the bench.
Plugging in with no bias, everything's ok; the DC voltages are ok (exactly +/- 22vdc, wohoo!). I adjust DC offset first to 0v and then slowly increase bias up to 1,7A... the sinks get hot :) All fine 'till here... each transistor has it's own sink and every one measured 67-70șC (mind this is one of the hottest days of the year here). I was happy!
So i smell heat... damn. I touch the sinks on the cap. multiplier and they're SCORCHING hot! I used small sinks, but i thought i was in te vecinity of 5șC/W (30șC worst case). Not only that, one of the drivers transistors has no sink (a BF258 replacing the original MJE371; this trans. has a really old case, like a "water tank"... think of a germanium one)... 100șC there!!! :confused:
Now, i'll have to rebuild the whole thing... a separate board for the two psu's, with the cap-x transistors on a bigger sink and another one for the pair of amps (replacing the BF258 with a BD140, sinked). I HATE this, the amp was running fine and i was dissipating more heat on the driver/psu transistors than the output ones!!!!!!! :mad:
Lesson: don't ever, NEVER, underestimate the power dissipation on small transistors, and use sinks. Even a rather small piece of Al will do the trick. 2w made one crazy, while the other, with a minimal sink, just stood warm. So did the 7815 regulator (i added a resistor to ground just to make sure i always had the minimal drive current).
There's a bright side through. I adjusted the DC offset to cero before starting... and it increased *only* to 2mV after the whole mess! Which is rather amazing, knowing how hot the drivers got. I once readed someone here asking for a DC servo for this amp... from my experience, it's VERY stable in that sense. I dunno about the 1969 JLH, but the quiescent current control used in the '96 can be used there too. The turn-on and off thumps were small and fast too, through my digital multimeter's not the fastest.
Back to the workshop :(
Sorry to hear that you have had problems with your JLH. As you have found out the hard way, the BF258 was not a good choice for Tr5. According to my databook, it is rated at 1W whereas Tr5 dissipates around 850mW at idle and has a peak dissipation of 1.9W (1.1Wrms) at maximum output. If you cannot get the MJE371, the normally listed equivalent is the BD436 though a BD140 should be OK.
If you are rebuilding your amps, please consider using a ccs in place of the 7815, as shown on my 'JLH for ESL' page. I have been concerned about the 7815 injecting noise into the emitter of Tr4 (not to mention the 7815 stability problems) which is why I introduced the ccs in the ESL version. Another constructor has just replaced the 7815 with a ccs in his 1996 JLH (built with premium components) and has reported that the ccs is "cleaner, smoother and weightier".
I think you will find that when your amps are working correctly, the output dc offset variation between switch-on (cold) and normal operating temperature will still be significant (>100mV) even with the 1996 version.
About the dc offset... yes, i was expecting that too :) But i think the amp WAS running ok (despite the enormous heat), and i rechecked the offset quite a few times. I didn't want to plug a loudspeaker as you can imagine... the thing could destroy itself at any moment.
I already built a good sink out of Al sheet for the cap-x psu (this one will be internal), and will be building that part in a short while. Wish me luck! :D
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