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Old 9th Jun 2017, 5:27 am   #199
Diabolical Artificer
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Join Date: Jul 2009
Location: Sleaford, Lincs. UK.
Posts: 7,639
Default Re: 807 (maybe) amplifier build. Now EL34

The fun continues ... yesterday after putting a tiny ferrite bead on the collector of the CCS transistor I turned it on, checked cathode I, all well apart from two valves have a little high quiescant current. No squeak.

Signal in, all ok until about 30v P-P on the OP, then a bit of thickening of the bottom of the sine was apparent followed by all hell breaking loose with the waveform almost unrecognisable.

Power on, no signal input, scope probe on anode of one side, volume slowly turned up, at about the same place, IE volume knob turned by 90 deg, where we had the problem above, a 100khz 20v P-P almost perfect sine appeared, what the? Got another scope probe out to see where it was coming from or starting. Whilst probing around I noticed that when I touched a part of the circuit, it dissappeared or was attenuated. Interesting. Also noticed that touching the chassis attenuated it somewhat. Haha! Grounded chassis with a jump lead, 100khz sine vanished. Job done (maybe).

So your all probably shouting "what you didn't ground the chassis you pudding ! " Yep, it got forgotten what with doing a hundred and one other things. The big box is grounded/earthed, just not the wee chassis. So, thinking about this the chassis is like one big plate of a cap and will form a capacitor with wiring, components, Aunt Murial's hat and of coarse the builder of the amp. Which in turn, turns our amplifier into a transmitter.

These problem's I keep getting are a bit of a drag. Solve one, get a new one. However I enjoyed solving that last one. It was a chance to use acrued knowledge and put it into practice. I'm learning and if I say so myself getting better at solving problems, though I keep making daft mistakes. It's good craic, though hopefully soon the amp will work for more than five minutes without developing a fault.

Onwards and upwards, Andy.
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