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|13th October 2005, 06:21 AM||#21|
Join Date: Jun 2004
Location: big smoke
Oh great, playing on my techno-paranoia now! I double checked this one, the waveforms still appear with the probes disconnected from the circuit and shorted to the ground clips. What finally tipped me off was the signal trace increased in size as the scope sensitivity was raised until the transition from 100 mv/div to 50 mv/div, then is disappeared. Never considered the AM possibility though, makes sense. There are two or three 50 kwatt stations pointed my way less than 10 km away. Crumpling up the disconnected probe leads almost completely eradicates the waveform. It's also one of those digital realtime scopes, borrowed from work. I'm not sure it's completely honest when the horizontal timing is reaching 5 ns/div.
This RF stuff is so ridiculuously finicky. My EL84 Mullard 3-3 plays radio stations when a Wima or silver mica are used in the two feedback positions and is dead silent when they're replaced with PIO k40y-9's (.01 uF and 430 pf versions are dirt cheap on Ebay.) More art than comprehensible science.
|12th July 2006, 11:12 AM||#22|
Join Date: Nov 2005
I just found this thread, and found the schematic, which
I have designed.
Well, I build about 8 Mono amps using this schematic.
I had never a problem with oscillation.
The amps are still running without a problem.
The idea for this amp I got from a old glass-audio article,
which used a Ecc81 for phase inverter, and driver.
I did a modification for ECC85 and build a prototype with el84.
With this prototype, I optimised the values for the
components, by using a signal generator and a oscilloskop.
I also tried a lot of ohter circuits, with 2 or 1 phaseinverter/driver
But my goal was to find a simple circuit with good bandwith and the best rectangle signal playback.
If you need less gain, you can omit the 1,5 k resistor
between the cathodes of the Ecc85.Or you can play arround with
different output taps for the feedback, or the values of the feedback -resistor(24K)and cap as well, of course.
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