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Old 24th June 2013, 12:40 AM   #1
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Default My amp is an oscillator !

I have just built up a 80 watt valve amp with tone, volume and gain controls.
If the volume is more than 10% off zero ohms the amp breaks into oscillation.
I broke down the amp removing output tubes and phase splitter tube and the oscillation was still there.
I have 2 gain stages on the front end using 12ax7's.
These combined seem to oscillate.
I shorted out the input to the 2nd 12ax7 and the oscillation stopped.

So the problem is in the first valve.

I had a look at the voltages on the first stage and for some reason despite cathode current being 2mA the anode voltage was at B+
I swapped out the valve for a new one and it then started working OK.
The original valve was supposed to be new but I guess it is just faulty.
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Old 24th June 2013, 12:56 AM   #2
roline is offline roline  United States
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Just be sure you have the transformer polarity right, If you jumpered a common cathode input stage, you switched the polarity......
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Old 24th June 2013, 04:13 AM   #3
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"My amplifier oscillates, my oscillator won't." - old engineering proverb.
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Old 24th June 2013, 06:03 AM   #4
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The schematic would be usefull to see. But is there still the same situation you mentioned earlier, when I asked for the schematic:
Quote:
...The cct diagram is too big to show...
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Old 25th June 2013, 06:13 PM   #5
hpeter is offline hpeter  Europe
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Quote:
Originally Posted by rfengineer2013 View Post
"My amplifier oscillates, my oscillator won't." - old engineering proverb.
... (small) pentode wired as klystron, add a piece of wire&cap between G2 G3 and voila- >600 MHz tunable osc, screwing up the neighborhood
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Old 25th June 2013, 11:19 PM   #6
multi is offline multi  Australia
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Default YAYI MINI Monoblocks

YAYI amps; when I first recieved these Chinese amps they where transmitting at about 810 Khz could pick them up on the radio 75 meters away. Added the 4.7k and 500pf cap across the input tube 6AC7, this stopped the broadcasting but now they turned into a radio receiver and you can hear broadcasting, changes with the length of the RCA cables any suggestions.
Thanks
Phil
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Old 25th June 2013, 11:38 PM   #7
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Grid stopper resistors usually take care of RF.

At which stage does it occur ?
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Old 26th June 2013, 12:48 AM   #8
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Thanks
I will check it on the Oscilloscope and try adding grid stoppers.
Phil
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Old 26th June 2013, 01:25 AM   #9
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You can also make the grid stopper a RC filter if you put a capacitor after it to ground.
A 10K grid stopper and 220pf capacitor is a good starting point.
This will at least get rid of "out of band" signals.
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