While going thru the Heathkit manual during the porcess of rebuilding two 1956 W-5M amps I was thumbing through the troubleshooting section and came across a way that I had never heard of to prove out if a amp is oscillating with out the use of a O scope, I will type it out here as printed in the manual for the most part.
Amplifier oscillation is usually indicated when performance seems " strained" or "muddy".
Intermittent performance of this type is usually a difinite symptom as is sudden change in apparent output level.
To check for oscillation, it is usually a good idea to connect an oscilloscope or wide range VTVM across the output with the normal speaker load left connected.
If these instruments are not available, checks can be made with a NE-2 neon bulb or a fluorescent bulb from a deck top lamp.
While holding the glass envelope of the neon bulb in one hand touch either of the bulb leads to the plate or the screen of one of the output tubes.
Keep your other hand in your pocket, or behind your back when doing this for safety.
If the bulb glows, the amp is oscillating.
If a fluorescent bulb is used touch one of the pins at one end to either plate, touching the bulb pins at the other end with a finger in a dark or dimly lit room will show a dull glow if the amp is oscillating "
I hope this helps some folks with issues now or in the future!
Oscillation prove out.
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Stevem
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Oscillation prove out.
When I die, I want to go like my Grandfather did, peacefully in his sleep.
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Re: Oscillation prove out.
I'm glad I have a scope. That sounds dicey at best. Even a casual hobbyist should own a scope. The performance/cost benefit speaks for itself. Literally the cost of a few transformers gives you the ability to see waveforms instead of guessing. Repairs can be faster, and design work is much faster when you can see what signals are riding anywhere in the circuit. Common wiring and noise issues are easier to cure when you recognize distortion patterns in waveshapes.
Last edited by TUBEDUDE on Thu Oct 13, 2016 3:22 pm, edited 1 time in total.
Tube junkie that aspires to become a tri-state bidirectional buss driver.
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Stevem
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J
I fully agree, but many beginners post on this site and others with a first build that has this issue and just do not have the funds for a scope at the moment!
When I die, I want to go like my Grandfather did, peacefully in his sleep.
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
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Malcolm Irving
- Posts: 139
- Joined: Wed Jan 14, 2015 12:06 am
- Location: Derbyshire, UK
Re: Oscillation prove out.
I've never tried it, but I think supersonic oscillation might show up as a noise burst in an AM radio held close to the amp?
Re: Oscillation prove out.
Srsly? Harder to find than incandescent light bulbs!Malcolm Irving wrote:AM radio
I build and repair tube amps. http://amps.monkeymatic.com
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Malcolm Irving
- Posts: 139
- Joined: Wed Jan 14, 2015 12:06 am
- Location: Derbyshire, UK
Re: Oscillation prove out.
If you haven't got an AM radio, a Crystal Set will do. 
Re: Oscillation prove out.
A NE-2 bulb might be a dangerous thing in the hands of a beginner, especially when poking at the plate and screen pins of an output tube.
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Stevem
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M
This part of how we all learn, by dealing with dangerous things but working safe!
And why could not extension leads be solder on with clip leads on the other end?
And why could not extension leads be solder on with clip leads on the other end?
When I die, I want to go like my Grandfather did, peacefully in his sleep.
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Re: Oscillation prove out.
If someone has a PC etc with a soundcard, and the wherewithal to build an amp, then they surely could rig up a software scope?
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