what causes oscillation and how to find it without scope?

[12AX7]

Can it be lead dress? What else? Any suggestions on how to look for it in the gain stages of the typical 2204 circuit but with a regular gain stage in place of the cold bias stage?

[PRR]

> what causes oscillation

Gain and sneak-back.

Amplifiers oscillate and oscillators don't. (sub-detail of Murphy's Law)

Theory is little help.

Look for big no-nos like OT plate leads going anywhere near small-signal stages.

Look for little no-nos like tone pot wires running up and back past high-level stages.

Poke around at the lead dress. If you put a pencil (graphite in dry wood) between wires and it gets worse, move those wires apart.

[12AX7]

Thanks, i'll give the pencil trick a shot. How does that work by the way? Is graphite some sort of conductor?

[PRR]

> Is graphite some sort of conductor?

Do you have an ohm meter?

[12AX7]

yes

[super&plexi]

I got about 14 Ω out of a six or seven inch remnant from ww.w dropout cabinets.com.... heck maybe we  could get a cathode Bias Resistor from one of those big Lumber pencils...Ha!  On a serious note graphite goes through probably hits the rubber eraser... is surrounded by that metal band... if somebody's been chewing on  the eraser...

And I see a lot of pencils with metallic lettering on the sides... if it's continuous so is the path I guess. Might be better to play chopsticks...or PRR, is benefit to low R, instead of non conductor?

[kagliostro]

This is the pencil lead

https://www.youtube.com/watch?v=h5zGn2sX5qE


Franco

[12AX7]

Why not a metal rod safely covered with a couple layers of heat shrink?

[sluckey]

Here's a bit of info I read over at TAG just yesterday. I have never tried this...

"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 "

[12AX7]

This brings up two questions for me...does oscillation only happen in the output stage? And unrelated but, is amp garage ok to visit? For a very long time now i have not visted it because i get the virus warning page when i try. Is it ok to ignore that?

Here's a bit of info I read over at TAG just yesterday. I have never tried this...

"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 "

[Willabe]

...does oscillation only happen in the output stage?

No. It can happen in/at any stage.

[2deaf]

Here's a bit of info I read over at TAG just yesterday. I have never tried this...

"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 "

What could possibly go wrong?  I'll pass.

[sluckey]

Need to pull your shoes off so you'll be well grounded. 

[sluckey]

Here's a bit of info I read over at TAG just yesterday. I have never tried this...

"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 "

Just a bit more on that quote... The source is the troubleshooting section of a Heathkit manual for a W-5M amp. 

[shooter]

a fluorescent bulb

works great for insuring an MRI is in fact transmitting, just don't hold onto the metal end!!!

[kagliostro]

May be is possible to use a Neon Bulb Tester ? (isolating the metal on the top)





Franco