Voltages and volume

[Bernardduur]

Hey all!

I have a small question.... lately I've been working on some amps I have here and this was the first time I noticed it..... maybe it is normal...... I tried searching although I don't know on what words to search

I have this Marquis amp that is basically a clone of the Marshall 100W plexi (4xEL34). I did some updating of the parts, added new tubes, etc. It works perfectly and the bias is easily set. Only, when I turn up the volume (w/ master volume on full) on the last 'turns' of the gain I get a sudden flash of blue light on the tubes and the current I measure on the kathode of the power tube double. I also notice that the main voltage in the amp drops.

I tried other ways to determine what and how and yet I get stuck by the volume; if the volume of the amp is set too high this happens.

Is this normal or is something wrong with this amp? Note that the volume this happens on is really high.....

[Albert Kreuzer]

Sounds like ultrasonic oscillation to me (not that I could hear it...).
Have you scoped the output?

Cheers,
Albert

[Bernardduur]

Shame to say my scope is not calibrated yet.........

I'll check!

[Enzo]

Does you scope even remotely work? ALl it needs to do is present something on its screen that more or less resembles what goes on in your circuit. It doesn't mater if an oscillation is 23.4v or 22.1v, if you have 20 some volts of oscillation going on. All you need to see if if there is some large amount of it or not.

Kinda like testing a wall outlet with a very poorly calibrated voltmeter. Even if the meter says 135v instead of 120v, you still could easily determine which outlets were live and which were not.

[Bernardduur]

Does you scope even remotely work? ALl it needs to do is present something on its screen that more or less resembles what goes on in your circuit. It doesn't mater if an oscillation is 23.4v or 22.1v, if you have 20 some volts of oscillation going on. All you need to see if if there is some large amount of it or not.

Kinda like testing a wall outlet with a very poorly calibrated voltmeter. Even if the meter says 135v instead of 120v, you still could easily determine which outlets were live and which were not.

Whaha, yes, when I wrote it down I thought the same...... indeed it was an oscillation. I added a cap and now it's gone. Tnx all!

[daz]

Can i ask, where did you put the cap?

[Bernardduur]

I put a 56 pF cap between the plates of the phase inverter...... there was originally a cap in there that I removed when I checked the circuit.