18W volume problem

[DOLBY]

I have built a simple 18W plexi without reverb and only tone and volume control. It works great except for a strange step in the volume at low volume.
If I'm cranking up the volume suddenly increases with a big step. You could also say that there is a dip in the volume range.
I've already replaced the pot but the problem remains
Has anyone got an idea what the problem is?

[Tom Phillips]

An Idea. It could still be the pot. Was the second pot the same brand as the first? Maybe both have the same problem. I've seen many modern pots that do not work well at the extreme low end of the rotation. (Although this is usually more of a problem in higher gain circuits than the plexi) You could disconnect the pot and then measure the resistance as you turn the shaft to see how it behaves. Measure between the center lug and the end lug that was grounded when the pot was in the circuit. If there is a jump in resistance when you turn the shaft through the same point where the amp jumps in volume, then it's the pot. In that case you can look for another / better brand for a replacement.
Let us know what you find.
Tom

[DOLBY]

Thanks Tom,
I will definitely try again with the pot. You were right guessing that it is the same brand.
In fact I was not sure of this pot either. I've ordered another one from another brand.
The problem with my ohm-meter is that it responds rather slow, so measuring a pot is not easy.
I will have a look this week and let you know the result.

I have read somewhere else that it could alo be caused by oscillation, if one of the pot wires is too close to a wire connecting a plate? This could cause oscillation with high frequencies (beyond the audio frequency) influencing the volume.

[Tom Phillips]

If you want to try measuring the pot response just turn the shaft in small increments and wait for your meter to take the reading. You don’t need a fast responding meter. It can be very interesting to plot the pot curve. You will find significant differences between brands and new production vs old pots.

The oscillation problem you are referring to is known as parasitic oscillation. It can be caused by poor wiring layout in high gain circuitry. When it happens the ultrasonic oscillation can suck up all the power and to your ear the amp can sound like it lost power. However, unless the ultrasonic oscillation is continuous, it usually starts when you turn the volume up and stops when you turn it down. Since you are observing the opposite effect (“when cranking up the volume suddenly increases with a big step”) I don’t think that ultrasonic oscillation is causing your problem.


Regards,
Tom

[R ski]

If the plexie you built does'nt have a gain between the stages, then the
volume will dump hard upon turning up the master pot.

A reverse audio taper pot will help

A regular audio taper works ok backwards.

A simple idea I incorporated on my project high gain amp is
a double volume control. Add a second pot at either 5K or 10K
between the chassis ground of your main pot. The taper will spread
out on a larger dail rotation. This is handy for lower volume situations

If a gain attenuator was in between the stages the master pot
doesn't dump as hard on lower gain settings.