The Amp Garage

Yeah, they are pricey. I have a 3424, which is about $1200 (I think). But it is used for lots of other audio work, so for building one or two amps it is a bit rich. There are a lot of other USB scopes out there, this was the only one I found that had the resolution for audio work.

d2

post a schematic - we can take a look from there...

He he - this should get some hackles up... I use a USB scope. A 4 channel jobby from pico scope made for audio analysis. I can do real time freq analysis, save the pictures, tweak and do it all again. I can put in 4 probes throughout the circuit to see how the signal changes as it progresses through...

As others have said measuring low resistances with a cheap meter is a waste of time. Even a lot of very good meters are not all that great at measuring low resistances. But why are you even bothering to measure the resistance anyways? All you really care about is the voltage drop. Your cheapo meter ...

An excellent article, simple to understand, on exactly your question can be found here:

http://www.audioxpress.com/resource/aud ... /index.htm

Select "Designing Your Own Amplifier Pt. 1: Voltage Amplifier Stages" tells you why and how you select different plate resistors and cathode resistors.

The "Source" setting is for triggering. Triggering is required to align the scope on a signal so the image is held still. CHA means "when channel A rises above a certain point, trigger of its waveform". CHB means to use channel B for triggering. The triggering level knob sets the amplitude at which ...

Are those a 22megohm voltage divider by the PAB switch?

It would be interesting to see scope pictures on the input of V1B with that switch on and off. If you have a dual trace scope, put the second channel on the late just after the coupling cap of V1B.

I had this on a few amps of my own design. They were always close to oscillation, so just a touch of the wrong lead dress and then hitting a low note would cause an oscillation at the leading edge of the waveform. To a hell of a long time to track down, but boiled down to a combination of too much g...

Hey Mat, I am not surprised it sounds rough when loaded (based on your scope readings). What kind of amp is it? Can you post a schematic?

BTW - what is wrong with the amplifier you are working on?

The oscillation can occur anywhere in the circuit. Basically any place one signal can be fedback into another area of the circuit. I see them most often when a strong signal lead is parallel or close to a weak signal lead. I have also had them with power tube grid wires too close to each other.

Here is the pic...

Mat,

no oscillation there! The square waves are very heavy distortion. Here is a pic of what an oscillation might look like. The red trace, right at the beginning causes a short oscillation to happen which sounds like marbles in a tin can.

Mat, I think all is OK, don't sweat the bias I think it is fine. To measure the voltage of the input signal - use your scope! Just connect it up to the input jack, adjust the scope to get a good reading and based upon your amplitude scale you can read it off the scope. For oscillation, just connect ...

Hey Mat, If the bias is good when idle, you are good to go. Don't sweat it when running a signal through. If you are using a signal generator, dummy load and crank up the amp, all amps make themselves heard - this is normal. OTs get hot, fry an egg hot? That might be a bit much. With loud guitar pla...