Oscilloscope/Signal Gernator Help

[gar13]

I finally got around to buying/making the right cables for my o-scope and signal generator. I have a Hitachi 60Mhz scope and an HP 204D Oscillator. I'm using it to see where the clipping is happening in my AB763 build. I've got the oscilloscope showing waves through the circuit and can follow the wave from input to speaker output.

My question is:
What settings do I use on my signal generator to estimate the output of a guitar. I beleive the proper frequency is 1KHz, but I aslo have an amplitude pot and Attenuationg switch. The Atten switch has settings for -70 to +10 dB.

I can see as I turn the atten switch up from -20 to -10, the speaker output goes from sine wave to hard clip.

[sluckey]

Set the 204D output to 0.5V_pp as viewed on the scope. (This is 0.25 V_peak or 177mV_RMS)

Use a BNC Tee to connect the 204 to one vertical input channel of the scope (or ext. trigger input). The other side of the Tee will connect to the amp. Set your scope to trigger on the signal from the 204.

Then use another scope channel to actually trace the signal thru the amp. Triggering on one channel and tracing with the other like this will allow you to trace the signal without having to fiddle with scope sync (trigger) and also allows you to see the phase relationship (with reference to the input) of the signal as it passes thru the various amp stages.

[Iannone]

I know that 1kHz is sort of the unofficial standard, but I like to set my input frequency to around 400 - 500 Hz, as this more accurately reflects the frequency range of the guitar (and this is, after all, a guitar amp).  The A note at the 5th fret of the high E string is 440 Hz.  C, played at the 20th fret of the E string is just a hair over 1000 Hz - how often do you play up there?  Clipping is going to occur first at the fundemental - that's why I prefer testing amps at lower frequencies.

[gar13]

Ok, can't quite figure out how to read the o-scope for 5 Vpp.  I can set the Votls/division to .1 to read, but there is a red knob in the center of this control which affects my reading. it increases/decreases the size of the wave and has a switch when turned full CW. This seems to be a calibration mode. Not sure where this red know should be when I'm trying to set the signal generator output. If it helps, it's a Hitachi V-650F scope.

Thanx for the help guys, I'm really making progress on understanding this setup.

[sluckey]

Ok, can't quite figure out how to read the o-scope for 5 Vpp.  I can set the Votls/division to .1 to read, but there is a red knob in the center of this control which affects my reading. it increases/decreases the size of the wave and has a switch when turned full CW. This seems to be a calibration mode. Not sure where this red know should be when I'm trying to set the signal generator output. If it helps, it's a Hitachi V-650F scope.

OK, red knob first... Turn it CW until you feel the switch operate. This is the calibrated position.  The switch must be in the calibrated position when making voltage measurments. All the scopes I use have a "UNCAL" LED that illuminates when the red knob is not in the calibrated position.

Making the measurment... Set the volts/div switch to 0.1v/div. This means that each major vertical division on the screen is worth 0.1v. Adjust the 204 output until the scope displays 5 divisions of signal, ie, the top of the sinewave is on some vertical division (use the vert. position to conveniently move up/down) and the bottom of the sinewave is 5 divisions below. This corresponds to 0.5v (peak to peak).

[gar13]

Thanks,
With your help, I think I've got it now. Found the Manual online and realized I have a 10X probe. I used th 10mV/div setting with that probe and got everything working great!

I currently have it set up with the sync out from the 204 to the trigger
Can't thank you guys enough, Seeing what I'm hearing is so much fun!

[Animatic]

One nice thing about using 400hz is;

if you halve the range control,
you get 40hz which is about where the
3db slopes are happening in many amps,


if you double the range you get 4k hz,
which is pretty much topping out on most guitar amps,

500hz gets a similar range of easy comparison.

1Khz gets your either 100hz which is effected by tone controls,
 but not really showing the amps roll off issues at all
like when you change caps etc.

And above is 10khz which is only good for triggering occilations,
or maybe some spurious harmonics from the strings.

So make use of that control.
I had a OLD HP 204 series,decades ago...
 and it WOULD change it's levels as the range was changed or the dial turned.
That's another reeason to run right to Chan A on the scope
and then compare it to Chan B.
You can be SURE what your reference is,
And it if needs adjusting to standard .5v, you can easily.

[sluckey]

My 204C output level is verrry flat across all ranges. That's what I remember from the '70s when I was calibrating them in PMEL. The ss WB oscillator and flat output characteristics were the main reason I chose the 204.

Still a good idea to directly monitor the oscillator output if you're plotting a freq response curve, etc.

[Animatic]

Slucky it might have been a 204A or B, certainly not a C,
I just saw a pic of it next to a 204D. But a bad picture.
We got it some time around 1966, and not even sure if it was
new then. But went with a VERY nice dual trace Techtronics scope.
it was for tube tape deck alignment and biasing.

I just got a nice function generator this week.
Dead even from 5hz to 3meghz. asine triangle or square.

[PRR]

> see where the clipping is happening in my AB763

Normally, only the output stage will clip.

So you want to dummy-load, set all gain/volume knobs pretty high, sneak the input level up until the speaker output is just-bent. Read voltage, note load, compute Power, compare to specs and expectations.

If this is a big late two-6L6 amp, you should be seeing 40W-60W at speaker terminals when the sine wave is just-bent. And it should be clipped near-equal top and bottom. And a slight back-off on source should give a very sine-ish wave.

And with all knobs full-up, your source should be around 20mV midband to make full power.

If it won't make expected Power without clipping, note the source level which puts it at the edge of clipping and work backward. At each lower level stage, a small increase of source should cause a small increase of stage output. If a stage won't rise, that's your bottleneck. (The driver stage in a NFB amp, when power stage is clipping, may show horrible distortion: it is trying to fight the clipping. If its output rises, it is doing the right thing. If its output is stuck, clipped, that's probably wrong. Note that many Fender drivers are tweaked to just-barely slam the outputs, may not rise much beyond that point.)