Making sense with an oscilloscope

[pbiagi]

Hello,

Trying to learn what an oscilloscope can do, is showing.
By making measurements I learned that my 5E3 output transformer blue/brown wires were backwards to the 6V6 plates, reversing the signal, fixed that.

Below is an image of 400 hz sine wave through normal channel with both tone at about 12 o'clock - I believe that was the setting.
The measurement was taken at the grid of the tone stack, pin 2 on V2.
From there the flat top - even harmonics - naturally continues through the circuit. But this appears to me the first stage where the sine wave signal changes.
It seems only the top of the wave form is affected here. Could that be a sign (pun intended) that the bias is too high, or too low?
I experimented by adding a pot in series with the 1500 cathode resistor, and noticed the higher the resistance, the top of the sine wave would start becoming curved - I used a 5K pot which changed the curve only slightly.
What would be required for the sine wave to remain in tact at this point in the circuit?
Why is only the top part of the sine wave changing?

I very much like the 5e3 sound, so the intention is not to change it, but rather to understand, and apply changes possibly on other amps, possibly hifi..
After learning some on the 5e3, I'll surely check out other amps..



Thanks in advance for the feedback

[Dave H]

I think that is the grid of V2 clipping the positive peaks of the sine wave. Try looking on the wiper of the volume control. It should be a full sine wave there if the pot is at 12 o'clock.

[teemuk]

Looks like grid conduction. Grid voltage in respect to cathode becomes high enough, grid impedance drops from several megaohms to much lower impedance, it begins to draw current and subsequently loads the previous stage harder further accentuating the clipping.

Could that be a sign (pun intended) that the bias is too high, or too low?

That depends. If you talk about bias of the preceding stage then not really since the DC bias point of that stage is isolated from the grid by coupling caps. If you talk about DC bias of the stage in question then either cathode or grid voltage could be altered to adjust at which voltage threshold grid conduction begins. Needless to say, original Fender design does not do it and it will change the sound as it will change the performance.

I experimented by adding a pot in series with the 1500 cathode resistor, and noticed the higher the resistance, the top of the sine wave would start becoming curved - I used a 5K pot which changed the curve only slightly.

Like said, altering cathode resistance will alter DC bias at cathode. Altering the cathode bias will also alter grid-to-cathode voltage difference and henceforth voltage threshold of grid conduction.

What would be required for the sine wave to remain in tact at this point in the circuit?

There are many options, which will at least decrease amount of grid conduction if not compeletely remove it.

- Reduce the overall amplitude at the point.
- Add a grid-stopper resistor (this will introduce series resistance to grid and decrease amount of current drawn by the conducting grid)
- Remove cathode bypass cap (without bypass grid signal will also exist at the cathode decreasing voltage difference of the two)
- Have the stage driven by a high-current, high input impedance, low output impedance source like cathode follower. (this will both portray a constant high impedance load to previous stage and provide more current feed so that distortionless signal can sustain at the grid despite increased current draw.)
- Transformer coupling + high current driver stage.

All these methods will affect the tone some way.

Why is only the top part of the sine wave changing?

Grid-cathode voltage difference is higher at the top portion than at the bottom portion of the waveform swing.

[Jazz P Bass]

It was never stated what the amplitude if the injected signal was.

You may be overdriving the input itself.

I would start at 100mv's for the input signal.

[pbiagi]

You may be overdriving the input itself.

I would start at 100mv's for the input signal.

Thanks for this point.
My sine wave generator has 3 signal outputs, 0db, -20db and -40db. I was blindly using -20db.
I just measured the output at -20db which is 1.125v.
-40db = 120mv.
That will likely make a big difference. Will re-check the audio path using -40db points.
Thanks Teemuk for the things to look into, just for learning.

[km6xz]

DB is a ratio between two values so it requires either both values to mean something. In audio, the measured values is assumed to be a ratio of some standard reference value. Which reference level, is the first question when using a db measurement. What reference level are you using for -20 is 1.125volts? Is the generator calibrated for a specific reference level? It is old it is probably calibrated in power, dbm, 0.775 volts RMS across 600 ohms for 0dbm=1milliwatt.
I suspect the generator is newer so referenced to dbv, 1 volt across 1000 ohms should be 0dbv. So -20dbv would be 100mv with is a decent level for mid gain amp testing. A high gain amp will clip earlier stages with 100mv in, so try -30dbv.
If you want variable output, you can attach a higher value linear pot across the the generator output, and the shielded test cable connected shield to the low out and the center conductor connected to the wiper of the pot. Since the amp input is very high impedance the voltage out, with, say, 50k linear pot, will not load the generator much so the voltage out will be proportional to the wiper position. This allows fine adjustment of level to prevent overload or excess noise when -40 is used. Setting the generator to -20 would give -30 if you set the pot rotation 1/3 the travel from the ground end. That will make your generator more versatile.

[pbiagi]

My signal generator is fairly old, I'm guessing 70's, but seems to be working fine..
It actually has a "Fine" attenuator similar that you described. Using the scope I've dialed it to 100mv.
Just need some time to look at the signal in the amp..
Thanks for the comments km6xz.