Pre-Amp Build

More issues here. I loaned the Gadget to @jmathers for a few days and while he liked the sound, he found it a bit too noisy when he had the volume up for his vinyl playback. When I got it home I put in on the oscilloscope and was distressed by this display.

aGgvT89.jpg

It is a new scope and a new function generator and when I run the output into the scope directly, I get a perfect sine-wave as expected.

XNBir8H.jpg


ecK2a8R.jpg

(Sorry for the mess)

Any ideas as to what I am seeing and the cause? Oscillation?
Could this have damaged input tubes in an amp and in the Salectric Phono-stage? If so, I feel responsible for the trouble in the rest of the system.
 
Someone told me to add some load resistors so I used my 47K ones and clipped onto the resistor instead of hooking up directly with BNC cables. Looks MUCH better but the sine waves don't really look alike. Gets worse as I increase the volume.
Any idea where to begin troubleshooting?
Thanks!
 
What's the frequency and magnitude of what you're looking at? Are the inputs shorted? Are you using the probes that came with your scope?
 
Forgot to post the picture. Sorry
Inputs are not shorted they are booked up to the signal generator.
The probes are the ones that came with the scope. Set to 10X as I read things are more accurate when using that setting.PXL_20210418_154404636.jpg
 
Operator error was responsible for much of this! The two probes were poorly calibrated. One was off in one direction and one in the other. Once corrected, I got nice waveforms in both channels at half volume:
fVcycIR.jpg


Both channels overlapping below:

COuu7at.jpg


At higher volumes, 2/3rds approximately, I get some separation at the bottom of the waveforms and the sinewave seems flattened at the top:

suYIo9v.jpg


Thoughts on this and how to troubleshoot?

Feeling better about the thing at this point.
 
The slight difference your seeing in the last pic is nothing to worry about. That is minimal, and most likely volume control tracking. The flattening is find as well. Measure the AC output, what's the voltage level. The preamps reached its limit.
The noise you see in your previous post seems to be the bigger concern. That looks like noise from your regulator tube.
Scope your HV at the regulator. Is that noise there?
Regulator tubes can be noisy, and your circuit dies not have particularily high PSRR.
 
Are you talking about this noise BD?
aGgvT89.jpg


If so, that cleared up with adding some loading resistors of 47K into the circuit before the probes are clipped on. I was just using BNC cables before but someone pointed out that the loading was wrong then. I am most concerned about the flattening of the top of the waveform at higher volumes.

BTW, I believe I heard your phono-stage and maybe your pre-amp at Jim's (in Canada) a few years back. His rig sounded lovely!
 
Glad you got tbe noise issue cleated up.
At what voltage level is the output when you get the flattening.

If it's the Jim I'm thinking of, he has the first 6sn7 preamp i built. The circuit has progressed a bit since then.
His phono is one of only 10 built.
 
Jim lives near Sarnia. Flattening occurs around 2.5V peak to peak. Weird that only the positive portion flattens.
 
Seems to be running out of gas pretty early.
I may have missed something here. Which schematic are you working with here?
 
Gadget2.1.jpg

This is @paulbottlehead's design with my mods to allow different VR tubes to be utilized. He has done an incredible job helping me through this build and I am very happy with the results. (the differences from the first build are outlined in red but they are really just different parts selection. The plate resistor switches are totally my mod.)
 
IDK. I'll open it up and see if that is how I wired it or just how I transposed it into the schematic.
 
The 100K resistor at the output needs to be on the other side of the cap.

I'm totally confused about your scope shots, you said something about noise, but then you posted waveforms that show 1kHz sine waves. Do you have 1kHz noise coming out of your amp? That would be really unusual.
 
I originally posted shots of the outputs directly hooked up to the scope via the BNC cables. This showed that distortion. Once hooked up with 47k load resistors I got the waveforms displayed.
I'm concerned about the flattened waveforms below.
 

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@paulbottlehead
Here were the original problem scope pics that seemed horrible.
PXL_20210417_194750709.jpg

After adding loading RCA plugs.
PXL_20210418_183216195.jpg
You can see the flattening of the tops of the waveforms and the lack of synchronicity at the bottom but at least the tracings seem reasonable.

At around 2.5V, the waveforms are showing the distortion of the upper peak.

Will change the wiring to reflect this.

Gadget V2.3.jpg
 
Paul and others will know better, but my guess is that the rounding is tube "cut-off" -- i.e., the tubes stopped conducting a bit prematurely (since tubes invert signal, the plate voltage is highest when the signal voltage is lowest, and vice versa). I think that can be tweaked by adjusting the bias point - but you'd need someone other than me to help with that.

What was the rms signal voltage from the signal generator?

That said, the rounding is very smooth and looks pretty modest to my nooby eyes.

cheers, Derek
 
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