Because oscillation under signal would explain everything I was seeing (shorting out various inputs remedies buzz, touching caps to various places, shorted input is quiet, etc). I thought I would 'cleave the amp in two' and see if the power amp was behaving properly in isolation (which it appears to not be).
This procedure is something I do on new amp builds (tune the feedback loop by injecting test signals into the power amp and tweaking the feedback resistance and capacitance until the amp is stable under various loads/gains). But I've never seen behavior like this on an existing amp.
I did try it in two other rooms, and not much of an audible difference that I can hear between any of the places.
I haven't owned an analog scope in probably 20 years. 🙁
If it helps, I generally run with a 10MHz bandwidth limit on all 4 channels when measuring tube gear.
If it helps, I generally run with a 10MHz bandwidth limit on all 4 channels when measuring tube gear.
Siglent makes fine digital scopes.
Since I have mine I haven't used my Tek analog scopes anymore.
But you should look up the zoom function in the manual.
Since I have mine I haven't used my Tek analog scopes anymore.
But you should look up the zoom function in the manual.
Or just change the timebase to look at the HF stuff directly, perhaps 1 to 10µs/div. Analog 'scope isn't going to make a difference really.
In fact an analog scope can make a huge difference. I swapped from analog to DSO Rigol 1054 some years ago.
Besides many advantages there is one major thing annoying me and that is the massive amount of common mode rf noise poisoning the probe inputs. There is some chance that the rf ripple on scope display is originated by the scope itself.
On the contrary old school analog scopes are deat quiet.
Besides many advantages there is one major thing annoying me and that is the massive amount of common mode rf noise poisoning the probe inputs. There is some chance that the rf ripple on scope display is originated by the scope itself.
On the contrary old school analog scopes are deat quiet.
Only severe oscillation will lead to mains hum and for me it seems to be something different. The scope picture in post 7 is probably hf noise added to the hum trace, pick up of wireless or something like it. This can be seen easily with an analog scope. We need a clear scope picture - do you own a millivoltmeter - I ask again. You are able to reply to simple questions.
Big advantage of zooming into a stored trace is that you don't have triggering issues.
E.g. I was able to clearly see a 440kHz oscillation riding on top of a 400Hz square wave and the scope even displayed the frequency.
With help of a miilivoltmeter (ask which model) you can have high input sensitivity and use the analog output for connection to the scope. This way you have resolution and useful filterering but hf oscillations will probably no longer be shown (dependent on the ac voltmeter used).
Instead of mV pre-amp and scope a soundcard using REW do the same: Filterung to audio bandwidth and display an oscillogram on the screen.