New Mojotone Deluxe Reverb noise question

[ARhodes]

In a related post I had asked about high voltages.  In this post I am asking about an odd noise that I can hear from the speaker with the amp idling.

The noise is very faint.  Ambient noise can mask it.  It sounds like a teakettle whistle or a fax modem doing its hand shaking when it starts.

I used an 8R dummy load and put the amp output on my scope.  Attached are the photos of the scope.  I am not experienced with pulling out individual signals from a modulation of several signal components.  But I see a 49 KHz, signal in there, a 2.4KHz, 120Hz and 60Hz.  The 120 and 60 Hz seem to be 90 degrees out of phase.

I pulled the preamp tubes section by section and it was unchanged until all the tubes were out except for the main amp tubes.

WHen I reinstalled just the phase inverter tube the signal returned

I removed the 820R negative feedback resistor and the noise was unchanged.

Is this noise simply normal for the deluxe reverb design?  If not, where do I look next for the source?

I am hesitant to stick my scope probe around the guts of the amp because the scope says the probe is limited to 400V and I have some voltages in excess of that

-Tony

[PRR]

> 49 KHz, signal in there, a 2.4KHz, 120Hz and 60Hz.

60 and 120 are "natural" in North America. 49kHz you can not hear. The 2.4kHz might be real audible and is not traditionally "natural". But we are living in a Future. Surrounded by electronics. Start turning OFF lamps, cellphones, WiFi, laptops, TV, etc and etc....

Or get the long extension cord and take it out in the driveway so you are not so close to your own e-chatter.

That being a "digital" o'scope, the unnatural tones might even be coming from it.

[ARhodes]

our sugestion to switch off lamps, other electronics, etc is a very good suggestion!  I Will take it far away from noise sources and see what I hear!  This could be the issue!  I have fluorescent lights in the shop and a spot light that could be emitting RFI.

As I was testing the amp, I did hear the typical RFI of my cell phone getting a text message.  That noise was louder than the little buzz I am hearing.

Your question as to whether the scope itself could be the cause.  And, I suppose anything is possible, but these are very well behaved scopes.  One of the issues with digital is the effect of quantization, both on the time axis and the amplitude axis.  It requires some care when there are signals that are tightly packed in a longer sweep.

In this case, that apparent 2.4KHz signal may not be as nice a sine wave as it appears.  I will show the capture in a different message on this thread.  But, as I increased the sweep speed, I saw that the 2.4KHz signal was actually the 49KHz sine wave being modulated.  On an analog scope, I suspect the 2.4KHz signal would have looked fuzzy, but that fuzziness got quantized away on the digital scope.  I am not positive, but I think this is the case here.

So, I have no idea what is modulating the 49KHz signal.  But it looks to me that it is a 49KHz signal mixed with nearly the same signal but 2.4KHz higher or lower in frequency, and the 2.4KHz is the beat frequency.  This beat frequency is in the audible spectrum.   That's my guess.  But I have essentially no experience with this kind of stuff.  DC type stuff? Yes.  Digital stuff?  Yes.  Analog audio?  Not so much.

So, aside from RFI, what could be causing this in a deluxe Reverb?  As I said before, the weird signal was not there when all the preamp tubes were removed, but was back when ONLY the phase inverter tube was added to the main amp 6V6's and the rectifier.

-Tony

[tubeswell]

The 120 and 60Hz being 90 degrees out of phase correlates with phase lag in transformers.