Burns B3 Deluxe only producing static/hissing

I need some help troubleshooting the Burns B3 Deluxe that I just picked up secondhand. The seller claims it was working for him earlier today. Currently it produces a crackling static/hissing sound - I can manipulate the sound by moving my hands toward the antenna but it’s not a theremin tone at all. 
I’ve tried it in two different rooms on several different stools (no metal) with the volume antenna clear of the top surface and lots of room around the instrument. I’ve tried various amp settings which change the sound slightly but still don’t produce a musical tone. I’ve disconnected and reconnected everything multiple times. The jack’s nut was loose, so I tightened it.
Possible culprits: the theremin didn’t come with a power cable. The adapter the seller was using is input 120vac 60Hz 6W output 12vdc, 100mA.The one i have is input 100-240vac 50-60Hz 0.8Amax, output 12vdc, 2A. Could this be the problem?
Also, I’m using a little 3 watt practice amp (this one: https://www.amazon.ca/gp/aw/d/B08LH8HJSB?psc=1&ref=ppx_pop_mob_b_asin_title ). Could this be the problem?
The seller has been very helpful and will refund me if I can’t get it working but I was super excited for this instrument and really want to get it playing. I’d be so grateful for any insight.

From your description it is clear that you, instead of direct current, feed the theremin with alternating current.
This is mistake.
Alternating current cannot replace direct current.

You need a power adapter (power supply) that outputs 12 volts DC. And it should be well filtered 12 volts. That is, a quality power supply.

From your description it is clear that you, instead of direct current, feed the theremin with alternating current.

Apologies, that is a typo! Its output is 12vdc. It’s an ac/dc adapter. 

Is the polarity of the power supply correct?  Is the Theremin grounded through the practice amp?

It's clear. Then clarify what it means:

[color=#333333][font="Helvetica Neue", Helvetica, Arial, sans-serif]"The one i have is input 100-240vac" ?[/font][/color]

I do not understand such a large input voltage range.
100 and more than twice as many. In my country, the standard is 220 volts, and maybe this is my problem in understanding these numbers, but still, please explain.
(I don't understand what is the reason for increasing the font size, I can't understand, sorry))

Is the polarity of the power supply correct?  Is the Theremin grounded through the practice amp?

The polarity is correct, but the amp isn’t grounded! It’s a 2-prong plug, or can be used with battery. Someone else mentioned this to me and I’m hoping it’s the problem. I’m going to try tonight with a proper amp. I’ll report back if this is the fix. 

It's clear. Then clarify what it means:"The one i have is input 100-240vac" ?I do not understand such a large input voltage range.100 and more than twice as many. In my country, the standard is 220 volts, and maybe this is my problem in understanding these numbers, but still, please explain.

Where I am, it’s very common for adapters to have the 100-240v range for ac input, so they can be used in different areas with different voltage standards. Usually, as long as the dc and polarity are correct, and you have enough amperage, it isn’t a problem. But I know theremins are finicky, so maybe it’s a problem for a theremin.

Lsgos,

Seems you use the SMPS (switch mode power supply) which create the hell of interferences.

Try 12v battery first, and the transformer adapter with internal 12v linear regulator next.

The Burns that I have uses an internal 78L12 regulator, so the external power has to be at least 14VDC.  If yours is the same, probably the seller used a linear power supply that produced a sufficient voltage under the light load of the theremin (I have measured 15-18V on supplies marked 12VDC).  You are using a well regulated switching supply that does not produce enough voltage to turn on the regulator.  So, try a crappier power supply   A 19V laptop power supply would also work.

P.S. Grounding is not required.

If the input range of the power supply is 100-240V, it’s definitively a SMPS which is not suitable for most Theremins. The Burns B3 Series with its RC oscillators and the corresponding low RF voltage at the antennas is extremely sensitive to interferences. Here in France, it’s even possible to listen to LW radio stations with these… 😂😂😂

Thus, you need 2 things: First, as told above, use a linear regulated power supply. Second, play the B3 through an amplifier which is grounded via the mains wall plug. Then, it might be stable in operation and without interference.