What does that "OVEN" sign mean?
This is from an audio compressor NTP 179-130 schematic.
Warning upfront: Service manuals of audio equipment often contain slightly manipulated schematics to at least stop the most incompetent counterfeiters. Don't assume everything in the schematic makes sense.
but here: the symbol is that of a variable resistor.
So, either this is just a variable resistor to set the power of an oven attached to (K) and (O), or, and that looks more likely seeing the thing is flanked by 5.6 Ω resistors on both sides, is an oven.
Ovens are used to keep components at a stable temperature, e.g. oscillators ("OCXO": Oven-controlled crystal oscillator. Used in cases where you need a very stable oscillator and don't care too much about power consumption.), or maybe a semiconductor that needs to be kept at the right temperature to behave like the device requires it.
If you look at the PCB layout in the 179-140 instruction manual for compressor amplifier 179-130 on page 9 you'll see R56 close by Q1 like this: -
And I suspect that Q1 has a heater coil around it that may be called "OVEN". Q1 is a JFET that is used to control the amplitude of the signal (hence form a compressor) and although the heating side of things isn't totally clear, I believe that it is warmed either to enhance the compression circuit (deeper compression) or to keep the circuit stable in different ambient temperatures. Q1 and "OVEN" connection (page 8 of the linked document): -
Yes, this is a component oven - its purpose is to keep the (highly selected) attenuation JFET at a constant temperature (80°C IIRC) - in order to keep its electrical parameters from drifting with ambient temperature, like the OCXO mentioned above.