Yes it is a big concern Vince, as tautech said if the dip is too big the MOSFET will turn off! EEVblog video #1119 describes this really well!
The graphs in my previous post are comparing different MOSFET's, the type of the MOSFET is the filename of the picture.
So the first one is IRF820PBF, that is like the original MOSFET (I have replaced it with a new one though) and even that has a dip down to around 7.5v.
That's not great but the point it will turn off is from the datasheet: VGS(th) min: 2.0v max: 4.0v, so we are "ok"
The second graph's MOSFET is a IRFBC30, this is actually quite close to what Tektronix changed to in 050-2242-03 as
flobydust wrote here.
The dip for this goes right down into the VGS(th) range, so it's not good.
What's interesting is that there is a change for the 2235 in one of it's later service manuals.
At the last pages they add component R909, a 39Ω resistor at the gate of the MOSFET.
Of course that will reduce the inrush into the gate and thus the dip will be much smaller.
Maybe even gone if the problem is with capacitance as I suspect.
The last graph is of the really low RDS(on) part STP15NK50ZFP that I bought, which was a fully insulated part.
It also has some nice features like built in zener diode protection from too high voltages between gate and source.
However it's capacitance is much higher than the original part, and it's gate dip is really pronounced all the way down to 2v!
C921 which I mentioned before sits on the TL494's PWM output, it's an AVX SA205E104MAA general purpose Z5A ceramic capacitor.
It has pretty horrible specifications, significantly dropping in capacitance with increased heat or lower DC bias, or AC component.
That said it's parallel with the 100µF electrolytic cap C913 so it might not be an issue at all.
The one installed on my scope is original, and it measures exactly 0.1µF (103nF) so it's still ok after 35 years.
I noticed it's not even present in the 2215's big brother 2235 schematic, so I took it out: there was a bit more ripple, but the dip was exactly the same.
So it has no effect at all, I even put another small electrolytic cap parallel above it but it did not make any discernible difference either.
Finally I tested to add the 39Ω gate resistor R909 right now, it improved the situation with the original MOSFET IRF820PBF quite dramatically.
I have attached a scope shot of that here, and also a picture showing where I installed the resistor.
As I understand it the 2235 also has a ferrite bead on the MOSFET, so I'll go see if I can find any pictures of that.
The eagle eyed might also notice I've replaced resistor R911 & zener VR917, mine read 161k & 6.0v respectively, so it felt best to replace them since they are 5% components.