If the problem is resolved without the use of a snubber I’d not bother. At this stage it’s more a matter of what’s causing the ring - you say it’s 250KHz? Reduced by change of diode type but at the same frequency?
Is the AC feed to the preamp from the transformer case grounded at the transformer end as it is in the box? Or is it isolated at the TF case and only grounded in the preamp box.
Still if there’s spiking after those caps the voltage could exceed 50V. I’m wondering what you would see across those front end caps with an ungrounded scope.
The addition of a front end resistor is certainly a good idea - low value would give not much voltage drop. I’d try a separate one for each supply.
Well, the spike and ring is much reduced. From the scope traces I posted above for example I'm not sure how I'd reliably quantify the reduction between pics 2 and 3. I'd estimate it's overall about a factor of ten, but the initial spike seems less consistent. The ring is still there in pic 3, for a slight ripple or two, and you can see it follows the pic 2 trace with a period of around 2 divs, so at 2 microsecs/div then ~250kHz before and after the diode change. (In the other thread discussing input tx damping this kind of ringing magnitude didn't seem to worry most people! Here it is uncorrelated with the audio signal of course. )
Another reason I had for stopping at this point (before Per in Denmark jogged a few more braincells back to life) was that the kind of disturbance visible in the last pic (3) occurred with the 24v psu branch disconnected from the ac supply completely, just the 48v connected. Since upgrading the diodes in the 24V supply produced behaviour about as good as with no 24V supply connected at all then I felt that particular aspect was fairly well dealt with.
Finally I also upgraded the diodes in the 48V supply but it made little if any visible difference.
The AC feed is floating - not grounded at the TX and just connects to the signal ground through the chassis at a single point - except for those 1nF decoupling caps. (1nF presenting about 600ohms to a 250kHz signal)
ok I see where you're coming from now re the spiking on top of the peak ac voltage at the cap. I don't think there is going to be anything significant there, now at least, but I'll take another look. I can't (at least won't) unground the scope as such but could do a differential probing across the input caps perhaps. I'm generally trying to avoid using both channels on my old scope as one channel is a bit flaky, though it can be cajoled into stable operation sometimes (worn out/corroded internal switch contacts I suspect, but not very easy to get at).
Re front end resistors "I’d try a separate one for each supply" - agreed.