Solid state drop in tube replacement?

No, there isn't. MOSFETs and BJTs (powerful transistorized amplification devices) do not work like old-school tubes; they don't use voltages the same way and their input capacitance is way too high to make a simple plug-in device with them.

Roland and Korg have tried a few different approaches to making analog 'tube' technology work on a PC board. Roland hasn't released a product yet, but Korg has; they call it NuTube. Here's a pretty good writeup on it:

diyaudiostore.com/products/korg-nutube

I do like the Weber Copper Caps. I have one in my Princeton RI and one in my 66 Champ.

They make an amp a bit more punchy, with less sag and sponginess.

Thanks for the link Mr. Goo.

I've got a WZ34, but it's rather more saggy than a real GZ34
78 ohms of resistance in it, whereas at high current draws, a GZ34 equivalent resistance is more like 20 ohms.
And the HT ramps up in a couple of seconds, rather than the wonderful slow 45 second ramp up of a real one.
I don't like how its metal case isn't / can't be safely ground to the chassis, seems a potential shock hazard if disaster struck inside it.
Of course the glass envelope of a real tube can break and expose live electrodes, but its live parts would be much smaller and less conductive than the copper case.

And then there's the Jet City Retrovalve 12AX7 substitute www.amazon.co.uk/Jet-City-Amplification-RV1A12-Replacement/dp/B017LTI0Z4
They had good reviews but were rather pricey and seem to be discontinued now.

pdf64 - those Jet City tube replacements are what I was wondering existed (well, at least on the preamp side).

Have you ever tried them/know much about them?