#1
I recently stumbled on Roberts Audio Technologies' Retrovalves.

http://www.robertsaudiotech.com/designs/retrovalve.php

I was just wondering if anyone had tried these?

I mean, I've done some research on the Web, and I know some UGers suggested they might be useful to test whether an amp's problem is because of a dead or dying tube.

I have seen what people THINK they would sound like. But the only thread I found where someone actually tried them had dead video links.

So...anyone have some experience with them?
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#2
Not very good. They have a weird edge to the tone that shows up even playing clean, and can get really annoying with distortion. I keep them for testing tube positions when the preamp is acting up, but besides that I have no use for them. They sound boxy in nearly any context. The high gain version is nearly unusable, and they are actually noisier than my regular tubes, especially using more than one in the same amp.

The low gain version could be usable in reverb driver or similar circuits where you don't really hear it. Some amps might tolerate it in the PI.

It's a good idea but there's a reason this particular design has been around for many years with no real excitement or interest.

Edit: here's what I wrote about them when I first bought the set:

Got a set of Jet City Retrovalves, the SS tube plugin replacements. Not impressed.

They sound like crappy tubes. There's something wrong in the high end, they all had a weak and brittle treble. The high gain version had some nice harmonics but overall they just don't sound great. The low gain one seems to be around a 5751 type output, and the high gain is pretty darn hot, beyond what you'd get from a 12AX7. They all had that weak top end and an odd mid scoop, not terrible sounding but certainly a notch below any decent current production tube. Tried them in the PI too, same sound issue. The high gain tube introduced a pretty nasty buzz in the PI slot. The amp I'm using is about as sensitive to preamp tubes as you'll ever get, with half of a 12AX7 driving a bright and the other half a normal input, straight into the phase inverter. Perhaps they're more bearable in amps with more preamp stages, but I wouldn't hold my breath.

I might consider using the low-gain or normal version as a replacement for a reverb or EQ driver tube, or maybe the high-gain version in the 2nd or 3rd position of a high-gain amp. They also may come in handy for diagnosing tube issues. As a replacement for a real tube in any tonally critical slot? Nope.