SET owners, need help with Rectifier tube

I am looking to replace the rectifier tubes in my Coincident Frankenstein monoblocks. The current tube which came with the amps is a GZ4P. I have learned the P designation indicates a Chinese tube.

Does anyone have experience with a Russian substitute which is a 5U4C?

Mullard has a substitute which is a GZ30 which go for $50.00 each. Is it possible these tubes can effect a sonic improvement?

Any input would be much appreciated. Thanks.
You might try writing to Israel. He'd certainly know the best rectifier tube for his amp. With my own Quicksilver SET amps which are also a 300B SET with tube rectification (5AR4's in my case), changing the rectifier definitely has an effect on the sound.

Good luck!
Try contacting Kevin Deal at Upscale audio. He is very respected and has extensive knowledgable of tubes. He will definitely be able to help you.
Definitely check with the manufacturer first.

And I second the Kevin Deal recommendation. I needed new GZ37 rectifiers for my Art Audio PX-25 and he delivered the goods. You get exactly what's advertised(no fakes or failures) with him and he stands behind his tubes 100%, not true of every tube dealer out there. Just be sure to do your homework and know what you are looking for before calling, things are usually pretty busy at Upscale and Kevin doesn't always have time to chat freely.
Thanks for the input. I will give Upscale Audio a call.
I have found Mullard GZ30's on ebay for as little as $22 per tube, they sound great, closer to the 5y3 than the 5u4 in voltage drop. However, they don't have the high voltage/current limits of the 5U4 or the 5AR4, you would have to be careful about using them in an SET power amplifier without knowing they are capable of the output, I would stick with the other types for power amps.
Third on calling Kevin at upscale, and I also agree on the Mullard rectifier(though Kevin has none listed for your amps at the moment). I've found them to affect the most improvement, and last the longest also(Blackburn plant).
I thought the Frankenstein uses 5U4G rectifier tube? That's what mine uses.