will
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Luis,
There is a big difference in some ways, depending on how deeply you are looking. I think you are referring to 5U4G-STs (coke bottle shape), and the GBs are straight bottle, right. Also both come in different vintages with different construction. 300 is a lot in my book for STs. I wonder if this is a nice 40's pair, which is a really nice tube, but 50s are good too. If you look on Ebay, you can usually find a nice NOS or near NOS matched pair "buy it now" for 70-100 delivered. And if you look carefully at construction, you can find RCA made rectifiers with different company labels, and these are almost always cheaper. I have a pair of Raytheon labeled Sts that look exactly like some RCAs and were brand new for 55. This was an extra good buy, but they are there if you take some time.
If you go shopping be sure they are at the top of the test range, matched, and have exactly the same construction...the same getters, plates, bottle shape, space from top micas to glass top etc... this is usually the case anyway, but some sellers will have electronically matched pairs that are not quite the same construction.
Generally, GBs are bigger and deeper low down and generally with a relatively sweet, open richness in the mids, with nice open highs. They also tend to have a more "powerful" vibe by a touch...a bit more push. I have seen and heard three basic types of RCAs, short bottle, tall bottle, and extra tall bottle , the extra tall with the inner construction just like STs. The later was my favorite for solid, open, detailed and extended sound with less color. For more richness and warmth, a couple of Raytheons I had of different construction were favorites. One was 50s and the other 40s with a strange stepped shaped base. Both were really nice for warmth without detail sacrifice. It seems that earlier is often better, but I had some great sounding tall bottle Sylvanias from the 60s too...I would not get GBs if you have bass issues in your system/room and no way to tune the bass.
I personally went back to STs after my affair with GBs finding them solid, but generally more neutral and smooth, and having nice, delicate inner detail...also with really good extension, but generally not as bass heavy as GBs.
That said, they both vary within these generalities. The RCA-STs with the the wires running between the plates from the bottom through the top mica, those are I think 40s, and they will generally be more delicate and revealing with a bit less bass than the later 50s tubes with thick plates and no wires between the plates. And the 50s tubes vary also, some denser and darker and other more delicate.
Hope this helps some. What you are replacing, what you would like to get for sound change based on your current sound, and if you setup tends to muddle or being too bright, these would weigh into what would be best for your setting. There are other interesting rectifiers too that have specific advantages depending on need like 5Y3GTs, 5R4GYs, and 5V4Gs...all in a similar sound range, but each type tending to do it differently.
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