5U4 GB with strange phenomenon

My VTA-ST-120 has a strange situation :

Last week, I had a completely disturbed stereo-image, as there was a extreme left-right accentuation.
One part of the voice was extreme left, the other extreme right..... A phase splitter failing?
I removed the three 12AU7 (6189W Philips JAN) tubes and changed them by a new same JAN 6189W in middle position
end two ECC82 Philips NOS on each side ..... all was well again.

I am using a 5U4 GB rectifier (Sylvania NOS - USA made) that (normally) has a clear top on the glass, allowing one to see clearly
two glows through the top of the tube;
In about 48 hours, the 5U4 GB started to form a dark marking on the top of the tube, now it has the same mirrorlike
dark top, a bit dull gray from, the inside, but perfectly mirror looked fromtop/down (same as on KT-120)

While the tube makes this change, I am wondering :
- what is this phenomenon?
- what is causing it ? Is it dangerous or just a vaporising of mercury ?
- should I take precautions ? Change the tube ?

With all this going on, the amp keeps playing a top musical performance. Biasing is correct at 0.52 VDC for every tube with nothing drifting off...

Could anyone explain?

Be interesting to see what would happen if you were to swap the original drivers back in. If the same problem develops, try switching positions? The center tube is what should set up your sound stage - the side tubes provide amplification of the split signal?

Also, wondering about using a 5U4 in the ST120 without any internal mods. You're talking much greater sag and filament current than a 5AR4. There's also the issue of direct vs indirect heaters to contend with ... with lesser amps, you could cook the iron ... don't expect that's an issue with the ST120, but ... I'd sub in a standard 5AR4 or copper cap till you get some time to research the issues ...

Possibly related ... I tried a 5R4 here with less than spectacular results. The amp biased up well and sounded good ... but that was short lived. Maybe a month of use, and it was fading fast. Can't win em all I guess, but it did give me the incentive to do some digging and I came up with a real nice NOS Mullard GZ37 at a reasonable price, so it's not all bad.

Thanks for your reply.
Why a 5U4G.....? I like the size, the GZ34 being such a dwarf next to the KT-120. I have several 5U4, also GZ34 (Mullard, Genalex Gold Lion, Sovtek, JJ,....) and even a new Russian NOS 5U3

In april 2013 Bob Latino wrote "No ... You can use a 5U4 as a rectifier in a VTA ST-120. The only minor issue is that a 5U4 has a higher voltage drop than a GZ34 and power of the amp will be very slightly lower .. The slight drop in power, though, may not even be noticeable if your speakers are fairly efficient. The advantage to using a 5U4 is that there are still many well made USA 5U4's floating around at reasonable prices. A 5U4 was used as the rectifier in many of the earlier tube color TV's of the 1960's so many of these tubes were made and many are still in circulation. Personally, I would consider a USED RCA, Sylvania or GE 5U4 (provided it tests as new) over the modern JJ version.
If you do switch to a 5U4 after having a GZ34 or a Weber WZ68 in there, you will have to rebias your amp.
Bob"

I am using a new NOS 5U4G Sylvania and the amp biases perfectly with it. Having rather sensitive speakers (91 dB/W/m), so the eventual drop in power is not important.

In the coming days I will follow your advice and swap the Original (6189W) driver tubes. See what happens. I do have a NOS GZ37 Mullard, but it was during my first test
with this that the "anomalies" with the driver tubes showed up..... making me a bit "uncertain" and "in doubt"....... it is a beautiful large bottle however :-)

I should mention that, although perfectly able to build a kit, I know nothing about tubes as such and notions like "drag", "filament voltage" etc are new to me... I will learn however .... slowly... :-))

OK then ... same reasoning for me trying the Philips 5R4GYS ... those big KT120's just dwarfed the other rectifiers. That, and the 5AR4 varieties didn't much like dialing up to 60mA, and for whatever reason, I prefer to keep the WZ68 copper cap as a spare ... I swear I can hear a difference in the sound quality using it.

Lucky me ... I stumbled across a NOS Mullard GZ37 that provides plenty of power AND stands proud amongst the big boys ...