Rectifier Tube Rolling in a Modwight LS 36.5DM


I was hoping to get advice, I have been rolling Rectifier tubes in my two box preamp I am blown away how different tubes change the sound ,I own two sets of Mullard GZ37/CV378 a set of GE 5U4GB both vintage early 1960's and a set of stock Sovtek 5AR4 also have a NOS set of 6H30 DR I am trying in the clean  box , both the Mullard's and Dr tubes only have about  five hours of run in time but the sound seems defused on the top end ok on the bass the GE Tubes are good in the mids but soft on the highs and bass so far the stock Sovtek seems to sound best ,Dan says try 5R4-GY Tubes, any one tell me about  the Mullard's in this preamp I am getting frustrated,  thanks 
128x128goofy
The Russian's have copied and used Western tubes since before WWII. The correct Western designation for the 6H30 is "6N30". Cyrillic " H" is the Western "N". I suspect that they are actually 6CG7/6FQ7's. Certainly the plate voltages (250) and GM are similar! As is construction (dual-triode). I will contact Roger Modjeski for his opinion!
I have found more data: same pin-outs and construction, equivalent electrical characteristics. So the Russians just copied the 6CG7 (including its internal shield). Shame on ARC and others for calling it a "super" tube - and boosting the prices! Another audio "myth" revealed as just clever marketing!
Haven't been able to find any online source that states that the 6CG7 is a drop in replacement for 6H30 ; what did Roger have to say?
Thanks for all your Advice,  I wouldn't have tried using GZ37 MULLARD'S if Dan Wright didn't ok this move  I am still using the Mullard's in both the PS 36.5 and the PS 9.0 with great success, I will watch out for glowing red tubes, thanks again all of you fantastic Audiogon posters 
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