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 
Different rectifier tubes drop the B+ voltage by different amounts. That's all! This will effect an electrical circuit's operating points. It is best to use the rectifier that particular circuit was designed for! For instance, a 5Y3 cannot be substituted for a 5AR4/GZ34 because it cannot handle the same current! It will overheat (glow red) and fail! You must use a rectifier tube matched to the voltage and current required for that particular circuit. Do not do such a stupid thing as using different rectifiers! Serious damage may result! 
Different rectifier tubes also draw different amounts of current from the power transformer - even though most are all 5 volt heaters. Too much current draw from the 5 volt AC winding will cause the power transformer to overheat and fail! Stick to the original rectifier that your preamp was designed for!
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If you desire a "different" sound you can try different makes of the other tubes in your preamp: 12A*7/ECC81,82, 83's/5751's, 7025's, 6DJ8/6922/ECC88's, 6SL/6SN7's ..  This is a SAFE method!
No experience with rectifiers on a
 Modwright , but you cant judge the sonic effect of a tube with only 5 hours burn-in.
@roberman, I love your making the point that what is commonly thought of as the "sound" of a tube is more a matter of it’s electrical characteristics and performance in any given circuit than some inherent subjective, "artistic" quality. That electrical performance is a result of its design and construction, which very much determines the sound it creates. I’m sure some will disagree with that assertion.
@bdp24  Amen to that! Tubes do not possess some "magical" property that gives them a distinctive "sound"! An electronic circuit connected to a transducer (speaker or headphone) has a "sound". Not the individual parts! I expounded upon this topic at length some time ago in an earlier post.
Tube brands DO NOT have distinctive tonal qualities! The "golden ear" crowd can throw stones at me! For the record, I have been around tubes and tube gear from young (the Fifties)!
I have tried to give the OP some practical advice - and maybe save him from disaster! 
" If you desire a "different" sound you can try different makes of the other tubes in your preamp: 12A*7/ECC81,82, 83's/5751's, 7025's, 6DJ8/6922/ECC88's, 6SL/6SN7's ..  This is a SAFE method!"
Not sure what you are telling him as his preamp doesn't take these other tubes.
" Tube brands DO NOT have distinctive tonal qualities"

So, all tube brands sound alike? Not sure what you have been listening to.

I use a NOS Philips 5AR4 rectifier in my Modwright LS100 preamp , and a NOS Mullard 5AR4 rectifier in my Modwright Elysee DAC , haven't had any sound "anomalies" using either; and found both to be better than the stock new production rectifiers
Not sure about sound differences but I've seen new Russian versions create fireworks when replacing an original tube of the same specs.

EAR-Yoshino's Tim de Paravicini has talked about how in the "old days" when one tube maker ran low of a given model (tube factories often made an entire years supply all at once, then switched over in production to a different model), they would buy some from a competitor, put their own silk screen label on it, and sent it out. Tube experts like Tim and Roger Modjeski (Ram Tubes, Music Reference) can carefully inspect an unlabeled tube and tell you what factory it was made in.

I don't know the tube complement the LS36.5 DM uses. Just thought I'd list the most common preamp tube types! 
they would buy some from a competitor, put their own silk screen label on it, and sent it out
This happened more often than people realise. Yet another reason to buy from trusted sellers who can identify these tubes.

"I don't know the tube complement the LS36.5 DM uses. Just thought I'd list the most common preamp tube types! "
He listed it in his opening -  6H30 DR . 
I have done some research. Apparently the 6CG7/6FQ7 will work as a substitute. Same pin-outs, similar GM and capable of 250 volts/plate. Good quality US-made ones are available for a lot less than the pricey 6N30's (correct designation). I use them in my NYAL Moscode 150.
The Cyrillic "H" is actually "N" - and is pronounced that way! I can read and speak Russian!
roberjerman -  tubeworld contradicts your finding that the 6H30 DR and 6CG7/6FQ7 are equivalents
" a 6FQ7/6CG7 is electrically similar to a 6SN7GT, but a 6FQ7/6CG7 has 9 pins versus 8 for a 6SN7GT "
and from fourwater
" To address this problem, the 6CG7 was created which simply put the 6SN7 elements into a 9 pin minature tube. The main difference is that the plates of the two sections are slightly closer together because of the narrow confines of miniature tube. They did add a twist though, since there were 9 pins available, they decided to add a shield between the two triode sections and bring it out on the extra pin. This was used to keep the two sections isolated, which can be especially useful for high frequency use. This made the tube slightly more expensive to make, so later the 6FQ7 was created which eliminated the internal shield. "

@facten : The 6N30 and the 6CG7/6FQ7 are both 9-pin bases. I haven't yet looked at basing diagrams so I can't say for certain that they are identical. I suspect they are! And the Russian tube is likely a copy of the 6CG7/6FQ7 type dual-triode. Not some secret "super" tube! 
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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Yes don't go all crazy with tube swapping but...well researched substitutions can make a world of difference.  I have a Modwright PS 9.0 that came with Sovtek 5ar4 tube. That one blew after a week.  I replace it with the same tube...6 months and another blow.  I put one last Sovtek in it and worked but sounded like crap. Edgy grainy unlistenable. I'm new to tubes so I started asking Mr. GoogleI settled on NOS Tung Sol 5R4GY which sound so much better. This was not an expensive tube and I learned what rolling could do.  Next I popped for more expensive NOS Phillips 5R4GYS and bang Jackpot!  Now lets see how long this one lasts

@goofy you are highly unlikely to exceed the current capacity of a 5Y3 rectifier with a preamp utilising 2 6H30 tubes and two shunt regs. OTOH the suggestion of substituting 12A*7/ECC81,82, 83’s/5751’s, 7025’s, 6DJ8/6922/ECC88’s is ludicrous as they are totally different types of tubes with MUCH higher gains and plate resistances and totally unsuited to your preamp (6DJ8 family are less bad but still unsuitable). God only knows how they would bias. Behause they draw far less current than the 6H30 they may also overload the shunt regulator in the pre which is supplied by a constant current source. 6CG7/6FQ7 have a similar gain, but their plate resistance is about 10 x that of the 6H30 and they run at about a third of the current so you would lose a lot of drive capability. The only sub I would suggest trying is the 6N6P, 60‘s Reflektor are very nice.

I don't know why all these rectifier tubes are so popular among manufacturers, they are usually overpriced and don't perform as well as two 2.5 volt half wave rectifiers.  I guess is because it's cheaper to implement than the 4 pin half wave rectifier tubes.

the 5R4GY draws an additional 0.1A filament current, and will drop more voltage at any given operating point. Their stated maximum current capabilities are identical. The 5R4GY is directly heated, 5AR4 indirectly: fast voltage ramp up vs slow. Purported sonic benefits of directly heated tubes.

draw your own conclusions