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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 
goofy

Showing 12 responses by roberjerman

roberjerman
4,319 posts
 
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!
roberjerman
4,319 posts
 
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!
roberjerman
4,319 posts
 
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! 
roberjerman
4,319 posts
 
@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.
roberjerman
4,319 posts
 
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)!
roberjerman
4,319 posts
 
I have tried to give the OP some practical advice - and maybe save him from disaster! 
roberjerman
4,319 posts
 
I don't know the tube complement the LS36.5 DM uses. Just thought I'd list the most common preamp tube types! 
roberjerman
4,319 posts
 
The Cyrillic "H" is actually "N" - and is pronounced that way! I can read and speak Russian!
roberjerman
4,319 posts
 
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.
roberjerman
4,319 posts
 
@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! 
roberjerman
4,319 posts
 
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!
roberjerman
4,319 posts
 
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!
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