Hi Roger:
I was looking at the tube store part of your website and saw that you have the 5751 and the 6FQ7 tubes in three different grades for CJ gear. I've got an MV-50 that was recently rebuilt and was wondering which grade of tube you would suggest for this amp?
Thanks!
Good question. In your amp the input tube is a 5751 and the 6FQ7s are drivers. The input tube should be LN or SLN (for low noise). The drivers can be lower grade like SG or T as they won't contribute to the overall noise of the amp. We sell all tubes in all grades so the intelligent user can put the quiet ones where they matter (at the input) and the not so quiet farther down the line where noise does not matter.
Sometimes a 6FQ7 is used for an input tube or line amp and then it should be low noise. Thus we bother to find quiet ones for those applications. The 12AT7 and 12AU7 are other tubes that are typically drivers or line cathode followers though are now getting popular for line amp input tubes. We provide SLN and LN grades for those applications but if its used as a driver SG or T is fine.
A good example is the classic 3 tube phono and line stage that Marantz designed and ARC, CJ and many others copied. You would use a SLN or LN as the input tube, then one grade lower for the next tube and one grade lower still for the last tube. There would be no improvement in noise by using all SLN or LN tubes which would be much more costly.
For those who don't know our grading system which I created in 1980 and has since been copied by many. We use the following grades in order from top to bottom for lowest noise and even ascribe values.
SLN = Super Low Noise. These tubes measure within 1 dB of their theoretical (absolute quietest noise level). Yes, seeming unknown to some designers each tube type has a theoretical noise limit that cannot be improved upon by cryogenic treatment, sandblasting, special materials, prayer or voodoo.
LN = Low Noise. Tubes within 4 dB of theoretical noise
SG = Standard Grade, within 7 dB of theoretical noise
T= Tested, within 13 dB of theoretical noise
I created a thing called the RAM Factor (clever name huh) to actually measure how consistent the noise is because steady noise, like tape hiss, is far less objectionable noise that comes and goes. We tune out steady noise. This factor is also scaled and printed out on each tube label.
Microphonics are measured (not by tapping with a pencil) and similarly scaled based on what that tube is generally capable of.
These three measurements determine the grade of the tube. To be a SLN all 3 have to be low. If one is high and the others low it falls down a grade or two till it meets all 3 specs.
We also measure Gain, not Gm because the gain of a tube and matching of it for channel balance is not based on Gm, it is based on Mu which is what we measure. Matching tubes for Gm on a Hickok does not match them in your preamp. BTW the best Hickok and other testers many use have absolutely no suitability for audio tubes, preamp or power, and were not meant for those purposes. Those guys can go back to their sandboxes as far as I'm concerned.
We give you all 5 measurements, for both sections individually. It's on the box, it's real, it's repeatable and not something scrawled on the box with a sharpie or some meaningless mass printed label.
There is a lot to say about tube matching, most of which I have covered in previous posts. Perhaps someone would give them a bump to the top.
