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In Reply to: RE: 5751 always safe instead of 12AX7? posted by Mechans on April 13, 2016 at 17:27:27
Not sure ruin is the right word here. Mu of 100 vs. Mu of 70 would not make much of a difference in MOST applications and in fact might be better in many.
About the only one I can think of that MIGHT be critical is in a phono preamp where the gain of the tubes are critical for properly establishing the RIAA curve.
Follow Ups:
There seems to be some misunderstanding regarding differences between the two tubes . The difference is NOT just the mu: the transconductance and plate resistance of the two tubes is different too. To say that the mu is different is an oversimplification that persists to the detriment of the understanding of using these tubes.
Perhaps ruin is too strong. I too was thinking of a tubed phono stage.
In addition I can't see them used in very low power SETs with their antique triodes(as a driver perhaps?). I have never seen a 5751 in an SET. I hear that SETs sound great I am not casting aspirations I would love to have one someday.
I run a GE triple mica 5751 in my rehabbed vintage Magnavox 196 SET amp, and I like it very much. I find that the somewhat lower gain really smooths out the sound of the audio. Now when I roll a 12ax7 thru the amp, I find it much too bright for my ear.YMMV of course
For my money a 6SL7 would be a better choice than a 5751 in most applications.
But that's just me.
I currently use a triple mica GE black plate 5751 (cryoed) in my phono preamp in a circuit design where the RIAA is not dependent on the mu of the tunes (supposedly).
The 12AX7 was never regarded as a particularly high quality valve, but there's nothing at all wrong with noval base valves.
It gives a massive amount more choice in circuit design.All this so called "superior" stuff about octal 'SN7, 'SL7 stuff from the 1930s, is crazy junk science.
As an example, I used the triode section of the UCL83 as AF amp recently to get rid of the half 'AX7 part of the 12DW7/7247 it had before.
(It has 38V 100m/a heater so can be run direct off the output valve DC bias supply).I could say the original 'ax7 af amp was far from satisfactory and they had even put a tube cap over the valve to try to contain the stray noise pickup.
The new 'CL83 result was superb, with much lower distortion, noise and hum pickup.
That often seems to be the main problem with poor performance,- poor design, and poor optimisation.Try as I might, I can't find an equivalent anywhere for the UCL83 triode,(same as in ECL83) and the pentode in the same compact length valve also doesn't have a match.
It's closer to some sort of TV valve
It's something like an absolutely ideal audio triode, which at the time was given a junk status as some sort of TV valve audio stage or SE budget distortion dustbin, like many other TV valves.
After reading this about the 5751, I checked if that were perhaps closer?
In reality, the UCL83 triode has a considerably lower anode impedance than the ECC83/12AX7, (roughly half in fact), while the UCL83 triode gain is half way between the reduced gain version of the ECC83 (the 5751 at 70) and the ECC83 (at 100).
It also has the improved cathode which seems to have been upgraded with other improved versions
This improved performance with mu of 85 means you get an extremely low distortion AF amp less miller effect, and lower impedance output.
In my opinion this put it in some of the top 5 audio valves with the rather rare unobtanium 12AD7,12DT7,12DF7, and even 12BZ7 which all have lower anode impedance but the same gain and uprated higher reliability cathodes/heaters.
FYI, - "it seems that one section of the 12BZ7 is equal to both sections of a 12AX7 in parallel"
I would never use 12AX7 again unless of course it were run in cascode, which effectively turns it into a pentode, then causes split heater large voltage differences.
I recently turned to USSR submin valves and these worked even better still.
You can literally solder them into place, and throw away the noval valve above chassis altogether.
Edits: 04/16/16 04/16/16 04/16/16 04/16/16 04/16/16
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