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I just bought a Chi-Fi amp and had some extra 12au7s laying around which are wheat the middle two sockets call for. They are NOS and sounded much better. I put two more in the 12ax7 sockets and is sounds even better. Smoother. The only difference I noticed was that I have to turn the volume a little higher to get the same volume level, but it still goes plenty loud. Are there any long term issues with this substitution?
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:-).
Warmest
Tim Bailey
Skeptical Measurer & Audio Scrounger
As the others have pointed out, the 12AU7 is a much different tube. It is designed to be relatively linear at a much higher current than a 12AX7 uses. If you plug it into a 12AX7 circuit it will likely be running at a much lower current than is optimal and hence will be distorted.
As a general rule, circuits are designed to work best using the tube that is specified in the design. Plugging in a tube of the same type but a different brand is perfectly OK. Plugging in a different tube type usually results in sub-optiumum performance, e.g. less gain, more distortion, etc.
The tube designers who I have read commenting on substituting one tube type for another generally have a low opinion of the whole idea. It's like assuming that a major league pitcher can replace an NFL quarterback because they both throw a ball.
hmm...i dropped in E80CCs in my Vincent SP331 Mk hybrid power amp where orininally 12AU7 should work.
the amp sounds extraordinary much better with the E80CC than it does with any other 12AU7 i tested! (and i tested many, many 12AU7 vintage stuff...).
so i do not longer think, doubt or wonder but just listen and enjoy!
N.
The times, they are changing...
The E80CC is a lot closer to a 12au7 than a 12ax7 is.
The E80CC does draw twice the heater current than the 12au7 and that could be a problem.
BTW Thinking is good. Not thinking is bad.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
surely no problem - the amp ist running for months now this way!
any problems should have become manifest right from the start but didnīt.
:))
The times, they are changing...
If it were me I'd be looking to find out the current limits on the filament winding.
The effects of over heating can take a while.
Warmest
Tim Bailey
Skeptical Measurer & Audio Scrounger
Well, as Tre points out there could be a problem if the heater windings are being stressed by the extra current draw - depends on how much the transformer is over specified. One thing you could do is place your hand on the power transformer after its been running for awhile and see if it is comfortable or too hot to touch. If the latter, you could be stressing the transformer which would shorten its life. Things don't always go bad immediately.
Further, all 12AU7s are high-distortion when used in a gain stage. (Unless a lot of NFB is used.)So you will have be listening to quite high distortion.
I recall that you used 12AT7s in place of 12AU7s in the VTA ST120, I for one am not at all surprised that you didn't like them. The circuit isn't designed for them. Of course they sounded wrong.
Why do you imagine changing tube types - as against changing brands - would be an improvement?
Do you have any understanding of audio reproduction circuits or are you just trying for effects - as in a stage/PA amplifier?
Warmest
Tim Bailey
Skeptical Measurer & Audio Scrounger
Edits: 04/02/16 04/02/16 04/05/16 04/05/16 04/05/16
The VTA ST120 was originally designed to take the 12at7. I later modified the driver board by changing out certain resistors at per instructions I received to take the 12au7 instead for lower gain.
I ended up switching my Chi-Fi amp back to the 12ax7s due to negative effects on distortion levels and the overall flatness of the sound. Will find some vintage 12ax7s eventually. Thanks for all of your input. `
The Tung-Sol 12ax7 is a really nice tube.....
A circuit built for and optimized for a 12ax7 will not be right for a 12au7 so the harmonic distortion will be higher than it would be if the circuit was meant for a 12au7.
It's hard to say how much plate current the 12au7 will draw when placed into a 12ax7 circuit.
It probably won't burn the amp up.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
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