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I have a friend who is considering installing 5751s in place of the 12AX7s in his integrated Fisher amp. I believe it's for his X-100-C. We're both aware of the drop in gain going from the 12AX7 (100) to the 5751 (70).
My question is....would the lesser gain be significantly noticeable?
Would it cause someone to just turn up the volume in order to compensate for the reduced gain of the 5751?
Perhaps it would tame a volume control that's to hot in the lower range?
I've never run 5751s in anything.
Your interest may vary but the results are the same (Byrd 2020)
I can't compete with the dead. (Buck W. 2010)
Cowards can't be heroes. (Byrd 2017)
Follow Ups:
5751 has a lower Mu than 12ax7, that is not the same as lower gain.
I use 5751 often in place of 12ax7 in the input stage of my Rogue amp. I have never noticed any group in gain for the substitution. That said, there could be an effect if they were substituted in a phono gain stage.
The general consensus is that outside of a phono stage, 12ax7 and 5751 are interchangeable in 99% of equipment out there.
Enjoy the roll!
I've used 5751's a few times in place of 12AX7.Based on my experience, I think the answer to all your questions is yes, maybe. Depends on where in the circuit or in the system you put them.
My preamp, in the final output, I did have to use a higher setting on the volume control to get the same SPL out of the speakers.
In my tubed CD player I noticed virtually no difference except a slightly softer presentation.
Tell your buddy to give them a try. It wont really hurt anything. Although what Ivan said about phono is good advice.
Edits: 08/24/21
Six 12AX7s in X-100C; different applications in different stages. V1 and V2 are phono preamp, using feedback EQ. Lower gain will result in less accurate equalization at the low end where gain is highest. Probably some loss in bass. V3 and V4 are line amps - each using both halves of the tube. Gain will drop by about half, maybe less. V5 and V6 are drivers. May not have audible effect since they're inside a feedback loop.
Tubes are engineered to have three significant characteristics: Mu (amplification factor), Gm (transconductance) and Rp (plate resistance). So to determine the correct tube type for an application, the design engineer takes into account all three characteristics before choosing the most appropriate tube for the application. Once the tube type has been determined, the engineer then determines the correct operating point for that tube in the particular circuit design. So all three characteristics e.g., Mu. Gm and Rp (not just Mu!) are involved in determining the best operating point for the tube and for the circuit in which it is operating.
Now it is possible to insert any tube type with the B9A noval pinout into any B9A socket. Will this cause any physical damage to the tube or circuit? No smoke will result. Will the device work as the engineer carefully designed it? Of course not. If the device is a phono stage what will be the result of replacing a 12AX7 with a 5751? You will create a random filter.
if the gain of the tube could change the RIAA correction.
But otherwise...
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