is there actually a difference in effect on headroom/fidelity using a 12AT7 in place of a 12AX7 in V3

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doctorunderhill

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i'm preparing to do some preamp retubing. the amp in question is an Egnater rebel 20. i have 2 Dutch made Amperex 12AX7's and 1 AT (also an Amperex) that i had tucked away. my question is will running the AT on V3 actually make a difference in headroom/fidelity vs running an AX in this position. i have seen posts that claim it Does make a difference, some that say perhaps a slight difference, and then that so it makes no difference. i basically run my amps clean, and if i need breakup/distortion i achieve that through pedals.
just curious.

and, i know what you're thinking- running these high end preamp tubes in this amp may be akin to putting a Christian Dior evening gown on a pig.......
 
Im not super familiar with design of the rebel 20, but the answer to this question comes down to the individual circuit of the amp. Some amps get their “sound” from preamp distortion, some from the power section, etc.
 
Yeah, not sure it would directly affect headroom per say but it will have less gain. And a 12AY will have even less - say 70% of a 12AX. The comporable 5751 will have less gain and be very clear, which is why they are popular in the PI position. But since you have something to try - try it and report back. Since you probably have to take the headshell off to swap the tubes please be sure to never touch the insides of an amplifier with your bare hands. Could be lights out.
 
Maybe if the TC of the AT7 is greater than an AX7? I’m simplifying this as there are currents to take into consideration as well, etc.
 
If your running the amp clean I doubt your going hear a huge difference with a 12AT7.

Dutch made Amperex 12AX7 sound glassy and clean to me. That I expect to make a voice difference.
How it would react in a Rebel 20 hard to say. I had a Rebel 20. Not bad for what it is. I gave it to a friend of mine years ago.

Try it and see what you think.
 
I think the amp needs to be designed for the specific tube. VHT uses a 12AT or U 7 (I forget which), partly because it has a lower impedance and drives the power amp differently, or maybe it allows the power amp to be driven differently. I’m sure this also has to do with the type of power tube and the intended design and therefore use of the amp.
 
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