5751s replacing 12AX7s

[jim777]

So, I decided to see if I could find out what was wrong with my '80 Acoustic G100T amp, and pulled all of the tubes. I hadn't played this amp in literally 2 years, because the MK IV is so well suited and reliable for what I need, and found 2 of the 3 12AX7s in the Acoustic were blown. When I last played it it would be fine for 20-30 minutes then fizzle out, so the preamp tubes should have been obvious really. The power tubes were perfect. So I went online and ended up ordering 3 5751 tubes because they are supposed to have about 70% of the gain of 12AX7s, which sounded to me like they'd be fine for no pedals jazz, and if they fixed the Acoustic that would be my 'jazz' amp.

So I put them in, dial the reverb up to about 4, and the sound is just the warmest, most perfect clean sound I have ever heard from an amp, literally any amp. This is with an old Ibanez FA-100, which is a Gibson ES-175 copy, so probably not a sound or kit you guys are looking for or have lying around, but damn, these are great tubes.

Have any of you ever tried them? I had thought in the past of going with 6V6s in the MK IV, but if you flip the power switch to full they all blow (evidently) so this seemed like a safer bet than messing with the power tubes. And now I've got this glorious warm, clean sound from an amp I couldn't get $200 for 10 years ago!

EDIT: I'll add here the Acoustic G100T is a 100Watt (with a half power switch) combo with an old EVM12L in it. It's a fairly blatant copy of a Boogie MK II, but you can use the 5 band graphic EQ in either channel, where the Boogie only had the graphic in one. Mesa sued them for that, and Acoustic went out of business.

[Budda]

Nicely done!

[Mike]

5751s have been a suggestion for a while for amps that needed taming. Glad it worked out for you - those old Acoustic Boogie clones are cool amps.

[Markus]

Interesting. Is it possible to mix and match, i.e. have a 12AX7 for one channel and a 5751 for another? This could be cool for the crunch channel in my Laney, but I wouldn't want to tame the lead channel.

[Tony]

Interesting. Is it possible to mix and match, i.e. have a 12AX7 for one channel and a 5751 for another? This could be cool for the crunch channel in my Laney, but I wouldn't want to tame the lead channel.

Yeah if you know which valves are for which channel.

I love 5751.

They also work really well in the phase inverter position.

It widens the tone so to speak fills it out and tames the fizz.

Especially good in stupidly high gain amps pi.

[Markus]

Yeah if you know which valves are for which channel.

I love 5751.

They also work really well in the phase inverter position.

It widens the tone so to speak fills it out and tames the fizz.

Especially good in stupidly high gain amps pi.

Cool - I'll have to look into it. I think I have the schematic so it's possible to find out which one is which.

[jacksonplayer]

I've used 12AT7s in rack gear to clean things up before.

[lespauled]

After reading this thread, I went on search of some more info, and found these:

https://toptubeamplifier.com/5751-tubes-vs-12ax7-tubes/

https://www.thetubestore.com/gain-factor

I'm a little confused about why you just can't turn down the gain to get a similar gain level as the 5751s.

[Budda]

After reading this thread, I went on search of some more info, and found these:

https://toptubeamplifier.com/5751-tubes-vs-12ax7-tubes/

https://www.thetubestore.com/gain-factor

I'm a little confused about why you just can't turn down the gain to get a similar gain level as the 5751s.

Probably the compression at the very beginning vs later on?

Axe fx 3 doesnt have that tube type as an option, i bumped the wish thread for it.

[jim777]

After reading this thread, I went on search of some more info, and found these:

https://toptubeamplifier.com/5751-tubes-vs-12ax7-tubes/

https://www.thetubestore.com/gain-factor

I'm a little confused about why you just can't turn down the gain to get a similar gain level as the 5751s.

I was replacing dead tubes and had a chance to get something a little different from what I already have in my MK IV, and my AX8/PA rig. The first article you quote says the following, "Nevertheless, in some cases, you might feel that the audio quality isn’t that impressive." And in the OP I said, "the sound is just the warmest, most perfect clean sound I have ever heard from an amp, literally any amp". I was looking for zero gain, as you might think of gain in a metal sense. Honestly I don't really feel a need to keep either the MK IV or the AX8 at this point.

[Tony]

After reading this thread, I went on search of some more info, and found these:

https://toptubeamplifier.com/5751-tubes-vs-12ax7-tubes/

https://www.thetubestore.com/gain-factor

I'm a little confused about why you just can't turn down the gain to get a similar gain level as the 5751s.

It's not gain in that sense. The controls on amps are cut controls and only affecting that one point.

The lower gain rating affects how the whole gain stage reacts which in turn affects what follows it as its expecting a gain factor of 100 but getting ~70.

Voltages can be lower it also affects frequency response and interstate loading where your gain control itself only affects saturation /drive.

[Drew]

I've used 12AT7s in rack gear to clean things up before.

I have a pair of those I've been sitting on for, well, since pre-2020, to try in my Mark V, as they're supposed to do good things in the phase inverter position and one of the preamp positions to the way the IIC+ channel in particular is voiced, less a decrease in gain than a change in structure. I really need to take a night or two to try that out.

[Tony]

I've used 12AT7s in rack gear to clean things up before.

It can work wonders.
If you have a triaxis a 5751/12at7 will also stop the dynamic voice from clipping

[zepp]

It can work wonders.
If you have a triaxis a 5751/12at7 will also stop the dynamic voice from clipping

In the phase inverter position?

[Tony]

In the phase inverter position?

V3. Channel mixer /master I believe.

That and changing ic3 should fully stop it happening.

Of course you can just keep the master below 5 which also stops it.