low-gain 12AX7 variant?

Hi All,
I just had an amazing amp made based on the Vox AC4 schematic, and I really love it but it has a wee bit too much preamp gain for my taste. I put a 12AU7 in the preamp and that helped, but the tremolo is tied to that tube and it almost disappears now.

I'm hoping someone can tell me of a low-gain variant of the 12AX7 that I can put in....

Thanks for any advice,
Chris

chrisb said:

Hi All, I just had an amazing amp made based on the Vox AC4 schematic, and I really love it but it has a wee bit too much preamp gain for my taste. I put a 12AU7 in the preamp and that helped, but the tremolo is tied to that tube and it almost disappears now. I'm hoping someone can tell me of a low-gain variant of the 12AX7 that I can put in....Thanks for any advice, Chris

Click to expand...

Hi Chris; the 12AX7 comes from a family of twin triodes; the 12A--T/U/X/Y--7 are all wired the same internally, perform the same function, are drop-in replacements for each other, but have different amplification factors; more on the subject Here. Since you already tried a low-gain variant of the 12AX7, there are a couple of other options.

Leave the 12AX7 in but increase the resistance of the plate resistor; this will reduce the DC on the plate and reduce the total gain however it will still have an effect on your tremolo.

If the cathode does not have a bypass capacitor, you might consider adding one; this will also change the gain characteristics of the tube but, depending on size, it will also have an impact on tone and this tweak is unavoidably a trial and error process.

Do you have any other 12AX7s around you can substitute in/out? You might find one whose tone is pleasing enough to set aside your objections about 'gaininess'. cj

Don't forget the 5751 capn. Not a huge gain reduction, but IMHO, much more character.

Thanks Don; good thought - gain characteristics at 70% of a 12AX7 and identical pinout. Can't help but wonder about gain changes v. tremolo. No schematic but if trem is operated by 1/2 of a 12AX7 and the trem function needs the 12AX7's gain, not sure what to do about the gain characteristics of the other or audio 1/2 of the 12AX7. Assuming high gain on the trem side and lower gain on the audio side, would either need a twin triode with different gain characteristics in each half or increase the plate resistor on the audio 1/2 to kill off some gain. ????

PS: Thank you so much for putting the BB back in play!

John

Thanks for the amp-factor link, cap'n-- that's exactly what I needed!
The AX is too gainy at 100, and the AU isn't gainy enough at 17, so I'll try an AT at 60 or an AV at 41!!
Perfect...

Hope it works; note that each 1/2 of the tubes in this group are 'gain-matched'. The question might wind up being how far down can you take the gain in one section before interfering with the trem function in the other. Good luck! cj

I'd try that 5751 first, having had good luck with it in other amps. Many tweeds seem to benefit from a 12ay7 in V1 especially, maybe also V2. I think many came stock that way in the 50s. The 5751 has a bit more gain than the ay (I think), definitely less than the 12ax7. You can get excellent sounding GE NOS ones for 20 bucks or so if you look.

May be some help.................

http://www.univalve.net/vbulletin/showthread.php?t=34

Here's a link to the original AC4 schematic: http://www.blueguitar.org/new/schem/vox/ac_4.gif

Terry used a 12AX7 instead of the EF86 for V1. This allowed for a gain control so I can dial in as much preamp gain as I want to overdrive the power tube (or the preamp tube, which is nice, but not as cool). It works great, but that's the tube that has too much gain for me. It's also tied to the trem, hence the effect it has when a 12AU7 is subbed.

He added a depth control for the trem too, which also has some interaction with the gain control.

I'll ask him for a schematic & post it for those interested... Meanwhile my gut says try the 12AT7, so I'll get one tomorrow & report back!

Hi Chris; yes, I'd like to see the revised schematic. Interesting design (that's 'code' for I don't exactly know how it works) ... don't understand how / why the cathode of stage 1 of the trem tube is tied back to V1. Good luck; hope the new tube does the trick. cj

OK, the 12AT7 did the trick: the gain is more manageable and the trem is still strong. Maybe someday I'll try an AY or AV...

Meanwhile, here's Terry's response to my request for a schematic:
"Well, I did not take the time to document it, like a dummy.
However, the tremolo circuit is direct from the Champ vibro.
The input 12ax7 circuit is close to a Fender princeton.
The rest was the Vox AC-4. Kind of a trio amp!
The tremolo circuit feeds pin 8 of the 12ax7.
Gain is set by the resistors feeding pins 1 and 6."

Hopefully that's helpful. When I get some pics I'll start another thread for the amp.

Good Chris - If it works, it works. cj

Try a 12AY7. Lower gain, smoother and mellower.

I like the Electro-Harmonix.

Oh yeah...put it in V1.

Don't have a schematic right in front of me, but from my recollection a Vox AC4 is a three tube amp with a EZ80 rectifier, EF86 preamp and EL84 power tube. There is a 12AX7, but for the tremolo circuit only.

From what I understand, an EF86 is a pretty gainy tube. I have only played my AC4 with single-coil p'ups, so I don't get into the snarl of say a baby-Marshal. The EF86 does impart a tone however, unlike a 12AX7, perfect for when I play at the Shadows-walk in the basement.

Interestingly, a local tech made me (his concept and design) a stand-alone EF86 preamp to go in line, like a pedal. Instant Bluesbreaker sound with ANY amp, even a solid state one...even my venerable old 66-J!

The hybrid you tech put together sounds like the kind of fun I am looking forward to in about six months when I retire! Bet it sounds great...