Question about EF86 and 12AX7 tubes [Vox AC15 Reissue]

[VoxRocker]

Hey guys

I ran across a schematic for a Vox AC15 (not sure of the model in the schematic, but mine's a 1996ish reissue) and the first preamp slot shows an EF86 (6267) tube, second and third ECC83 (12ax7), and the fourth and fifth vibrato/tremolo ECC82 (12au7). My question is, what are the tonal differences between using a 12ax7 instead of an EF86 in the first preamp slot. Should I just buy an EF86 to try and see if I like the sound, or can I even use one? I've heard that 12AX7, 12AY7, 12AU7, 12AT7s, etc are all mostly interchangable and just changes the gain on tap (right?).

If anyone has a link to a good read regarding tubes and the tonal differences, especially if it includes anything for Vox type amps, I would LOVE the chance to read it.

Thanks in advance!

Matt D

[John Phillips]

No, you cannot put an EF86 in a 12A*7 position - don't even try it, you may cause damage.

These amps are NOT reissues.

Sorry, it's one of my pet peeves that they're constantly described as such. They are a modern amp based loosely on the top-boost channel of an AC30, with half the power. They have next to nothing in common with an original AC15. I can't remember the exact order of functions of the preamp tubes in the modern amp, but you can be sure that it isn't the same as the original either.

The 12A*7 types are mostly interchangeable, and change both the gain and the frequency response, ie tone (sometimes drastically so). Some swaps don't work properly, and a few (anything other than a 12AT7 in the reverb driver position in Fender BF/SF amps for example) will lead to problems, but mostly you'll be OK.

This amp is quite a pain to get at the tubes on BTW.

[VoxRocker]

Thanks for the quick response John!

Looking more closely at the schematic, it is definitely the original ac-15 because it has seperate inputs for the vibrato/tremolo channel. In that case, I'll stick w/ the 12a*7s.

And yes, you're correct, the tubes are DEFINITELY a pain to get to. And sorry about the reissue thing. Even though they aren't exactly (or remotely) the same as the originals, I still consider it a "reissue". ;)

[bizzwriter]

These amps are NOT reissues.

Sorry, it's one of my pet peeves that they're constantly described as such. They are a modern amp based loosely on the top-boost channel of an AC30, with half the power. They have next to nothing in common with an original AC15. I can't remember the exact order of functions of the preamp tubes in the modern amp, but you can be sure that it isn't the same as the original either.



Hey, John -- have you had a chance to check out the new AC15 heritage handwireds? If so, what are your impressions and how do they stack up to the originals? Thanks!

[Doug H]

The original AC15's used an EF86 pentode for the preamp. There was a volume control between this and the phase inverter. There was no tone control in the preamp, except for a "brilliance" switch. There was a "top cut" control between the phase inverter and the 2xEL84 power stage to control high frequency response. It was a simple and elegant design, supposedly the "holy grail" of Vox tone, due to the EF86 and the way it essentially directly drove the output stage.

As John mentioned, the current AC15's are not "reissues" of the originals at all. They are essentially a "top-boost channel only" 15W version of an AC30.

[Swarty]

Another item of note is that of the EF86 variety AC15s, only the early versions ran it as a pentode. Around '61 the circuit was revised and the EF86 was wired as a triode. My guess is that this was to tame some stability (oscillation) issues. I had a customer bring me one with the triode circuit with the request to revert it to the pentode circuit. The amp sounded good before the change, but after the change it really had some gonads! Such a striking improvement that I was inspired to fit an EF86 in one of my 20w Marshalls (with excellent results too!).

[fullerplast]

The new AC15HW with EF86 sounds really, really good. FWIW.

[VoxRocker]

Hm, thanks for all the info guys. I am by no means an electrical wizard and realized after i posted that I was looking at a schematic for an earlier (original) version of the ac15.

Does anyone know a lot about the late 1990s reissues? I've been messing around with it (Tone-Man modded when I bought it) and have got 2 Tesla (pre-jjs) el84s then a mix of GT mullard reissues (SUCK) and have messed around with a CV4024 (12at7) in V1. I've heard of a lot of people using the mullard CV4024 in the PI slot on old fenders; does the "reissue" vox have a PI slot? I've got a couple Telefunken 12ax7s on the way to test out in it, but i've heard mixed reviews. Is anyone here a reissue vox owner that loves their tone and could give me some pointers on where to head tube-selection wise? I'm playing a PRS singlecut and dig the whole U2 "warm" breathy tones. Any advice would be greatly appreciated!