6EU7 vs. 12AX7

[EETStudent]

Anyone have any knowledge in this area? I know the gain factor are both 100 for each tube, but tonally how do they compare? I know Gibson was fond of them in some of their amps in the 60s, and I was curious as to how they might sound in a Fender or something

[jrfrond]

Because the pinout of the 6EU7 differs from the 12AX7, I've never A/B'ed them in the same amp, but because they sport identical specs, I suspect they will sound similar. Something you WON'T have: the opportunity to swap with many other brands to find the right-sounding ones. The only MAJOR difference is that the 12AX7 has dual heaters, one for each cathode. Because the cathode of the 6EU7 share a heater, which is not as close to the cathodes as in the 12AX7, the plate voltage and heater-to-cathode voltage are a bit higher.

[Rob Mercure]

The 6EU7 and 12AX7 - as well as the 20EZ7 - are exactly the same "guts" with different heaters and pinout. If you calculate the heater wattages for each bottle you'll see that they're identical. Not an uncommon situation with output tubes designed for line connected euipment such as the 25L6 - 35L6.

[capnjuan]

Anyone have any knowledge in this area? I know the gain factor are both 100 for each tube, but tonally how do they compare? I know Gibson was fond of them in some of their amps in the 60s, and I was curious as to how they might sound in a Fender or something

Hi EETStudent; As indicated, hard to A/B 6EU7s w/ 12AX7s. Last year, side-by-side I had a Gibson GA18 (2X12AX7 / 2X6V6) and a Gibson GA19RVT (3X6EU7, 1X7199, 2X6V6); both tweed era, both with 5Y3 rectifers, all the 6V6s were new JJs, new 10" Weber 10A125 in the GA18 and a used 12" Weber 12A125 in the GA19.

Both circuits refurbished with the same type resistors and caps, both had trem, the GA19 w/ reverb (using the 7199), both w/ upgraded #14 OFHC speaker drops. The GA19 still had its original Gibson-logoed 6EU7s and I had a set of new Sovtek 6EU7s and both speakers and passives w/ enough time to break in.

Except for two things, the amps's tone, range ... output almost identical; the exceptions were that the GA19 had a little more noise at idle which at least I think was a function of the 7199 (new JAN Phillips) and the GA19 was noticeably darker. Not identical amps but the 6EU7-driven amp was audibly darker than the 12AX7-driven amp. Not science here and your mileage may vary .... John

BTW: I bought a total of four new Sovtek 6EU7s; of the four, one had the noise that has dogged the Sovtek/6EU7. j

[Enzo]

Not an uncommon situation with output tubes designed for line connected euipment such as the 25L6 - 35L6.

Careful there Rob. The 25/35/50L6 is NOT a 6L6 with a different heater. The 25/35/50 ARE the same as each other inside though. Those tubes are only rated up to 200v and about 4 watts. Some tubes are the same with different heater voltage, but not all. Having the same letter/number after the heater is no guarantee of similarity.

The 50C5 is another power pentode like the 50L6, just a different bottle. But the 6C5 is nothing like it, it is a triode.

Never assume.

[Rob Mercure]

Hey Enzo,

I can throw the "never assume" back at you cuz there is no mention of the 6L6 in my response - I've been well aware of these tubes not being the same for over 30 years and note that I referred to them as "line connected" tubes which usually doesn't include the 6L6! You might accuse me of choosing a bad example of letter sequence though as the 6L6 is so familiar. Anyway my point was about heater wattage not consistency in the RMA/EIAA tube numbering system <grin>.

Rob