Right 12AU7 and 12AX7 combination

[lth1]

In a previous post by WA4SWJ he stated that he used a GE5751 and a RCA 12BH7. I had quite a few of these lying around and installed them into the ST35, they replaced two Mullard tubes. This has really opened up the ST35 and it now really competes with the 300B. Wonderful combination, highly recommended. Thank you WA4SWJ.

[WA4SWJ]

You're welcome!

Enjoy.

[TerrySmith]

I also like the 5751, you should try it in a PAS-3!

[RobertWStephens]

Hello! I ordered a set from Jim McShane on Sunday per Ed's suggestion. I can't wait to check it out. I will report back after I get them in. Has anyone tried Mundorf SIO caps in the St-35? I added them to my phono stage and they made a big difference in detail. I am not sure if that would be the same on the ST-35. I put in Orange Drop 716's. Peace. Robert

[erichayes]

Hi All,

If you're going to use 5751s in a preamp with series-wired heaters, such as the PAS-3, use only 5751s--don't mix them with 12AX7s. With a heater current of 350/175 mA, the 5751 will cause premature heater failure of a 12AX7 wired in series with it. Yeah, you can throw a 510Ω 1W resistor across the AX's heater, but only if pin 9 is not connected to anything (like ground). Preamps that have multiple 12A*7s like the Citation I and IV or the Mac stuff have strange heater wiring; arbitrarily stuffing a 5751 or 6072 here and there can cause problems you might not even be aware of.

[EWBrown]

Some of the integrated amps/receivers use the filaments of the 12AX7s or 6EU7s in the phono stage, as a cathode "resistor" for the output tubes.

It's just a cheapo way to get DC for the filaments of the higher-gain stages. So, replacing 12AX7s with 5751s or 6072s will make the output tubes draw somewhat higher plate current.

Are the Sovtek 5751s any good, or should I stick to NOS? I will be using two of thsm in a yet to be completed 300B SET amp project. I have a single NOS, which I'll try along with a 12BH7A in one of the DIY-ST35s.

/ed B in NH

[erichayes]

Hi All,

Ed, When Bud and I were doing our insane Ampenstein experiments a year and a half ago, we discovered that the Sovtek 5751s sounded even better than the NOS JAN tubes we'd been using. The downside is that their cathodes are not as emissive over the long haul--they ain't yer daddy's 1952 Mullard.

[TomMcNally]

Eric - What are the octal equivalents of 12AT7, 12AU7, 12AX7 ?
I am thinking 6SL7, 6SJ7 and 6SN7 ?

Thanks - I can't find this info anywhere.

... tom

[Francois_G]

12AT7 does not really have an octal equivalent. The 5694 has a bit lower gain, but that is the closest I know of. Unfortunately it is quite rare.

6SJ7 in triode connection is more like a 6SN7

12au7 > 6SN7
12ax7 > 6SL7 (actually 5751 is the counterpart of 6SL7)

[rrd]

Are there any modifications that need to be done to use the 5751 and the 12BH7? I have the rev D board with Hammond iron.
Thanks

[EWBrown]

Just plug the 5751 ansd 12BH7(a) in and run with it, plug 'n' play, no modifications needed. Another nice 12AX7 alternative is the 7025, though these usually don't come for cheap :o

With a little etch "surgery" to allow using pins 4 and 5 separately for the 6.3 VAC filament power, 6CG7/6FQ7 or 6GU7 could be used for the phase splitter.

6CG7/6FQ7 is identical in character to a 6SN7, and 6GU7 is basically a 6V 12BH7A.

Now, my question is, is there an 8V filament equivalent or close equivalent to a 12AX7 or 5751?

If there is, then one can go the "plinker" route, and use 8CG7/8FQ7, or 8GU7, and four 8BQ5s. and save a lot on the NOS tube costs. Of course, the 12AX7/5751 could simply use a couple of 2.7 ohm, 1W resistors to drop the filament voltege from 8V to 6.3V, with a little bit more etch surgery Yellow_Light_Colorz_PDT_20
And of course, an 8V, 3A AC or DC power source would be needed to do this .

/ed B in NH

[erichayes]

Hi All,

Tom, the 6CG7 was one of very few miniature tubes to transit directly from an octal predecessor (the 6AQ5, 35C5 and 50C5 also come to mind). Most of the miniature twin triodes were developed during WWII for specialized applications in radar, and in the '50s for VHF and UHF avionics and television. The 12AU7 and 12AT7 were civilian equivalents of several different military types, but don't have octal counterparts. I've dropped 6SL7s into 12AX7 circuits and they've worked okay. The same would probably be true with a 6SL7 in a 12AT7 circuit. A 6SN7 would be an improvement over a 12AU7, even though they're not that close on paper. A couple of 6J5s could also be used wherever 6SN7 type dualies are used, and with a little tweaking, two 6SF5s could go where 12AX7s are found.

Ed, the tubes you mentioned were developed for 450 mA heater strings, so yyou need to go in the other direction. Unfortunately, it looks like 3AV6s and 4AV6s are the only plinkers that could be forced into 12AX7 service, although there might be some compactrons that have a section or two that might work.

[paart]

In his book "Valve Amplifiers", Morgan Jones shows the results of extensive distortion testing on a large sampling of 6SN7 equvalents and other types of medium mu triodes commonly used in audio circuits. The book is worth buying just to see these results. In short, the 12AU7 exihibited the greatest amount of distortion of the 18 differant types that were tested. The clear winner was the "carbonized CV1988" followed by a "carbonized 6SN7GT" (with weighted 2nd through 6th harmonic distortions of -58db, and -55db). The 12AU7 came in dead last with a weighted figure of -36db. The British author refers to the 12AU7 as "ghastly". Another well known tube authority, Eric Barbour, has written that the 12AU7 does not belong in quality audio circuits.
Both the 6SN7 and the 12AU7 are medium mu glass envelope dual triodes. Beyond that, there is little similarity.

---ART---

[SteveH]

Glad ya could make it Art Yellow_Light_Colorz_PDT_12

[RobertWStephens]

In my system, the 12BH7 made a nice improvement over the 12AU7. I did not care for the 5751 that I got over the 12AX7. I am not sure if it had to do with the lower gain or the actual tube that I got. Thanks for keeping this fun! Peace. Robert