Right 12AU7 and 12AX7 combination

for the DIY ST35, the Dynakit and every other PP EL84

Postby HiFiGuy » Tue Feb 09, 2010 2:28 am

EWBrown wrote: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

You could also use a 6AX7, 7AU7, 9AU7
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Postby nyazzip » Wed Feb 10, 2010 12:36 am

i just put a bid in on eBay for some 12bh7s because of this post. currently 5 bucks for 4. (lol) they actually have a slightly lower amplification factor than the au7s...?
ignorance made me afraid to mess around with different tube types till now... tonight i stuck a 12ay7 in PI position with the 12ax7 in front. bass was improved but i was detecting some clipping. then i tried leaving the 12ay7 as PI, and used a 5751 in V1....wow! really seems to sound great to my iron ears, at low levels. i just am wondering about the au7 as PI; i don't think it provides enough bass
next experiment will be a 12at7 as phase invertor(inverter?)...i read transconductance has a bigger impact than mu in a PI

by the way i was listening to "Bachelor Pad Royale" a mix of 50s and 60s loungy stuff...it is really dynamic music, and has a nice blend of electric guitars and organs mixed with orchestral instruments and brass, and voices. and, it was all recorded really really well, with top notch equipment (the height of the hi fidelity craze). fantastic stuff for critical gear testing
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Postby nyazzip » Mon Feb 22, 2010 2:19 pm

"I put an EH 12BH7 in my rev D ST-35 the other day and it sounded
kind of nasty. The 12AU7 went back in after a half hour or so."

"My son and I tried an RCA 12BH7 greyplate with a Sovtek 12AX7LPS and it was quite bright compared to the RCA blackplate 12AU7. I like the 12AU7 with the speakers we are using.

I have some amperex and mullard 12AX7's we might try with the 12BH7 and see if that tames it down some."

"I think the 'BH7 really brings out the detail in the mids and highs, but the bottom nearly suffers. It loses some of the richness that it had with the 'AU7. I'm guessing the 'BH7 is more "accurate", and overall I prefer to have the detail. But, I can't help but miss what I'll call the warmth of the 'AU7."


just received my 12bh7As in the mail. surprised at how tall they are, no wonder they require more heater current...
using a 12ax 7 and the 12BH7 as PI, anyway my initial, right off the bat impressions seem to fit with the above quotes. maybe the midranges are scooped out a bit more...?also with no program material playing i thought i could hear a high pitched whine or ring, immediately upon tube "heat-up". don't seem to notice it 10 minutes later though
i find it very difficult/impossible to objectively A/B subtle stuff; seems like after more than 1 or 2 seconds elapses, the imagination takes over and the ears "re-calibrate"
i need to play with other recordings/V1s...
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Postby HiFiGuy » Wed Apr 21, 2010 9:38 pm

I just finished my ST35 build. The tube combination I tried and liked was a Telefunken Smooth Plate 12AX7, and a GE 1958 5814 (12AU7), the power tubes are a quad of reissue Mullards. This is a great combination. Extreme detail and clarity.
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Postby nyazzip » Fri Oct 29, 2010 8:38 pm

tonight i just tried a "serious" listening comparison between 2 diferent phase inverters in my st-35 clone, complete with note pad and pen. [:)
i listened to the same track, and picked out different instruments to focus on, and took some notes. for the record, the 12AX7 was a contemporary chinese thing.
the comparison was between an RCA 12BH7A, versus a "clear top" USA 12AU7A(perhaps Conn branded?) inverter:
my notes indicated that the 12BH7 was a clear winner, even though it was second and hadn't really been given a chance to warm up. while i am not generally a fan of shimmering sparkly high end, the BH7 seemed to deliver it in a well-proportioned way. the mids seemed clearer too. everything with the AU7 seemed "rolled off", treble-wise, with the exception of shakers(mexican music), in which there seemed to be a spike. stuff like toms and cowbell seemed muddy and undefined, and even the main vox seemed "rolled off".
with the BH7, overall everything seemed clearer, and the shakers didn't stick out as beeing overly bright in the mix. even the bass appeared "more crisp", but perhaps suffered in level a bit...
i need to try this a bit more with some better 12ax7s....cheers
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Postby kmolles » Thu Nov 11, 2010 12:10 am

What I have settled in on is a RCA Command black plate 5751 and an RCA Command black plate 5814. They have replaced the Telefunken ECC83 smooth plate and RCA clear top 12AU7A.
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Postby Rex Everything » Thu Nov 11, 2010 7:06 pm

I'm running a Tele ribbed plate AX7 up front and a nice Black plate UA7 behind in my new ST35 build.

Happy with this combo but it may be time ta do some rollin'
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