12axu as PP for 18w pre.

NOTLOUD18 · Post by **NOTLOUD18** » Thu 04/17/08 11:13 pm

Hello all, I am new to these discussions, so please bare with me if I ask a simple or odd question.
I built an 18 watt TMB preamp and wanted to quite it down to living room volume and am using a 12ax7 as a push pull power tube. The "Normal" channel sounds very good, but the TMB sounds muddy on the low strings. I have switched out the cathode caps & resistors, but the problem remains. Does anyone have any suggestions for this configuration? Any help would be appreciated, Thanks.

NOTLOUD18 · Post by **NOTLOUD18** » Thu 04/17/08 11:14 pm

I'm sorry , a correction. It's a 12au7, not a 12ax of any sort.

Alexo · Post by **Alexo** » Fri 04/18/08 8:59 am

I built a 12AT/U7 amp too, and had a heck of a time getting it un-muddied. Maybe it's just a "thing" about 12AT/U7 power stages.... But check out the Firefly on AX84.com, people seem to like the sound of it and it may give you some ideas.

davent · Post by **davent** » Fri 04/18/08 10:15 am

Here's a couple schematics with the 12au7 used as the output tube. Maybe there's a clue in there that might help solve your problem. Wasn't able to attach them in this forum but attached no problem in the 18watt Technical Discusssion forum.

http://www.18watt.com/modules.php?name= ... 566#189566

dave

Post by **zaphod_phil** » Fri 04/18/08 1:13 pm

DennisF built one of these "Marshall 1.8W" amps, and reported that it sounded pretty good. IIRC according to some of the posts I read, having a separate PI rather than the Firefly's self-split power stage, improves the sound quite a lot. Anyway, that's just some old second-hand information. Here's a clip by DennisF - http://members.cox.net/dennisfweb/sound/sos.mp3 , based on this schematic - http://members.cox.net/dennisfweb/images/18w_AU_3.jpg

martinw · Post by **martinw** » Sun 04/20/08 8:10 am

I've built a couple of these and had no problems with the sound...in fact they sounded fantastic.

Phil_S · Post by **Phil_S** » Sun 04/20/08 9:19 am

You might consider a ECC99 tube. This is a dual triode that is plug compatible with 12A?7 family of tubes. The audiophool community seems to really like this as an output tube.

NOGE · Post by **NOGE** » Sun 05/04/08 3:57 pm

Try a 12BH7. It's a direct swap for the 12AU7 and un-muddies the sound a little.

I think maybe the 12AU7 doesn't overdrive very nicely.

Also I think with preamp distortion you need a clean (and therefore somewhat higher powered) power amp to get a good sound. Power amp distortion on top of preamp distortion sounds muddy to me. Just my .02

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12axu as PP for 18w pre.

12axu as PP for 18w pre.