12AU7 output tube

[jeff]

My brother wants me to build a low wattage amp with a 12AU7 as an output tube.
Anyone ever built one? Anyone have a schematic or any info? What transformers to use?

Thanks
  Jeff

[kagliostro]

I never build one, but here they are



http://www.projetg5.com/projets/g1

K

p.s.:
on the link for the G1 you'll find also many samples
I don't know if the Firefly layout respects exactly the schematic, give a look as to be sure about this

[jeff]

Thanks.
  Jeff

[smackoj]

This amp doesn't use a 12AU7 for power, but it's a well-documented 'Gnat Killer' amp that many guys have had great success with....just in case your brother can be nudged towards a tested type of tube comp.

http://diy-fever.com/amps/ax84-4-4-0/

Big smile  

[Deric]

I built this and it's a lot of fun....

[kagliostro]

Deric

12au7 final tubes and the other tube ?

K

[jeff]

Thanks for the ideas guys
I was just wondering, these seem to be PP. I have an extra reverb OT. Has anyone ever built a small SE amp using either a 12AU7 or a 12AT7 as a power tube using the reverb OT? I'll buy a PP OT if I need to but just wondering if there's a way to use the SE reverb OT I have to make a small SE 12AU7 or 12AT7 amp?

Jeff

[sluckey]

Do you have a Fender blackface amp with reverb? If so, try this. Unplug the reverb tank from the reverb transformer. Now plug your favorite speaker into the reverb transformer jack (use adapters or whatever you need). Next pull the PI tube just to kill the main amp. Plug your guitar into the VIB channel and set controls as desired. Basically what you have is a Fender preamp playing into a SE/parallel 12AT7 power amp connected to your speaker. If you think this has promise, play with the 500pF coupling cap to get a fuller sound.

Search for PRR's third watter on the forum. It's based on the Fender reverb driver. Can't remember which front end he used though.

FWIW, I've troubleshot reverb problems by connecting a speaker to the reverb OT. It produces a nice bedroom volume just as is.

[DummyLoad]

Can't remember which front end he used though.

he's a diabolical that one, he is...used a driver tube as the output tube, or was the output tube as a driver tube? 

http://www.el34world.com/Forum/index.php?topic=5039.0

--DL

[sluckey]

Can't remember which front end he used though.

Apparently I couldn't remember which PA he used either! 

I seem to remember this amp spun off from someone talking about the standard Fender 12AT7 reverb driver. I prolly have that wrong too! 

[jeff]

Cool man. Always wondered what replacing the 8 ohm tank with a 8 ohm speaker would do. Pull the PI-Genious! Unfortunatly I don't have a reverb amp right now to try it out. I use speaker driven reverb so instead of building reverb into every amp I use that for my reverb for all my amps.

Don't think I have any 6AU6s.
Can I just dupe the 12AT7 circuit values out of a twin and throw any preamp in front of it?

Thanks
Jeff

[Deric]

Deric

12au7 final tubes and the other tube ?

K

Doh!  Didn't realize that they weren't on the schematic....  12au7 output, 12ax7 PI and Pre.

[DummyLoad]

Don't think I have any 6AU6s.

fun tube 6AU6 is... makes some serious gain. 6136 rigid constructed industrial/military 6AU6. another low micro-phonic variant is the sylvania 7543 which is also incorporates rigid construction and helical wound filament and improved hum performance. EF94 is the euro equivalent. a variant i built with 6AU6 in P-P; link here. you could easily graft in 6FQ7/12BH7/12AU7/6SN7, etc..

the AX84-4-4-0 sounds nice. i wouldn't mind tweaking on one. hmmm... no pete, standel 1st... 

PM if you'd like a sample or two for for a couple of bux + postage.

--pete

[PRR]

> used a driver tube as the output tube, or was the output tube as a driver tube?

I think someone was talking about 6AU6, and I joked that it could be preamp OR "Power", then Xeroxed the early Champ and changed 6V6 pin numbers to 6AU6 pin numbers.

[7string]

With a 12AT7 and a reverb OT you could build a Thundertweak Heavy Watter

http://sonicdeli.com/ThunderTweakWeb/links.htm

Except the Hammond 269EX might be a more appropriate lower voltage PT.

I just finished a Firefly type 12AU7 self split except with a Marshall preamp. Very happy how it turned out. I can vouch for PRR's 6AU6 amp too. That one has been working good for me for a few years now.

 

[stratele52]

How come your amp have 18 watts with 12AU7  and Kagliostro's amp with same tube give 1 watt ?

[stratele52]

I never build one, but here they are

 
K

p.s.:
on the link for the G1 you'll find also many samples
I don't know if the Firefly layout respects exactly the schematic, give a look as to be sure about this

 Site très intéressant , merci ,

Interesting link in french , thank's

[jeff]

 I have a transformer that would be good voltage wise for PRRs 1/3 watter and I did find a 6AU6 in by box of tubes. The only thing is that I don't know the transformers ratings. It came out of a mic mixing unit that ran two 12AX7s and one 12AT7. I was gonna use that transformer for a preamp but can I use it for this?

-Would the current draw for this amp be significantly more than running the preamp mixer?
Doing the math:
 1.2V across the 100 ohm resistor cathode resistor = 0.012A, plus whatever the first 6AU6 draws.
-Should I chance it?

[DummyLoad]

How come your amp have 18 watts with 12AU7  and Kagliostro's amp with same tube give 1 watt ?

he means the circuit is based on a marshall 18 watt amp and has the 18 watt pre-amp, but uses 12AU7 instead of EL84. he does not imply that 12Au7 are making 18W.

--pete

[Deric]

How come your amp have 18 watts with 12AU7  and Kagliostro's amp with same tube give 1 watt ?

he means the circuit is based on a marshall 18 watt amp and has the 18 watt pre-amp, but uses 12AU7 instead of EL84. he does not imply that 12Au7 are making 18W.

--pete

Exactly!   

[kagliostro]

Jeff since you posted your question I had on my mind one amp that can be a good candidate for your 12au7 amp build, but till now I didn't remembered nor the brand nor the model

Now I've remembered, it is a Traynor DarkHorse DH15H

This amp has a particular architecture and can switch between 12au7 final tube and 6v6 final tubes

of course if you want you can limit your build to the 12au7 without the 6v6 tubes with a much smaller PT & OT

Owner's Manual:
http://traynoramps.com/downloads/ownersman/omdh15h.pdf

Service Manual:
http://traynoramps.com/downloads/servman/smdh15h.pdf

K

[roseblood11]

Does anybody know the specs of the output transformer in the Darkhorse? Especially the primary impedance?

I'm planning a similar power amp with switchable power tubes. I've got a Heyboer transformer with 6k6, perfect for two 6V6, but maybe too low for a 12au7?

[PRR]

The Darkhorse with both 6V6 and 12AU7?

Clearly the OT is picked to use the 6V6es to best advantage. 6k, 8k, 10k.

The poor 12AU7 can't pull such a load with authority; but the 12AU7's job is "small sound".