Single ended 12a_7 ???

[walkman]

Any designs out there for a SE 12a_7 amp .. what sort of transformer speck s
would be needed .... what sort of power output enough to be audible clean???

cheers

Walkman

[pvsage]

I'm not sure about single-ended 12A_7's, but check this out:
http://ax84.com/index.php/oldproject...ect_id=firefly

The Firefly has a 12AU7 as a "self-split" PP power tube. I've heard rumors about threads where the output had been re-worked as parallel SE, but so fat those have been vapor threads. Shouldn't be too hard if you have the right math.

[Chuck H]

IIRC he regular old Fender combo reverb is a single ended 12a_7 into an 8 ohm load. So you can go by the load R in the tube data or just duplicate the primary for a reverb unit, they're probably close anyway. Sorry, no numbers for you. You can look up that stuff anyhow.

I would think one of the Hammond 3 watt universal single ended OTs would work for this. They're said to sound pretty good.

The 12au7 has been used by a couple of known builders and many home brewers with good results. It's supposed to be a good choice because it's a lower mu tube than the other 12a_7s. And, again IIRC, the 12au7 can handle the current better.

The 1 watt or so you'd get from such an amp is said to be much louder than you might expect. I've read it's about as loud as a loud speaking voice or a quiet shout.

I think the Moonlight, on the same site as the Firefly, is a single ended 12au7.

HTH

Chuck

[tubeswell]

If you want it louder, you can stick some more triodes in parallel in the output stage

[pvsage]

Moonlight doesn't have a page at AX84 at this time (actually, it does, but it says "under construction"), but a quick Google search confirmed that it has the PP 6SN7 output stage that you can find under AX84's "Component Amp Projects".

The suggestion to use a reverb transformer is an excellent idea - the typical Fender reverb unit is driven by a 12AU7 wired in parallel. If anyone has a good source for replacement/aftermarket reverb transformers, I'd love to see it.

[Shea]

Blackface and silverface Fender combos generally use 12AT7s for reverb drivers.

I'd pick whichever 12A_7 has the highest transconductance. And I wouldn't rule out the 12BZ7.

Shea

[MWJB]

What about 12DW7?

[HTH]

I'd be interested in doing this on my Epi VJ to have a switchable option to use the 12AU7 for very low cranking.

If anyone susses out which OT to use, I'd interested to find out.

[pvsage]

If it doesn't *have to* be SE, you could probably simply lift the self-split PP power stage from the Firefly. All the info is there under "Archived Projects". The recordings I've heard at AX84 and on Youtube sound pretty good, if a little trebly - probably a combination of the small speakers used and the preamp voicing.

You'll almost certainly need to use a separate OT for the dual triode output stage. Switching between the two power amps will be a little tricky, but still easier than if you used the same OT for both.

Be sure to use the right voltage on the 12AU7 - perhaps the rail that supplies the pentode's screen grid voltage?

[walkman]

I've some single ended transformers from old radio's about 7000/15 for 6gw8 or 6m5 output. Would this be suitable for 12a_7 , i'd use the power transformers too , so rectified voltage may be round 270v DC.

Currently i've built and rebuilt an amp with el84 running in triode mode and with a a conjunctive filter and it sounds quite good, with an old rola alnico 12" speaker, but is too loud for others when cranked , or even just warm.