2W 70.7 V audio transformer question(s) for the build anything anyhow people

[murrayatuptown]

Hello:

I have a bunch of tiny 70.7V audio transformers with primary taps for 2W, 1W, 0.5W and 0.25W, and 8 ohm secondary. (actually, there are 7 taps, 4 for 70.7V, 4 for 25 V, with the middle one common to both)

Primary impedances at 70.7 V for 2W, 1W, 0.5W, 0.25W are
V                      P       R (Z)
70.71067812   2   2500
70.71067812   1   5000
70.71067812   0.5   10000
70.71067812   0.25   20000

They are pretty tiny (roughly 1.25"x1"x1"). Some are still mounted on a 4" 10W 8 ohm speaker.
One side of the primary is a common lead and the other lead goes to push-on terminals on the transformer circuit board.

I'm pretty sure I verified (I've forgotten & have to look for notes) the Common-1W-2W form a center tapped primary (0-5000-20000...half the primary is 1/4 the impedance). I wouldn't put money on my explanation as it doesn't make complete sense as written. Assume for the sake of progress the primary does offer a center tap for the full primary.

Now, digging through piles of basement treasure, I am intrigued at the possibility of a tiny push-pull tube amp.

 

The following is NOT the type I have, but the power rating is the same, and I offer this link for a visual aid.

http://www.alliedelec.com/m/d/d8292cf7f11c601de4c651a61586c0eb.pdf

Any thoughts other than laughing?

Like maybe a pair of 12AU7's, PP?

Thanks

[trobbins]

Suggest you look at link,
http://www.ozvalveamps.org/optrans.htm


and search through linked forum for threads on the Altronics line transformers such as MM1900 for how they have been used:
http://www.guitargear.net.au/discussion/index.php?board=8.0


There were number of different models from Altronics over time - the latest small model is M1112.


As you have a 0.25W and a 1W tap, then you appear to have a 10k P-P primary to work with, but it sounds like you have lots of taps.  Always worth checking the voltage turns ratios to confirm that you are choosing two equal primary half-windings for a PP connection.  If you apply a signal to the secondary, just keep it very low (perhaps less than 2Vrms and appreciate that the primary taps will have high voltage on them.


Ciao, Tim

[murrayatuptown]

Thank you.

I think I identified the wrong taps but had the right idea.

They are good for something...

TBD

[PRR]

You could surely put 6VAC on the lowest-W (highest Z) primary taps, then look for the 3V each-way point.

I have doubts about how much clean power you can get with tubes this way. OTOH it is interesting how *little* power it can take to annoy the whole house.

[jjasilli]

How about the AX87 Firefly?  The latest Rev shows an OT primary impedance of 22,500 ohms for paralleled 12AU7's (into 8 ohms).  I don't know how they chose such a seemingly high primary impedance, but they say it sounds good and has good frequency response.  Seems your .25W tap about matches that.

Re freq. response: it seems line-matching iron is all over the map.  Some limited to 150 or 300 - 9000Hz.  Others full frequency.

[Willabe]

The latest Rev shows an OT primary impedance of 22,500 ohms for paralleled 12AU7's (into 8 ohms). 

Isn't that around what a Fender Reverb tank driver OT spec is?

[2deaf]

Isn't that around what a Fender Reverb tank driver OT spec is?

I used 70V line transformers for reverb drivers many years ago when I had no source for real reverb transformers.  I used the 10W Realistic 32-1031 from Radio Shack and the Calrad 45-723L from my local electronics store.  Later on when Mouser sent me a catalog that was no more the 1/2 inch thick, I used the 42KB001. 

I connected the E_bb to the 0.625W tap and either a strapped 12AT7 or 12AU7 to the common and then connected a 4A tank to the 4 ohm tap for an impedance ratio of about 16,000 : 8.  They all were 10W models and all had a restricted frequency response, which was perfect for reverb.  It looked a little cheesy to have a Radio Shack transformer with exposed lugs on an amplifier, but they made a rather impressive reverb.   

[PRR]

> I don't know how they chose such a seemingly high primary

12AU7 plate resistance is around 6K. For happy loading we would double that, 12K per side. So 24K plate-to-plate. Not at all critical.

> Isn't that around what a Fender Reverb tank driver OT spec is?

Similar argument. 12AT7 is near 20K, so both halves is 10K. Load with 20K. Since this is not a power-critical application, take whatever is close.

Freq response on small distributed sound transformers "only" has to be as good as the speakers. (Ignoring stress on the power amp.) An eight-inch speaker in a small can isn't much use below 150hz. Also you can't hear bass at these 1W-0.25W levels. And these school/factory systems are often competitive-bid, so every penny is squeezed-out.