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70v line transformers

6/3/1999 10:51 PM
70v line transformers
I'm doing my annual "what the heck is in this box" thing at the shop this week and I found a few hefty 15-20 watt 70v line transformers.  
The voltage ratio is about +16.5:1, or a Zed ratio of around 273:1.  
Besides their obvious use, can anybody think of a good use for them.... there is no center tap!  
SE class A parallel 6V6GTs with a couple 10" 8 ohm speakers in series?  
Mission Amps
6/4/1999 6:13 AM

It is likely that the core stack is not gapped, so they would saturate very easily with the class A bias current in a SE amp.  
Unfortunately, there aren't a lot of things you could use them for  
as is, other than audio distribution transformers - and neato high  
tech doorstops...
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6/7/1999 1:18 PM
Don Symes

To carry GFR's point a little further:  
Part of the reason the B+ supply runs through the primary is that it's a free choke, right?  
So, what if you AC couple the primary to the power tube, and send B+ through a separate, hefty choke (like a whole 'nother one of these puppies with primary an secondary in series)?
6/4/1999 8:33 AM

Not disregarding what R.G. said, I allowed my mind to wander a bit (low-level creativity). I know how you feel, "There's got to be a way to utilize these babies".  
You can build a low power SE guitar amp with these. Since they're rated for 20w w/o a standing DC current on the primary, they should be good for a 3-5w blues amp (for practice or recording), or use a pair for a nice hifi headphone amp for listening to CDs (tubes & iron round off the square edges that grate on your nerves).  
Based on your posted figures, I take it these paperweights don't have multiple output taps, only a dedicated "8ohm" secondary winding. At 273:1, an 8ohm load would reflect 2184 on the primary. For a 16ohm speaker load, you could use one of the common pentodes or beam tubes that would do well with a 4.3k load. For an 8ohm speaker load, you could use a tube with a lower plate resistance, like a 6Y6GA, or even use a triode like a 6EM7 for a watt or two audio (or an excellent headphone amp).  
You could use the transformers as is, if you don't expect good bass response through a speaker, or you could modify them slightly. Take them apart, and reassemble the E-I laminations as a complete E-core with an I-bar at the end for closure. Use a paper shim where the E contacts the I for gapping. You'll need external frames or end bells to hold the thing together via thru-bolts at the corners. Now you have a SE output trans. While you're in there, you could peel off a few turns of the secondary to increase the turns ratio.  
Well....that's what I'd do with them if they were mine. I already have enough doorstops, compact, beatifully made Mil-spec power transformers, potted, sealed in smooth flat black rectangular cans with the connection schematic printed in white, glass-sealed solder post terminals, for use in 400 cycle AC. I'll have to wait a long time for the power company to raise my line frequency!
6/7/1999 6:06 AM

What about a "single ended push-pull" amp (similar to srpp)? You would need two separate power supplies (+V and -V) or you can use a capacitor to decouple the transformer. Just rambling...
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