Please help to decide what to do with these transformers. I have this pair of nice-looking NOS Altec-Lansing transformers that carry a #15170, apparently a model number. They are slightly bigger than Dynaco ST70 transformers so I would think they could handle 30-50 watts. On one side with two terminals it is marked “70.7 VRMS” between the terminal and on the other side it has two terminals with “Pri-Com” marked at one and “Pri-4Ohm” at the second. I assume they are distribution transformers but could not find any references that made sense to me.
I’m not sure if they are good for any hi-fi amp building project, tube or other-wise, and if so what primary impedance would be reflected from a 8 Ohm speaker load. Any clarifications or suggestions of how to put these to good use would be very welcome.
I’m not sure if they are good for any hi-fi amp building project, tube or other-wise, and if so what primary impedance would be reflected from a 8 Ohm speaker load. Any clarifications or suggestions of how to put these to good use would be very welcome.
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The data written in the traffo says few because it has been done for a specific use.
Place a small transformer from the 110V line, say, with 6.3V in its secondary, and wire it to the secondary to the transformer under question. Measure both voltages (Primary or the 70.7V and secondary, 4Ω) with an AC voltmeter (Analog or digital), and take the ratio between them (Upper value div the lower one). Square this value to take the impedance ratio, this is a more relevant value.
Place a small transformer from the 110V line, say, with 6.3V in its secondary, and wire it to the secondary to the transformer under question. Measure both voltages (Primary or the 70.7V and secondary, 4Ω) with an AC voltmeter (Analog or digital), and take the ratio between them (Upper value div the lower one). Square this value to take the impedance ratio, this is a more relevant value.
Hi Kokoriantz, I just checked and it is definitely a transformer, not an autoformer. There is no continuity between the primary and secondary sides.
Osvaldo, thanks. I hope to get an answer here, but if not I will measure the impedance ratio as you suggested.
Comparing the size of the transformer to known Dynacos, this is actually bigger than MkIII transformers.
Thanks for your assessment. It has a single coil on both primary and secondary side, like most distribution transformers I have heard of (no experience with them - that why I’m asking the experts). I don’t know if it is gapped or not. So, does that mean I can’t use this transformer for tube anything? Maybe transistor output? Or how about the Phillips EL86 cascade output stage meant for their OTL 800 ohm speakers (while driving 8ohm speakers)?
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70 Volt transformers are made for use in multi-speaker distributed sound systems like restaurants. A big one like yours connects a (mono) commercial amplifier to the distribution line, and smaller ones at each speaker match back down to speaker impedance. Sounds wacky, but it allows for small gauge speaker wires and lots of speakers safely in parallel. Non-technical installers need only set each speaker transformer's tap, marked in Watts (!), so that they all add up to the amplifier's rated power.
All good fortune,
Chris
All good fortune,
Chris
As Chris says, this OT is designed for the 4 ohm winding to connect directly to the 9441A amplifier output - the 70V winding is then used to power long distribution cables, where a smaller speaker with its own step-down transformer taps off enough power for the speaker at any location along the 70V distribution cable.
It seems like the OT has a 50 ohm impedance secondary for 4 ohm impedance primary, and has a 100W rating, and is probably no slouch with frequency response.
You could use it in reverse, if for example you had an old valve PA amp that only had 50-100V line output levels, but such amps are not common and the larger ones (eg. 50-100W) are likely to have OT secondary sections that allow typical speaker impedances to be connected.
That type of impedance ratio and power rating is sort of what you would find in a solid-state PA amp which ran from 12V or 24VDC - it would likely have an output stage needing a 2-4 ohm primary, and would have a secondary with multiple taps to cover 50-100V lines as well as local monitor 4-8 ohm speakers.
It seems like the OT has a 50 ohm impedance secondary for 4 ohm impedance primary, and has a 100W rating, and is probably no slouch with frequency response.
You could use it in reverse, if for example you had an old valve PA amp that only had 50-100V line output levels, but such amps are not common and the larger ones (eg. 50-100W) are likely to have OT secondary sections that allow typical speaker impedances to be connected.
That type of impedance ratio and power rating is sort of what you would find in a solid-state PA amp which ran from 12V or 24VDC - it would likely have an output stage needing a 2-4 ohm primary, and would have a secondary with multiple taps to cover 50-100V lines as well as local monitor 4-8 ohm speakers.
A 70V rated winding wants 100V peak. This suggests a 120V-150V supply.
There are few tubes which make large power at 150V. Your winding is single so you can't do push-pull. You can't do SE anywhere near the power its weight implies because the DC would saturate the core (also you would need a LOT of 25L6 to make big power at 150V supply).
No, there isn't much good use for a 100W 70V:4r line transformer, unless you work with 70V systems or 100W 50 Ohm speakers.
No, there isn't much good use for a 100W 70V:4r line transformer, unless you work with 70V systems or 100W 50 Ohm speakers.
There are few tubes which make large power at 150V. Your winding is single so you can't do push-pull. You can't do SE anywhere near the power its weight implies because the DC would saturate the core (also you would need a LOT of 25L6 to make big power at 150V supply).
No, there isn't much good use for a 100W 70V:4r line transformer, unless you work with 70V systems or 100W 50 Ohm speakers.
No, there isn't much good use for a 100W 70V:4r line transformer, unless you work with 70V systems or 100W 50 Ohm speakers.
Thanks for your very informative reply.
No use for a transformer of this sort in hi-fi? You sound like I should rather use the nice laminated I-E cores and try to rewind a new transformer!
Some transformer applications are that unique that they have pretty much no other use.
I certainly looked at what could be done with ss PA amps from the 1980-90's that get thrown out. They at least often had 50V and 100V taps, and so could be used in a push-pull configuration. And the impedances could be scaled up by circa 2x or 4x to say match a 16 ohm speaker, and provide a PP primary with many hundred ohm impedance. An application that may suit that is to use TV power pentode tubes rated for low B+, so if you had a sextet or more of tubes such as 6CM5 or 6CD6G or 6DQ6A, and had a suitable power transformer for heater current (the ss PA amps often have that) then that may be a worthy diy project - but you would be on your own pretty much.
I certainly looked at what could be done with ss PA amps from the 1980-90's that get thrown out. They at least often had 50V and 100V taps, and so could be used in a push-pull configuration. And the impedances could be scaled up by circa 2x or 4x to say match a 16 ohm speaker, and provide a PP primary with many hundred ohm impedance. An application that may suit that is to use TV power pentode tubes rated for low B+, so if you had a sextet or more of tubes such as 6CM5 or 6CD6G or 6DQ6A, and had a suitable power transformer for heater current (the ss PA amps often have that) then that may be a worthy diy project - but you would be on your own pretty much.
That was what i was afraid of.
I expected with all the 70 V transformers out there some smart person would have figured out how to use them to good benefit. Looks like, maybe I’m that one, except I’m not that smart.
That was what i was afraid of.
I expected with all the 70 V transformers out there some smart person would have figured out how to use them to good benefit. Looks like, maybe I’m that one, except I’m not that smart.
In the early wild and wooley days of hifi, some free-thinking folks would try to make amplifiers without any output transformer. (There was even a commercially made one, called a Futterman). These had lots of problems for obvious reasons, but would have been improved greatly with a low impedance, low ratio matching transformer like your pair. The original idea was a little shakey, but might give you some ideas down the road.
All good fortune,
Chris
All good fortune,
Chris
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