Greetings Tubulars, So I have this crazy idea to build a little tube amp just for fun. The transformers are quite expensive though. I will be looking for old ones probably because I can't justify spending loads on new ones. So considering my almost 60 years old ears, 39 of which have been spent in noisy environments, and my propensity to listen to loud music, what should I look for that will sound just a little better than I can hear? Maybe not an audio xmfr will be OK. There are some old tube amps in a couple local thrift stores. How can I tell if they had good xmfrs? Or can I? Eric
I've built a dozen or so, all point to point wired.
The top quality transformers usually were potted into a square can.
It's important maintain plate-to-plate impedence, so best to use the same output tubes that the transformers were originally paired with, or at least look into your tube manual and select a tube a set that is relatively close.
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Or you could wind your own...really no reason to use E laminations you have a lathe...simply trepann out the coil cavity, leaving a "post" in the center drop in the coils and screw on a round cover to complete the flux gap
If you do decide to wind your own, I would recommend using laminations from a power transformer before using a solid chunk of material. Audio transfo rmers use thinner laminations than a power transformer, but ones from a pow er transformer would be better than solid.
If you want I could keep an eye out for kind of suitable transforms at the local scrap yard. The shipping via a flat rate box would not be too bad. The last heavy stuff I shipped using a flat rate box , got separated from the box. But since then I got some stronger tape.
"PrecisionmachinisT" wrote in message news:PNydnddXCtyB9kbPnZ2dnUVZ snipped-for-privacy@scnresearch.com...
This is why transformer cores aren't solid iron:
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"Early transformer developers soon realized that cores constructed from solid iron resulted in prohibitive eddy current losses, and their designs mitigated this effect with cores consisting of bundles of insulated iron wires."
A large, powdered iron toroid works even better. Like an old 3A variac. I needed a modulation transformer for a AM mobile radio back in the '70s. I wound a second winding on a surplus 88 mH telco toroid, and had excessive audio bandwidth.
This is sort of like the permanent magnet assembly in at least one speaker I destroyed as a kid. (Well, the cone was already a goner.) The center pole and backing plate were one piece (really the pole was staked into the plate), and the outer housing was a ring Alnico permanent magnet). Great construction for a stable permanent magnet field, but not at all good for a transformer.
Now -- for really high frequencies (Ultrasonic, not RF) a ferrite pot core is really nice. Formed in pretty much the shape defined above for lathe construction, except that there were two of them, mounted cup to cup. The cups were available in a large number of sizes and a number of different ferrite compositions for different frequency ranges and maximum flux densities.
Agreed! Thinner laminations are better at higher frequencies.
I do remember some special transformers for really old telephones which were wound on a core of a number of lengths of straight iron wire bundled together.
Woah! A Variac did *not* use a powedered iron toroid -- nor did the Superior Electric "Powerstats", nor any other variable autotransformer by anyone else that I ever saw. :-)
Take a strip of the proper alloy (permalloy? Some other alloy?), and wind it into a hollow cylinder (with insulating coatings between layers) and *that* is the toroid that a variable autotransformer is wound upon. Ideally -- pot that in an insulating material, and use that to wind the coil on.
Powdered iron (ferrites) are much better at high frequencies, while most variable autotransformers were optimized for 60 Hz or 50 Hz. (I have held in my hand some optimized for 400 Hz -- and for those, the length of the cylinder is about 1/3 that of one for the same current at
The cores I'm talking about are the same powdered iron used for power line filters. I've used a Variac to match a speaker to the speakers on a factory floor after a fire. No idea what was in the box, it was in the company's boneyard and in a steel box. There are literally tons of surplus ferrite cores on Ebay, in almost any shape you can imagine.
The OP will waste a lot of time and gasoline finding and testing cores that don't work well enough.
Do you have the test equipment to measure the frequency response and distortion of a homebrew transformer at high power levels? You need a clean sine-wave signal generator and an amp that's better than the one you are trying to build.
I used a home-made Super Tiger modified into a wideband op amp.
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It was useful as an RF transmitter at Fort Monmouth when the Signal Corps students started competing to build and run pirate radio stations and jammers. Electronic surplus was cheap and plentiful in northern NJ and NYC's Canal Street in 1970 and we learned a lot through the pranks we concocted. jsw
On Sat, 18 Jan 2014 22:41:05 -0600, Martin Eastburn wrote: Greetings Martin, I don't have any specs yet. Or a design. I just decided I would like to do this. 20 watts per channel would be the max output, though I imagine I will end up with something that puts out much less. I will amplifying the output of some sort of CD player or mp3 player . Eric
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