Heading for article: Rewinding the Power Transformer for an Eico ST-84 Preamplifier

Here is a hands-on article which takes you step-by-step through the process of rewinding a burned-out power transformer. The example chosen was in an Eico ST–84 vacuum tube stereo preamp from 1962. Only simple tools and equipment were used, except for the electronic turns counter. (That could be improvised differently, though.)

The article is profusely illustrated with colorful photos and includes links to sources for materials. You can see it here (free!):

Thumbnail link for Rewinding Power Transformer article

View the transformer rewinding article (2MB)

Eico ST-84 preamp with the completed transformer

Eico ST-84 with the completed transformer

Disassembling the original transformer
Winding a new coil
Testing the completed windings
Adding external leads to the new windings

Disassembling the original transformer

Winding a new coil

Testing the completed windings

Adding external leads to the new windings


Reader Comments

Posted by Steve L. December 05, 2016 - 02:36 pm
Hi David, Very good. I look forward to hearing how it goes.

Posted by David Q December 05, 2016 - 10:51 am
Thank you so much for your reply Steve. Indeed, some math should be put in place to try to find a middle ground (the schematic does call for a tapped primary 0-10-210-230-250[which I omitted since my wall puts out 220V...even though the HT is set at 250v]). My initial thoughts on this were to either add a resistor before the load (to keep the V but change the current) or try a different gauge wire between 6.3 and 12.6. Probably none scientifically accurate. I'll put some numbers on paper and keep you up to date.


Posted by Steve L. December 03, 2016 - 08:06 am
Hi David, I appreciate your kind comments. Of course, the wire thickness for that tapped winding will need to be chosen for the highest current supported. For current/voltage calculations, I might take an intermediate value, with some consideration of which are most important. In this case, our main options are 5V @3A, 6.3V @3A and 12.6V @1A. I would tend to discount the 5V case, because rectifier testing is less critical. So it's between the 6.3 (call it "6V") and 12.6V (call it "12V") cases and they specify different power and current!

Designing for 6V would leave 12V a bit high and designing for 12V would leave 6V a bit low. There are two effects driving this: The 6V case loads the primary with up to 19W and the 3A current drops more in the secondary. [However, if your tube tester includes a line-voltage adjust from the same transformer, the primary variation goes away.] For 6V, I would factor-in consideration that a 19W heater is pretty unusual---the beefy 6550 burns just 10W. Of course, if you're a radio amateur concerned with transmitting tubes, maybe your priority would be different.

One approach would be to calculate the design for each of the two cases and compromise. Another method would be to do one calculation for an intermediate case. However, since I would rarely be concerned with higher power tubes, I might target the 12V case, directly. After all, even 12.6W heaters are rare. Would I really want to degrade 95% of my measurements to reduce the error for the others? Your choice might be different.

Wish I could provide more definite recommendations but the fact is, accuracy is going to suffer for one or the other, so it's a matter of preference. At least, in doing "what-if" calculations, you can get a handle on the trade-offs. If you're into writing (say) Basic code, you could do a small program to help. Else, perhaps a spreadsheet could be used. The only trouble with the spreadsheet is that the calculations, as presented in the article, involve iterations. To handle that in a spreadsheet, you could include a fixed number of iterations, based-on the number of iterations used in the example. Looking at the results would indicate whether the accuracy was sufficient. I will be happy to help if you have further questions. Best wishes with your project. I hope you will share the results with us!

Posted by David Q December 02, 2016 - 05:27 pm
Hi Steve, I'm still a newb when it comes to build Power transformers and I got to say your article saved me from a lot of mistakes. Thank you. I'm currently trying to build a PT for a sussex tube tester. Primary 230v and 6 secondaries. One of which is tapped 0-4-5-6.3 @3A - 12.6@1A. I'm trying to figure out how should I go about winding it. I mean first, how do I calculate "I" (Watts) in a secondary that holds different A and second, how do I go from 3A to 1A in the same winding?

Hope you can help. No one seems to know the answer elsewhere.


Posted by Steve L. March 01, 2011 - 08:23 am
Hi Joey, I truly appreciate your message.

Posted by Joey December 09, 2010 - 07:55 am
Thanks for a handy article!

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