Tubelab SE - alternate power transformer

[Evenharmonics]

Originally posted by tubelab.com
I have added two more filament windings to an AN4TK400. This is a nice transformer that has two 400 volt windings with a bias tap on each at 70 volts. This makes it easy to get -100 volts, +100 volts, -500 volts and +500 volts all from the same winding. It also has a 5 volt winding but only ONE 6.3 volt winding, I need 3 so I wound two more windings through the core using ordinary 18 gauge hookup wire (Belden 9918). I simply added turns until I got enough voltage. It looks like about 14 turns each, but it could be 13 to 16.

It seems like toroidal power transformer fits diy community really well. It can be very economical and fun.

[Evenharmonics]

Questions to gurus, by reducing filament tap's voltage as I did by unwinding turns, will it increase the amperage of that tap? If it does, will it be directly proportional to the voltage reduction?
Thanks.

[Arnulf]

Originally posted by Evenharmonics
Questions to gurus, by reducing filament tap's voltage as I did by unwinding turns, will it increase the amperage of that tap? If it does, will it be directly proportional to the voltage reduction?
Thanks.

Mostlikely no and no. Amperage is usually limited by wire diameter. Technically core crosssection and primary current limit secondary current as well but since transformer was designed with its existing capabilities in mind it is unlikely that either of these two would be limiting fators in your case.

Add another winding in parallel and in phase with the existing one in order to increase current. This way you can count on getting exactly as much power (VA) from the beefier winding as you would from existing one. I.e. if existing winding gives you 6V * 1A (6 VA), you could get 5V * 1.2A (again 6 VA) without affeting the rest of transformer in any way.

[tubelab.com]

It seems like toroidal power transformer fits diy community really well. It can be very economical and fun.

Many toroidal transformers have the center of the core (the hole) potted for easy mounting. Others are completely potted. Obviously these are not suitable for extra winding experimentation.

Most, like the Anteks are only wrapped in mylar tape. It is easy to add aditional windings to this type. I have added additional windings to isolation transformers for filament use. I added the extra windings right over the existing tape covering, then added a layer of Kapton tape over the windings. Wrap two or three turns through the transformer (any kind of wire) and make a voltage measurement. Then divide by the number of turns (2 or 3) to get the volts per turn. Then multiply that number by the desired voltage to get the number of turns needed. Remember that the primary and the core determine the total power that the transformer can supply (the VA rating). Adding windings does not change the total VA rating. Anny additional windings will compete for a share of that power so the current drawn from the other windings must be reduced.


I got the wild idea a while back to try one of these isolation transformers with extra windings as an OPT and it works great. The original toroid has 4 120 volt windings. Two were intended to be used as 120 or 240 volt primaries and two were the secondaries. I had added several additional windings, each producing 6.5 volts when used as a power transformer. I tried various combinations of series connection using all 4 120 volt windings. This became the center tapped primary in a push pull OPT. Various combinations of series an parallel connections of the new "filament" windings were used as the secondary until I found something that worked right. I have used a few "mains" toroide as P-P OPT's with varying success. I have found that the god ones work well and others often distort the bass or have limited frequency response. Current imnalance through the primary is an issue so forget using one for an SE OPT and matching tubes and bias current may be a requirement in P-P. Expect to get about 1/4 the VA rating in power output capability in OPT duty.

It is possible to cut away all of the secondaries from the original toroid leaving only the primary. Then you can add any number of secondaries that you need. I have only done this once because it is a real pain for windings of more than a few volts. Maryland Toroid sells toroidal cores with only the primary winding just for this purpose. They also have transformers for tube and SS audio amplifiers. They work very good, but aren't cheap.

http://www.toroid.com/standard_trans...ormer_kits.htm

[Evenharmonics]

Here are some photos of what went on to get the filament heater voltage I needed.

This is after already taking some wire off of tap 3 to make it from 12.6V to 5 V 2A and re-wrapped the mylar tape. The leftover wire is on the left and the one on the right is from 6.3V 4A tap to be reinstalled (notice the difference in thickness). There's some Mylar tape to the left of transformer to be reinstalled.


This is after reinstalling wire for tap 4 (6.3V 4A). I wrote numbers to keep track of turns and keep the spacing even. Mylar tape goes on top of it.


Here is a shot of finished TPT and testing. It gets 5V, 6.2V (close enough) and 348.6V.

[Arnulf]

Originally posted by Evenharmonics
Here is a shot of finished TPT and testing. It gets 5V, 6.2V (close enough) and 348.6V.

Test it under normal load. Readings will always be higher (say ~10% or so) without any significant load and this might put the output outside of recommended range for your tubes once operating in your intended application.

[Evenharmonics]

I did the same test before the modification and got 12.9V, 6.9V and 348V (untouched tap). After the modification, I did the usual "check out" per Tubelab website for my TSE and the voltages came out almost same as the setup with my previous PT. I'm not sure if that qualifies as "with the load" test though.

[Mush]

When testing for voltage, is it good idea to put a load onto a secondary?

Are the multimeters serving that purpose?

Thanks

[Arnulf]

Originally posted by Mush
When testing for voltage, is it good idea to put a load onto a secondary?

It is a good idea to load the transformer with same load it will be loaded with when operating in application, otherwise the readings will be too high. If you don't mind the unrealistic readings then you don't need a load.

Are the multimeters serving that purpose?

No, multimeters are hi-Z devices and present negligible load to the transformer.

[Ty_Bower]

I just had a strange thought... when adding another secondary to an existing transformer, the new winding is going to couple with the original secondaries. This should have the effect of raising all of their voltages slightly. Since we are talking about adding another 5V or 6.3V secondary, I wonder if the effect is even noticeable.

Has anyone taken precise measurements of all the secondary voltages, both before and after adding an extra winding?