Transformer schematic  - Page 1
Forum section: electronics / projects

author: iamnothim
author_rank: Regular Contributor
timestamp: April 02, 2013, 12:40:21 am
content: Hello,I am trying to understand the voltage sources on the schematic below.It is a combination of voltage from Mains and the transformer secondaries.I matched the schematic to all the parts of my Working Unit (before I took it apart).My project is to replicate the unit.My normal testing of a tube heater pins 4 & 5, is to hook up 6.3 V from a transformer secondary.I have not yet progressed to grid, cathode and plate. That's where I'm headed.As you can see the transformer from the unit had many taps.I only need one heater, cathode, plate voltage because I am only testing one tube type.I'm going to use multiple transformers for simplicity.One for the heaters of the tube under test, one for the heaters of the 6BN8 in the test circuit, and one for plate/grid power.I don't see how the schematic works. with mains going to pin 4&2 and the 6V secondary lead (yellow / black) going to pin 5.Then it shows the complimentary secondary lead (yellow / black) going to the grid test at 30VHow can one half be 6V and the otter 30V? (special windings?)Finally the other half of mains goes to the 6BN8 Plate / Grid circuit. I can see that because those voltages are much higher.But shouldn't other taps on the secondary be used rather than mains?I need to test this in LTspice, but I am clueless about setting up AC power, let alone a transformer.I'm going to work on learning that part, but I'm not getting general concept of this power source.Thanks

author: C
author_rank: Super Contributor
timestamp: April 02, 2013, 01:55:30 am
content: You need to remember that some of the old timers had as many tricks as you see today.Some of the old stuff just does not match what we use and think today.If you look up auto transformer, you should find that it is just wrap some turns of wire on a core, bring the wire out to a contact and repeat. To me that transformer is a back to back auto transformer. The primary is done as an auto transformer and used as such and at the same time drives the secondary also done as a auto transformer. Now remember if you are making a transformer nothing says you have to use the same number of turns between each contact, so the contacts do not have to all be the same voltage step.For some tubes the heater did more than just heat. The tube might work better if the heater was at a specific DC offset, Some tubes worked better with DC heater voltages. Also remember this a tube tester. Cheap all purpose tube tester. Almost a good/bad test. When you really needed to know, you would test the tube in the actual circuit, but some tests even the cheapest tube testers did were easer and quicker with a tube tester. Test a lot of the same tube and you might see the hint that saves time. Today we may match transistors or resistors, in the tube day was tube matching was common.C

author: iamnothim
author_rank: Regular Contributor
timestamp: April 02, 2013, 03:55:55 pm
content: Thanks C,I looked up autotranformer and indeed sounded a lot like whats in the schematic.A secondary lead and a shared primary. Times 2. Bizarre.Wikipedia DC bias definition actually stated that's it was used for grid/plate in vacuum tubes.I have the old trans to test with.First I'll see if I can create something in LTspice with 2 different voltage sources.However LTspice requires a ground. Since there isn't a ground, I'm having trouble seeing how to test 2 different halves of a circuit with different voltages.

author: C
author_rank: Super Contributor
timestamp: April 02, 2013, 04:16:58 pm
content: I am probably wrong,For LTspice you want want to think, multi-tap inductor not far from a multi-tap inductor.C

author: iamnothim
author_rank: Regular Contributor
timestamp: April 07, 2013, 10:03:37 pm
content: It's an Output Transformer. It can run mixing primaries and secondaries.I'm learning.The reason I posted again is to share this really great old book on making transformers.esp. for valves. "Coil Design and Construction"First pub. 1960 Last update 1974.It has a lot of info and formulas as well as a how-to on windings.http://ia700301.us.archive.org/13/items/CoilDesignConstructionManual/Babani-CoilDesign.pdf

author: IanB
author_rank: Super Contributor
timestamp: April 07, 2013, 10:24:24 pm
content: What you have there is something from the Old Days. In those times people didn't worship the God of Protecting People from Themselves quite so much and so live mains connections in circuits were quite common. These days Personal Responsibility is frowned upon and people are not allowed the same freedom to learn how to preserve their own safety.If you wanted to recreate a similar test circuit today you certainly ought to keep the mains on the primary of the transformer and put all your test supplies on the secondary side. The high secondary voltages are still dangerous, but they are less dangerous when you have to touch them with both hands to get a harmful shock.

author: C
author_rank: Super Contributor
timestamp: April 07, 2013, 11:43:21 pm
content: iamnothim,Some of the old days stuff was amazing.But as IanB says, not as safe as it could be.if you are rebuilding that circuit, you should make it safer. Remember, that schematic is for a cheap tube tester, the how cheap can we make it design, just like some things are done today.C

author: iamnothim
author_rank: Regular Contributor
timestamp: April 09, 2013, 12:17:03 am
content: Are we talking sparks, or flames too?Thanks for your concerns... I will heed your advice.I will not be mixing high voltage on the 6BN8 6.3V heater circuit.My pea brain can't resolve that concept.I'm using separate trannys for the 6BN8 and the valve under test.That leaves the high voltage plate and the -5 to 0V grid power.There are output transformers available. I will investigate.What I could understand of it, that book was amazing. I'd read some other transformer documents but not as detailed as that.Thanks again. I'm going to look for a "recommendation" tread for benchtop power supplies.If I don't find one, I'll start a thread.... Suggestions?