It's possible that what you're looking at is a marginal transformer on the left side. Considering that it's a device that must perform over a wide bandwidth, the "winding" of a good output transformer is actually a composite of separate windings, in an effort to balance the series and parallel inductances inherent in the device. The approach of "these numbers don't make any sense so the data is meaningless" doesn't really fly, because in reality important information winds up being thrown out. In making a winding for a transformer, you will have a given number of turns of a particular gauge wire, which has a resistance of R ohms per foot. In a given winding, you will have a particular length of this wire, ± a nominal amount for slop. In a high quality transformer, part of the goal is to have the amount of slop to be at a minimum, though in the real world it is impossible to eliminate entirely. As a practical matter, the DC resistance will vary a small amount from unit to unit, though a difference as large as a factor of 2 is important information, and should be questioned. Looking at the windup schematic for the A470 (ST-70), we see that the primary of the transformer is really two primaries in parallel. The Z-565 is most likely wound in a similar fashion. So if you're getting that high a reading in one plate circuit but not the rest, it's likely that one of the parallel windings is open.The best way to test this would be with real world powers and loads.www.diytube.com/phpBB2/viewtopic.php?t=3694
I agree with Sarah. The transformer with the higher dc resistance reading is going to be out of specs. If someone looks up a service manual of old tube console's it will give dc resistance specifications for the transformers. So if you were a technician that is how you would determine if the transformer was bad. I had a Fisher x100c with bad output transformers, so I unwound them and the primary windings were dual wires. Obviously the transformer in question has one of the paired wires open.
Thank you for this video. I just purchased a pair of these but the lead wires are very short and unusable. How would I extend the wires? Or could you do a video on extending the wires?
I noticed that when measuring inductive loads with a DMM strange readings, DC is applied in pulses looking at a scope. It would be interested to see what reading is obtained with a VOM. Other than that the high resistance could be caused by a poor carbonized connection in the transformer.
Hi BGE, Very interesting video, the 1st thing I thought of when you got the anomaly in d/c resistance between the one set of wires was just what you said... transformers step-up or down a/c. However, since you did end up getting proper a/c voltages for both the left and right sides, it must mean that for whatever reason, the lead that measured the higher d/c resistance has a longer wire or more turns to read that higher resistance correct?? I'm really curious as to why that is..... Maybe someone who winds their own transformers could chime in with the answer... Take care, Tom
I have two vintage output transformers. No wires are labeled. I know the primary by measuring resistance, but how do I know B from P? Or would it even make a difference? Likewise on the secondary, how would I tell ground from 8 ohms?
How would you suggest I cover the transformer wires inside ( under the bell covers) after I changed the wires. The sticky shellac like paper was covering the wires which I had to remove to change the wires. I'm not sure what to do. Thanks
I think I'd still be worried about resistive losses under load. You're measuring with a 10 meg input impedance meter. With a load it's the same as having that resistor in series with the transformer tap.
I'm sorry. Resistance matters at ALL frequencies. DC coil resistance is only irrelevant when it is low, not when it is high. When it is higher it becomes a major factor. Put a resistor of that value in series with a transformer winding under load and observe the waveforms with a scope at the different nodes if you don't believe me. 300 ohms resistance will be a very significant loss. It's exactly like ESR in a capacitor.
It wasn't meant to be negative - I think you're a little defensive.I've made a few dozen videos. Not set up to do that at the moment. I'm not a critic. If you don't want comments, just disable them. I like your videos by the way - and you're much better with tube amps than I am. I'm mainly a solid state guy. I think it would be fun to work with you!
I have a pair of unmarked output transformers, how do i identify secondary ground and positive 8ohm tap or does polarity not matter With nfb. And a stereo amp. Transformer has one output impedance.
I got a quick question on a old power transformer, it's got the 120v in and two greens for the filments, then it's got yellow and red that runs to the rectifier, then the confusing part is it's got two blue wires and a center tap blue and yellow, when I tested them to the center tap, they where 15 volts, 30 volts together, any ideas what this could be for? I think it may have went to a selenium diode on the old amp it came out of, I want to use it on a small amp build, but I don't know what to do with these blue wires. Thanks
I have many transformers like that some at 44v 80vac you have to change them to dc you can get a neg bias board on eBay with adjustable settings for each grid usually 4 it looks like yours is for a single ended amp el84 tubes or 6v6
A 'variac' or variable autotransformer is not an isolation transformer. You are telling people to wire their audio output transformer directly to the power line. If you must, may I suggest a 6 volt step-down transformer with a 1K resistor in series. If you get a wacky reading with your Ohm meter on a transformer's DCR, just switch meter leads. You assume to know at least one lead other than the CT by color. If not this method gives a 50/50 chance of wiring it out of phase.
