Hello everyone, got a potentially ignorant question here please don't judge
I have bought a few old PTs from a radio parts site. They have great stuff and the owner has been really helpful. I used a PT, OT and choke I got from him recently in an amp that started w/5E3 circuit as reference and the results were great.
Anyways, included w/my order was a sheet explaining some things about PTs. (one of the suggestions was to install a fuse! I guess most people who buy from him are restoring old radios many of which did not use fuses) On this sheet a piece of advice to help identify windings was that the HV secondary will usually read "between 50 and 250 ohms from CT to either end depending on the milliamp rating of the transformer" Basic caveman thinking told me then that the resistance somehow corresponds to the milliamp rating. I measured one PT whose HV secondary was rated for 120 mA and the resistance from CT to either end was around 175 ohms. I then measured another which was rated for 150 mA and it showed about 105 ohms. A giant PT rated for 250 mA showed me around 65 ohms. So, the higher the current the secondary is able to provide, the less resistance it has? Or am I wrong? Is there some other way these two things correlate to each other?
Short answer: wires with bigger diameters have less resistivity and can bring more current. So yes, the lower the resistance the highest the current capability.
Long answer made short: it depends on the VA of the trafo, so on the size of the core, and then on the number of turns on the core, but as a general rule you can consider that a low resistance corresponds to more ampères.
DC resistance is related to potential and current, but the VAC that a transformer secondary puts out is related to the VAC ratio of the primary-winding to the secondary winding, which is a direct function of the primary-to-secondary windings' turns ratio.
The power-handling of the PT is related to several aspects including; the size of the wire used in the windings; the current drawn through them; the amount of iron in the core (in iron-core PTs); and the frequency and potential of the VAC inducted from primary to core and core to secondary.
The current drawn through the PT is related to the load on the secondary side, which is a function of the product of the resistance between the high tension supply and the ground return, and the impedance across the heater circuit, and (in amps with tube rectifiers) the impedance in the rectifier filament (and the potential of HT supply, which is related to the VAC on the high-tension winding, and the type of rectification, and the smoothing and filtering).
The load on the high tension supply is related to the amount of signal supplied to the amp's input (which affects the overall current drawn through the PT at any given instant), the gain of each stage (which affects the amplification of the signal through subsequent stages); the attenuation between each stage (which also affects the signal strength); the voltage of the B+ supply throughout the amp (which determines the headroom and the maximum clean signal strength); the number, and types, of tubes (which sum to the total tube current drawn from the HT supply); and the load resistance presented to the output tubes (which affects the peak current draw); and (in pentodes and tetrodes) the screen voltage (which affects total tube current and gain of output tubes).
So its not just a matter of the DC resistance of the wires in the PT windings
It's a bit too simple to try and compare and correlate PT parameters like secondary resistance, without actually doing a good assessment of how each compared transformer was designed, and what application it was intended for, and how it performs in that situation.
Design and manufacture decisions change over time, with materials, with standards, with customer requirements, and with end application.
Even the simple spec that may be obtainable, like the OP's 120mA, 150mA, and 250mA 'rated' is fuzzy and should be taken with sceptical caution, and further questions asked as to the PT's application and winding voltages.
For amp PT's, the output current 'rating' is often the DC loading, and not the winding rms current rating. And the PT manufacturer is likely designing for very high crest-factor secondary currents - some even indicate what the load current rating can increase by for a low crest-factor choke filter loading. And manufacturers often designed and rated specifically for onerous doubler use (otherwise a 'load' current rating is ambiguous).
trobbins and tubeswell, thank you for your more in depth explanations. I'm still very much a beginner and am learning new things all the time, so this info is helpful.
The main reason I was curious about this relationship is because one of the PTs I acquired from this source is a rather large Stancor PC-8413. It is black, kind of old and beat up looking, and the leads are all spliced in some way or another (meaning the original leads are short and have had length added to them, which would indicate that it has been pulled from its original installation, used, then pulled again). I have a couple of Stancor PC-8411s which on paper have slightly smaller ratings than it's bigger cousin the PC-8413, but the 8411s I have are grey and brand new. I don't have any reservations building with them, whereas with the burly 'been around the block' 8413, judging by it's looks, I am a little concerned that if I use it in a build I could very well fire it up and have the thing fry in some way or another, and it would be difficult to replace. However, I know a lot of old used gear can look very beat up and still function properly.
So, in short, I wanted to find out if the low resistance reading I was getting on its HV secondary winding was any indication of its health being good or bad.
I'm late to the party. In my experience, the primary can have a very low DCR. The secondaries are very dependent on wire gauge and number of turns as DCR is a function of the particular length of wire. As long as it isn't "open" I think I wouldn't be concerned. This is especially true if you have 2 or more of the same with like measurements.
According to the 1961 Stancor catalog, the PC 8411 and PC 8413 are "power transformers for use with choke input filter, VR-tube regulated power supply, speakeer field in filter, or high voltage with condenser input filter."
I got distracted this morning and couldn't finish posting the specs from the catalog. Here they are:
PC8411: 375-0-375 @ 150mA DC, 5v @ 3A, 6.3VCT @ 4.5A
PC8413: 400-0-400 @ 250mA DC, 5V @ 4A, 6.3VCT @ 5A
