There is a reason for the indication of resistance of windings in some PS ?

[kagliostro]

I noticed it on the Silvertone 1484 but I can remember the presence of this indication also in other schematics

Usually secondary winding AC voltages of PT aren't indicated on schematics, but sometime

happen to see resistance on the windings indicated (like in the Silvertone 1484 schematic)

Which can be the reason for this particular information reported on the schematic ?

Thanks

K

EDIT: Previously I linked an image of the schematic that has an error, the dot connecting the 330k resistor to ground must not be there, it must be positioned to connect cathodes to ground
I corrected the image and attached here - note that the schematic you find have that error also the schematic present in the database of the forum

[John]

To help identify which windings go together? 

[stratele52]

No usefull , that's why we never see it

[sluckey]

Back in the '60s it was common for manufacturers to annotate DCR values for transformers and chokes on a schematic or service manual. Not everyone did. Sam's Photofact service folders often included this info, especially in TV circuits where you have many transformers. Sears was probably just trying to be helpful, especially for their own service department. Knowing the DCR of a transformer winding can be useful when troubleshooting. Sears seems to have forgotten that listing the voltages is also quite helpful, especially for this double-double power supply.

[kagliostro]

Thanks to all for the attention

of course

Many Thanks to Steve

seems you are something like my guardian angel 

Ciao

Franco

[terminalgs]

Is that a DC resistance or the Impedance Ohm of each winding?  With 3.7ohm on the primary, and 5.75ohm on the first secondary, that would make it a  145v winding (from 117v on the pri) (which seems right for that amp).,

although I'm confused by the 3.7ohm on the bias supply, producing 36V for bias with a 100K across the windings....  in fact, I'm really confused by the bias voltage, the way the schematic is drawn (big dot on the 330K/330K grid leak R's that ties it to ground?? must be wrong).   plus, at the bottom there is -1.5V pulled off at the bottom 2.2K of a 102.2K R to ground.  If it was -36V at the top, I'd think that -1.5V take off would actually be ~0.7V, and if both pri and sec are 3.7ohm, isn't that a 117v sec??

[stratele52]

Is that a DC resistance or the Impedance Ohm of each winding?  With 3.7ohm on the primary, and 5.75ohm on the first secondary, that would make it a  145v winding (from 117v on the pri) (which seems right for that amp).,

 

DC in my opinion.  Hammond transformer give DC resistance on their spec sheet . I do some reading on DC ohmeter to test .


www.hammondmfg.com/pdf/EDB290AX.pdf

[jazbo8]

(big dot on the 330K/330K grid leak R's that ties it to ground?? must be wrong)

Yes, it's drawn incorrectly, the dot should be on the cathodes of the power tubes.

[kagliostro]

Thanks Jazbo8

I deleted the link to the wrong schematic and loaded an image I've corrected
(only about that dot, I didn't searched for other errors)

K

[sluckey]

With 3.7ohm on the primary, and 5.75ohm on the first secondary, that would make it a  145v winding (from 117v on the pri) (which seems right for that amp).,

...and if both pri and sec are 3.7ohm, isn't that a 117v sec??

I don't think you can make an assumption about the voltages based solely on winding DCR. Voltage would be directly related to turns ratio but turns ratio can't be determined from just DCR. There may be different sizes of wires on the various windings and that would have a big effect on DCR.

[kagliostro]

I agree with you Steve

as to be sure of what is happening they must give the DCR and the thickness of the wire

Franco

[jojokeo]

as to be sure of what is happening they must give the DCR and the thickness of the wire

To help w/ clear understanding, I don't think this is true Franco. DCR would be helpful if you were thinking about the use of a choke and wanted to substitute using a resistor in the power supply to mimmick the choke's resistance in the circuit for similar behavior in order to save money on production of many amplifiers. The DCR is helpful but w/out that and knowing the actual Henry's @ a given current is much more useful.
For PTs and OTs DCR and thickness of wire are relatively insignificant. What IS important are the turn ratios (if using unknown transformers) because then you can actually calculate with reasonable certainty what voltages and/or what impedances you're going to actually get and want whether coming up with a replacement or use in new design work.
*Of course, all of the tranny's current and/or power ratings are very important too (let us not forget this also).

[kagliostro]

I agree with you Jojokeo

Franco

[PRR]

Back around 1929, John F. Rider started collecting and publishing radio repair information. At one point he wrote-up his thoughts about what IS and is NOT useful to the guy with a dead Philco on the bench and a customer waiting.

He felt that coil inductances were not much use, but coil resistances were useful. They are easy to check, often in-circuit. Open windings (due to sloppy work and poor varnish) were a common fault. So most of his diagrams show a nominal value for most windings (RF IF audio and power).

He also promoted the use of better ohm-meters by selling meters. While this may seem like double-profit, his way *was* better, and he didn't stay in the meter business once others started selling similar tools.

That particular drawing is not in the Rider style, but the idea of noting winding resistances on repair drawings became somewhat common.

[kagliostro]

Many thanks for your historic lesson PRR

Those news are very interesting 

K

[terminalgs]

With 3.7ohm on the primary, and 5.75ohm on the first secondary, that would make it a  145v winding (from 117v on the pri) (which seems right for that amp).,

...and if both pri and sec are 3.7ohm, isn't that a 117v sec??

I don't think you can make an assumption about the voltages based solely on winding DCR. Voltage would be directly related to turns ratio but turns ratio can't be determined from just DCR. There may be different sizes of wires on the various windings and that would have a big effect on DCR.

So, I didn't realize these were DCR values when I first studied this schematic,  I thought they were impedance. So I converted impedance to winding ratio like this: 

pri:sec = sqrt(3.7/5.7):1 = 0.81:1

pri voltage is known to be 117v,  so the sec. would be 145.2v

I guess it just works out to be conveniently 'about right' enough, that I thought those were winding impedance, not DCR.

[HotBluePlates]

Look at this McIntosh MC-30 Sams Photofact.

There is a resistance chart giving resistance to ground from each tube pin. Rather than guess about open/shorted resistors/caps, o take time to ponder what kind of reading you might get in those cases, they put the resistance measurement. You should be able to quickly go through those and confirm/deny gross errors or parts failures before spending more time troubleshooting.

[sluckey]

So, I didn't realize these were DCR values when I first studied this schematic,  I thought they were impedance.

Don't think about it as you would an OT impedance ratio. Think about it like this...

The primary is 3.7Ω. If that number was impedance rather than DCR, you would have 120VAC across 3.7Ω. That would be like 32 AMPS flowing in the primary! Kinda high, especially on a 20A house breaker.  

The actual primary impedance must be much higher. The schematic says the input power consumption is rated 117VAC at 100W at 1A. Apply OHMS Law and R works out to be about 117Ω. I know ohms law is not the proper formula to solve this AC circuit, but it's close enough to know that the primary Z cannot be 3.7Ω

[PRR]

> I guess it just works out to be conveniently 'about right'

There's a reason.

The ideal DCR for most PTs and OTs is *zero*.

But zero Ohm wire is always out of stock.

We gotta buy stuff with Ohms in it.

And the lower the Ohms the higher the price (and bulk).

How many ohms can we get away with?

Say you had a 6V 6A heater load. 6V/6A= 1 Ohm. Say you used a wire that wound-out to 1 Ohm at 6V while delivering 6A. There's obviously 6V*6A= 36 Watts of heat in the transformer, HOT; and 50% efficiency. Undesirable.

It works out, for a wide range of transformers (50Hz-100Hz, 10V-250V, 10W-100W) that you can probably work out a design with DCR about 5%-10% of load resistance/impedance, with reasonable cost.

That SilverTone OT is probably 4K primary? And marked for 170 Ohms DCR. 170/4000 is 0.04 or 4%. A bit low but in-sight of the usual 5%-10% goal.

There's major exceptions which make this a very un-certain "rule".

The 4K winding is thousands of turns of fine wire. It breaks. To reduce rejects the designer may choose to up-size the plate winding wire size.

OTOH high voltage windings need a lot of insulation, which tends to reduce the wire-size that can fit.

The low voltage (heater, speaker) windings are a *few* turns of *fat* wire. This tends to get lumpy. Which endangers the fine-wire windings. The designer will fiddle the wire size to get full-width layers of a not-too-fat gauge. For some reason it is much more common to down-size speaker windings even if the DCR goes past 10%.

And "extra" windings can be oddballs. We see there a 110V 100W winding with 3.7 Ohms and a bias winding which seems to be 24V at less than 1 Watt, also with 3.7 Ohms. If they used the same gauge for both, the 24V should be 0.8 Ohms. But since the bias is less than 1mA, they could use a gauge 1000 times smaller, for 800 Ohms. Since it feeds a 100K resistor that would work fine. But that might be too small to wind. It seems they simply used the same spool as used for the 84VAC windings (quad-stacked to 480VDC) so they only had 3, not 4, supply spools to stock and keep straight on the winder.

[terminalgs]

Ah!  thanks sluckey and PRR!!  many light bulbs went off as I read your replies.  now I get it (a little bit more, anyway...)