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Doing a FleaBay buy ST-70 pre-power check and I find a substanial difference between the 2 OPT's primary DCR. One reads @200 ohms and the other is @235 ohms. Tubes out. The difference seems to be between the grid taps, perhaps shorted windings or is this difference a standard deviation?

Thanks much!

Are you measuring from A to A or A to the B+ centertap?

There can also be a deviation in DCR readings between transformers, if they are from different manufacturing lots.

That is normal, the layered winding technique means that one side of the primary will have about 15% to 20% more wire length (thusly resistance) than the other side. Each successive layer has a slightly greater radius.

This phenomenon also is measurable on the Z565, and most PP OPTs, except for the higher-cost ones with bifilar primary windings.

The A470 has layer wound primary windings, and bifilar wound secondaries.

Refer to this: http://diytube.com/phpBB2/viewtopic.php?t=3694



I had one Z565 which had one side of the primary measuring at about twice the DCR of the other side - that is the one that "Geek" used for the Z565 autopsy / post-mortem.

/ed B

Ed, Thanks,

Measuring A to A for those numbers.

Between A and grid taps all @70'ish ohms.

I think this ST-70 spent some time out in the trunk of the Rambler.
Perhaps some moisture in there? I'm pulling the trannies now and I'll have the bells off in a while to see what's in there? This will be the first time I've taken the skirt off of one of these farm girls.