OT and power tube bias

[C Moore]

I have a friends Soldano HR 50. She said it was noisy, and I told her I would take a look. The preamp tubes were pretty micro, so I changed those. I checked the power tube bias, and there was a difference of 19mA between the two tubes. I installed a new set, and that helped with the noise issue a lot.
It has Merc Mag trannies. When I measure from the OT center tap to each power tube plate, I get 100 ohms on one side and 78 ohms on the pother. Is this OK.? I usually see a difference of more like 2-5 ohms when I bias an amp. Is this a problem.?
Thank You

[Andy Le Blanc]

DCR, or the resistance you measure with a little dc with a meter isn't the
same as the impedance that the tubes see in a push pull circuit.

I haven't yet heard anything bad about MM products except for the high price.

MM are usually modeled on something, so unless the opt shows other signs
of abuse like leaking coil potting or smelling like its been cooked, its probably OK.

[C Moore]

Yeah.....no doubt you are correct. I have no idea what the actual impedance might be. But like I say, I have never seen an OT that was that far apart when measuring resistance.
I have not been doing this anywhere near as long as most of you guys. I have only measured maybe 50 OT, for bias, in my time. But this is the first OT that was 100/78. Every other one I have seen was much, much closer.
Just made me wonder......
Thanks

[eddie25]

I've wondered the same thing a while back after checking out an Ampeg with mysterious red-plating on one side only. The OT measured like you say yours did, maybe even a little farther off. The red-plating was on the side with the much less resistance. I didn't have a lot of time with the amp unfortunately, I'm sure I could have biased it colder, but I always kinda suspected that the OT might have been damaged. Does that kind of thing even happen? An OT losing resistance on a side?

[Cliff Schecht]

Yes! Sometimes windings arc which will will short out a section of the windings. This will measure ok on a low voltage DVM but not with high voltage, especially in Ampegs and such with super high plate voltage.

[martin manning]

Ok, so if you can measure a difference in DC resistance, then you should be able to put a low-voltage AC signal in and measure the primary-secondary voltage ratio for each half and see if they match.