6V6 Output transformer resistance between primary ends

[dinkotom]

Hi folks,
I have four OT's for 6V6 tubes and they all have very different readings between primary ends:

1. OT15PP (from Musical Power Supplies) - reads 74 Ohms
2. Locally wound trans. - reads 192 Ohms
3. Trans. in my Deluxe Reverb (from 1979: http://www.webphix.com/schematic%20h...oost_schem.pdf) - reads 400 Ohms
4. and this one: Fender 5E3 Tweed Deluxe Style 20W Output Transformer 8 Ohm ClassicTone # 40-18022 - reads 680 Ohms

So, is this okay, or some of the readings are off?
My tech says that every 6V6 OT he's measured had about 140 - 170 Ohms between OT primaries ends. He's working on my DR right now (which starts breaking farts on low string at 3 on the volume, even with new tubes and filter caps), and needs to know if 400 Ohms is correct value for this amp. Could any owners of the same amp measure and confirm?

Thanks!

[trobbins]

Your problem may be bias.

Two 12-15W vintage OT's to suit 6V6 that I have meassure 340 and 420 ohm for 8-10k PP.

[mort]

I could easily be wrong here, but the varying resistance could be a result of higher quality transformers(lower resistance) OR lower quality or well used transformers(higher resistance). I wouldn't think that the actual resistance wouldn't have any effect on the function of the transformer other than the fact that if it's too high then heat would become a problem.

Somebody please correct me if I'm mistaken.

[Steve Conner]

Output transformer design for musical instrument amps is something of a black art. Having said that, the DC resistance measurement has a similar meaning as in guitar pickups. The higher the resistance, the more turns of wire used, which means more magnetizing inductance, and so more bass output, for a fatter tone.

More turns also means thinner wire, unless the whole transformer is made bigger and more expensive. The thin wire wastes some power, so you lose a little volume. But sometimes the extra bass can be worth it. Like everything else in electronics, it's a tradeoff that can only be broken by throwing more money at the problem.

I think all of the readings are reasonable, except 74 ohms which is a bit low for a 2x6v6 OT. They probably used too few turns. Unless it is a 5lb monster with a really big core, but if that were the case it would be expensive.

No, the resistance doesn't increase with use.

[mort]

Just to clarify, by "well used" I mean somewhat degragated. I didn't mean to suggest that simply using a transformer would cause the resistance to increase.

[trobbins]

mort - the resistance doesn't 'age' (ie. degrade) with use or any other service life factor - although if the OT uses oxygen free copper then maybe slow infusion of oxygen over the years could increase resistance! ;-)

[mort]

how about if I phrase it differently once more....


...if the OT is partially F-ed up...


better?

[Steve Conner]

Not really!

Even one shorted turn can ruin an OT (weak, distorted sound) without affecting the DC resistance measurement.
The other kind of failure is an open circuit, but that's "infinity ohms".
You can also get breakdown between the primary windings and the core, resulting in clouds of smoke and blown fuses.

So yeah, transformers don't wear out, they either work or are completely shot.

[dinkotom]

Not really!

Even one shorted turn can ruin an OT (weak, distorted sound) without affecting the DC resistance measurement.

Will tell this to my tech.

Thanks everyone for replies.