EL84 Fixed Bias resistance check and tube

[RFMann]

I learning tube amp troubleshooting and maintenance. I am checking bias or first time in my EL84 push pull amp. While checking resistance on my 20W center tapped output transformer. Assuming I am checking correctly, by measuring from B+ to pin 7 on each tube (blue wire and then brown wire). My readings I come up with are:

V6 Resistance = 71.9 Ohms
V7 Resistance = 58.0 Ohms

Question 1: Did I measure resistance at the correct points

This amp is currently biased

V6 = 23.6mA
V7 = 18.9mA

I would have thought the two sides of the transformer would have ohmed out a lot closer to each other.

Question #2: Is this indicative of a potentially unmatched set of power tubes?

Thanks

[Justin Thomas]

As for your transformer, it's not unusual at all to have the 2 halves be uneven as yours is. It just depends on how they manufacture it. As for your numbers, sure they're not perfectly matched, but how does it sound? I consider 3mA to be well-matched; this is good enough. As long as it's not humming or redplating, I'd go with it.

We don't have any voltages so it's hard to tell what the plate dissipation is...

Justin

[Enzo]

Yes^^^^

perfectly normal for some output transformers to measure uneven. The transformer runs on turns ratios, not resistances anyway.

[Mick Bailey]

That's the thing to remember - its the turns that matter. Plus, the DC resistance difference is inconsequential in a guitar amp. Some output transformers are wound so that both the turns and the resistances are equal. Hi-Fi enthusiasts love that kind of thing. Guitar amp output transformers are more commonly found with different resistances and I can't remember ever measuring one where they matched. The reason is that when you wind out from the centre of a bobbin, as the turns increase so does the diameter of the winding and it takes more wire per turn. So the winding with the lower resistance is the one closest to the core.

[alexradium]

low cost OTs are wound with half primary,then the secondary,then the other primary,it is obvious that the outer coil has bigger dimensions hence longer wire and resistance in DC,but talking about overall impedance it is just a tiny difference.

[g1]

Were the tubes you measured a matched pair?
That's not a big difference, but if you want, you can swap them around and recheck to see if the higher current one is the tube itself, or the circuit. As has been said, the OT resistance numbers you gave are pretty normal.

[RFMann]

Thank you all for your responses.
The amp sounds good to me. I am fairly new in the tube amp world and I just wanted some questions answered as I attempt to learn and better understand how they work. You all have helped me gain a little better understanding of my transformer. Again, thank you all.