OT Impedance

[J Rindt]

Say you have an OT, and the specs are for a reflected Impedance of 5k Ohms.
Is that 5k what each half of the CT shows the power tubes.?
That is to say...assuming a Fender BF Twin, does each pair of power tubes see 5k Ohms.? I would imagine so, because that is how the OT will be used...in a push-pull scenario.
I know this is probably another beginner question, will be happy to read a link that explains this "stuff".
Thank You

[HotBluePlates]

Let's adjust the scenario to be a Super Reverb with a 4kΩ OT. That means 4kΩ plate-to-plate, or from one end of the primary all the way to the other end (or from a tube plate to the other tube plate in this 2-output tube amp).

While all tubes conduct (class A) in our Super Reverb, each 6L6 sees from its end of the OT to the center-tap where there is a filter cap to ground. So each tube operates as though it has a 4kΩ/2 = 2kΩ load.

Once one side cuts off (begins Class AB operation), the tube which remains on sees the plate-to-plate load impedance divided by 4. So the 6L6 which is still one sees 4kΩ plate-to-plate/4 = 1kΩ load. The tube is still operating from its end of the primary to the center-tap (and filter cap), but the conditions have changed (see below).

• The easy way to remember that is "Only half the primary is conducting, so the load to the tube still on is half what it is when all tubes are on."
• The accurate way to remember that is "When one side cuts off, its half-primary is not conducting and is 'invisible' to the secondary; the effective turns ratio is one with half-as-many primary turns, and the impedance ratio is the square of the turns ratio. So with a fixed secondary load, reflected primary impedance has dropped by (1/2)² = 1/4.

[HotBluePlates]

... assuming a Fender BF Twin, does each pair of power tubes see ____ Ohms?

Let's revisit the Twin Reverb. Mojo's wiring diagram says it's 1050Ω per half-primary, so 2.1kΩ plate-to-plate.

There are 2 tubes per push-pull side. Each side sees plate-to-plate/2, or 2.1kΩ/2 = 1.05kΩ per side while all tubes conduct. Since there are 2 tubes in parallel on a side, the designer used half the load needed per tube (like 2 loads in parallel), so each tube acts as though it has a 2.1kΩ load (compare to the load-per-tube for the Super Reverb above during class A operation).

When one side is driven to cutoff (class AB operation), the remaining side sees the plate-to-plate load divided by 4, or 2.1kΩ/4 = 525Ω per side. But there are 2 6L6's in parallel, so the load to each tube is double, and each tube acts as though it has 525Ω * 2 = 1.05kΩ load (compare to the load for a single tube during class AB in the Super Reverb above).

Fender kept the supply voltages largely consistent between the various 6L6 amps, so they doubled tubes and halved overall primary impedance to double output power. That said, each tube is still doing essentially the same work into the same load per tube (which makes sense, as double-tubes then give double-power).

[J Rindt]

I did not understand why you were dividing by 4.
But you explain that with math in your last sentence.
Wow..!
I never would have thought that...
... it is not the most intuitive observation. (not for my intuition anyway  )
If I did not own some books, and have taken a basic AC-DC class at out local college, I never would have followed this.
Anyway.....I appreciate  the explanation. Not quite  the "beginner" question I thought it was.Transformers never cease to amaze (or confuse) me.

Thanks Again

[HotBluePlates]

I did not understand why you were dividing by 4.
But you explain that with math in your last sentence.

Yeah, the "easy way" to remember that I gave is meaningless but easier to recall. The "hard way" explains why it happens. But if you're just trying to remember the facts, any memory aid is probably sufficient.

It was a long time before I stumbled on the reasoning for the divide-by-4 situation, and even when I did I had to re-read it numerous times (and not all at once) for it to sink in so I could understand.

[J Rindt]

Yes...I understand it right now, but I am also printing your (very well written) explanations, for future reference.
9 months from now, your info will be ready for me, when I am ready for it again.
Thanks