OT Primary to Secondary ratio

[Willabe]

I have a Classic Tone 40-18030 5w SE OT set up in my bread board. 

Primary;
Red- common/start/plate
Brn.- 5K
Blu.- 8K

Secondary;
Blk.- common/start/ground
Yel.- 4 ohm
Grn.- 8 ohm.

If I hook up the Brn. primary 5K lead to the 6V6 plate what do the 2 secondary's become? Do they shift up or down?

Yel.- 8 ohm and Grn.- 16 ohm  OR  Yel. 2 ohm and Grn.- 8 ohm?

[HotBluePlates]

... If I hook up the Brn. primary 5K lead to the 6V6 plate what do the 2 secondary's become? Do they shift up or down? ...

It don't work like that.

What you connect to the secondary will dictate what impedance is reflected to the primary. So if you attached 8Ω of speaker between Green & Black, you will get 5kΩ reflected to your Brown & Red primary wires.

Impedance Ratio from Brown/Red primary to Green/Black secondary is 5,000:8 or 625:1. If you wanted to reflect 8kΩ to that Brown primary wire you'd need 8,000/625 = 12.8Ω of speaker load between the Green & Black secondary wires.

Same math process for the other taps, both primary & secondary. If working from a known secondary load, then you'd multiply by 625 in the example above instead of dividing.

[Willabe]

I have a 16 ohm speaker I want to hook up, so........?

16x625=10K?

[HotBluePlates]

Yes, you doubled the impedance on the secondary, so it doubled on the primary.

[Willabe]

So 1x6V6, 365vdc plate, 470R K, 10K primary (Brn./Red) 16 ohm speaker on secondary (Grn./Blk.), close enough? 

[DummyLoad]

So a 6V6, 365vdc plate, 470K R, 10K primary (Brn./Red on my OT) 16 ohm speaker on secondary, close enough?

470K??? Rgrid-leak? Rk? - eek!   

10K:16  w/ 365V works.

you want about 15V at the cathode or around 35mA for 90% of max Pdiss, e.g., 12.6W. so 430R or thereabouts. use a 2-3W cathode resistor.
if you don't have a 430R
parallel 820R 1W and 910R 1W
parallel 680R 2W and 1.2K 1W
parallel 470R 2W and 4.7K 1/2W

--pete

[Willabe]

470K??? Rgrid-leak? Rk? - eek!

 

  Sorry, my mistake, I fixed it. 470 ohm cathode R.

10K:16  w/ 365V works.

Thank you Pete. 

[trobbins]

Remember that the nominal operating impedance imposed on the primary winding will somewhat dictate the load line followed from the idle point, and hence what likely anode voltage swing is seen for an ideal load.

[HotBluePlates]

What Tim just said.

The unstated notion I was countering in my original response is that the tube has an internal impedance that somehow sets the load attached to the primary, thereby dictating the secondary impedance.

With pentodes and beam power tubes, the internal plate impedance is so high that the transformer primary impedance is almost always very much lower. So for these tubes it's all about the secondary load impedance and how that dictates the primary impedance.

And we assume the output tube is an ideal device for figuring output stage setup, and the % error (almost always guessing a little too-high output power) due to that assumption is small in DIY work.