Help with OT impedances

[thehoj]

Another clue is that the 8 ohm terminal is a winding tap and you can see the two wires from the winding going to that terminal in you photo above.

Ahh, good point. So this photo is wired correctly then. SO, I guess I've still got things not quite right.. lol.
I've been running an 8ohm load on the 16ohm tap, so effectively 3K on the primary side. Now that gets me thinking about this a bit more.. If the primary side is actually 4.5K (as that other thread indicates) to the 8/16 secondary, then 3K may actually be no better or worse than 6K.. I guess if it is indeed 4.5K then I could go either way. But if it is 6K then I should use the correct tap.

I would test this to find out for sure, but I don't have an isolation transformer of some sort to apply AC voltage to the primary side. Would it be a bad idea to just apply 120VAC right out of the wall to the primary winding? I guess I could also apply the voltage from my heater winding .. but that's pretty low voltage, I'd worry about my multimeter's accuracy at such a low voltage.

[nickb]

I would test this to find out for sure, but I don't have an isolation transformer of some sort to apply AC voltage to the primary side. Would it be a bad idea to just apply 120VAC right out of the wall to the primary winding? I guess I could also apply the voltage from my heater winding .. but that's pretty low voltage, I'd worry about my multimeter's accuracy at such a low voltage.

Using the heater would be the safer way to go. Don't forget you'll still get a high voltage on the 'primary' side if you feed the secondary so be cautious. You'll meter will be plenty accurate enough. This is only a rough measurement anyway but's lets not get into why that is.

[eschertron]

I would test this to find out for sure, but I don't have an isolation transformer of some sort to apply AC voltage to the primary side. Would it be a bad idea to just apply 120VAC right out of the wall to the primary winding? I guess I could also apply the voltage from my heater winding .. but that's pretty low voltage, I'd worry about my multimeter's accuracy at such a low voltage.

I don't work on amps for a living, so take M2CW with some skepticism:
The OT is designed to take hundreds of volts on the primary side, and will suck a few milliamps of current. Just plugging it into the wall will not hurt the OT. Just plugging it into the wall leaves YOU exposed to lethal current, however and you should take appropriate precautions.
I believe the usual way to test a transformer like this would be to hook a up a low volts source (like your heater winding) to each speaker pair and measure the high voltage generated on the primary. Again, a fuse or other precautions would still be advisable.

edit: nickb beat me to it!

[thehoj]

Makes sense, thanks for the info.

If anyone's curious, this is the amp I put together. Sounds just absolutely perfect for me.. http://s14.postimg.org/l2ayzrrlb/Bogen_Conversion.jpg
It's kind of a rip-off of the Tonemaster 214 / Magnatone 213 preamp (minus vibrato) with 6SF5's instead of 12ax7's, and then sort of a Fender 5E3 PI / Power section (6K6's instead of 6V6's) with a few changes.

Fairly different than any other preamp I've done before, the tone control is very interesting, and I really love what it does to the sound. Also the last leg of the power supply being that 0.047uF cap was odd I thought, but I put it in there and whatever effect that low value cap has, I really really like it. The amp get's extremely crunchy, but not overly messy sounding.. Even with no bypass cap on the power tube cathode resistor, I actually prefer the sound with no bypass cap there.

I tried to build this all with stuff I had lying around, and the PT being 145-0-145 wouldn't give me enough rectified voltage to do a FW rectifier, so I left the CT disconnected and did a FWB rectifier instead.. Of course that gave me a bit too much voltage, so I used a few zeners to bring it down by 40V. The PT is fairly beefy though, so I don't think there'll be a problem with too much current. Also the PT actually has 2 seperate 6.3V secondary windings, so I used one for the power tubes, and one for the preamp tubes.. After playing the amp for a while the PT gets warm-ish to the touch, but not overly hot, so hopefully I'm all good there. It's an old hammond PT, and I can't find any actual specs for it online, but it is decently big. We'll see how it goes.

[eschertron]

There's some fairly nontraditional bits in there; the tone control and the feedback (?) on the PI are what I see right off the bat. You've given me my homework for this week!

[thehoj]

Alrighty, so I did some measurements.
Firstly, measured resistance of the two taps.. The upper most one, or the one that we would assume is 16ohm, I measured 1.3ohms. The middle one, or the one that we would assume is 8ohm, I measured 1ohm.. So that clears up which is which.

Next I applied my heater voltage to the secondary. With power on, I measured 6VAC exactly on the secondary taps.

With 6VAC applied to the 16ohm tap I measured 96VAC on the primary side.
With 6VAC applied to the 8ohm tap I measured 137.2VAC on the primary side.

So the turns ratio for the 16 ohm tap is 16:1, and the turns ration for the 8ohm tap is 23:1.

To calculate the impedance ratio, I calculated 256:1 for the 16ohm tap, and 529:1 for the 8ohm tap.

If I calculate the primary impedance ratio based on both taps I get:
8ohm tap: 8x529 = 4232
16ohm tap: 16x256 = 4096

So I guess that clears THAT up.. Primary impedance ratio is more like 4500, not 6000.. Not sure what those 6K replacement OT's are all about.

So that all being figured out, with my 6K6's wanting 6000ohms, I guess I could use the taps as they're supposed to be.. Connecting an 8ohm load to my 8ohm tap will reflect 4232ohms, where the power tubes want 6000.
And connecting a 16ohm load to my 16ohm tap will reflect 4096ohms, where the power tubes want 6000.. That's not too bad of a mismatch I don't think..
Probably better than connecting a 16ohm load to my 8ohm tap, which would reflect 8464ohms where the power tubes want 6000.

Did I do this right?

Thanks for all the help everyone.



EDIT:
Just gave the amp a run hooked up to the correct tap. Sounds just absolutely great. I guess up until this point I had been running 8ohm load on 16ohm tap, or a primary impedance of 2048 where the power tubes wanted 6K.. :S.. lol. Sounded good like that too, but I can't imagine it's too easy on the power tubes like that.

[Gingertube]

Yes you did it right.
So Raa is 4K Ohms. A qud of 6K6 will work into that OK (Remember the 6K comes from the data sheet as being 1/2 the recommended 12K for a pair of 6K6).
That recommendation is for general audio (HiFi) use . 4K in lieu of 6K will give a bit more power and a bit more distortion. You could convincingly argue that that is better for Guitar anyway.

Running an 8 Ohm load on the 16 Ohm tap would have been reflecting a load of 8K Raa. That actually would have been easier of the output tubes (less signal current swing for same output power).

The amp will certainly deliver more power with the 8 Ohm load on the 8 Ohm tap. Whichever sounds best to you is what you should use.

Cheers,
Ian

[thehoj]

Running an 8 Ohm load on the 16 Ohm tap would have been reflecting a load of 8K Raa. That actually would have been easier of the output tubes (less signal current swing for same output power).

Really?.. Wouldn't it be 8(speaker load) x 256(16ohm tap impedance ratio) = 2048 ?

[Gingertube]

NO not really,
Must have been typing with the brain in neutral.
Sorry,
Ian