OT primary matching?

[nateflanigan]

I'm working out parts for an AC4 build.  The schematic calls for an OT with 5k primaries, I have a champ OT with 7k.  Would there be an issue with using that?

Thanks

[HotBluePlates]

Higher OT impedance results in less power output and more distortion. If your schematic doesn't call for screen resistors, you may want to add small screen resistors, as higher OT impedance might result in higher screen current with signal.

Less impedance than some optimum value generally result in less output power, maybe more (or less distortion).

Bottom line, the amp will still work.

[Leevi]

I don't know for which load (speaker) your 7k OT is specified but by decreasing the
load the impedance on primary side will also decrease.
The primary impedance = load x ratio^2.
/Leevi

[bobmegantz]

Also, you can raise the supply voltage (if that's a possibility) to the the output tubes and drop more for the preamp...

[nateflanigan]

Thanks that's all really interesting info.

The Champ OT I have is meant for a 4 ohm load, so it wouldn't be very convenient to try and reduce that.  None the less I don't understand the equation.

The primary impedance = load x ratio^2

Does ^2 mean squared?  Is "ratio" the primaries to the secondaries?

Higher OT impedance results in less poser output and more distortion

Is that a typo?  I hope not, I really want to know what poser output is

Messing with the voltages is also a possibility as I don't have a PT yet for this project.

Thanks for the help

[Leevi]

Does ^2 mean squared?

Yes, you can measure the ratio if you have a signal generator:
- connect a low AC voltage (e.g 20v) to the primary
- measure the secondary voltage
- the ratio = primary voltage/secondary voltage

In that way you can determine the impedance of any unknown OT.

More information:
http://www.radioremembered.org/outimp.htm

/Leevi

[sluckey]

Is that a typo?  I hope not, I really want to know what poser output is

poser should be power. S sits just below W on most keyboards. I do that more and more these days. I think my fingers are growing!  

[HotBluePlates]

Obviously, I can't touch-type. So I often don't see my dumb spelling mistakes.

Also, you can raise the supply voltage (if that's a possibility) to the the output tubes and drop more for the preamp...

You don't really want to muck supply voltage a lot, because we're not dealing with a resistor as the plate load. Raising supply voltage (and taking the subsequent steps to insure the tube doesn't melt) will almost always result in more power output. But if we do nothing else and just jack up supply voltage, we might move the EL84 operating point to a worse place.

Let's try to answer the first question again.

It's funny to me that Vox's AC4 schematic says "5k anode to anode" when it's a single-ended amp. Oh well... Where did they come up with 5k to begin with? I'd bet money it wasn't through lots of calculation and trial & error. Check out the Mullard EL84 data sheet; the first page has conditions for a single-tube amplifier. ~5k load gives 5.7w output. Vox simply copied the condition (near-enough) given by the sheet, because the tube manufacturer has already figured known-good operating conditions. The bias even lines up close enough, after you consider that cathode current is screen plus plate and will be a little bigger with 270v on the screen vice 250v.

That's all you get with Mullard, but the G.E. 6BQ5 sheet tells a little more.

On the second page, there are "Characteristics and Typical Operation" for a Class A1 Amplifier. This is single-ended operation, and has 3 different cases with 250v plate and screen. Loads are shown from 4.5k to 7k. At 7k, the power drops slightly from 5.7w to 4.2w. But this is still a little misleading, because notice the distortion is constant at 10 percent. I think that was used as the primary consideration, especially since the case with a 7k load uses a smaller driving voltage than the others, despite having higher bias; that's all about keeping the grid-drive small enough to hit a specific distortion number.

Anyway, don't sweat it, and just use your Champ OT. I'd bet that while you can measure 1w of reduced output, you can't tell it by listening. I also bet that the desire for a SE EL84 amp is more about power tube distortion than maximum clean power output.

[nateflanigan]

Well, would ya look at that.  That's really cool thank you, I often struggle with putting together all the data on data sheets into a complete picture but that really helps.

neat-o