Possible dumb question alert:: OT Primary/Secondary impedance ?

[captntasty]

What sets the impedance on the primary and secondary, mostly the secondary. I'm using a Hammond 125CSE - here's a link:

http://i27.photobucket.com/albums/c1...l23/125cse.jpg

Sorry for what might seem a dumb question.... here's the link to the schematic it is in...

http://i27.photobucket.com/albums/c1...turboChamp.jpg

[Chuck H]

Not having looked at either of your links...

The primary and secondary impedances are determined specifically by the the tubes you intend to use, how you intend to use them, the plate voltage, and the speaker load you will be running the amp into.

There is some lee way with this also. As in, you can sacrifice some fidelity for higher output. Many AB1 guitar amps do this but most class A amps seem to stick to the tubes prefered impedance.

Everything you need to know is in the tube data. Available from many sources online.

Chuck

[captntasty]

Yeah... I don't quite comprehend the specs... that's why I'm asking. Help me decipher the datasheets... ex.

Class Va Vg2 Vg1 Ia Ig2 Ra S Zout Pout THD
A S/E 315 225 -13.0 34.0 2.2 80,000 3.75 8,500 5.5 12

pardon my ignorance but how do I decipher this?

[Chuck H]

It really doesn't have to be that complicated. Just look at the class of operation you want in the tube specs. You will see they indicate an ideal primary impedance as "load resistance" and voltage under "typical operation". If your plate voltage will be higher that's ok. It is sometimes a good idea to up the primary impedance with highish voltage but I wouldn't sweat it for a guitar amp since the practice of running a lowish primary impedance is common in guitar amps.

Secondary impedance is determined by the speaker load you intend to use.

For example:

The el84 tube data for class A indicates several operating scenarios. But most of them are for a plate voltage of 250V. Since you will probably be higher than that I would suggest that you go with the higher "load resistance" example of 5.2k or 7k. Anywhere in between is also fine. Different primary impedances will alter the amps efficiency, THD, reaction to speaker damping and probably several other things I can't think of. But since every amp is different in it's goals I cant really suggest a specific primary impedance for this, or any other scenario except for my own builds. Since you have a universal transformer you could experiment for yourself to determine which primary impedance gives you what you want. Just use the "load resistance" range indicated in the tube data for your tube.

Figuring for the secondary impedance is where some math comes in. I hate math. But I'll do my best here:

Transformers have a "turns ratio". It's all about the ratio. If you were to use an OT with specs of "4.2k primary" and "8 ohm secondary" but you used it with a 4 ohm secondary, you would be reflecting a 2.1k primary impedance to the tube/s. It's a ratio. Now, how to figure the ratio:

The turns ratio will be the square root of the desired primary impedance divided by the secondary impedance. 4.2k (primary) divided by 8 ohms (secondary) is 525, the square of 525 is roughly 23. So the turns ratio for this scenario would be 23:1.

Your universal OT schematic indicates what leads to use for the desired impedances so it's even simpler. But in case it gets more finite, now you know the math.

Chuck

[Wakculloch]

Single ended 6V6's have an optimum load of approximately 5K, so two in parallel should halve this to 2.5K, so use the data sheet for the transformer to match this to your speaker load, with the appropriate tap.

[Enzo]

I would counter argue this. The impedance is determined by the load. The transformer has no impedance of its own, only ratios. The tubes only see the rated impedance IF the load is as specified.

You would select the transformer or transformer windings that transform your load to the desired primary impedance. SO I may be nitpicking, but the tube data tells you what impedance they would LIKE to see, but they don;t affect it. It is up to you to select the transformer that reflects your load back in the amount the tubes want to see.

[Chuck H]

I did try to imply that in a previous post. Along with the simple math (the only math I can do) that applies. I think that "understanding" is a far better thing than simply knowing. Something the American culture seems to shun on the macro view.

Your a better educator than I am by a stretch. But I do try.

Chuck

[Amp Kat]

And just because the numbers say it is the best match doesn't always mean it WILL sound the best. This is why selecting OT's is such a black art.