amplifier output transformer question

[9 volts]

Hey there, I have an output transformer (valve) for a 6v6 push pull amp. The primary has a choice of two sets of wires 8000 and 10000ct. I looked at the hammond site and noticed they sell both an 8000 and a 10000 ct primary transformer. What is the difference between these two? (off topic but I figure someone will know about this) Thanks

[MusicAudio]

The primary impedance for a P-P output transformer will affect how hot or cold you bias your tubes. Bottom line is that either will work, it just changes the other components you use with the tubes

I don't know how much of the rest of the amp you have built, but I'd draw up some load lines and see which one you want to use. If you're not familiar with power amp design the link below is to the Valve Wizard site, I'd suggest going through all of his articles; they're very good for beginners. The link below is specifically to the push pull power amp article, this will tell you a little bit about what effect the primary impedance is going to have.

http://www.freewebs.com/valvewizard1/pp.html

If you've never seen a load line before, I'd read the articles on the triode gain stage and single ended power stage.

Hope that helps.

mike

[9 volts]

Thanks for the link. I'm about to read through it. This is my second amp but I'm trying to get a greater understanding this time around to aid in design.

[R.G.]

Here's another way to look at it. Music, no disrespect intended, but that's too limited a view.

OTs do not have impedances, they have impedance ratios. They are like a lever or pliers which allow the high voltage but low current tubes to "grip" the low voltage but high current speakers, just like pliers let the long-motion but weak human hand grip short motion but high force mechanical objects.

The OT you're looking at has the choice of two ratios, speaker to 8Kct or speaker to 10Kct. The 8K leads are probably taps on the 10K winding, not separate windings, by the way.

The reason this is done is that power beam tubes (6L6, 6V6) and true pentodes (EL84, EL34) have two different sweet spots in their loading characteristics. One, generally the lower impedance, is the maximum power point. At that loading, a pair of tubes will turn out the highest audio power they can for the given power supply available to them. The other is the minimum distortion point. At this load, the combined second and third harmonic distortions are lower than they are either higher or lower.

Guitarists want lots of power and kind of like the distortion. Hifi people want low distortion even if they can hardly hear the music. So the two different impedances are there for musical instrument versus hifi use - primarily.  (N.B.- Sorry, I couldn't resist.    )

The transformer does not force a change in bias. It may allow a change in bias as a result of different use fo the available power supply voltage and current, but it's a minor effect, and wide, overlapping bias ranges are available to you in both loading situations. In all cases, the output tubes will be operating with their cathodes at essentially ground, their plates at a few volts below B+, and their currents set by their bias voltages. The bias voltage changes the operating Class, from Class C (both tubes off with no signal) to Class B (each tube turning on just as the other turns on) to Class AB (tubes overlap in conduction but each still turns off for some portion of the signal cycle) to Class A (both tubes remain conducting at least a little all the time).

If the B+ voltage remains constant, the output tube static power dissipation is set by the bias voltage. Class C is not used for audio; Class B is hardly used and sounds bad in practice. Class AB is mostly used and as bias voltage decreases you go from near Class B to near Class A. Distortion decreases and power dissipated increases. The tubes get hotter. If the B+ is so high that the static power limits of the tubes is exceeded, you can burn them up by lowering the bias magnitude. As the standing current in the tubes increases, it increases in the OT as well, so you have to have a bigger, burlier OT to stand the increased DC currents from tube bias in Class A. You can burn up an OT intended for class AB by running it too hot in Class A.

After digging through this for years, I've come to view load lines as an interesting but not necessarily critical aspect of output stage design for tubes. It's almost the last thing to check, not the first. That's just my opinion, of course.

[9 volts]

Thanks for the response RG. This helps me out, I'll be going with the 8000k ct!

[brett]

Hi

We should put this message in a special location:

OTs do not have impedances, they have impedance ratios.

And it's true of all transformers, not just OTs.  It would hopefully signal (no pun intended) that if you were to change from an 8 ohm speaker to a 7 ohm one, the load seen by the valves would reduce by more than 10%.  That's a fairly big change.  Of course, our 8 ohm speakers are only 8 ohms (maybe!) at some nominal frequency, and are probably 6, 7, 8, 9, and 10 ohms at different frequencies (and this will even change with things like cabinet type!).

So, like RG said, the 8k vs 10k thing is quite a minor thing.

After digging through this for years, I've come to view load lines as an interesting but not necessarily critical aspect of output stage design for tubes. It's almost the last thing to check, not the first

+1, for sure.  I feel that the chances of actually knowing what load line you're on must be quite low. 

Things like badly biased tubes (especially too "cold") and can make any p-p amp sound awful.  And, of course, the design basics need to be "right" in the first place.

cheers