Impedance and output transformer selection question

[johan]

Hi everyone

It's about 15 years since I studied electrical engineering and I am trying to remember some basic things again. I have some questions about the selection of output transformers for a tube amplifier and I would appreciate if someone could aid me in the right direction. The circuit I am looking at is a Williamson type parallel push-pull ultra-linear with 6L6GC tubes.

Most similar designs that I have seen use one pair of tubes in push-pull UL, which with a pair of 6L6GC tubes makes about 6k6 output impedance. In this design there are two pairs, or so called parallel push-pull. A similar circuit by Acrosound is here: http://oestex.com/tubes/acrowill_1.gif The Acrosound amp uses a 3k3 output transformer.

Instead of using an output transformer designed for 3k3 primary impedance, the circuit author uses a 6k transformer. I'm now asking, what is the benefit of using a 6k OT here? Maybe to make it easy to switch to other tubes? If I were to build this amplifier, wouldn't it be better to use a 3k3 transformer? I am trying to understand how it would affect power output and characteristics. I would appreciate some inputs on this.

I have been looking at some available transformers and for instance Edcor has many transformers in their CXPP series, but none with 6k impedance. I would have to choose between 6k6, 5k or lower (there are 4k2, 4k, 3k8, 3k5, 3k3 etc.).

Here is the circuit in question:

http://img855.imageshack.us/img855/1221/parts100dpi.png

Note.
The author of the above circuit is Jukka Tolonen, which has published articles in Glass Audio Magazine and a Finnish Hifi magazine. His homepage is here: Jukka Tolonen Home Page

[hemgjord]

Loadlines, loadlines, loadlines! education+diy or Steve's Tube Pages

After digging that through you'll be able to figure out yourself.

For triodes it is pretty straight forward, higher load gives lower output power, lower distortion and lower output impedance.
For pentodes there is a pretty narrow optimal load span which gives reasonable THD and power out, output impedanece is always high and needs to be treated with nfb.
UL is somewhere inbetween depending on UL percentage. It is a little trickier since there is seldom plate curves avilable in the datasheets. But if I remember correct there should be an example in the GE 6550 datasheet.

/Olof

[johan]

Hej Olof

Thanks for replying. Does this tell me what I need? It is from the KT66 tube data. It should be almost identical to the 6L6GC. The lower graph specifies 450V, which is what I will use. Otherwise I will try to figure out loadlines (I have some vague memory how to do it).

[hemgjord]

Yes, that's all the work done already.
So according to this the optimal loading is somewhere btwn 7-10 k(a-a) (depending on how you value thd vs Pout)for a single pair.
In your parallell PP 3k5 to 5k should be good. I would go close to 5K but more seems to be giving up power for no good reasons.

/Olof

[johan]

Thank you.

The Edcor 100W 5k transformer CXPP100-8-5K is probably the cheapest but still a good option.

[johan]

Another question. Suppose I'd like to temporarily remove one pair of output tubes and connected the 8 ohm speaker to the 4 ohm tap on the secondary (assuming I had such an OT). Would this load the output transformer in some weird way or affect bandwidth or something?

[hemgjord]

Should not be any problem. BW will be less than with all tubes in place since your drive impedance will be higher. But a reasonably decent OPT will cope.

/Olof

[johan]

Thank you. Good to know. Now I can proceed buying the transformers.