Choosing an UL output transformer

[Rootz]

Hi all, I just bought a pair of 600 Vct @ 250mA power transformers for an upcoming project. The idea I have can be seen in the recent thread about bias servo's for class AB amps.

Now that I've got the power transformers, it's time to choose some output transformers. There are some blanks for me to fill in.

Here's what I want/have:
- B+ approx. 420V loaded
- Ultra linear
- 8 Ohm nominal speakers
- EL34 power tubes, if possible other big bottles as well
- fixed bias, source follower power drive

Here's what confuses me:
- are my 8 Ohm rated speakers actually 8 Ohm? (apparently no, probably more 5 to 6 Ohm around 1 kHz
- Must one take this knowledge into account? I mean, I rarely see a designer state: my speakers are 6 Ohm in reality, so I adjust the reflected primary impedance accordingly.

All of which leads me to the question: which primary Z to design around? Higher seems to lower 2nd harmonic at the expense of some power, lower impedance is more powerful. I haven't found any EL34 characteristics UL connected. I'm guessing about 5k5 tot 6k6 should do the trick, am I right on this? The LL1663 seems like a nice candidate to me and is used in the 4-30 by Claus Byrith. The combination of a lower primary Z and 8 Ohm secondary Z worries me though. Also, this transformers seems a little light for anything more than a pair of EL34's.

 

[kodabmx]

TDSL lists a UL Raa of 6k6 but cathode bias. I would guess in fixed bias 4k would be suitable which is the same for triode strapped operation.

The Hammond 1650NA is a good candidate. I'm using them with KT88 Class A triode strapped and I'm happy with them.

http://www.hammondmfg.com/1608A.htm

TDSL Tube data [EL34]

 

[PRR]

420V and 0.25A suggests total p-p load of around 3.4K, so stereo 6.6K loading.

420V and 0.25A is just over 100 Watts DC, which four EL34 can dissipate steady; which means self-bias is to be preferred for self-protection. (Sorry this conflicts with your goal to play with bias servo.)

For any speech/music application short of guitar amp, 420V and 0.25A will support the higher power 2Kpp loading (two 4K OTs) but must be run fix-bias (or servo).

The 2dB difference in maximum output may matter in a stage amp, or on a spec-sheet, but maybe not so much in domestic reproduction.

Any of these conditions can just as well be handled by KT66, 6L6GC, KT88, or any of the big bottles. It is not insane for eight total 6V6 (hey, you can have a V-8!).

 

[Rootz]

Thanks both for chiming in. Let me first clarify two things:
- the power transformers have multiple secondaries. 500, 550 and 600 Vct;
- this is for a hifi amplifier, not guitar;
- already mentioned, but repeated for the sake of clarity: fixed bias, but not with a servo perse.

That being said, I've drawn some load lines. With 420V plate and 1/2 and 1/4 Ra-Ra for a LL1663 much of the load lines are over the 25W plate rating of a EL34. I suppose loading the secondary with a speaker that frequently dips to 5 Ohms makes that even worse. Is that a problem?

I 'could' drop the B+ to a lower voltage tap. Or I could just rely on the fact that e.g. the A470 is 4k3 Ra-Ra and works perfectly fine for 2xEL34 UL. I guess the question still is if I should take into account that my speakers are not resistive and not 8 Ohms, but more like 6 Ohms.

By the way, power transformers are std EI96 cores. 6.3V @ 5A. Around 5 lb./2.5 kg. PRR are you suggesting to run 4 EL34's on one power transformer? To me it looks under sized for anything more than a pair of el34's, 6550's, e.g..

 

[Rootz]

Here's what the speaker impedance should look like. Not measured it yet to confirm these calculations, because I'm rebuilding the cross overs as we speak.