a.c. heaters

[abbey road d enfer]

1:10 is very common in tube designs. It is the only way to achieve a reasonable noise figure.

OK, that makes sense. I am so used to thinking of regulated DC heaters -which leave you with the irreducible noise of the first valve- that it wouldn't occur to me to use more than 6dB of step-up. Not so for traditional AC-heater designs where you have hum to think about too.

Anyway, using a high step-up input xfmr also optimizes the contribution of the intrinsic valve noise. I don't see any believable justification for using a 1:2 or 1:4 xfmr vs. a 1:10. The only limit there is the HF response which suffers from the higher leakage inductance. The practical limit seems to be about 1:12, although there have been examples of 1:14.

 

[ruffrecords]

1:10 is very common in tube designs. It is the only way to achieve a reasonable noise figure.

OK, that makes sense. I am so used to thinking of regulated DC heaters -which leave you with the irreducible noise of the first valve- that it wouldn't occur to me to use more than 6dB of step-up. Not so for traditional AC-heater designs where you have hum to think about too.

The irreducible noise of vacuum tubes, even with dc heaters, is high enough that a respectable EIN is not achievable without an input transformer ratio in the region of 1:10.

What surprised me about the tests with ac heaters was that figures as good as with dc heaters were obtained.

As you know, the Lindos MS10 I use for noise measurements meets IEC268 ,which can give noise measurements as much as 8dB higher than more common rms measurements. The Lindos measurement of -121dBu EIN with ac heaters could arguably be -129dBu rms which would be an exceptional result.

Cheers

ian

 

[PRR]

> That's 20dB of step-up even before you start!

Mike hiss is say 0.2uV.

Transistors "can" get down in this part-uV zone.

Tube hiss is closer to 1uV. Unless we do something heroic, like a TV-tuner tube running 13mA.

1:7 to 1:15 are usual ratios in tube-work. The higher ratios are either PA-quality (limited/ringy top end) or come from very clever designers (Beyer for one).

OTOH there are studios with all high-level condenser mikes and any darn ratio is ample.

 

[emrr]

What surprised me about the tests with ac heaters was that figures as good as with dc heaters were obtained.

What has always surprised me, as you may have noticed, is people feeling the need to use DC heaters.    I keep running all these antique tube amps with AC heaters, and never hearing any hum, and no one believes me. 

 

[ruffrecords]

What surprised me about the tests with ac heaters was that figures as good as with dc heaters were obtained.

What has always surprised me, as you may have noticed, is people feeling the need to use DC heaters.    I keep running all these antique tube amps with AC heaters, and never hearing any hum, and no one believes me.

When I first started designing tube mic pres over ten years ago I began using a triode strapped 6AU6 (some old BBC mixers used this configuration). I had dreadful problems with hum and finally had to resort to powering the heaters of my test rig with four AA batteries. Since then I have always used dc heaters. It is only now with ten years experience under my belt and some designs I know work well that I have fortuitously tried ac heaters again and as you know been pleasantly surprised. Consider me a convert.

Last night I was listening to a CD of tracks from the early days of Sun studios. Several tracks had breaks in them. Putting my ear right next to the speaker I could hear some hiss in these breaks but not a trace of hum.

Cheers

Ian

 

[DerEber]

What surprised me about the tests with ac heaters was that figures as good as with dc heaters were obtained.

What has always surprised me, as you may have noticed, is people feeling the need to use DC heaters.    I keep running all these antique tube amps with AC heaters, and never hearing any hum, and no one believes me.

I do  ;D