basic valve questions

[TobWen]

Hi there,

I've got some basic valve-questions.
Of course, I have searched the board, but I got various answers:

What's the best DC-voltage to power a ECC83 / 12AX7?

Some guys are writing 210V, some says 280V and even some power them with 350V.

How much ampere needs a 12AX7 maximum? About 0.3 A at 280V?

What's needed for heating? I think, 6.3V AC is best?
How much ampere does it need for heating? About 1.8 A ?

Best regards,
Tobias

 

[bcarso]

Check out the wiki: http://en.wikipedia.org/wiki/12AX7

There is a link to a Sylvania datasheet at the bottom as well.

 

[DerEber]

It helped me a lot to read this articles about tube loadlines:

http://www.audioxpress.com/resource/audioclass/index.htm

You will be able to figure it out by yourselfe, depending on your needs.

 

[PRR]

> What's the best DC-voltage to power a ECC83 / 12AX7?

You are starting with an answer.

12AX7 may not be the "right" tube for your job.

Start with the question. What do you need a tube to do?

How much gain? How much output voltage? How much output current? How much noise can you accept? How much money do you have?

> What's the best DC-voltage to power a ECC83 / 12AX7?

What is the "best" voltage to power a 60K 1W resistor?

12AX7 work very well with supply voltage below 90V and over 400V. The choice depends on what the tube must DO, and also of course what is handy.

> How much ampere needs a 12AX7 maximum? About 0.3 A at 280V?

Not even close.

> What's needed for heating? I think, 6.3V AC is best?

Many ways to heat.

Figure out what you need the tube to DO. Find some similar known-good plan, and study it, even steal it.

 

[analag]

EDIT

 

[TobWen]

[quote author="PRR"]Start with the question. What do you need a tube to do? [/quote]

I'm doing a refresh design of a discontinued device for ProPro.
Since it uses a 12AX7, I need to keep it.

[quote author="PRR"]How much gain? How much output voltage? How much output current? How much noise can you accept? How much money do you have? [/quote]

Okay, according to this fact, I have to do some reading.
The datasheets helped me a lot already.

[quote author="PRR"]12AX7 work very well with supply voltage below 90V and over 400V. The choice depends on what the tube must DO, and also of course what is handy.[/quote]

So the 12AX7 does other jobs on different voltages?
I thought, the harmonics/distortion and sound is changes with different voltage?

@90V, there is pretty much distortion (4-5%)
@300V, the distortion is pretty much lower (1%)


[quote author="PRR"] How much ampere needs a 12AX7 maximum? About 0.3 A at 280V?

Not even close. [/quote]

Much less, as I've read from datasheets. Nice device

[quote author="PRR"]Figure out what you need the tube to DO. Find some similar known-good plan, and study it, even steal it.[/quote]

Okay, I will find it, study it, mix it and ask here gain

 

[PRR]

> I thought, the harmonics/distortion and sound is changes with different voltage?

Think: Ratio of signal voltage to supply voltage.

> @90V, there is pretty much distortion (4-5%)
> @300V, the distortion is pretty much lower (1%)

At what Signal voltage????

If supply is 300V and signal is 0.1V and the circuit is good, THD will be <<0.05%.

Actually I can guess your signal voltage from the above info. A well-used resistance-loaded triode can make a peak signal of 20% of its supply voltage before clipping or THD>5%.

THD falls as the Ratio of signal voltage to supply voltage falls, and nearly in proportion.

> @300V, the distortion is pretty much lower (1%)

At 300V you should be able to swing 60V peak @ 5%THD, or about 12V peak at 1% THD.

Is that approximately the signal you were testing with?

> I'm doing a refresh design of a discontinued device for ProPro. Since it uses a 12AX7, I need to keep it.

But you don't know the original supply voltage?

If in doubt, if power is available, if you want large signal with low THD, 250VDC is usually a good starting place.

300V will give 20% more peak signal OR 20% less THD (from 1% to 0.83%), which is not very different. Still we often use 300V-400V just because it costs little more than 250V. Past 400V, standard 450V electrolytic filter caps make costs rise suddenly, so we rarely do that. Also the 300-330V maximum steady voltage on 12AX7 means that a supply higher than 450V-600V is too high to use safely.

"Sound" scales with ratio of signal voltage to supply voltage, and circuit balance, not with absolute supply voltage. A triode is a very linear thing, and "linear" here means proportions stay the same at any voltage. In prcatice, most small vacuum triodes have a bend near 50V, and may lose some control past 500V. Still, AM radio audio volt amps ran fine on 90V supply because they only had to deliver 5V peak to the power tube grid (and 1%-2% simple triode distortion is not a big deal).

 

[Gordy]

[quote author="TobWen"]Hi there,
I've got some basic valve-questions.
[/quote]

Tobias, I too have recently started to investigate vacuum tubes.
Here are some sites that you may find of interest:
http://www.aikenamps.com/ (see tech info)
http://www.valvewizard.co.uk/ (pre amp stages)
http://www.pmillett.com/technical_books_online.htm (...keep you busy)
http://www.tubecad.com/ (...also keep you busy)

Good luck,
G.

 

[Nishmaster]

Aiken Amps is indeed a great resource, especially for guitar amp specific theory.

Another broader resource can be found here:

http://mysite.du.edu/~etuttle/electron/elect27.htm

 

[Gordy]

If anyone is looking to build up their schematic library, this fella seems to have harvested a lot of tube power amp circuits...
http://www.simpletube.com/databank/data_down.htm