Tube Square-Law, what and how to set up?

[ervinl]

Hi,

What is square law in tube amp/pre amp stage?
How can we get range/region of square law for some known valves, e.g. 5687, 7119, 6072, 12AU7 etc? I can not determine this in the datasheet/load curve.

And what is the effect on sound signature, distortion etc?

Thanks,

Ervin L

[Poindexter]

May I ask you to show me this tube square law? I've been doing this for quite a long time, have a couple of degrees in this stuff, and I have no idea what you are referring to.

Aloha,

Poinz
AudioTropic

[Miles Prower]

Originally Posted by ervinl

What is square law in tube amp/pre amp stage?

Idunnow. The only square law device I'm aware of is the FET. VT's are more like one-and-a-half law devices.

[aardvarkash10]

the only ^2 law I have come across involves the rate at which $ get spent.

COme to think of it, its actually logarithmic...

[Globulator]

Originally Posted by aardvarkash10

the only ^2 law I have come across involves the rate at which $ get spent.

COme to think of it, its actually logarithmic...

Yes I think it is logarithmic, with odd peaks of noise usually centred around hi-fi shows or DIY articles. Quite a controversial issue with wives and girlfriends too sometimes.

Also in general valve stuff at shows tends to be about 5-10x more expensive than solid state, and as a rule the more you regress technology toward the 50's (transformers, tubes, paper cones, alnico etc) and the less components used the more the price increases too.

[DF96]

Some valves, such as ECC82/12AU7, do have a part of their gm characteristic which is a reasonable approximation to square-law. EC90/6C4 too (=half an ECC82), which is why it is used as an RF mixer.

Look on the data sheet for a plot of transconductance versus grid voltage (not all sheets have this). Pick a bias point where this is roughly a straight line. Use a very low anode load resistor - much smaller than usual, as you want to avoid any linearising feedback from the anode voltage.

[kenpeter]

Schade claimed it so for RCA 6L6 at the very least.

Total idiot's speculation here, no facts involved:
I suspect its carefully shaped remote cutoff of Gm?
May or may not be square in the larger sense, but
only what happens in AB crossing really matters.

I don't know how faithfully this feature is copied in
off-RCA-brand clones?

[DF96]

Remote cutoff valves have a roughly exponential response - just like a BJT!!

The OP didn't say what he wants a square-law response for. An effects box, or a communications receiver mixer? These are the only useful things to do with it.

[kenpeter]

Browse this page: Tube Data
Then find RCA "Beam Power Tubes" By OHSchade 1938

See Page 168 of the original document, or page 38 of PDF.

[kenpeter]

For tube without square law, you can reshape Gm
(reduction by NFB) by placing a square law diode
under the cathode.

A stack of Schottky diodes, FET or small MOSFET
that has been strapped as a diode. These have
square law function. But you need make sure the
square law diode function is scaled appropriately
for your quiescent current. A diode too big won't
curve the signal enough to matter.

I like Schottky, no recovery glitch as it turns on/off.

See Post#97 this thread:
van Dorn's Octode design - recommendations for a 10W/ch version