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Variable Transconductance tubes

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aldovan

Member
Joined 2005
#1
Friends

I see some preamp designs using transconductance valves a few times.
If I connect a variable transconductance pentode in triode mode, can it be used in audio, or will it generate a lot of distortion?

Thanks

Aldovan 😛
 
#2
I fell out with some tube rollers on Head-Fi over this ,I tried to explain that valves/tubes DESIGNED for RF/IF stages of radio/communications equipment are not meant to be used for audio .


It didn't go well ,even quoting all major UK/USA manufacturers of the time in radio formulae was not accepted by them .


They could not understand that the grids (for example ) are designed for varying RF signals and directing them to websites containing manufacturers engineering data did not persuade them.


I learned my lesson ,never again will I get into an argument on this subject as its impossible to reply to those fixed views of--- BUT ---it SOUNDS good.


You will gather I don't hold a positive view on this but don't tell me the manufacturers agree with them .


If that was the case an old IF valve like the 6k7 would be used in the

IF-AF-output saving the manufacturer $$$$ in build costs .
I tried to mention AVC/AGC (RF variety ) etc ----no that didn't work.


Read-


6K7 @ The Valve Museum
 
#5
audiowize said:
Head-Fi isn't exactly a technical forum and hard core tube rollers even less so.
Click to expand...


I was on it because I had Stax electrostatic headphones and I had modified their solid state energizer .


The website went downhill -IMO when it changed its website -kicked off those in business who helped the posters and made it hard to post (for me due to my safety set up browsers ).


I still get emails inviting me to London conferences where their own -handpicked business people sell their $$$$$$ products.


It took me a while for it to sink in that science wasn't top of their agenda and that trying to bring reality wasn't a good idea.
There was only one guy who was good ,he was a business man living in Iceland but he was "removed " .
 
#6
I can't remember which manufacturer it was, but I remember someone discovering that a resistor value had changed in someone's product. It may have been something like going from 27K to 22K. I don't even think anybody ever said what the resistor was even for, but that manufacturer was lambasted for having poor quality control, consistency, etc.

I don't remember there being any technical discussion about what the resistor did in that particular amp, why the new value was worse than the old one, or why the collective at HeadFi thought they should be in control of someone else's designs.
 
#7
You may get linear operation out of variable μ type, like the 6ES8/ECC189, at low signal levels, such as found in a MM phono preamp. The only other audio use I can conceive for such stuff is as the "top", current setting element, of a cascode. Just about anything will "work" as a cascode current setter. The transconductance of a cascode's "bottom" element is what determines gain.

Don't prod the "flat Earthers", as it's a complete waste of time. 😡
 
#8
The way grids are constructed in variable mu tubes meant for RF as a means of automatically controlling signal level .
Variable wire pitch (of grid ) is used allowing some electron flow as the bias moves towards cut-off the denser electron field areas of the grid cut completely off.


Only in more open areas ( of the grid ) which has less control determine current --bias up -less grid influence.


You still wont get me using them in audio but I have built a few communications receivers with plenty of them .

Primarily used in AGC stages ( of a receiver )--allows smooth AGC control without serious distortion (of the RF signal ).
 
#9
What Eli says. A BJT transistor is as non-linear as it gets... but if signal levels are well under 20mV it is fine. (Note the magic "26mV" number in BJT math.) Tubes have loss and run hot, the magik number is nearer 100mV at the worst, >300mV for "linear" tubes at happy supply voltage.

Most (not all) very-variable-Gm tubes have higher max Gm than "linear" tubes. If you have very small signals and want low-low-hiss, a vary-Gm may be interesting.
 
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