I've had instances where I've gotten radio pickup and noise from cell phones or wifi routers, which caused signal degradation either from rf 'detection' (not sure the correct terminology) or maybe from intermodulation distortion into the audio band.
So now I use an array of precautions by default: No cables that are not well shielded, star quad speaker wire (I have feedback from the output transformers, so the output is also an input), ground lift circuit, and small rf shunt caps from signal ground to chassis ground at inputs.
I'd put grounding the unused elements in a tube in the same category of 'best practice', that is not worth the effort of determining if it makes a difference or not in the individual case.
So now I use an array of precautions by default: No cables that are not well shielded, star quad speaker wire (I have feedback from the output transformers, so the output is also an input), ground lift circuit, and small rf shunt caps from signal ground to chassis ground at inputs.
I'd put grounding the unused elements in a tube in the same category of 'best practice', that is not worth the effort of determining if it makes a difference or not in the individual case.
Floating metal is exactly what makes it an antenna. Sometimes unconnected metal can be more of an issue than metal connected to your circuit. You definitely don't want floating metal anywhere near an audio circuit if you can avoid it (and you can almost always avoid it).
But I did not want a higher gm, it introduced a huge amount of microphonics on my design and whatever measures I took
(and trust me I am not a beginner), I only got rid of it when I separated that second triode. Calculations
afterwards did show that I did not need that extra triode in parallel.
And I still could have them in parallel by increasing the cathode resistance but.... anyway... I did, in the end, not went that way.
But OK, I am wasting a triode but having to re-calculate the whole DC operating point. No that I did not want.
Not THAT much worried by antennas, although certainly they approach wavelengths used in ubiquitous Cellphones/WiFi/Bluetooth etc. which did not exist in the 40´s/50´s when these tubes were invented, but certainly having floating large metal parts inside glass envelopes, high vacuum conditions, with electron clouds floating around and strong fields present scratch me the bad way.
Plus grounding them is easy peasy, can´t find any justification for not doing so.
Plus grounding them is easy peasy, can´t find any justification for not doing so.
That is a good argument (my opinion) when dealing designing an IM frequency amplifier but is that valid for designing an audio amplifier?
JMFahey wrote:
"Plus grounding them is easy peasy, can´t find any justification for not doing so"
Alright, but in the start of this thread I just wanted to know if there were any arguments to "dead" this second triode.
"Plus grounding them is easy peasy, can´t find any justification for not doing so"
Alright, but in the start of this thread I just wanted to know if there were any arguments to "dead" this second triode.
I would decrease values of both anode and cathode resistors to get the same amplification factor.
It won't. However, the charge build up on the floating metal parts will worsen distortion performance. That and RF pickup are the reasons to ground the floating parts.
Or you could use tubes with the same gain and the same quality that have only one triode inside them and that also cost much less.