Why are 12ax7 triode sections numbered 'backwards'?

More knowledgable people will most likely add to this, but IIRC, due to how the tube is constructed, section 1 (pin 6,7,8) are considered lower noise and therefore preferred (or something along those lines)

 

Fuzzy memory here, but I think it is related to humbucking filiments.

 

Because we can....

Seriously tho, I only commented cuz I'm curious about this too.

 

Yup, right. Found it in the data sheet. When 9, and 4+5 are used for heaters and the filament tranny center tap is connected to earth, 6,7,8 are more favorable with respect to hum

 

From the top V1A would be to the left front. Seems to be because the oldest gear tubes went in from the top. When Leo started building amps, first he put them in back; then he hung them from the top, so he could use the same chassis in heads and combos.

The pinouts are shown from below the chassis, because that's how they were wired. I guess.

 

I am not an expert in history of valves. Did the older iterations of dual triode such as 6sl7 share this phenomenon?

 

https://www.google.com/url?sa=t&sou...FjAPegQICBAB&usg=AOvVaw393q_WKWCkUbQoT4ZkxH0T

 

There seem to be all sorts of valve / tube data sheets there

https://frank.pocnet.net/

 

So why would a 12dw7 have the 12au7 side on the favorable side for hum? Seems counter intuitive.

 

Modern usag3 sometimes names pins 1,2, and 3 as the A triode. Here is a Fender schematic that does this....it confused me when I saw it and made me say Whadahey.....
https://schematicheaven.net/fenderamps/blues_deluxe.pdf

All of the old spec sheets indicate 6,7,8 as being the A section or section 1. That is interesting about the hum rejection. If that is so, then it seems one would want to use the 1st or A section for the earlier gain stage rather than as the second gain stage as in those Modern Fenders, correct???
The Bruce Zinky designed Pro Sonic uses the triodes in this manner...and labels them according to the old spec sheets. The B sections are used as the first gai; in the clean mode and as the first two gain stages in the Gain mode. https://schematicheaven.net/fenderamps/prosonic.pdf in a high gain amp tis might be an important consideration. But...the .blues/hot rod amps are based on the same basic approach as the Pro Sonic, and Fender uses the 1,2,3 triode as the first gain stages. So...does it really make no difference???.
Vintage .fenders used 1,2,3 as the first gain stage, as well. Again......whadahey?

 

10 9 8 7 6 5 4 3 2 1?

 

More important to consider:
Why does half the world drive on the left, the other half on the right?
Why do we have metric and imperial measurements (as well as US and British imperial).

Seems the human brain is too large for requirements, so dumbs itself down to compensate.

WRT to this post, possibly just some guy sitting at the drafting table, who could chose whatever.

 

I have noticed the shared pentode/triode tubes have the triode on what would be the "A" side. The triode would usually be first in a circuit. Food for thought.

 

Rocket science.

 

Simple! That's it! Thank god it's not music theory!

 

King Fan, I suppose the only way to ascertain if there is a preferred order for those triodes is to note operation in one arrangement and then change the input preamp stage to other triode. I am going to think that there may be no difference. I checked the Mesa Boogie Dual Rectifier, which is a ‘clone’ of the Soldano SLO circuit. Mesa uses the B triode...pins 1,2&3...for the first stage of the clean mode and also uses the B triode of V2 for what is the second gain stage in the high gain mode. Of course, Mesa labels those triodes as the A triode, which is contrary to the old school thought. There are no PIN numbers or triode labels in the schematics of the SLO that I have seen. The Peavey 5150 labels the triodes A and B but does not designate PIN numbers. I would think that IF there were noticeable differences in noise levels, these high gain amps would use the B triode...pins 6,7,8....in the earlier stages??? But....then there is Zinky and the Pro Sonic, right? Why did he go against the grain? I don’t know.
I know enough to be confused...for a second....and then just walk on down the path. My main concern is the modern thought that decades of established terminology must needs be overturned. Many years ago I was confused as to why the engineers who designed these tubes labeled the triodes A and B in the order that they did...but I accepted it. Then I started seeing modern amp designers change the engineering terminology that the designers of the tubes used.
I have a Bogner Telos open on the bench. I think I will see how he went about using those V1 triodes. Fwiw, this is one well-built PCB amp.

note the parallel heater filament supply. And...though you cannot see it, the first two 12AX7s are supplied with 12 volts.