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I have had an amp builder (who shall remain nameless) try to convince me that, when using only one triode of a dual triode such 12AU7/12AX7, with the filaments run in 6.3 parallel, it is necessary to use pins 6,7 and 8 to reduce hum.
In other words, he claims using pins 1, 2 and 3 (as opposed to pins 6,7 and 8) will induce hum.
A 12A_7 has two identical triodes, so this makes no sense to me.
Am I nuts, or is the amp builder nuts?
Follow Ups:
Perhaps you could ask your builder how he arrived at that conclusion, and whether he has a hypothesis to explain it. It does beg the question as to whether the AC signals induced onto the cathodes by each side of the filament are necessarily in phase. It seems like a simple thing, but I've never tested for it.
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Buy Chinese. Bury freedom.
Here is a quote from the builder:
"Please check any tube chart and it will tell you that a 12ax7 run on
6.3v/parallel must be hooked up to pins 6,7,8 to reduce hum. Not your
idiotic suggestion for saving tubes."
You can tell I am not doing any business with this builder.
I have other quotes if someone is looking for a giggle.
Let me say, first, that nearly all my DIY work is with octal-based triodes. As a result, I'm not familiar with all the idiosyncracies of the 12AX7 series. However, a quick Google search turned up a reference to his exact thing in the AX84 forum. I'll leave it to you to research this further, or perhaps someone here is more familiar with it.
http://www.ax84.com/bbs/dm.php?thread=209122
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Buy Chinese. Bury freedom.
That link does make a little sense; however, it does not apply to all 12A_7 types.
I have been examining a number of different 12A_7 types (new production and vintage) and, in the VAST majority, the wiring for the #9 pin runs closest to 6, 7 and 8 because the plates are oriented "parallel" to pin 9 (along the 9-6 and 1-4 planes).
I do have a Telefunken ECC83 smooth plate, a Philipps Miniwatt ECC83 and an older Ei ECC82 where it appears that the wiring for pin #9 may be closer to 1, 2 and 3 because the plates are oriented "perpendicular" to pin 9 (along the 1-7 and 3-6 planes).
So, if that article is correct, in most cases you would actually want 1, 2 and 3 instead of 6, 7 and 8 (it would all depend on what tube you are using).
I believe the concept being discussed has to do with capacitive coupling between one of the two grids and a filament wire carrying 60Hz. A more important point at the moment is that the builder is clued in on this detail of 12AX7 operation. He knows something that those judging him here do not, and you may have written off someone whose experience is valuable.
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Buy Chinese. Bury freedom.
Knowing something and knowing what to do with it are two different things.
Another one of his quotes:
“I am an ear person, not an engineer type.”
While he quoted 12AX7, the amp in question used a 12AU7.
Also, That means no matter what, using one half of a tube or both halves, that one side is inherently quieter than the other. I do not think that tests out very well.
Extremely easy to prove with a simple test.
Hum would probably be due to improper ground schemes or wiring issues.
You could parallel triode sections for a lower Rp, but the "choir effect" might come into play if you have high EF speakers.
Your builder likely has run into a situation where in his efforts to kill a hum, made a swap like this and it worked. It was likely a situational solution that involved many possibilities like a cold solder joint, wire routing, tube location, etc. that combined to kill the hum. Once it worked, he declared a theory that the cause of the hum was the particular side of the bottle and he as stuck with it. I do not have similar experience using one side or the other.
More importantly, why does he want to light the unused side of the tube when he has a split heater? Just light the one side you are using. If one channel uses the left side triode and the other uses the right side triode, when a tube burns out you can swap the tubes between channels and get two free new tubes out of the deal. If you just burn up the unused side, you get no benefit at all.
~!
The Mind has No Firewall~ U.S. Army War College.
My point to him exactly (after which he called me something akin to "uneducated nuts").
Once you pick a builder, you kind of have to let him do his thing. Some (most) get really miffed if they perceive you as a backseat driver. It may be time for you to open a door and try building stuff on your own. It can be very rewarding.
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