Tube Gurus please advise

pitdogg2

Would you consider using a 12sn7 in place of a 6sn7 ? Some seem to think they (12sn7) sound better in the mid-range and top ranges. I can't get past if it was made for 12 volt's then how does it work getting half to voltage?

I'm perplexed by this, any thoughts?

heiney9

Have no idea how a 12v tube would work in a 6v system. I surely wouldn't try it unless I was 100% sure. Might damage the power supply components since the tube is trying to draw twice the voltage and who knows how that affects the current draw.

Probably have to modify the circuit a bit to use it, but I can't say for sure.

Use caution

H9

mhardy6647

Powering the heater with "6 volts" (typically 6.3 VAC) will result in low (probably very low) emission. It shouldn't hurt anything to plop a 12SN7 in, though -- it can only use the voltage that's there (it can't try to "over-draw" voltage), and, thanks to Ohm's law, the filament will draw half the current at 6V than it would at 12. Look up the specs on the two
tubes and you should see the filament current for the 12SN7 is rated at half that of a 6SN7. I'd paste 'em in here, but I have do do somehting else at the moment, sorry

"12 volt" (12.6 VAC, typically) filament voltage would have to be supplied to a 12SN7 "for best results". I cannot imagine one heated at 6V would work very well at all... but, no, I haven't tried it! Do you have a reference for someone who has?

By the way, a few volts low on filaments is generally OK (which is why, e.g., some folks have used 8BQ5 as a "cheap" substitute for 6BQ5/EL84).

On the other hand, a tube like the 12AX7 has a center-tapped filament, which allows it to operate on either 6 or 12 volt filament supply voltage, depending on whether the filaments are wired in series or parallel.

marvda1

i'm thinking you have to use a socket adapter.

pitdogg2

marvda1 wrote: »

i'm thinking you have to use a socket adapter.

socket is the same for both octal's

and I'm right there with you Brock. Here is where I first seen it and yes I know it is eBay. I may research it to see like Doc just haven't had the time and to try and read lots of stuff on the phone is tedious at best.

https://www.ebay.com/itm/12SN7GT-Matched-Pair-JAN-CHY-HYTRON-SYLVANIA-1951-RARE-NOS-PAIR-SUB-6SN7GT/323391856165?hash=item4b4ba81e25:g:NzgAAOSwZ~hbDtgi&_sacat=0&_nkw=12sn7&_from=R40&rt=nc&_trksid=p2380057.m570.l1313.TR12.TRC2.A0.H0.X12sn7.TRS0


seems there are some good tubes to be had cheaper than the same tube in 6sn7.

heiney9

Ivan, do your research as I don't believe anything I read in an Ebay sale. It probably will work, as I trust Doc Hardy's analysis, but it may not sound good or as good as if the tube was in a proper 12v circuit.

I suppose if the cost is cheap enough experimentation is the best option.

H9

pitdogg2

yes I will Brock. just thought I'd check first with those much more knowledgeable than myself.

[Deleted User]

"On the other hand, a tube like the 12AX7 has a center-tapped filament"
I believe that is true with the 12AX7A, but not the 12AX7. And operating a filament at reduced voltage will significantly reduce filament life.

heiney9

12AX7: the original version of this tube. This can only be used in parallel filament circuits. This is not a big deal as virtually all audio equipment is of a parallel filament design. These often have large rectangular plates with several horizontal ribs. The older versions have blackplates, which are often preferred by audiophiles.

12AX7A: This version can be used in series or parallel filament circuits. These usually date to the 1960s and have greyplates


Copied from Brent Jesse's website.

[Deleted User]

For what it's worth department, I prefer filaments connected in series in groups of three. When I select tubes I have to measure the filament resistance of each tube so the voltages balance. You'd be surprised how much difference there is from one tube brand to another. I find filament supplies are very important to the noise level.
I believe there is a connection between filament supplies going from 6 to 12 volts and automobile electrical systems making the same change.

mhardy6647

Re: the 12AX7 I did not know that. Thanks, gentlemen!
My own fault for throwing in an irrelevant aside to make it look like I knew something (that I didn't).

So... let's get back to the 6SN7/12SN7

Here's the datasheet for a 6SN7GTA:
http://www.tubebooks.org/tubedata/HB-3/Receiving_Tubes_Part_2/6SN7-GTA.PDF


The filament (more accurately, "heater") is rated 6.3V (AC or DC) @ 0.6A

Here's the "datasheet" for a 12SN7GTA, in its entirety
http://www.tubebooks.org/tubedata/HB-3/Receiving_Tubes_Part_2/12SN7-GTA.PDF

mhardy6647

Tell ya what -- I know there are some 12SN7s downstairs. I'll plop one in my trusty Sencore Mighty-Mite III* and measure the emission at filament voltages of 6 vs. 12 volts.

Just because I happen to have one handy here's a picture of a 6SN7 in the Mighty-Mite.

DSC_0623 (2) by Mark Hardy, on Flickr
_____________________
* The Mighty-Mite is a simple, basic emissions-type tester. It is useless for, e.g., matching tubes, since it cannot measure transconductance and it doesn't test tubes at anything like their typical operating points, but it does measure electron emission, which will be the primary variable as a function of applied heater voltage

mhardy6647

Jeepers! I guess I am gonna have to mail my tube guru credentials back to the Acme Tube Guru School™

So, I dug out a 12SN7GT from some box in the basement and tested it at the proper filament voltage. Emission was "good" @ ca. "93%".




Dropped the voltage to 6 and let it 'cool down' (so to speak) for a while.
Result? Emission only dropped a few percentage points, to about "87%"!




Well, I'll be.

So... I guess it could work -- and (last time I looked, at any rate) 12SN7s are cheap. Ken's advice is well taken, though... but, of course, if the tubes are cheap enough, shortened lives won't matter too much (unless they're really short!).

PS Not shown, but I also checked both triode sections (emission and grid leakage) and checked for shorts at both voltages; all was well.

heiney9

Excellent work Doc!

I wonder if 12SL7's could be used in 6SL7's place? Lots of them on Ebay. Although most 6SL7's don't cost an arm and a leg so I'm not sure there'd be must cost savings like with the 6SN7/12SN7.

pitdogg2

what do you know first thing that popped up....

http://forum.polkaudio.com/discussion/138031/12sn7-vs-6sn7


these are a good read

http://www.fourwater.com/files/hist6sn7.txt


http://www.effectrode.com/signal-tubes/the-6sn7gt-the-best-general-purpose-dual-triode/



12sn7 was made for vehicles and their 12v systems.

I'll just pass on these 12sn7 tubes.

thanks guys..