The Reference 6SN7 Thread

Aug 24, 2024 at 10:21 AM Post #10,261 of 10,668
Does it matter if the voltage sent to the heaters from the external supply is AC or DC? Or is it tube dependent?
The selection of AC or DC was made by the designer, usually on the basis of signal to noise ratio, as DC eliminates the effect of heater-cathode leakage. It should also be noted that many circuits elevate the AC heater supply above ground with a DC bias. Obviously, if you want to preserve the noise, distortion, and other factors, the external heater supply should replicate the circuit it is replacing, except, obviously, the new heater voltage. To do this, you need to understand how the unit works, and what you need to do to properly implement the change. This is a job for an experienced tech.
 
Aug 24, 2024 at 12:50 PM Post #10,262 of 10,668
The selection of AC or DC was made by the designer, usually on the basis of signal to noise ratio, as DC eliminates the effect of heater-cathode leakage. It should also be noted that many circuits elevate the AC heater supply above ground with a DC bias. Obviously, if you want to preserve the noise, distortion, and other factors, the external heater supply should replicate the circuit it is replacing, except, obviously, the new heater voltage. To do this, you need to understand how the unit works, and what you need to do to properly implement the change. This is a job for an experienced tech.
So what your saying is we shouldn't just connect the heaters to a car battery?:wink:
 
Aug 24, 2024 at 2:26 PM Post #10,264 of 10,668
Does it matter if the voltage sent to the heaters from the external supply is AC or DC? Or is it tube dependent?
I'd assume that DC would be less prone to generate noise.
As far as the points others have brought up… if you're using an external power supply for the heaters, aren't they essentially isolated from the rest of the amp circuitry now and doesn't that make how the internal heater power supply works irrelevant?
 
Aug 24, 2024 at 10:24 PM Post #10,266 of 10,668
Call back to several posts back re: 7N7 variations.

My original impressions between the "common" 7N7 and the oddball with smoked glass/chrome top found the latter to be superior. This was several years ago and several gears ago in a Cary preamp. My use of them now is in the output stage of a Lampizator DAC and having spent a couple days with the smoked and a good part of today with the common one I have to say I can't tell the difference in this application which, to my ear, is far more revealing than when in the preamp. So I became curious and looked closely at my small supply of 7N7 and now see more variation than I had initially noted. Poor pictures as a result of my general uselessness, sorry.

  1. Grey glass, chrome top, black parallel plates, rounded corner rectangle bottom mica, oval top mica, extra support rods and copper rods in the plates, too hard to see getter holder almost flat top.




  2. Heavy Chrome top, black parallel plates with rough finish, oval bottom and top mica, copper rods in the plates, too hard see getter holder, round top.




  3. Thinner chrome top with less "drip" down the sides, more refined looking black angled plates, dimpled square foil getter holder, round upper and lower mica, copper rods in plates, round top.




  4. Chrome only on the very top of glass, refined black angled plates, round "spiked" upper mica and round lower mica, halo getter holder, copper rods in plates, round top.


The reigning champ had been number 1 and "common one" I mentioned listening to at the start of this post is number 3 - so one parallel plates and the other angled. To be honest I'm not sure which of 2, 3 & 4 I had compared with 1 back when I had that preamp. I plan on checking them all out this weekend.

Oh, I vaguely recall briefly listening to a pair of 7AF7 I had back then (still have) with round black plates and disliking them. I may dig them out of the bottom of my drawer and give them a second chance too...

Cheers,
Robert
Don't get @bcowen started on 7N7.. almost forgot I have a few of those.
 
Aug 24, 2024 at 10:31 PM Post #10,267 of 10,668
So what your saying is we shouldn't just connect the heaters to a car battery?:wink:
The thing with a car battery is that usually the nominal voltage is higher than 12V - the voltage of a car battery is between 13.7-14.7V.
Very often power supplies for portable phones and games that are rated 12V put out a higher voltage which may drop under load to around 12V. It is advisable IMHO to use a regulated power supply that keeps the voltage constant.
 
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Aug 24, 2024 at 10:54 PM Post #10,268 of 10,668
Further to my last 7N7 local post, and apologies for dwelling on them, but between last evening and this one I've had at least a few hours with each of the four 7N7 types I have which I described in the earlier post. Plus 2 hours with a pair of 7AF7.

Before I start, just a note on my usage in the output stage of a Lampizator DAC which allows the use of a wide variety of power tubes run in triode strapped configuration, but also accepts the 6J5 family of tubes directly and dual triodes with a parallelized adaptor. So this parallelized usage in a DAC does probably contribute to a different sound character than may be found as a driver for a power tube or as part of a line stage.

As I mentioned earlier I found essentially no difference between #1 and #3, which surprised me given plate differences and others elsewhere generally panning the angle plates of #3. To my ear these are both some of the best 6SN7 I've heard, up there with the Sylvania 6SN7W I also own and the Melz 1578.

#2, which have the closest physical construction to #1, are an excellent tube with a lightly darker character. Not dark, just not as crystalline sounding on top and a very slightly warm midrange, particularly on male vocals. It's not an entirely different tone from the others, just very slightly subdued in comparison to the others here.

#4, looks to me likely to be the last version of the 7N7 due to its refined finish on plates and crisp small getter flash with a halo holder. You'll think I'm taking the easy way out saying this, but it's closer to the #1 and #3 than I can distinguish. I very much doubt I could tell any difference between these three on a blind quick-switch, nevermind guess which is which. To my ear these are equals.

7AF7
The getter flash is such that a picture wouldn't be of much value, much like the grey glass one where only a tiny strip of clear glass is available. However they are black round plates, no extra support rods and, unlike all the 7N7 here, the cathode rods are not copper in these, they appear shiney silver coloured.

This tube surprised me the most. When I tried them in my preamp a few years back I didn't care for them, though I don't specifically recall why. I suspect it was just bland, but I truly don't know. Note that the 7AF7 has a mu of 16 vs 20 for the 7N7, so slightly less output. It was noticeable but only just. What surprised me the most was just how crystal ball clear and transparent this tube is. The clarity is super and its a significantly more airy tube than the others here. Everything is just slightly tilted brighter. This is excellent with some material and a bit much with sunlit recordings. It never, though, was edgy or sharp sounding, just cristaline. I also found this tube was also just slightly forward sounding, independent of the increased clarity. It sounded just a little more urgent - sorry, that's a crappy description but it seemed to be true, perhaps lively is a better description.

ALL of these local tubes did very well with deep bass. It never seemed artificially enhanced like some KenRads I've used and it was textured, nuanced and punchy.
All had excellent presence and scored high in 3D realism, which is something that drew me in to the #1 (smoke glass) 7N7 a few years back and placed it as one of my top three 6SN7 tubes. I'm really surprised and pleased its apparently a family trait.

Cheers,
Robert
 
Aug 25, 2024 at 2:50 AM Post #10,269 of 10,668
All this talk about 7N7 has gotten curious me. Which version do you guys think that I have?

IMG_7369.jpeg

From my observation this has T plates and tall envelope like my Sylvania 6SN7W but without the support rod. Mica shape looks similar and there are 4 umbrella spokes from the top mica too.
 
Aug 25, 2024 at 12:08 PM Post #10,271 of 10,668
I'd assume that DC would be less prone to generate noise.
As far as the points others have brought up… if you're using an external power supply for the heaters, aren't they essentially isolated from the rest of the amp circuitry now and doesn't that make how the internal heater power supply works irrelevant?
As far as I know the internals of the amp work as they should with external heater power. All that you did is to disconnect the heater voltage from the amp and instead you are using an external source.
All you need is to ascertain that the current draw from the external power supply exceeds the current draw of the tube.
 
Aug 25, 2024 at 12:51 PM Post #10,272 of 10,668
As far as I know the internals of the amp work as they should with external heater power. All that you did is to disconnect the heater voltage from the amp and instead you are using an external source.
All you need is to ascertain that the current draw from the external power supply exceeds the current draw of the tube.
What about the heater pins on the main device? I realize they shouldn’t be seeing any load at all but is there any downside to just omitting those pins from the adapter altogether?
 
Aug 25, 2024 at 1:18 PM Post #10,273 of 10,668
All this talk about 7N7 has gotten curious me. Which version do you guys think that I have?

IMG_7369.jpeg

From my observation this has T plates and tall envelope like my Sylvania 6SN7W but without the support rod. Mica shape looks similar and there are 4 umbrella spokes from the top mica too.
Here is a Unicorn 14N7 that I don't remember seeing before:
IMG_2924.jpg

The markings on the glass are (nothing on the base):
MR
SA96
H 4 on top under the 14N7 designation
MR = Manufacturers Replacement. This dates the tube to 1942-45 when vacuum tubes were rationed and mainly going for military use. A small portion was allocated for civilian replacement use.
SA96 - don't know. Anybody?
H 4 looks like Sylvania codes and could be August 1944.
This is a tall tube and possibly the loctal equivalent of the Sylvania metal base 6N7W. Looks like T-plates and has a single copper support rod with oval bottom ractetrack mica. It sounds excellent!
 
Aug 25, 2024 at 1:32 PM Post #10,274 of 10,668
What about the heater pins on the main device? I realize they shouldn’t be seeing any load at all but is there any downside to just omitting those pins from the adapter altogether?
In my experience it is perfectly safe and I haven't noticed any untoward effects. But I do not have any technical knowledge and would suggest to check with the manufacturer of your device to make sure that it is safe.
 

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