Puzzling problem with 6SN7 adapter

[Zimmer64]

The noise could also be the tube itself. Have you tried another one? I had some NOS russian 6SN7 that were noisy. I was looking for all sorts of technical issues until I replaced them with Sylvanias from the mid 50's, which solved the problem for me. Just a thought as I dont have a crack, but another preamp using 6SN7s.

Best of luck

Michael

 

[w0lfd0g]

Ok, now it's my turn to have some problems with the tube adapter. I don't know where to post this, so Adam, forgive me if this post bothers you.

I got a Chinese adapter of which I took photos and posted above the page. As you all know, the Crack has plenty of power, so you'll have no problem to reach a preferable level before it's too loud at approximately 9 o'clock position ( the minimum volume is at 7 ), and me too. Today, I inserted my 6SN7 Raytheon VT-231 into the Crack using the Chinese adapter. The initial impression was amazing, dynamic and fantastic sound, no hum at all. But when I shut down my DAC and turned the volume pot to about 10.30 to 11 o'clock position ( which is far beyond my reference volume level ), the hum started to appear. It varies on the volume level, the louder the volume is, the louder the hum. But when the volume pot reached about 3 o'clock, it's dead silent again, no hum. So as long as I turned my volume of my PC ( DAC ) to max and turn the Crack's volume pot to about 9 o'clock, it will be fine. But this hum bothers me a little bit.

I tried my E80CC back to my Crack and it's back to silent again. So I assume this was due to the fact that the Crack is not designed for using 6SN7? Because of higher gain or heater current of the 6SN7's?

Thanks a lot!
Kratos.

Hi Kratos

I have the same behaviour exactly with my adapter.  When using 6SN7 tubes I set the Crack volume to maximum and control the volume through my preamp.

 

[adamct]

The noise could also be the tube itself. Have you tried another one? I had some NOS russian 6SN7 that were noisy. I was looking for all sorts of technical issues until I replaced them with Sylvanias from the mid 50's, which solved the problem for me. Just a thought as I dont have a crack, but another preamp using 6SN7s.

Best of luck

Michael

I've tried 5-6 different 6SN7s.

 

[Aeolus Kratos]

The noise could also be the tube itself. Have you tried another one? I had some NOS russian 6SN7 that were noisy. I was looking for all sorts of technical issues until I replaced them with Sylvanias from the mid 50's, which solved the problem for me. Just a thought as I dont have a crack, but another preamp using 6SN7s.

Best of luck

Michael

Hi Michael,

I have tried several 6SN7 tubes, and the noise is still there, perhaps it's slightly different depending on tubes, but just very slightly. All the noise is very noticeable and unacceptable. As soon as I turned the volume pot to about 11 o'clock, it started to hum no matter what tube I was using.

Hi Kratos

I have the same behaviour exactly with my adapter.  When using 6SN7 tubes I set the Crack volume to maximum and control the volume through my preamp.

Hi wOlfdOg,

It's weird, because I am the opposite to you. I set the 'preamp' volume ( Foobar volume ) to maximum and then control the volume through my Crack. So I just needed to turn to about 9 o'clock to reach my preferable volume level.

I guess I'll clean up the tube pins and try it again.

So we have possibilities of the tube or the adapter.  I'll throw in another variable, short the inputs with jumpers or shorting plugs before testing hum. 

It could be the interconnects.  It could be that it was there but the new tube brought it out.  So you need to eliminate anything before the Crack.

Hi Grainger,

The hum was still there even without the source. So it can't be the interconnects. As soon as I turned the volume pot to 11 o'clock, the hum appeared.

Best,
Kratos.

 

[w0lfd0g]

Hi Kratos

I have the same behaviour exactly with my adapter.  When using 6SN7 tubes I set the Crack volume to maximum and control the volume through my preamp.

"Hi wOlfdOg,

It's weird, because I am the opposite to you. I set the 'preamp' volume ( Foobar volume ) to maximum and then control the volume through my Crack. So I just needed to turn to about 9 o'clock to reach my preferable volume level."

This is what I would normally do, except when this dodgy adaptor.  My method results in no hum.  A workaround.

 

[Paul Birkeland]

Also, what did you mean by wire the Crack to use 6CG7/7AU7?

If you break the connection at pins 4/5, then move the black wire from pin 9 to the unoccupied pin (4 or 5) on the 9 pin socket, then you can use the 7AU7.  This will also allow you to plug in a 6CG7, which is roughly equivalent to a 6SN7.

-PB

 

[adamct]

Thanks, PB. I think I'm going to leave it as-is. I'm very happy with how my Crack sounds with 12AU7s. The 6SN7s were an experiment. If the next adapter doesn't work, so be it.

Unless I decide to build a separate power supply for the 6SN7...

 

[Paul Joppa]

The volume control behavior suggests that there is some electric field coupling capacitively to the grid of the 6SN7. When the control is at either end, the impedance to ground is low so the high impedance of the capacitive coupling is shunted to ground - but at maximum impedance (around 3/4 of the way up for a logarithmic volume control) that interference does create an audible grid voltage.

My guess as the likeliest culprit is a grid wire inside the adapter is too close to a heater wire.

If anyone has a pile of adapters including the reverse (use 12AU7 in a 6SN7 socket), you could stack two so you test the 12AU7 with and without adapters, to prove whether it is the adapter or not. A 6-volt wall wart power supply feeding DC to the 6SN7 heater would also determine whether the heater power is the source. These are diagnostic suggestions, not long-term solutions of course.

 

[w0lfd0g]

The volume control behavior suggests that there is some electric field coupling capacitively to the grid of the 6SN7. When the control is at either end, the impedance to ground is low so the high impedance of the capacitive coupling is shunted to ground - but at maximum impedance (around 3/4 of the way up for a logarithmic volume control) that interference does create an audible grid voltage.

My guess as the likeliest culprit is a grid wire inside the adapter is too close to a heater wire.

If anyone has a pile of adapters including the reverse (use 12AU7 in a 6SN7 socket), you could stack two so you test the 12AU7 with and without adapters, to prove whether it is the adapter or not. A 6-volt wall wart power supply feeding DC to the 6SN7 heater would also determine whether the heater power is the source. These are diagnostic suggestions, not long-term solutions of course.

In my mind it is definitely the adaptor.  I have tested thirty odd 6SN7s with the same outcome in each case.

 

[2wo]

Have any of you considered installing an Octal socket? Not hard to do.

If I was considering the 6SN7, and I would. That's what I would do. Give it a fair chance.

We can help...John 

 

[corndog71]

Have any of you considered installing an Octal socket? Not hard to do.

If I was considering the 6SN7, and I would. That's what I would do. Give it a fair chance.

We can help...John

No kidding.  I would've changed the socket if I were going to test 30 tubes.

 

[w0lfd0g]

Have any of you considered installing an Octal socket? Not hard to do.

If I was considering the 6SN7, and I would. That's what I would do. Give it a fair chance.

We can help...John

No kidding.  I would've changed the socket if I were going to test 30 tubes.

Hah - Didn't buy the tubes to test the Crack.  Had the tubes anyway.  Thanks for your suggestion though.

 

[adamct]

Adapter #4 was the worst of them all in terms of aesthetics (ugly as sin), build quality and usability. It took me about 40minutes just to get one of my 6SN7s into the adapter, because either the holes are slightly too small, or the pins are too tight.

Once I got the 6SN7 inserted far enough, I plugged it into my Crack. Same problem as with the others. Four adapters, and far too much money and frustration later, I'm done. I have plenty of 12AU7s and related variants I can use in the Crack, and I love the way they sound, so I don't even know why I went down this 6SN7 detour. I'll save my 6SN7s for my other amps.

Yes, I know that putting in an octal socket would be fairly easy, but its just not worth the effort to me. I'll stick with the stock socket and 12AU7s. Now I just need to spend another hour or so trying to get my 6SN7 out of that #%^

 

[Paul Birkeland]

You can heat the driver tube with a DC supply, and your socket noise issues should go away.

-PB

 

[captouch]

So no one has found an adaptor to use 6SN7's in the 12AU7 socket with no added hum/noise, huh? 

If anyone has, a link would be appreciated.

 

[hopeful]

Some folks do claim that they have noise-free 6sn7 adapters, but my experience has been the same as Adam's.  Granted, I gave up after two failed attempts. 

My second adapter (from lowther), was way better than the first but still noisy enough that my ears felt considerable relief when I switched back to a 12au7.

It may also be that the frequency of the noise is a lot more audible to those of us who primarily use our cracks with hd800's.  When I have a chance, I'll bust out the better of my adapters again and try it with my hd650's.  Their polite trebles may well serve to mask most of the noise.

 

[captouch]

Thanks Mike.  I have Hd650's and may have a chance to get some 6SN7's for cheap, but I'd prefer not to spend the $15-20 on the adaptor if it's just going to be noisy.

 

[Grainger49]

It sounds like that if you like the 6SN7 over the 12AU7 you should put in an octal socket and convert.  There are others who have done this and have had no noise problem.  After all, all wiring is under the chassis plate and soldered.

Then maybe a 6SN7 to 12AU7 adapter will be quieter.

 

[Paul Birkeland]

I'll repeat (for about the 3rd or 4th time) that heating the driver tube with DC will eliminate the noise issue of these adapters, and can be done very inexpensively.

 

[hopeful]

PB - that was my original understanding, but in response to one of my posts in the crack thread on headfi (as "skeptic"), I understood Doc as saying that ac shouldn't really be an issue for purposes of using adapters.  (see posts 1532-1533 https://www.head-fi.org/t/476650/crack-bottlehead-otl/1530_30#post_9222640 )

Have you guys had a chance to do any in house testing on this?  If so, it would be great if someone would post a dispositive conclusion on HF to end any remaining confusion on this point.