Little Bear Tube Phono Pre??

[Devon Dancer]

Just some updates on my experiences with my LB.
The sound has been great. Very open, wide soundstage. Seems to come out in front of the speakers. No matter the volume it never seems 'shouty'.
I decided to try some Russian valves and because of the sparsity / cost of 12AX7 valves I purchased some 6N2. On installing them and resetting the switches I immediately noticed a VERY loud hum which I did not have before. I checked all my earths and connections, removed the mains earth for a while, grounded the transformer case etc. Eventually I replaced the Russian 6N2s with the original Chinese 12AX7s and lo and behold hum was gone and you have to place your ear right up to the speaker cones to hear any background noise.

Anyone else experienced this or have thoughts on the cause ?

 

[Lavane]

Are the pin outs the same for both tubes? It's possible the 6n2 has the same pinout as the 6n1? There is a switch for each tube to select between 6n1 and 12ax7's. Do a search for the data sheets for both tubes you have and make sure the pinout is the same. I used some Sylvania, GE and JJ 12ax7's without any hum problems. If the pinouts are the same try one of the Rusdion tubes at a time using the Chinese tubes in the other two sockets. You may have just one bad tube.

 

[T68]

I did the same thing. Replaced the chinese 12AX7 valves with russian 6N2:s with the rocket logo. They sound just great in my LB T10 and Shure M75 with a Jico elliptical and I have not gone back to the chinese walves. But at first upon changing them I missed to set one of the tube switches in the 6N2 position and got no sound. Can´t remember if there was hum though...
Maybe one of the switches didn´t fully engage. Or maybe you got a lemon tube from Russia. Hope you get it sorted. Good luck!

 

[Devon Dancer]

Thanks Lavane and T68.
As you can probably appreciate, my first concern was to return to the original set-up which was hum-free so I put all the Chinese 12AX7s back in place. Now this has confirmed the problem is most likely to do with one or more of the Russian valves I'll try what you suggest, swapping one at a time, when I've got an hour or two to spare. I've got 5 Russian tubes to try so fingers crossed !!

 

[Lavane]

Thanks Lavane and T68.
As you can probably appreciate, my first concern was to return to the original set-up which was hum-free so I put all the Chinese 12AX7s back in place. Now this has confirmed the problem is most likely to do with one or more of the Russian valves I'll try what you suggest, swapping one at a time, when I've got an hour or two to spare. I've got 5 Russian tubes to try so fingers crossed !!

The first tube in the circuit makes the most difference in sou d and is the most sensitive to noise and microphonics.

 

[T68]

The first tube in the circuit makes the most difference in sou d and is the most sensitive to noise and microphonics.

Thanks for that information! Is it possible to mix 12ax7s and 6n2:s if one has the switches right for each type of tubes?

 

[Lavane]

Yes

 

[T68]

Thanks, might give that a try.

 

[Devon Dancer]

Thought I'd bring you up to date on my experiences. I replaced the 6N2Ps with the original 12ax7s and the hum disappeared ! Relief !
I then looked carefully at the 6N2Ps supplied (I had 5) and noticed that one of them had different markings to the others (hard to make out but the key was the markings were grey/silver whereas the other 4 were black). originally I'd just picked three out of the pack. This time I chose 3 of the black marked (clearly the 6N2P markings), re-installed and fired up. Bingo! no hum! He'd clearly thrown a random valve in which I'd managed to pick out. I'm now running it without the supplied screening cans and no problems.
Sound now is brilliant, very clear and seems to separate from the speakers giving a real three-dimensional feel.
I've also worked my way through the mega-thread on VinylEngine and have undertaken one of the mods to the RIAA correction components with stunning results !
I've also got on order a batch of 'Rocket' branded 6N2Ps so will be trying those when they arrive. This is so much more fun than transistors ​

 

[tubeamp57]

just pickup a v2 on graiglist ,it allready had the russian tubes only the rectfier was stil the 6z4 ,first inpresiun wasn,t bad at all ,only a hum and some white noise ,moved the trafo 10cm from the print and put a steal shield arround the first tube ,was allready better,today i istalled a socket for the ez80 rectifier on top of the 6z4 socket and wired to fit the ez80 philips rectifier ,that made a big diferents ,no more hum , no more noise better dynamics

 

[nonsuchpro]

It's been a while since I've visited this thread so, I figured I'd see if there was any real progress. I've tried a bunch (not all) of the recommendations over the years with little to no effect on fixing the hum issue. I hoped by now there would be a definitive answer to the problem. In my particular case, If I touch the top of tube V3 (the one closest to the I/O), the hum goes away. A tube shield doesn't do it, only my finger on the glass of the tube. I've tried MANY different tubes in all sockets and in different configurations but, nothing changes. I just threw my hands up and stopped trying at some point. I bought a "Ground Loop Isolator" and the hum went away 100% but, at the cost of some low end. They are not all the same though, I've bought 3 different isolators and only 1 of them gets rid of the hum completely. Still, I wish I didn't have to use it. I really do love the sound of the T10 v2.5 I have. So, without standing there with my finger on the tube while listening to an album, is there anything anybody can think of to try in my particular situation? It's so weird to me that touching the glass of tube V3 gets rid of the hum for me. Everyone talks about shielding V1 but, that doesn't do anything for my problem nor does shielding any of them. Thanks in advance!

 

[neevo]

I can’t offer any real advice but what I can say is that there must be some ways to improve the noise of the preamp. I have a v10 that was improved slightly with solid tube shields on the front tube. Certainly stopped some noise that I used to get from my router in the same room.

But I also bought a LB from a chap in Melb once for my mate. It had a lot of work done on it. Replaced wire would resistors and other hot rod stuff. Looked a bit rough but the chap was an electrical engineer apparently.

The difference in noise floor is dramatic. I tested his unit on my system before it went up to his house and I thought at one point it wasn’t working properly the noise floor was so low.

 

[Poultrygeist]

It's been a while since I've visited this thread so, I figured I'd see if there was any real progress. I've tried a bunch (not all) of the recommendations over the years with little to no effect on fixing the hum issue. I hoped by now there would be a definitive answer to the problem. In my particular case, If I touch the top of tube V3 (the one closest to the I/O), the hum goes away. A tube shield doesn't do it, only my finger on the glass of the tube. I've tried MANY different tubes in all sockets and in different configurations but, nothing changes. I just threw my hands up and stopped trying at some point. I bought a "Ground Loop Isolator" and the hum went away 100% but, at the cost of some low end. They are not all the same though, I've bought 3 different isolators and only 1 of them gets rid of the hum completely. Still, I wish I didn't have to use it. I really do love the sound of the T10 v2.5 I have. So, without standing there with my finger on the tube while listening to an album, is there anything anybody can think of to try in my particular situation? It's so weird to me that touching the glass of tube V3 gets rid of the hum for me. Everyone talks about shielding V1 but, that doesn't do anything for my problem nor does shielding any of them. Thanks in advance!

By now you've probably tried these?

 

[knockbill]

It's been a while since I've visited this thread so, I figured I'd see if there was any real progress. I've tried a bunch (not all) of the recommendations over the years with little to no effect on fixing the hum issue. I hoped by now there would be a definitive answer to the problem. In my particular case, If I touch the top of tube V3 (the one closest to the I/O), the hum goes away. A tube shield doesn't do it, only my finger on the glass of the tube. I've tried MANY different tubes in all sockets and in different configurations but, nothing changes. I just threw my hands up and stopped trying at some point. I bought a "Ground Loop Isolator" and the hum went away 100% but, at the cost of some low end. They are not all the same though, I've bought 3 different isolators and only 1 of them gets rid of the hum completely. Still, I wish I didn't have to use it. I really do love the sound of the T10 v2.5 I have. So, without standing there with my finger on the tube while listening to an album, is there anything anybody can think of to try in my particular situation? It's so weird to me that touching the glass of tube V3 gets rid of the hum for me. Everyone talks about shielding V1 but, that doesn't do anything for my problem nor does shielding any of them. Thanks in advance!

Try isolating the jacks from the chassis,,, I've found some amps won't tolerate metal input jacks in contact with metal chassis, it forms a ground loop right at the start of the signal ckt...

 

[Bunty2]

So, what is the latest in Little Bear tube phono pre-amps? Has that bear of a problem with the hum been taken care of with the latest model?

 

[Lavane]

So, what is the latest in Little Bear tube phono pre-amps? Has that bear of a problem with the hum been taken care of with the latest model?

What's the latest model? The T-11 was the last one I've seen.