Domesticating & Documenting the Little Bear

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Coffee Phil
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Domesticating & Documenting the Little Bear

Post by Coffee Phil » 31 Mar 2014 07:35

Some of you may recall that a Little Bear Tube RIAA phono stage came into my custody about one month ago.

viewtopic.php?f=19&t=64786&hilit=Little+Bear&start=75

The owner was having hum issues and I offered to sort it. In addition to helping a fellow vinylengine member my goals are to document the piece and evaluate if I may want to own one.

It arrived with the bottom Plexiglas plate broken and the transformer loose. Normally I would fire up the equipment to verify the complaint and find the cause before starting to work on it. In this case I wanted everything tied down before starting so I started on a new plinth first.

Below is a picture of the broken Plexiglas base and the piece of ~ 0.19" thick aluminum I cut using the old part as a template:

[img]28820[/img]


Below is the drilled, tapped, and painted new plinth ready to use with the old part:

[img]28821[/img]

I did not get too carried away in sanding a preparing the metal before painting as all that will be seen is the edges.

Here is the little bear mounted on the new plinth.

[img]28822[/img]

Note the short and twisted AC wires. If you don't want hum this is important. Note also the green "safety" ground wires from the IEC power connector to the plinth and to the transformer. The IEC connector is connected to the PC board ground. I wanted the plinth and the signal ground connected at only 1 point. That point is the IEC connector. One of the board mounting standoffs contacted the ground trace so I chamfered the hole and used insulating washers between the standoffs and the board. The plinth will provide electrical shielding at the bottom of the board. It will also provide AC magnetic shielding. Trust me on this. Magnetic flux passing through the aluminum produce eddy currents which in turn produce an opposing magnetic field. I highly recomend this aluminum plinth to Little Bear owners.

With the Little Bear as you see it, I just had to try it. I ran it's output to the AUX input of my Hafler DH 100 and the output of my Sonus Blue cartridge into the Little Bear input. I would say this is a nice sounding stage and there were NO hum issues which I noticed with no additional shielding. This stage has ~6 to 10 dB more gain than the internal Phono Stage in the Hafler.

Taking the Little Bear to the test bench I looked at the output with an oscilloscope with the inputs open. I found ~ 100 mv peak to peak hum. I then took a standard tube shield from my Citation IV and placed it over the input tube with a clip lead connecting it to ground. The hum dropped to ~10 mv peak to peak. The hum level seemed high based on what I heard listening to it. I need to repeat the test with a phono cartridge plugged into it. I'm pretty sure the hum will drop very substantially. I think a sheild on the first tube is a good idea in any case.

I found the tube heater voltages to be a bit high. This could be because the maker takes our 110 volt number literally. The transformer primary is marked 110 Volts.

I did trace out the circuit and drew schematics of the left channel, the right channel, and the power supply. The reference designators on the schematic of one channel which I posted before came from the Chinese schematic and do not match the markings on the board. On the following schematics I used the designators from the board for the resistors. The designators for the other components were not marked so I did not provide them other than for the vacuum tubes. I assigned designators to the tubes in the order of signal flow. I gave the rectifier the designator V4.

Below is the left channel:

[img]28932[/img]


Next is the right channel:

[img]28933[/img]


The power supply is below.

[img]28931[/img]

Note the labels B+1, B+2, and B+3 are mine and do not appear on the board.

More later.

Phil

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Re: Domesticating & Documenting the Little Bear

Post by spittenkittens » 01 Apr 2014 05:08

I have been waiting for your thoughts on this little guy. Hum was never any kind of issue with me(knock on wood) I have also noticed many Asians use the term 110v even though 121 volts might be more accurate. You would know this, but I thought any device was designed to be run at + or - 10% of the voltage. If that was true, shouldn't it still be in the acceptable range? I have really grown to like my little bear and have gotten used to the sound. I took out another ss preamp I had used for awhile to compare the sound again. The little bear seems to have much more depth to me for lack of a better explanation. I was going to make a case for it, might still someday but I kind of like the raw look of it and since I am single I can get away with that.
Will be interested to hear more derails when you get them.

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Re: Domesticating & Documenting the Little Bear

Post by Coffee Phil » 01 Apr 2014 07:56

Hi Spittenkittens,

If you had no hum issues you must have done a pretty good job with the lead dress of the AC wires from the transformer. In the unit which I'm working on the leads were long and not twisted. Also the fact that a three wire cord is used can lead to ground loops if something else is grounded in your system. If the amp following the ot Bear has a two wire power cord and there is no tuner connected to it and a grounded antenna system the three wire cord on the Little bear may not be a problem.

Running on my mains the heater voltage to the 6N2s is about 17% high and the heater of the 6Z4 is about 15% high. I was not able to find the tolerence of heater voltage on the 6N2 data sheet. 17 % excessive heater voltage won't be catastrophic but I expect it will shorten the life of the tube by prematurely boiling the oxide coating of the cathode. For the 6N2s at least I plan to drop the heater voltage a bit. There is a jumper between the filter caps in the 6N2 heater supply which can be replaced with a resistor. The AC heated 6Z4 will be more trouble as there are no jumpers which can be replaced with a resistor.

I will connect a record player to the input of the Little Bear on my test bench as I think the hum voltage which I measured seems pessimistic based on what I hear with the unit in my system.

More results as I get them.

Phil


spittenkittens wrote:I have been waiting for your thoughts on this little guy. Hum was never any kind of issue with me(knock on wood) I have also noticed many Asians use the term 110v even though 121 volts might be more accurate. You would know this, but I thought any device was designed to be run at + or - 10% of the voltage. If that was true, shouldn't it still be in the acceptable range? I have really grown to like my little bear and have gotten used to the sound. I took out another ss preamp I had used for awhile to compare the sound again. The little bear seems to have much more depth to me for lack of a better explanation. I was going to make a case for it, might still someday but I kind of like the raw look of it and since I am single I can get away with that.
Will be interested to hear more derails when you get them.

Coffee Phil
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Re: Domesticating & Documenting the Little Bear

Post by Coffee Phil » 02 Apr 2014 22:37

There are some typos in the paragraph at the bottom which was part of my reply to Spittenkittens and I'm past the time which allows editing so I'll try again.

What I am trying to say is that the three wire power cord with safety ground connects the Little Bear signal ground to earth ground at the receptical where it is connected. If it is connected to a receiver which which is also grounded either by a three wire power cord or the coax shield of an FM antenna a ground loop may result. Three wire power cords and unbalanced audio (RCA connectors) are not a happy combination. This is one reason pro audio uses balanced audio with XLR connectors.

If the Little Bear is connected to a turntable and an amplifier both with two wire power cables the three wire cord on the Little Bear should not be an issue.

Phil



Also the fact that a three wire cord is used can lead to ground loops if something else is grounded in your system. If the amp following the ot Bear has a two wire power cord and there is no tuner connected to it and a grounded antenna system the three wire cord on the Little bear may not be a problem.

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Re: Domesticating & Documenting the Little Bear

Post by spittenkittens » 03 Apr 2014 01:39

I learned about ground loops way back in jr high being part of the stage crew. That was great fun running spot lights projectors and powerful amps. About the only way we ever got in trouble was to on any equipment overnight. In high school I had basic electronics. I don't remember all of what we learned but I learned to keep one hand in my pocket when playing with electrical stuff. Back then everything was tubes and arc lamps and heavy duty. Maybe that's where I got my love of tubes from. And also running the projector in my friends dad's theater.

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Re: Domesticating & Documenting the Little Bear

Post by Coffee Phil » 12 Apr 2014 08:33

Here is the start of the tube shield for V1. It is the only tube which needs the shield.

[img]29047[/img]e

Before adding any more value to the mesh shield I tested it against the standard shield from my Citation IV shown next to it. The mesh shield was found to be equally effective and will permit observing V1 in all of its tube glowing glory.

Phil

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Re: Domesticating & Documenting the Little Bear

Post by jc_the_trucker » 13 Apr 2014 19:40

Hey Phil,

Good to see that you have made progress on the little bear! I always thought mine sounded good, if the hum could be brought under control. Shielding V1 as you said brought it down to "acceptable" levels with my old yamaha receiver, but on the new VTA amp, it just bugs me.... Yes music will cover the small hum I still have, but I know in the back of my head that it's still there. And as silent as the VTA is its very obvious when changing records.

I still haven't decided if it's worth my time to go further with it. It has after all served it's purpose of being an introduction to tube gear in both cost and sonic presentation. I did not intend for it to be the last phono stage I would ever buy though, and I'm toying with the idea of going full steam ahead on complete VTA system over the next year or so. But on the flip side of that, VTA's top of the line phono stage is about 5 times what the little bear cost and I may need some convincing as to whether or not it would actually be 5 times as good.

Either way, I'll be watching with interest over the next four weeks to see what you come up with, and what would be easy to accomplish from a modification standpoint. When I get back to the house after that, I'll make a decision on whether to pursue better sound from the little bear, or if I will start stacking pennies once again.

On a side note about over-voltage on the heaters, I don't see reduced tube life to be a huge problem with this amp as Chinese 6N2's and Russian 6N2P's are still cheap and easy to come by. As long as I get a year out of them, a replacement tube set at roughly $10-15 wouldn't upset me too much. If I had a 300B amp that was burning it's tubes up that fast, I'd be a lot more concerned :lol: !

Coffee Phil
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Re: Domesticating & Documenting the Little Bear

Post by Coffee Phil » 18 Apr 2014 06:40

Hi JC,

It is hard to say if you should put more effort into the Little Bear or just go with the VTA preamp. As you will see from the pictures below I have gotten the hum to better than 40 dB below the music. While that is not state of the art, it just may be good enough. The numbers which I have are not weighted but just the amplitudes measured with an oscilloscope. An RIAA stage has more than 20 dB more gain at 60 Hz than at 1 kHz. Human hearing is not all that sensitive at 60 Hz. In my system I did not notice hum even before I added the tube shield. The tube sheild made ~ 12 dB improvement over the configuration which I last listened to. That configuration included the aluminum plinth and the wiring as you see it. I do need to do more critical listening when it is quiet.

I hear you on the tube life thing, but for the 6N2 tubes it is a matter of replacing a jumper with a resistor. No big deal and there is an added benefit of less ripple on the heater supply. For the rectifier tube a cut and jump is required. I may accept possible reduced life for the rectifier and just do the 6N2s. We don't want to evaporate cathode oxide coating onto the control grids and get grid emission.

Below are some pictures:

[img]29087[/img]

This is the Little Bear with the shield on the input tube which I call V 1. Note the wire from the shield to the ground screw at the input connector.

Below is a close up of the shield in place:

[img]29088[/img]

Here is the top Plexiglas installed with the shield in place:

[img]29089[/img]

Below is the right channel output with the tube shield lifted and the input open:

[img]29091[/img]

The vertical scale is 10 mv / div and the horizontal scale is 5 ms / div.

Same as above with shield in place:

[img]29092[/img]

Here is the right channel output with some music playing.

[img]29093[/img]

The vertical scale is 1 V / div and the horizontal scale is 5 ms / div.

The 12 dB number for shield on vs. shield off comes from the ~4:1 ratio of hum amplitudes in the pictures. The 40 dB un-weighted number for hum with shield vs music is from the ~ 100 to 1 amplitude ratio.

More later.

Phil



jc_the_trucker wrote:Hey Phil,

Good to see that you have made progress on the little bear! I always thought mine sounded good, if the hum could be brought under control. Shielding V1 as you said brought it down to "acceptable" levels with my old yamaha receiver, but on the new VTA amp, it just bugs me.... Yes music will cover the small hum I still have, but I know in the back of my head that it's still there. And as silent as the VTA is its very obvious when changing records.

I still haven't decided if it's worth my time to go further with it. It has after all served it's purpose of being an introduction to tube gear in both cost and sonic presentation. I did not intend for it to be the last phono stage I would ever buy though, and I'm toying with the idea of going full steam ahead on complete VTA system over the next year or so. But on the flip side of that, VTA's top of the line phono stage is about 5 times what the little bear cost and I may need some convincing as to whether or not it would actually be 5 times as good.

Either way, I'll be watching with interest over the next four weeks to see what you come up with, and what would be easy to accomplish from a modification standpoint. When I get back to the house after that, I'll make a decision on whether to pursue better sound from the little bear, or if I will start stacking pennies once again.

On a side note about over-voltage on the heaters, I don't see reduced tube life to be a huge problem with this amp as Chinese 6N2's and Russian 6N2P's are still cheap and easy to come by. As long as I get a year out of them, a replacement tube set at roughly $10-15 wouldn't upset me too much. If I had a 300B amp that was burning it's tubes up that fast, I'd be a lot more concerned :lol: !

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Re: Domesticating & Documenting the Little Bear

Post by jc_the_trucker » 18 Apr 2014 15:03

I wasn't aware that grid emission existed in the first place! Learning is fun! Anyway, I have a friend that is into electronics repair as a hobby, and he wants to check it out too. Mostly because of its relatively minute cost. We'll get it squared away (two heads are better than one after all), and maybe I'll sell it to him when I get a suitable replacement. Or maybe as fair trade for his spare o-scope. Not that I'd know what to do with it, but I can figure it out.

I plan first on cleaning up the wiring and making a more aesthetically pleasing tube shield for v1. I know, I should have done the wires sooner because it's easy and quick, but I was side tracked by other projects.

The one thing that is messing with my head is that when I put a finger near the caps close to v1 (tube is shielded), my hum increases substantially. It's not something I do often, but it seems to me that there is something more going on in that area of the board than RF messing with the tube. I'm curious if you have observed the same thing on yours, and what could possibly cause that?

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Re: Domesticating & Documenting the Little Bear

Post by Coffee Phil » 25 Apr 2014 19:21

Hi JC,

In small signal tubes grid emission is usually not a problem. If you check the data sheet for power tubes such as the 6L6 you will find that they specify the maximum grid circuit resistance usually ~100 k-Ohms for fixed bias and ~500 k-Ohms for cathode self bias. The reason for this is if there is current from the grid to the plate the grid will get more positive increasing the plate current and therefore the temperature. This can lead to thermal runaway. Cathode bias to a large extent mitigates that, hence the higher permitted grid resistance. I think that grid emission gets worse when the grid becomes contaminated with cathode coating which is why I get nervous about excessively high heater voltage which might make coating sublime off the cathode and get on the grid. 10% high should be fine but the Little Bear operating on 120 VAC seems to be pushing it a bit.

I do recommend that you shorten and tidy up the AC wiring as that can produce AC electrical and magnetic fields. I also recommend the grounded metal plinth. If you are looking for just electrical shielding, covering the OEM plastic with foil will do but the heavy aluminum plinth also gives some Eddy current AC magnetic shielding as well. The shield should be grounded. I grounded the plinth to the IEC connector and used insulating washers on the standoffs to isolate from the ground traces on the board. I want to keep any possible AC ground currents low.

The caps which you mentioned couple the plates of V1 to the summing nodes at the grids of V2. I would expect them to be very sensitive to AC voltage fields. On my example this is not a big problem since any AC voltage field will terminate at the plinth not far from the caps. I still do notice it however and am considering shields for the caps if I can show at least 10 dB more improvement and can come up with an aesthetically acceptable solution.

I have fallen into some contract work which is paying so my progress on the Little Bear has slowed a bit.

More later,

Phil


jc_the_trucker wrote:I wasn't aware that grid emission existed in the first place! Learning is fun! Anyway, I have a friend that is into electronics repair as a hobby, and he wants to check it out too. Mostly because of its relatively minute cost. We'll get it squared away (two heads are better than one after all), and maybe I'll sell it to him when I get a suitable replacement. Or maybe as fair trade for his spare o-scope. Not that I'd know what to do with it, but I can figure it out.

I plan first on cleaning up the wiring and making a more aesthetically pleasing tube shield for v1. I know, I should have done the wires sooner because it's easy and quick, but I was side tracked by other projects.

The one thing that is messing with my head is that when I put a finger near the caps close to v1 (tube is shielded), my hum increases substantially. It's not something I do often, but it seems to me that there is something more going on in that area of the board than RF messing with the tube. I'm curious if you have observed the same thing on yours, and what could possibly cause that?

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Re: Domesticating & Documenting the Little Bear

Post by Laila1 » 27 Apr 2014 01:05

Please, keep up your good work Phil!

I got one and I tamed the filament voltage with diodes.
The first tube are now wrapped i aluminium foil(earthed)
and the whole thing sounds pretty good to my ears.

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Re: Domesticating & Documenting the Little Bear

Post by jc_the_trucker » 27 Apr 2014 19:25

Very good Phil. I think I'll see how far I can go with it; the modifications you've come up with so far are neither difficult nor expensive. I like the little bear well enough to keep it around a while longer. And by all means, make money before giving out free advice, I'd do the same thing!

Looking online right now for a suitable aluminum plate. Anyone able to give me rough dimensions on the little bears bottom plate? I don't exactly carry it around with me on the road :wink: . I want to get as close as I can to original dimensions as I don't have any way to cut down a 2 foot X 2 foot plate. I've found various dimensions, usually in 6" increments, on eBay that are fairly cheap, and I'm more likely to get it done if it's waiting for me when I get home. I'd also need to know what size drill and tap kit to get, as a full set is quite expensive.

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Re: Domesticating & Documenting the Little Bear

Post by Coffee Phil » 28 Apr 2014 07:16

Hi Lailal,

Thanks for the encouragement!

I'm glad you got your heater voltages within reason. I guess the 240 volt version has the same issue as our US 120 volt Little Bears. Sometimes in the US we refer to the heater in tubes with "indirectly" heated cathodes as the filament however more commonly as just the heater. The cathode in directly heated tubes is commonly refered to as the filament.

I hope to work on this Little Bear a bit more but it is not mine and its owner has been without it for more than a month so I will have to bring my efforts to a conclusion soon. I would like to get another 10 dB of hum out of it and measure the RIAA accuracy, but paying work beckons and I have to keep clients happy if I want to see more. I do hope to have more to report before the little Bear Has to go home.

Phil



Laila1 wrote:Please, keep up your good work Phil!

I got one and I tamed the filament voltage with diodes.
The first tube are now wrapped i aluminium foil(earthed)
and the whole thing sounds pretty good to my ears.

Coffee Phil
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Re: Domesticating & Documenting the Little Bear

Post by Coffee Phil » 28 Apr 2014 07:36

Hi JC,

I'm pleased with your continuing interest in my Little Bear "research".

I'm fried tonight but tomorrow I'll to get the dimensions of the bottom plate and the drill and tap sizes. The only holes which I tapped were for the transformer mounting screws and for the safety ground lug. Those screw holes were 6-32. The tap drill for 6-32 is 7/64" or in number size #35. The standoffs are metric, I think close in diameter to a 4-40 machine screw. I will verify that.

I cut the aluminum plate with my table saw using a 10" cut-off wheel.

I was hoping to have the heater voltage stuff done soon but tomorrow better be a "real" work day if I want to see more of it.

More soon hopefully.

Phil

jc_the_trucker wrote:Very good Phil. I think I'll see how far I can go with it; the modifications you've come up with so far are neither difficult nor expensive. I like the little bear well enough to keep it around a while longer. And by all means, make money before giving out free advice, I'd do the same thing!

Looking online right now for a suitable aluminum plate. Anyone able to give me rough dimensions on the little bears bottom plate? I don't exactly carry it around with me on the road :wink: . I want to get as close as I can to original dimensions as I don't have any way to cut down a 2 foot X 2 foot plate. I've found various dimensions, usually in 6" increments, on eBay that are fairly cheap, and I'm more likely to get it done if it's waiting for me when I get home. I'd also need to know what size drill and tap kit to get, as a full set is quite expensive.

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Re: Domesticating & Documenting the Little Bear

Post by jc_the_trucker » 28 Apr 2014 17:22

Good deal, Phil. Looking forward to it.

By the way if you don't get everything done that you want to do with the one you're using now, I'll be happy to let you borrow mine. After my week at home of course. It will hopefully have all your current mods done to it at that point. You'd be able to have it for a month or so, I usually stay out for six weeks so I wouldn't be missing it during that time. Just let me know and we can work out the details.

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