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Old 7th January 2014, 08:14 PM   #1
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Default Little-Bear B1 modding..

Picked up one of these portable amps to play with. My first impression was a bit of a shock at how hot it ran, i almost couldn't pick it up after a bit of use. I had a poke around the pcb and discovered the heater supply resistor was a .1ohm. Yup .1ohm so the element was getting 12v across it and ran hotter than the surface of the sun. If anybody buys one of these double check the resistor they have installed as its way too small..

I tried a 7806 linear regulator to output 6.35v which worked ok except the regulator itself ran at ~70-80c with a small heatsink clipped on. I have since replaced it with a Traco unit, much better battery life and less heat, definitely the way to go. If you study the PCB there are two positions for transistor packages that are unused, i assume they had planned some sort of power manangement circuitry. By chance they are perfect fit for the Traco unit pin outs when mounted sideways, as per attached pic. Coupling caps are laid out as per datasheet.

I've also attached a schematic i found on another forum, full credit to whoever actually created it as i have no idea. So far i have done the following,

-Replaced Ratheon 6111 tube with a NOS G.E. 6021A
-Replaced both 10k resistors with 20k adj pots and set to 6v DC. (9.3v with 6021A tube)
-Replaced all resistors with known quality metal film.
-Replaced all capacitors with Elna Silmic II.
-Added 10uf lytic / 100nf to Opamp power pins.
-Replaced blue led with red. ....i hate blue leds with a passion.
-Replaced NE5532 with a genuine LM4562.
-Replaced volume pot with a genuine Alps, cures crackly volume control.
-Installed Traco based heater supply.

The original tube (Ratheon 6111) got roasted alive with 12v across its heater so i replaced it with a NOS G.E. 6021A. Vs the 6111 that was installed it sounds much better, lower distortion, greater clarity, and more gain. I also have a 6BF7W and Russian 6N16b tube to try at some stage.

Basic amp can be bought from this seller, portable valve from woshixionghui2010 | eBay auctions typically end for 10-15. Traco regulator is ~5, and a few quid for a few decent caps/resistors and you will have a very nice sounding little amp, highly recommended.
Attached Images
File Type: jpg LBB1.jpg (203.9 KB, 1015 views)
File Type: jpg IMG_5886.JPG (825.1 KB, 978 views)
File Type: jpg B1b.JPG (105.4 KB, 880 views)
File Type: jpg B1.JPG (41.2 KB, 840 views)

Last edited by mcandmar; 2nd May 2014 at 01:56 PM.
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Old 10th January 2014, 12:29 AM   #2
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Split supply: Not much of a point here.

Opamp voltage follower with direct feedback usually works fine, only a few VFB types are fussy (e.g. AD797). Can't do that with a CFB opamp, obviously.

Blue LEDs: Me too.

Adding the 1k8 seems like a good idea, it makes the pot a fair bit more logarithmic.

The note on the schematic re: heater voltage actually isn't correct, underheating isn't exactly healthy either AFAIK.

Not sure why the output needs a pulldown as low as 2k2, is the 1000 that leaky? Anyway, 2k2 >> typical loads, so who cares.

As in many of those cheap tube designs, plate current and voltage are quite low, which is not doing noise and distortion a favor. And they could've left out the 10 ohm cathode resistor entirely, it drops Vgc to a whopping -6 mV.
I like the idea with the transistor CCS instead of the plate resistor and variable cathode resistor (AC bypassed if needed) for DC adjustment.

If there's an opamp in there to begin with, I'd want to look into adding a negative feedback loop wrapping the OP and tube. That would require a non-unity gain on the OP though, and as such some sort of virtual ground.
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Old 10th January 2014, 07:29 PM   #3
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Quote:
Originally Posted by sgrossklass View Post
Split supply: Not much of a point here.

Opamp voltage follower with direct feedback usually works fine, only a few VFB types are fussy (e.g. AD797). Can't do that with a CFB opamp, obviously.

Blue LEDs: Me too.

Adding the 1k8 seems like a good idea, it makes the pot a fair bit more logarithmic.

The note on the schematic re: heater voltage actually isn't correct, underheating isn't exactly healthy either AFAIK.

Not sure why the output needs a pulldown as low as 2k2, is the 1000 that leaky? Anyway, 2k2 >> typical loads, so who cares.

As in many of those cheap tube designs, plate current and voltage are quite low, which is not doing noise and distortion a favor. And they could've left out the 10 ohm cathode resistor entirely, it drops Vgc to a whopping -6 mV.
I like the idea with the transistor CCS instead of the plate resistor and variable cathode resistor (AC bypassed if needed) for DC adjustment.

If there's an opamp in there to begin with, I'd want to look into adding a negative feedback loop wrapping the OP and tube. That would require a non-unity gain on the OP though, and as such some sort of virtual ground.
I've heard that before about running low heater power, just measured mine again and its got 6.37v at the tube.

I thought the 1000uf cap unusually large but have yet to replace it and measure the output for DC. I have a bunch of WIMA caps ~1uf i might try in its place.

I picked up a NOS G.E. 6021A tube to try so at the moment the 6v DC out of the opamp is at 9.3v, thats the lowest i can set with the 20k trimmers so i need to work out how to calculate the bias and cathode resistor values and sort them out. Can anybody help a tube newb with that?

Vs the 6111 that was installed its much better, lower distortion, greater clarity, and more gain. I think the original 6111 was partly fried with the 12v accross the heater as there are dark stains on the glass around the places the heater was exposed.
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Old 2nd May 2014, 01:58 PM   #4
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Update: Image attached showing current configuration with Traco regulator.
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File Type: jpeg IMG_6857.jpeg (114.3 KB, 698 views)
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Old 25th May 2014, 11:26 PM   #5
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Hiya, I registered for an account just to make this post:

I'm very interested in performing the mods you've so awesomely documented here.
If you'd be so kinda as to go into some more detail about the caps on the opamp and how you've got the regulator hooked up (and which model) I'd really appreciate.

I consider myself pretty proficient at electronics, but can't totally infer the right way to do these mods. Thanks in advance!
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Old 26th May 2014, 05:45 AM   #6
Junm is offline Junm  Philippines
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Since you're using 12v supply I might as well suggest to use 12AE7, 12v dual triode for the tube.

Or jack up the supply voltage using voltage multiplier of the tube and use a coupling cap at the buffers input.This will make tubes work in their linear region and lower distortion.
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Old 22nd August 2014, 05:41 PM   #7
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Quote:
Originally Posted by Junm View Post
Since you're using 12v supply I might as well suggest to use 12AE7, 12v dual triode for the tube.
Yeah, using a dual triode with different amplification factors between halves is probably not the best suggestion.

To get acceptable performance from something like this, you either need a space charge tube (resistant to non-linear grid current at low plate voltage), or a separate high voltage plate supply and your current tube (raising plate voltage will raise effective input impedance and reduce distortion, but at the expense of bulk).

You could probably also abandon the tube in favor of a pair of fets.
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Old 21st September 2014, 01:42 AM   #8
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Hi, just wondering, what model potentiometer are you using to fix volume crackle?, thanks
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Old 21st September 2014, 02:15 AM   #9
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alps rk097 10k right?, also what traco module you using, thanks
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Old 3rd December 2014, 02:35 PM   #10
redshot is offline redshot  Portugal
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Hello, this is my first post here, and hopefully not the last I have been watching this thread for a long time, and I finally bought one of these babies, and it arrived today.

Having been warned about the heater voltage problem, I immediately opened it up to find out a green PCB from May 2014, so it has been redone. Unfortunately, the voltage problem persists, I measured about 9-10V across the heater, and immediately removed the 0.1 Ohm (Yes, 10^(-1)) resistor, thinking about replacing it with a 20 Ohm 5W.

However, I had a tiny 12V to 5V DC-DC converter from a car phone charger, and after connecting it to the heater instead of a resistor, it measured 6 to 6.5V, stabilizing around 6.3V, which is exactly the requirement for the heater!

Having that, I installed it inside and assembled the whole thing, and now I'm going for the stress test until it runs out of battery, and also checking for the temperature.

I also replaced the blue led for a red led from the same phone charger (my lucky day )
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