Little Bear B-1 v1.2

[Mikkel]

Hi.

I recently bought a Little Bear B-1 v1.2. I saw you had made some suggestions to changes on another forum, but I suck at reading schematics and also suppose that Little Bear made some changes from the first version to v1.2.

My main concern is one resistor that gets very hot (the one marked on pic.)

Should it get so hot or should i change it for something else?
Also. The gain is very low. I have to put the potmeter all the way to max for it to play reasonably loud.(32ohm headphones) Would the log. potmeter fix this?
Thanks
Mikkel

[Ostebaronen]

The resistor is one of those resistors which can take a higher load. I would guess that one is a 5W resistor, so it is prone to get hot.

[Solderdude]

Yes, it is supposed to get quite hot, and thereby wasting lots of battery energy in the form of heat.
It lowers the (12V) supply voltage for the heater, which needs about 6V, but in a very inefficient way by generating a lot of heat.
For this reason I recommend to replace it with a switching regulator which is MUCH more energy efficient and doesn't get hot.

A log potmeter won't give more gain, it only controls the volume more naturally.
A lin potmeter reacts strongly in the lowest part of the volpot range and hardly from 50% to 100%.

In order to increase the gain you would have to modify the circuit substantially and alter the feedback loop of the opamp.
The gain is determined by the tubes 'gain' together with the 10k resistor in the anode path.

You could replace those resistors with the small potmeters (as recommended) and maybe get 1 or 2 dB more gain perhaps but not much more.

Here is the schematics:
http://www.mediafire.com/view/ycy89y499dbqe81/Llittle%20Bear%20B-1%20schematic.pdf