300B resistor

[miguelnoda]

Anyone know where I can find the resistance of 880 ohm-25W to
the 300b. I have a put a wound adjustable, but I think they are
the metal best inductivas.Les not sent photos.
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[Sandor]

Originally Posted by miguelnoda

Anyone know where I can find the resistance of 880 ohm-25W to
the 300b. I have a put a wound adjustable, but I think they are
the metal best inductivas.Les not sent photos.

Image - TinyPic - Free Image Hosting, Photo Sharing & Video Hosting

I think that for a single 300B these 12W non-inductive resistors are sufficient:
Mills resistors page

I use them in my 300B SE amp and the work wery well.

Paul

[azazello]

With Mills resistors You need cathode cap about 1000 uF with very good quality, like Black Gate with U = more 100V, that has expensive price.
If You have possibility connect 300B with fixed bias, this is better idea with better sounding.

[tomchr]

If you want the bias to be adjustable, you won't need the entire 880 ohm range. I suggest using a 200 ohm wire-wound pot in series with a larger power resistor (750 ohm should be about right). You can make a large power resistor by putting many resistors in parallel. For example two 10 W 1500 ohm resistors in parallel would make 750 ohm, 20 W.

The power dissipated in the potentiometer can be calculated from P = I^2 * R. As the 300B is limited to 100 mA, the worst case dissipation with a 200 ohm pot is 0.1^2 * 200 = 2 W. If the pot is closer to the center of its range, you'll get 1 W. A 5 W wire-wound pot is probably OK here...

~Tom

[DHT112A]

Here:

Angela Instruments Online Catalog - Resistors - Wirewound

or here:

Huntington TMC 880 ohm - Handmade Electronics

Both in US

[Troncones]

If the 300B (or any tube) is operated in Class A and has a bypass cap does the cathode resistor actualy see AC? Does it need to be non-inductive?

[tomchr]

Originally Posted by Troncones

If the 300B (or any tube) is operated in Class A and has a bypass cap does the cathode resistor actualy see AC? Does it need to be non-inductive?

That's a good point, actually... The time constant of the cathode cap and resistor is typically set to form an LF pole in the 2~10 Hz range. So the resistor shouldn't really see much AC in the audio range.

That said, I generally find that many people obsess over the inductance of resistors. I've measured some general purpose 0.25 W leaded axial thin film resistors on an impedance analyzer. The impedance curve was completely flat (resistive) out to 40 MHz.

I'm also amused by the people who recommend placing non-inductive resistors right at the tube pin only to have the other end of the resistor connected through 50 mm of wire. Mind you that the self inductance of a piece of wire is around 1 nH/mm (rule of thumb).

~Tom

[Troncones]

Thnaks Tom,

I quess I should have included "in the audio range" in my question.

At times there does seem to be a lot of obsession on the minute. Seperating the wheat from the shaft is one of the challanges for a Newbie.

Tom