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Old 11th January 2012, 09:07 PM   #1
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Default 300B resistor

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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Last edited by miguelnoda; 11th January 2012 at 09:29 PM.
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Old 11th January 2012, 09:30 PM   #2
Sandor is offline Sandor  Italy
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Quote:
Originally Posted by miguelnoda View Post
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.
Click the image to open in full size.
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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
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Old 11th January 2012, 10:39 PM   #3
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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.

Last edited by azazello; 11th January 2012 at 10:43 PM.
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Old 11th January 2012, 11:47 PM   #4
tomchr is offline tomchr  United States
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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
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Old 12th January 2012, 02:17 AM   #5
DHT112A is offline DHT112A  United States
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Here:

Angela Instruments Online Catalog - Resistors - Wirewound

or here:

Huntington TMC 880 ohm - Handmade Electronics

Both in US
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Old 12th January 2012, 01:50 PM   #6
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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?
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Old 13th January 2012, 02:35 AM   #7
tomchr is offline tomchr  United States
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Quote:
Originally Posted by Troncones View Post
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
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Old 13th January 2012, 02:34 PM   #8
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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
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Old 28th August 2013, 11:06 AM   #9
FSK is offline FSK
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Default 880 ohm resistor

I have same problem. While 880 ohm resistors are available from USA, the postage to Australia is ridiculously expensive.

Any issues if I parallel a more commonly available 1000 ohm 30W wire wound resistor with a 7500 ohm resistor to get 882 ohm?
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Old 28th August 2013, 12:52 PM   #10
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Hi!

There is no reason why a design would call for exactly 880 Ohms cathode resistance. In cathode biased designs the value of the cathode resistor is not that critical. Individual tubes can vary too. Just pick the closest value you can get.


Quote:
Originally Posted by azazello View Post
With Mills resistors You need cathode cap about 1000 uF
Why would a cathode resistor in the range of 800 Ohms need such a high bypass capacitance? 100uF should be sufficient.

Best regards

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