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Using fixed bias on a 6J5G

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Posted on November 1, 2016 at 17:28:41
DAK
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I am just wondering if there is any advantage to doing this. I already have fixed bias on the power tube so what the heck? Add some resistors and 2 pots and 2 caps. It is definitely doable.
The 6j5g is the driver tube in a 2 stage amp and is capacitor coupled to the power tube.

 

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RE: Using fixed bias on a 6J5G , posted on November 2, 2016 at 05:13:12
Chip647
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Most will use a battery in this position to provide negative bias. The problem with using a power supply derived fixed bias on a driver tube is that there is a lot of gain so any noise gets amplified, very small changes in the voltage 0.1 v will have an impact on bias versus the wider tolerance for output tubes. This can lead to a signal compression. You could use a small volt reg, but that will have an impact.

It is possible, but you will need a cap on the grid of the tube so your negative bias voltage does not go upstream. Usually the cap offsets any advantage of fixed bias when resistors or diodes are pretty capable.

 

RE:How do you use a battery for bias?, posted on November 2, 2016 at 10:59:14
DAK
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I have not tried that yet. It seems like you would be changing batteries regularly. Dak

 

RE: Using fixed bias on a 6J5G , posted on November 4, 2016 at 09:43:10
Triode_Kingdom
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These tubes are most linear (albeit at the expense of gain) when used with an unbypassed cathode resistor. From that perspective, neither the typical self-bias (bypassed cathode) or fixed bias will be as clean. I agree with Chip647 regarding the noise issue, although I'm not a fan of batteries for this use.





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RE:How do you use a battery for bias?, posted on November 4, 2016 at 10:40:02
Chip647
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Since there is almost zero current to support the bias they last a long time. Need a low leakage cap on the input.

 

RE:How do you use a battery for bias?, posted on November 4, 2016 at 11:22:32
kyle
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John Broskie of Tube Cad wrote a pretty good article outlining battery bias:

 

RE: Using fixed bias on a 6J5G , posted on November 4, 2016 at 12:36:14
hifipaul
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diode bias is better.

 

RE: Using fixed bias on a 6J5G , posted on November 4, 2016 at 13:32:14
DAK
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You mean using a LED?

 

RE: Using fixed bias on a 6J5G , posted on November 4, 2016 at 13:41:58
Paul Joppa
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You can use many kinds of diodes. LEDs are often used because they have a larger voltage drop, and are available in many voltages.

 

RE: I have not tried a diode, posted on November 4, 2016 at 13:49:06
DAK
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Hi Paul, do you have a favorite that you would care to share with me? thank you

 

RE: I have not tried a diode, posted on November 4, 2016 at 18:21:12
Paul Joppa
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We (i.e. Bottlehead) use the HLMP-6000 which is "standard red," i.e. one of the earliest chemical formulations. It has an unusually low ESR, and at one time was known to be quieter than most others. Runs about 1.55v drop per diode. I suspect that the inefficient formulation produces less ESR, but I have not yet learned enough to defend that notion.

Some people use Schottky diodes, but they will have a very low voltage drop per diode, unless you get the high voltage silicon carbide diodes.

Don't use Zeners - they are much noisier because they operate in reverse-breakdown mode. To reduce that you need a huge bypass cap, in which case one might as well use a resistor and get the DC feedback stabilization.

 

RE: I have lots of red leds., posted on November 4, 2016 at 19:37:19
DAK
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I have a bunch of red, blue, green, yellow and white or clear. The red ones i have are bullet shaped with one lead shorter than the other and the base having a flat side associated with the shorter lead. right now with my resistor of 380 ohms, i am getting about 3.8vdc on the cathode with a plate voltage of 360 and a plate resistor of 100k. Plus there is a plate to plate feedback resistor of 100k(from plate of power tube to plate of 6j5).
So, for the 6j5 i would use it on the tube's cathode. with cathode of the LED grounded? I would find it informative to see if using a LED is of significant difference to the sound. How many LEDs do i need per tube? regards, Dak

 

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