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Old 22nd January 2013, 03:50 PM   #1
chip647 is offline chip647  United States
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Default LED cathode bias noise issue

This is concerning my 801a preamp. When I tried using a string of green LEDs the bias was correct but I got a lot of noise. When I put back in the resistor, all was quiet again. The specific LEDs were selected by ear by some pretty smart audio people for cathode bias service.

The preamp is choke loaded with a 50H choke boosted by a 1.8K Mills 12w resistor. The chokes are very nice potted units from Electra-Print. I had them on hand, not really enough inductance but the Mills helps out to give the preamp bass. The output is just from a 1uf cap from the plate of the 801a.

Is this noise pickup from the LEDs due to the high-ish output impedance? Other suggestions?
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Old 22nd January 2013, 03:57 PM   #2
SY is offline SY  United States
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How many do you have in the string? Could your noise pickup be due to something simple like lots of leads (read: antennas)?

Green LEDs tend to be quiet, but they're not the best in that respect. The best selection process is measuring impedance, not "selected by ear by some smart audio people." Depending on what voltage you need, a different color may be a better choice.

edit: Looking at the datasheet for the 801A, it seems to be a power tube requiring a very high bias. Putting aside its odd use as a preamp tube, you would appear to need a LOT of series devices to bias it...?
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Old 22nd January 2013, 04:12 PM   #3
kevinkr is offline kevinkr  United States
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Without cathode degeneration the 801 may be oscillating, install a grid stopper resistor and consider a ferrite bead on the plate.

As SY indicated make sure that you have the required number of leds to bias the tube correctly based on vf.

Good build techniques are a must.

In terms of selecting leds I either measure or select based on data sheet parameters.

I don't know too many well calibrated audiophiles so I prefer my test equipment or that of the diode OEM through their spec sheets..
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Old 22nd January 2013, 04:16 PM   #4
chip647 is offline chip647  United States
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Default Using a string of 8 LEDs to get about 9 volts bias

I kept the LED leads very short. The power transformer is on a separate chassis. The noise is harsh sounding. I use DC heaters. The LEDs could be accentuating any residual AC riding on the DC heater?

I am using a 1K carbon Comp on the grid. (2 watt Allen Bradley). Will try upping it to 10K. The noise sounds like switching noise rather than oscillation. Square wave sounding.

The dudes that picked out the LED were a bit more than "audiophiles" ;-)

Last edited by chip647; 22nd January 2013 at 04:20 PM.
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Old 22nd January 2013, 04:45 PM   #5
tomchr is offline tomchr  United States
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"More than audiophiles" or not, I strongly suggest looking at the spec sheet for the LED to figure the dynamic impedance. Pick an LED with low dynamic impedance at the chosen operating point.

I've been using red (HLMP-D101) and green (HLMP-3507) in the input stage of my 300B amp. They work well at currents in the 10~20 mA range and are less noisy than a resistor would be in the same position. I've run them lower than 10 mA. That works too, but I'd probably rather choose a low-current/high-efficiency LED for that low current.

Do you have measurements that show the noise you're talking about? A scope shot would be handy here...

If you have residual AC on the DC heater supply, I suggest fixing it. Or use switching regulators so you push everything out of the audio band. I use switching regulators on my 300Bs. They're nice and quiet.

But it sounds like you need to take a couple of measurements here... Figure out what's going on.

~Tom
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Old 22nd January 2013, 05:44 PM   #6
euro21 is offline euro21  Hungary
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What is your operating point?
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Old 22nd January 2013, 06:48 PM   #7
chip647 is offline chip647  United States
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I am running the tubes pretty lightly, around 200v on the plate and 15mA for a plate current of around 3 watts. Since it is just a preamp, I am not asking it to swing much signal.

I have a Tektronix oscilloscope sitting in the closet that an EE neighbor gave me. Time to take another run at it. It could be busted as I cannot make it respond to my signal generator.
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Old 22nd January 2013, 07:11 PM   #8
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Hint: try to increase value of the grid stopper. Using LEDs instead of resistor you increase amplification factor, so on higher frequencies it may start oscillating. Sometimes it is heard as a hiss, sometimes as microphonics, sometimes as nasty distortions.
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Old 22nd January 2013, 08:02 PM   #9
euro21 is offline euro21  Hungary
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Belongs to this point (200V, 15ma) Ug is about -15V, thus you need 6-8 LEDs connected in series.
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