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301a hum

70.50.47.4
Posted on May 11, 2012 at 07:03:30
vetmedrobert@gmail.com
Audiophile

Posts: 189
Location: Ontario
Joined: September 20, 2007
 I am using CCS for the plate set for 3.5 ma and am using one of Pete Millett's filament supplies to feed the filaments. Using a 5 volt drop LED to bias the tube. Ground of the filament supply is attached at pin 4 (the negative pin for battery supply according to some data sheets) and the positive filament supply connects to pin 1. LED connects to pin 4. All the LEDs in the CCS light up and I get ~ 100V on the plate. Unfortunately I get very loud hum or buzz from both channels but sounds very promising when playing music loud enough to mask the noise. Any ideas what I am doing wrong? I suspect that it has something to do with the filament supply, but I need some help. Thanks.

 

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RE: 301a hum, posted on May 11, 2012 at 07:20:39
Chris O
 I built a 10/801 preamp and I first tried a LED diode string to set cathode bias. I have a LT1085 current source on the filament and a choke on the plate. That hummed like a son of a pup. I switched to a resistor bias and the noise went away. I believe that either the LEDs were interacting with the current supply or that they are broadcasting a signal to the plate choke. I could probably fix it with a big filament choke after the current source, but I do not have any room left below deck.

You have a lot going on at once. For testing:

1. Ground the grid.
2. Measure the AC on the plate supply with only the plate supply running.
3. Measure the AC on the filament supply with only the filament supply running.
4. Use a resistor to bias the tube.
5. With everything running, measure the AC on each pin.

Once you go through the 5, something should pop out as a culprit.
It could be as simple as you forgot to ground something.
If it is quiet with the resistor bias, try adding the LEDs. If the noise appears, you will know why.

 

RE: 301a hum, posted on May 11, 2012 at 09:20:02
vetmedrobert@gmail.com
Audiophile

Posts: 189
Location: Ontario
Joined: September 20, 2007
 Thanks for your input, I really appreciate it. I had it in mind to change to resistor bias and having done so the noise is gone and this is a very quiet circuit! Really am intrigued as to why the LED was so noisy in this circuit as I have other gear that uses LED for bias and it is dead silent.

 

RE: 301a hum, posted on May 11, 2012 at 11:41:55
Chris O
 I think the problem is the interaction between the LEDs and the regulator. I tried putting in a 50mH hash choke after the reg and it made the regulator oscillate and actually shut down. I think the LEDs present a load for the regulator that it hates.

 

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