September 22nd, 2018, 11:02 am
September 22nd, 2018, 11:15 am
Grover Gardner wrote:Curious if someone can answer this. It's quite basic but I have trouble with some of the basics, as we know.
So I'm playing around with single-ended 300B again. Decided to try DC on the filaments to reduce noise. I used a full-wave bridge block and a filter cap, very simple. I left the 25 ohm balance pot in place to create a center tap. Voltage dialed in just right and I listened for a while--still some residual DC, needs more filtering, blah blah. Then I wanted to compare to AC filaments again, so I removed the filter cap and simply connected the AC leads to the output tabs on the bridge, thinking I've just bypassed the bridge.
But have I? Are those diodes still doing something? Because now, when I use the hum balance pot I can null out all noise--it's almost dead quiet, even the DC component. I like it, but the question is, what am I doing by leaving a full-wave bridge across the AC filaments? And should my wife take my soldering iron away from me?
September 22nd, 2018, 11:22 am
Grover Gardner wrote:Curious if someone can answer this. It's quite basic but I have trouble with some of the basics, as we know.
So I'm playing around with single-ended 300B again. Decided to try DC on the filaments to reduce noise. I used a full-wave bridge block and a filter cap, very simple. I left the 25 ohm balance pot in place to create a center tap. Voltage dialed in just right and I listened for a while--still some residual DC, needs more filtering, blah blah. Then I wanted to compare to AC filaments again, so I removed the filter cap and simply connected the AC leads to the output tabs on the bridge, thinking I've just bypassed the bridge.
But have I? Are those diodes still doing something? Because now, when I use the hum balance pot I can null out all noise--it's almost dead quiet, even the DC component. I like it, but the question is, what am I doing by leaving a full-wave bridge across the AC filaments? And should my wife take my soldering iron away from me?
September 22nd, 2018, 12:05 pm
September 22nd, 2018, 4:07 pm
GaryB wrote:Grover,
When you connect the transformer to the output tabs of the bridge rectifier, you have 2 diodes in series across the transformer. On one half of the AC cycle, these diodes are reverse biased and effectively out of the circuit. On the other half of the cycle, the two diodes clip the voltage at ~ 2 diode drops or ~ 1.2 volts. A 5V AC transformer has peak to peak voltage of 5v x 1.414 = 7.07v or referencing things to ground, the AC voltage will swing +- 3.535v. So on one half of cycle you'll be clipping the AC, which is why you're measuring the reduced voltage.
---Gary
September 22nd, 2018, 4:09 pm
SoundMods wrote:Grover Gardner wrote:Curious if someone can answer this. It's quite basic but I have trouble with some of the basics, as we know.
So I'm playing around with single-ended 300B again. Decided to try DC on the filaments to reduce noise. I used a full-wave bridge block and a filter cap, very simple. I left the 25 ohm balance pot in place to create a center tap. Voltage dialed in just right and I listened for a while--still some residual DC, needs more filtering, blah blah. Then I wanted to compare to AC filaments again, so I removed the filter cap and simply connected the AC leads to the output tabs on the bridge, thinking I've just bypassed the bridge.
But have I? Are those diodes still doing something? Because now, when I use the hum balance pot I can null out all noise--it's almost dead quiet, even the DC component. I like it, but the question is, what am I doing by leaving a full-wave bridge across the AC filaments? And should my wife take my soldering iron away from me?
A full wave bridge provides 120-Hz. pulsating DC that needs to be filtered to smooth to straight DC.
September 22nd, 2018, 5:12 pm
September 22nd, 2018, 7:07 pm
September 23rd, 2018, 12:28 pm
September 23rd, 2018, 12:56 pm