"Can he go the full eight seconds?"
...CQ CQ CQDX UNIT BR549 In Phuzzzzzzzzzzzzzit
um , can anyone hear me ?????
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Tube Amp adjustment.
- Thread starter 190
- Start date
"Can he go the full eight seconds?"
...CQ CQ CQDX UNIT BR549 In Phuzzzzzzzzzzzzzit
um , can anyone hear me ?????
Losing a diode in a bridge rectifier basically cuts the output to less than half. Not only that but the filter (caps) has to contend with a seriously asymmetric input . In short , kaput.
The most common reason for diode failure in this application is a a bad (internal short) cap, closely followed by a shorted tube killing a cap, preceded by enough capacitance that the bridge is looking at more or less a short during current inrush on turn on and hap hazard bleeder resistor values. All are a possibility in an older sweep tube amp.
Have fun and keep one hand in your pocket . 1KV "MAY" not kill but it can be seriously annoying and painful.
The most common reason for diode failure in this application is a a bad (internal short) cap, closely followed by a shorted tube killing a cap, preceded by enough capacitance that the bridge is looking at more or less a short during current inrush on turn on and hap hazard bleeder resistor values. All are a possibility in an older sweep tube amp.
Have fun and keep one hand in your pocket . 1KV "MAY" not kill but it can be seriously annoying and painful.
Think of the diodes as a way of "steering" the AC current. On the positive side of the AC wave, one pair of the four diodes conducts, feeding a pulse of DC voltage out to charge up the filter capacitors. This side of the AC voltage dies down, and the polarity now reverses. This cause the OTHER pair of diodes to conduct now. Because of the way the other pair is installed, a pulse of the same DC polarity comes out of the second pair of diodes and into the filter capacitors.
The 60 Hz AC from the transformer has two peaks every 1/60th-second cycle. One positive, one negative. The bridge circuit steers each side of the AC into a DC output. This causes the pulses that charge your filter caps to be 120 per second from the 60 Hz AC out of the transformer's secondary winding.
The phrase "full wave" means that you will draw current from the transformer on both sides of the AC waveform, the positive and negative half both.
I would think that there is some wocka-wocka animation on YouTube that explains this in a more visual way, but that's the short version.
73
The 60 Hz AC from the transformer has two peaks every 1/60th-second cycle. One positive, one negative. The bridge circuit steers each side of the AC into a DC output. This causes the pulses that charge your filter caps to be 120 per second from the 60 Hz AC out of the transformer's secondary winding.
The phrase "full wave" means that you will draw current from the transformer on both sides of the AC waveform, the positive and negative half both.
I would think that there is some wocka-wocka animation on YouTube that explains this in a more visual way, but that's the short version.
73
wow ok nice explanation, i guess that explains the hum and no output, ok if the tubes are ok and i put the new diodes in, then it still dont work whats next ? could i have burned up something else ? hope it passes the next "smoke test" lol
The only way to be any more sure of what you'll get replacing the parts that you already know to be bad would be to test every other part in the amplifier.
Before you power it up next.
Fine advice if you have the stuff to do that.
I have been using a "Variac", or variable-voltage AC power supply for a very long time.You use it to raise the AC voltage feeding the power cord slowly, starting at zero. The amplifier draws only an Amp or less of current on standby, so you put a breaker or fuse rated for only that much in line with it. Naturally, the power-on surge would pop this breaker if you just plugged it in and flipped the switch. But cranking it up slowly from zero prevents this surge. If the amplifier will power up on standby, this represents a test for everything that runs in standby mode. Now install the normal-size fuse and see what happens when you key it.
And if the 1-Amp breaker trips before you can turn it up to full voltage, this limits any surge damage that could be caused by remaining faults you didn't know are there.
A tube tester is a no-brainer. If you have one, you can avoid blowing out parts that can be damaged by a bad tube.
If you don't have one, you're stuck with the traditional "smoke" test method, crossing your fingers and flipping the power switch.
Troubleshooting is a process. Identify a fault, fix it and see if you get the desired result.
If you don't, next identify the symptom.
Next, theorize what fault in the circuit can cause that symptom.
Test the parts that can cause the fault in your theory.
If they're bad, replace them. Power it up to test.
If it works, great.
If not, lather, rinse, repeat.
The more tools you have, the easier it tends to be.
And if you have to change a clutch with only a few wrenches and a scissors jack, it's a lot more work than doing it with all the right tools.
73
Before you power it up next.
Fine advice if you have the stuff to do that.
I have been using a "Variac", or variable-voltage AC power supply for a very long time.You use it to raise the AC voltage feeding the power cord slowly, starting at zero. The amplifier draws only an Amp or less of current on standby, so you put a breaker or fuse rated for only that much in line with it. Naturally, the power-on surge would pop this breaker if you just plugged it in and flipped the switch. But cranking it up slowly from zero prevents this surge. If the amplifier will power up on standby, this represents a test for everything that runs in standby mode. Now install the normal-size fuse and see what happens when you key it.
And if the 1-Amp breaker trips before you can turn it up to full voltage, this limits any surge damage that could be caused by remaining faults you didn't know are there.
A tube tester is a no-brainer. If you have one, you can avoid blowing out parts that can be damaged by a bad tube.
If you don't have one, you're stuck with the traditional "smoke" test method, crossing your fingers and flipping the power switch.
Troubleshooting is a process. Identify a fault, fix it and see if you get the desired result.
If you don't, next identify the symptom.
Next, theorize what fault in the circuit can cause that symptom.
Test the parts that can cause the fault in your theory.
If they're bad, replace them. Power it up to test.
If it works, great.
If not, lather, rinse, repeat.
The more tools you have, the easier it tends to be.
And if you have to change a clutch with only a few wrenches and a scissors jack, it's a lot more work than doing it with all the right tools.
73
Ok i took took the amp to a fellow hams and we replaced the diodes and tested the caps they good but we discovered the wire leaving the caps goes to no where, take a look at the pic is there a wire missing , we applied 80 volts ac current and heard hum and the new diodes started to get hot, so he things the wire on opposite side of the high current goes to ground maybe, he said if he had the schematic he can figure whats wrong and may help me get it going.
trying to post pics not letting me how do i add pics
trying to post pics not letting me how do i add pics
At the top of the text window where you type your reply, there is a toolbar directly above it. The one to the right of the happy face is the 'insert image' button.
73
73
Bottom right hand of where you are posting there should be a button that says "Upload File" It should be to the right of the "Post Reply" button. That allows you to upload an image directly from your computer. You must then choose either thumbnail or full sized image.
Whups! My bad.
I use ImageShack for posting pics. The capn's suggestion works better if you don't use an online image host.
Oops.
73
I use ImageShack for posting pics. The capn's suggestion works better if you don't use an online image host.
Oops.
73
I see rectifiers turned around the wrong way in the top picture.
Not sure how to illustrate where they ought to go with my limited graphic-arts skills.
Only the NON-banded end of diodes in a bridge circuit like this should go to the ground lug. You have the banded end of one pair going to the ground lug.
That's wrong. Just not sure how to illustrate the correct setup clearly.
I'll look for a pic of the "eight-diode" hookup in a D&A amplifier that's clear enough to use as an example.
73
Not sure how to illustrate where they ought to go with my limited graphic-arts skills.
Only the NON-banded end of diodes in a bridge circuit like this should go to the ground lug. You have the banded end of one pair going to the ground lug.
That's wrong. Just not sure how to illustrate the correct setup clearly.
I'll look for a pic of the "eight-diode" hookup in a D&A amplifier that's clear enough to use as an example.
73