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The Christiansen "DG" 300B Amplifier Build Thread - Page 31

post #451 of 631
Thread Starter 
I ride on the short bus. What's a ground loop breaker?
post #452 of 631

A 10 ohm 5W resistor in parallel with a 0.1uF 250VAC/500VDC capacitor.  This connects to mains ground, and then you connect the output XLR to it.  If the buzzing goes away, then you know you've got ground loops and we can try to fix it.  The ground loop breaker is generally a patch, but it works.  The is allows the voltages between the 2 grounds to be different.  You can also wire 2 1000V diodes in opposite directions in parallel to these as well.

post #453 of 631
Thread Starter 
Excellent !
Thanks.
I will see what I can put together.
Edited by sceleratus - 9/1/13 at 9:23pm
post #454 of 631
Thread Starter 

Holland,

 

Is there a preference?

 

I have three large 3R3 10W and the 0.1uF 630VDC cap

It will be an ugly assembly.... Or I can build it outboard on a Proto-Board

 

I also have two general purpose axial diodes.  1000V 1A  DO41

post #455 of 631
Quote:
Originally Posted by sceleratus View Post


Is there a preference?

 

 

No.  You can just use the resistors and cap right now.  IT doesn't need to be pretty, just a quick test right now.

post #456 of 631
Thread Starter 

I just this moment finished using the 100V diodes, reversed, in parallel.

 

There was not improvement.

post #457 of 631
Thread Starter 

I'm making the Cap / Res config now.

post #458 of 631
Thread Starter 

I have reached Max-Q of my abilities.

mad.gif

Shipping it to an expert.

smile.gif

post #459 of 631

I guess this is too late to help you, but since it used to work OK, and then a hum started, I would have looked for cold solder joints and/or any bad connection.  I would guess something came slightly loose, or you have a bad joint somewhere, and the temperature cycles made it open up.

 

I think the noise you are seeing with the scope isn't the real problem, since you see the noise with things disconnected, sound more like a measurement issue which is diverting your attention.

 

Randy

post #460 of 631
Thread Starter 
Quote:
Originally Posted by randytsuch View Post
 

I guess this is too late to help you, but since it used to work OK, and then a hum started, I would have looked for cold solder joints and/or any bad connection.  I would guess something came slightly loose, or you have a bad joint somewhere, and the temperature cycles made it open up.

 

I think the noise you are seeing with the scope isn't the real problem, since you see the noise with things disconnected, sound more like a measurement issue which is diverting your attention.

 

Randy

Perhaps,

 

I certainly checked the wire connections as well as the board work.  Nothing looks suspect.

The noise comes on fro the moment the power is thrown then the left escalates after the valves have energized.  I have been looking at this for a week.  Regardless of if it's on the scope or not I'm still in the same place.... not being able to identify it.

 

I thought it might be the power transformer but I'm told it's a stinking mess when one goes bad.  That's not the case.

 

 

Whatever it takes, it's going to be fixed.


Edited by sceleratus - 9/3/13 at 4:52pm
post #461 of 631
If it worked once, it will work again.

Put away the scope. You may be mistaking problems of measurement technique for amplifier issues.

Randytsuch had exellent suggestions. I'd reflow every solder joint in the left channel.

Get a can of electronics freeze spray. While listening through your headphones, follow the left channel from beginning to end with the spray. If the hum changes when you spray a particular area, you've found the problem.

Get a small wooden dowel or chopstick. While listening to your headphones, use the dowel to move wiring around to see if the hum changes.

Simplify the signal wiring by going back to a single-ended signal path. You may be inducing a ground loop by your pseudo-balanced signal path.

I doubt the power transformer is an issue. It would affect both channels equally and would probably have shown up right from the start. If you think it might be.disconnect one output transformer and remove it from the chassis. With the output transformer attached only to the headphones, move the output transformer around the energized power transformer and listen for induced hum.

Put the whole project away for a few days and don't think about it. When you return, you'll have a completely different frame of mind and will see things that previously eluded you.

The design is good, it worked before, and you will fix it. When you do you'll feel a sense of triumph and accomplisment that is much greater than if everything had simply fallen effortlessly into place.
post #462 of 631

I may have misread, but it was my understanding that he disconnected the output transformers from tube output and b+ (only connected on the secondaries to the headphones and to mains ground via a ground loop breaker), and he was still getting the same noise.  If that's the case, I don't think reflowing is going to do much.

post #463 of 631

I'm not sure about what happened with the output tranny's disconnected.

 

In general, I agree with Frank, I would concentrate on the left channel, and reflow all the solder joints, even if the look good.

I was also going to suggest pulling on wires to test wired connections, but if you reflow the joints then maybe you can skip this step.  But, I always do a "pull test" after soldering a wire down, found more bad connections than I care to admit with this method.

 

Another test I wanted to suggest is to swap left and right circuits.  For instance, you could swap the left and right inputs to the driver board, to make sure the problem is not before the driver board.  Eliminates some connectors and the stepped atten from the mix.  You could also swap the heater boards (I think I remember separate left and right heaters).  With this kind of troubleshooting, you don't need to worry about looks and such, just make sure you have good connections and no shorts, and see what happens.  I think you already swapped the output trannys?

 

Randy

post #464 of 631
Quote:
Originally Posted by randytsuch View Post


Another test I wanted to suggest is to swap left and right circuits.  For instance, you could swap the left and right inputs to the driver board, to make sure the problem is not before the driver board.  Eliminates some connectors and the stepped atten from the mix.  You could also swap the heater boards (I think I remember separate left and right heaters).  With this kind of troubleshooting, you don't need to worry about looks and such, just make sure you have good connections and no shorts, and see what happens.  I think you already swapped the output trannys?

Randy

X2
post #465 of 631

If the issue is only present in one channel, then it is NOT anything in the power supply so bias circuit as those are shared between the two channels. 

 

No amount of shielding will do anything for 60 Hz hum induction. Unless we're talking shielding using µ-metal. EMI is not likely to downconvert to 60 Hz by chance. 

 

With all the messing and custom silver wiring that's been going on the the input end of the amp, I bet you have a broken wire or cold solder joint somewhere in the left side. 

 

Turn the volume all the way down. Measure the anode voltage (pin 6) of the input tube in the left channel using your o'scope (high-impedance inout, AC coupled). If you have hum there (compare with the corresponding voltage in the right channel), then the hum comes in through the input of the left channel.

 

If it worked before, it'll work again. The two amps I built of that design have hundreds of hours on them by now. And that doesn't account for the time they spent in the DeathTrap prototyping stage (i.e. point-to-point wiring on a piece of plywood). But then, I haven't done all sorts of modding on mine either. I do that during the prototyping stage.

 

~Tom

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