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Old 28th February 2006, 01:01 AM   #1
MlinarS is offline MlinarS  Canada
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Default Problem, please help !!!!!!

Hello everyone,

I would appreciate all and any help in solving the problem I have with the amp. I'm working on this problem for a week now and can't find what is going on here.

I built prototypes of 300B SE. Two monobloks. One channel is working allright and the other has some serious problem. Here is description of the problem:

The voltage from the power supply is all corect. The regulator is working corectly. There is 420V on the plate of 300B. The voltage on the cathode is only 24V and the voltage on the grid is NEGATIVE -65V. If I disconect coupling cap between driver and output stage I get the corect voltage on the output stage.

There is one thing that I newer experienced before. I have 50V on all the screws, socet bracets, screws that hold output trafo, even on the cans of the capacitors. The only screws that don't have this voltage on them are the screws that hold the Mains Trafo. The box is built of plywood and all the valves are instaled on a three separate plates made of copper clads. The terminals that are used to screw into the boards are not used for any conection. The capacitors and the transformers are not installed on the copperclad plates. They are screwed into the playwood directly.
I think that this two problems are related but I can't find out where this 50V is comming from. Did any of you guys ever experienced something like this ?

Thank you in advance for any leed in solving this problem.

Here is the schematic of the amp.
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Old 1st March 2006, 12:46 AM   #2
MlinarS is offline MlinarS  Canada
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Problem solved.
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Old 1st March 2006, 11:44 AM   #3
SY is offline SY  United States
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What was it?
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Old 1st March 2006, 04:36 PM   #4
kevinkr is offline kevinkr  United States
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Default Big Safety Issue Here!

Umm, one really important thought comes to mind when reading the original post - all transformer and capacitor cans should be at ground potential which means that there should be a GROUND wire from each one of these devices back to your star ground point. Normally a metal chassis would take care of this issue, but since you are using a plywood chassis you need to provide a path for leakage/fault currents to ground other than yourself.

If one of the output transformers should experience an insulation failure to the core anyone touching it could get electrocuted, the same is true for the capacitor cans as well.

Incidentally the speaker common should also go back the star ground point for safety again for the possibility of insulation breakdown between the primary and secondary interleaves..

A three wire cord with safety ground connected to your star ground is also a really good idea. I don't build anything anymore without effective safety grounding through the power cord!



edit: fix typo, add ac safety ground comment
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Old 2nd March 2006, 03:28 AM   #5
MlinarS is offline MlinarS  Canada
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Hello,

What was it ?
Whell, while soldering some components, small droplet of solder fell down on one of the sokets and made conection with copper side of the board. I didn't notice this until I removed soket from the board.

Kevin, thank you for your concern and advise.

Actualy after I realized that there is voltage on the parts where it shouldn't be any, I went back and connected all this components to the star ground.
I'm using 3 prong cord as you sugested. My star ground is connected to the safety ground trough 100 Ohm resistor. Is this OK or the resistor should be smaller ?

Thank you again for help.

Bob
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Old 2nd March 2006, 03:37 AM   #6
kevinkr is offline kevinkr  United States
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There really shouldn't be a resistor in the safety ground - if a fault occurs most likely this resistor will go open more or less instantly and then anything connected to the star ground will be hot as well.

I have seen instances where anti-parallel diodes of fairly high current ratings were used in commercial amplifiers to break potential ground loops, but I sure wouldn't want to rely on them not failing under fault conditions - usually they short but under very high current conditions you can't count on this.

I recommend you figure out how to eliminate ground loops in your various components without defeating safety ground. I used to believe this wasn't possible, but now all of my components are grounded and hum is not a problem.

Watch out for ground loops with catv connected sources though - you should use an rf isolation transformer between them and the cable if you connect to your hifi..

Kevin
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Old 2nd March 2006, 07:13 AM   #7
MlinarS is offline MlinarS  Canada
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Hi Kevin,
Thank you for the reply. As I mentioned in my original post this is only a prototype. A final product will be made out of some metal and will have all the safety rules followed. I need to think more about ground loops and how to break them. Originaly I had star ground connected directly to safety ground without resistor but the hum was real issue.

Bob
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Old 5th March 2006, 12:34 PM   #8
lndm is offline lndm  Australia
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If you disconnect the driver to o/p grid coupling cap and the problem is fixed, maybe the cap is to blame. It may be damaged or dried out. I have had caps that behave more like resistors than caps. I don't have them anymore...
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