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Old 8th March 2008, 04:24 PM   #1
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Default need to discharge psu safely

Hello tube people.
I am working on a Laney LC15 guitar practice amp.It was dead except for the power LED.It was bought from ebay and got a real rattling by the courior,a speaker wire had fallen off,all the valves were crooked and I suspected a cracked PCB or dry joints.
When opened I found a huge dry joint where the main HT comes back onto the PCB on a wire from the fuse.I touched the wire with my finger and got a good dig from it,even though it was unplugged for about 2 hrs before I opened it.I have 2 questions.

#1 Can someone tell me what sort of resistor value to use to discharge the main smoothing cap(50u,500v).Schematic shows 300v on this main HT line.
#2 Even after soldering this bad connection the amp does not work.I suspect more dry joints which is why I want to discharge it so I can handle it .Is it possible the bad connection on the main HT line caused spikes which damaged the valves?(I have no experience with tubes,but am aware of the dangers and want to learn more)
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Old 8th March 2008, 05:53 PM   #2
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Aidan,

If I remember correctly, I use 220K or higher as cap bleeder resistors. I solder them and leave them in place. If you assume 300V, then you need around a 1/2W resistor. However, I use something like 2W as I simply feel better about the large headroom and they don't cost much anyway.

You can use the following website to calculate power dissipation easily, if you don't have a calculator on hand.

http://ourworld.compuserve.com/homep...en/ohmslaw.htm

One thing you could do is to use your voltmeter to watch the voltage drop when you switch off the amp before you add resistors, then again after. It may give you a little confidence that the bleeder resistors are functioning. However, I always try to avoid touching any cap contacts even with the bleeders in place.

I have also heard that a lightbulb conected across the caps can be used to discharge the caps. I used this method when building my AKSA amp. You could even use this method with bleeders in place.

As you can tell, I am very cautious. Afterall, I do like to listen to my modifications, something that is impossible from six-feet under!

Finally, if your caps are old, then even after discharge, they may develop reisidual charge. Depending upon how good the caps look, you may simply want to replace them as the cost to do so may be pretty reasonable.

Good Luck,

Charlie
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Old 8th March 2008, 06:33 PM   #3
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Another little thing to consider - instead of getting out the multimeter, get a high voltage lightbulb, small little thing. Attach it across the cap terminals and when the bulb goes out.....



*edit* genius here didn't spot the above post
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Old 8th March 2008, 07:33 PM   #4
cuibono is offline cuibono  United States
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The above advice is good - a permanent bleeder resistor is a good idea. First off: tube circuits, being high voltage, are particularly 'dangerous' - they need special caution when messing with them. I highly suggest you read up (if you haven't ). Second, you can make sure things are safe with a voltage meter - just measure across the caps terminals, and if the voltage is zero (or less than .5V), you're fine (as long as no other caps are charged).

Third, the answer you want - I've got a 47ohm 1/2W resistor permanently soldered to insulated alligator clips on a lead to discharge caps with. It discharges large caps within a few seconds, and (I think) safely. Just watch the voltage across the terminals, and remember big caps recharge on their own after being discharged...

Good luck with your amp.
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Old 8th March 2008, 07:39 PM   #5
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Thankyou all for the advice.
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Old 8th March 2008, 08:29 PM   #6
poynton is offline poynton  United Kingdom
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Default Re: need to discharge psu safely

Quote:
Originally posted by Aidan135711
.Is it possible the bad connection on the main HT line caused spikes which damaged the valves?(I have no experience with tubes,but am aware of the dangers and want to learn more)
Very unlikely. you are probably correct in thinking more bad contacts.

Do the tubes light up ?

Andy
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Old 8th March 2008, 10:03 PM   #7
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Hey Andy
Just now checked tube heater supply,fuse was not blown as such but was "cooked"and brownish at one end and was open circuit.
I looked at this earlier but the filament looked intact but before measuring with the power on I had one more check with the ohm meter and found this.
Now the wee amp makes a good sound but the master volume has no effect!Will have another look tomorrow,have had too much beer for tonight.
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