need to discharge psu safely - diyAudio
Go Back   Home > Forums > Amplifiers > Tubes / Valves
Home Forums Rules Articles diyAudio Store Gallery Wiki Blogs Register Donations FAQ Calendar Search Today's Posts Mark Forums Read

Tubes / Valves All about our sweet vacuum tubes :) Threads about Musical Instrument Amps of all kinds should be in the Instruments & Amps forum

Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Thread Tools Search this Thread
Old 8th March 2008, 04:24 PM   #1
diyAudio Member
Aidan135711's Avatar
Join Date: Feb 2008
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)
  Reply With Quote
Old 8th March 2008, 05:53 PM   #2
diyAudio Member
Join Date: Oct 2006

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.

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,

  Reply With Quote
Old 8th March 2008, 06:33 PM   #3
diyAudio Member
Join Date: Jul 2005
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
  Reply With Quote
Old 8th March 2008, 07:33 PM   #4
cuibono is offline cuibono  United States
diyAudio Member
cuibono's Avatar
Join Date: May 2006
Location: City of Angles
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.
  Reply With Quote
Old 8th March 2008, 07:39 PM   #5
diyAudio Member
Aidan135711's Avatar
Join Date: Feb 2008
Thankyou all for the advice.
  Reply With Quote
Old 8th March 2008, 08:29 PM   #6
poynton is offline poynton  United Kingdom
diyAudio Member
poynton's Avatar
Join Date: Mar 2005
Location: UK
Default Re: need to discharge psu safely

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 ?

  Reply With Quote
Old 8th March 2008, 10:03 PM   #7
diyAudio Member
Aidan135711's Avatar
Join Date: Feb 2008
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.
  Reply With Quote


Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off

Similar Threads
Thread Thread Starter Forum Replies Last Post
Safely using large router bits? ShinOBIWAN Multi-Way 25 21st April 2008 08:10 PM
how much capacitance can i add safely? Clipped Car Audio 8 28th September 2007 10:40 PM
gainclone PSU- how to safely discharge caps? sansbury Chip Amps 11 27th March 2007 09:05 PM
Can I safely increase capacitance by 47%. Cyrus PSX. Sonusthree Power Supplies 4 17th February 2007 10:17 PM
Please Help: How to safely bring to life NOS Capacitor? dexter Solid State 8 6th July 2001 11:57 PM

New To Site? Need Help?

All times are GMT. The time now is 05:10 PM.

vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2017 DragonByte Technologies Ltd.
Copyright 1999-2017 diyAudio

Content Relevant URLs by vBSEO 3.3.2