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Bleeding power out of tube amp??

  • Thread starter 1Misplaced Yankee
  • Start date
1

1Misplaced Yankee

Guest
Hello All,

I have a maco 200. I need to change the relay.

What's the proper way to bleed the power out of a tube amp?

Thanks, MpY




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It is hard to believe that this question was on here for almost 38 years and never was answered. I wonder if the guy is still alive waiting for an answer. I am no tech, but here is what I do. I have a jumper with a large resistor 1 meg that I clip to ground and then to the top cap then wait a little while for it to discharge. I usually leave a couple of jumpers grounded while I am inside the amp.
 
I think there may have been a goof in the date, or maybe it's a re-post from somewhere. This forum hasn't been around for 38 years. Neither have computers and/or the inter net (no matter what 'whats-his-name' says)...
- 'Doc
 
It is hard to believe that this question was on here for almost 38 years and never was answered
Click to expand...

This Question was part of the transfer form the old forum, and there is a good chance that the answer was lost in the transfer.....things were kind of buggy at that point.


73
Jeff
 
If you have to ask this question and don't know how an VOM meter works, leave it to an qualified technician to do the job.
The high voltage inside an tube amp can kill.
 
And just to string this along a little, and to maybe clarify a misconception, there is no voltage stored in a tube. There can and usually is a lot being applied to that tube from the capacitors (which do store power) in the power supply. Those capacitors are doing just what they are supposed to do, nothing wrong with them. And just like people, even when you stop feeding them (turn the power supply off), it can take a while to get rid of what they have already been fed, sort of. Putting your fingers in that "discharge" can make you very, very unhappy, so don't! Even if you live over it, it's a real 'crappy' experience.
Discharging high voltage requires that the 'hot' conductor should be grounded. The chassis ground is usually where that grounding is done. The 'biggy' is to use something that is insulated, has an insulated 'handle', to do that grounding. An unused (or maybe over-used) screw driver is the most common tool used to do that. There is also a definite 'order' in making that ground connection! Touch the chassis ground (and KEEP that contact), THEN touch the plate cap(s) of the tube(s). Do it the other way around means you will probably get a very nasty surprise! And just to be sure... do it again. It's fairly common to hear and see when that discharge occurs, snap/bang/whatever, maybe even a bright spark. And if you are as 'chicken' as me, you'll wait a minute or two and do it again just to make sure, sort of. That stuff really does bite! It's also a sort of good idea to keep one hand in your pocket while doing that discharging, but don't 'hold' anything in that hand while you're doing that. Muscles contract when subjected to electricity and can produce some interesting results, sort of.
Hey, it's your neck (or other anatomical parts), treat it like you want it to be treated.
- 'Doc


Have I ever... Just once with a 6146, which has quite a bit less voltage than typical final tubes, and I will absolutely guaranty I will never do it again! No, I wasn't 'holding' anything... thank goodness!
 
I have a nice big long screwdriver with an electrically insulated handle, I then clamp a wire and resistor and a big strong battery cable clamp together. I clamp the chassis with one end and touch the leads of the cap banks with the other, in amps that are wired point to point i touch the lead, causing a spark, then push the screw driver forward more touching the chassis under the lead making sure its empty.

big big HV caps can also be charged by static electricity. When not in use you should have big caps jumpered out/shorted across the terminals for safety. don't just blindly trust the bleeder resistors
 
Last edited:
linearone said:
.....don't just blindly trust the bleeder resistors
Click to expand...

VERY good advice. While working alone on a broadcast transmitter that ran 2700 volts on the plates ( it was a callin in the wee hours of the morning) I found that the 100K ohm 100 watt bleeder was physically broken in half and offered NO protection whatsoever.Matter of course dictated that I ALWAYS used the shorting stick inside the TX cabinet anyway but never saw a spark any other time. That time it surprised me and made me think about what could have happened if I broke protocol just that one time. :eek:
 
AFTER you have taken their advise about how to short the high voltage out, do this: Locate the B+ end of the capacitor string and solder a couple of 25 kilohm (strung in series to make 50 Kilohm) from there to ground, that will instantly end the need to manually bleed it down.
 
Slam Dunk said:
AFTER you have taken their advise about how to short the high voltage out, do this: Locate the B+ end of the capacitor string and solder a couple of 25 kilohm (strung in series to make 50 Kilohm) from there to ground, that will instantly end the need to manually bleed it down.
Click to expand...

There have been many fatalities involving "technicians" who trusted the bleeder resistor and didn't use a VOM to make sure the HV was discharged.
 
Slam Dunk said:
AFTER you have taken their advise about how to short the high voltage out, do this: Locate the B+ end of the capacitor string and solder a couple of 25 kilohm (strung in series to make 50 Kilohm) from there to ground, that will instantly end the need to manually bleed it down.
Click to expand...


Apparently Slam Dunk did not read my post directly above his. Either that or has no regard for life even if it is his own. :angry:
 

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