Bleeding power out of tube amp??

[W5LZ]

The one thing I've learned in the time I've spent 'playing' with this sort of stuff is that you can NOT trust things to be as they should be (if they were, what are you doing poking around in there anyway?). I seem to learn 'best' by making mistakes. So far, those mistakes have not been fatal (I think?). Painful (as in 'ouch' and $$$), yes, fatal, not yet! All I can say is that it's your 'neck', treat it as you want it to be treated. If it absolutely has to be fixed right now, you are probably going to regret it. "Hurry" always has a price. It may not be much, but then, it certainly can be...
- 'Doc

Oh, yeah. If you don't know what you are supposed to be doing, don't do it. It's much harder to think of things before you have to, naturally. But that 'thinking' can save you all sorts of stuff. If you wonder, I will have lots of regrets if your goof is fatal. That'll make it all worthwhile, right??

 

[Slam Dunk]

While you guys were busy telling him how to stick a screw driver in and possibly send a spike back to his rectifiers and weaken them, I took the trouble of looking at the circuit and saw that it is built without the benefit of balancing resistors or a bleeder-load resistor (I build ALL my B+ suppliers with both and a B+ meter). What I suggested was that after he made it safe as you guys had instructed, that he put a bleeder system in.

A 50 kilohm resistor to drop the 900 volts would draw .018 ma and generate 16 watts of heat, 25 watt power resistors would suffice.

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.

I would question the builder about why he only used a 100 watt bleeder; the 2700 volts through the 100 K resistor would generate 81 watts of heat. It was irresponsible of the builder to use such an under rated part in a critical situation.

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.

I always build a HV metering circuit in any thing I do that runs B+ to tubes.

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.

Loooook, I have PTSD after getting hit by a JB-2000 Blackcat still to this date, 20 years later. I can be walking down the street and out of the blue, I am reliving the feeling of getting hit with over 3000 volts DC. I was out cold for 10 minutes, when I came to, I looked to see what I had left; It burnt holes in me.

The next day, I called everybody and told them to come and get their stuff NOW, I didn't do any radio work for at least 6 months.

The one thing I've learned in the time I've spent 'playing' with this sort of stuff is that you can NOT trust things to be as they should be (if they were, what are you doing poking around in there anyway?). I seem to learn 'best' by making mistakes. So far, those mistakes have not been fatal (I think?). Painful (as in 'ouch' and $$$), yes, fatal, not yet! All I can say is that it's your 'neck', treat it as you want it to be treated. If it absolutely has to be fixed right now, you are probably going to regret it. "Hurry" always has a price. It may not be much, but then, it certainly can be...
- 'Doc

Oh, yeah. If you don't know what you are supposed to be doing, don't do it. It's much harder to think of things before you have to, naturally. But that 'thinking' can save you all sorts of stuff. If you wonder, I will have lots of regrets if your goof is fatal. That'll make it all worthwhile, right??

AMEN!

 

[Captain Kilowatt]

While you guys were busy telling him how to stick a screw driver in and possibly send a spike back to his rectifiers and weaken them, I took the trouble of looking at the circuit and saw that it is built without the benefit of balancing resistors or a bleeder-load resistor (I build ALL my B+ suppliers with both and a B+ meter). What I suggested was that after he made it safe as you guys had instructed, that he put a bleeder system in.

A 50 kilohm resistor to drop the 900 volts would draw .018 ma and generate 16 watts of heat, 25 watt power resistors would suffice.



I would question the builder about why he only used a 100 watt bleeder; the 2700 volts through the 100 K resistor would generate 81 watts of heat. It was irresponsible of the builder to use such an under rated part in a critical situation.



I always build a HV metering circuit in any thing I do that runs B+ to tubes.



Loooook, I have PTSD after getting hit by a JB-2000 Blackcat still to this date, 20 years later. I can be walking down the street and out of the blue, I am reliving the feeling of getting hit with over 3000 volts DC. I was out cold for 10 minutes, when I came to, I looked to see what I had left; It burnt holes in me.

The next day, I called everybody and told them to come and get their stuff NOW, I didn't do any radio work for at least 6 months.



AMEN!

Shorting out the filter capacitor with the gear turned OFF should not in anyway harm the rectifiers.I have seen commercial gear that automatically shorts out the filters everytime the gear is powered down and no additional protection is offered to the rectifiers.My RCA broadcast transmitter in my basement (80m project) shorts out the HV every time one of four panels is opened. If the power is still on then that is another story.Actually it has survived that many times as well. I had a hidden faulty safety switch locked in the closed position and the HV was shorted out causing the plate overload relay to trip everytime I turned it on.With 3000 volts that HAD to be hard on the rectifiers however the resistance of the HV windings on the transformer offer some protection albeit little.I would suggest that anyone that has to ask how to "bleed the power out of an amp" does not posess the skills to properly add a bleeder and HV metering circuit in the first place. No offence meant to the original poster BTW.


Better redo the math. It works out to 72.9 watts for 2700 volts which really ran about 2450 under load so that becomes 60 watts of continous heat dissapation which is well within the limits especially considering the air flow that was present.Since that was the first and only case of a broken bleeder without ANY signs of overheating I have even seen I would NOT say that the resistor was underrated but rather was faulty,perhaps cracked from physical contact at some time.


My basic point was and still is that no one should EVER just blindly trust a bleeder to do it's job.When you 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." you made NO mention of monitoring the HV. It is for that reason I posted what I did when I said " Apparently Slam Dunk
did not read my post directly above his. Either that or he has no regard for life even if it is his own".
Remember, the original poster has NO apparent experiance with HV.


I spent 22 years as a station engineer in the commercial broadcasting industry and over that time I have learned to have a healthy respect for high voltage.Take nothing for granted and leave nothing to chance. Only once did I ever get bitten badley and that was not even at work but rather when my hand slipped while testing the plate supply on my Heath DX-60B transmitter.I got the 600 volts in my right thumb and fingers and out my right elbow.I have gotten many RF burns however.