I am going to install a bleed down resistor on the Fisher 80-AZ, but am curious if I use a relay then bleed down, do I need 4 separate relays, or can that actually cause damage. Here's the schematic of the 80-AZ: Fisher 80AZ Photofact
If you are saying and its only a guess,
You are going to use a relay with NC contacts to connect bleed resistors across the B+ at power off.
Not a good Idea.(Why)
If the coil fails on the relay it will connect the bleeders to the B+ with power on.
If the current draw is high for quick discharge then it could damage the power supply Tx winding.
The next point is if the relay sticks then you could expect B+ to be down and it is still up.
The next point is the bleeder resistors should be in circuit all the time to ensure B+ comes down if the heaters fail on the amp. Because it will have a slow discharge (bleeder only) no hot tubes drawing current.
The other thing is ensuring the relay contacts can open (break) the voltage across them..how are you going to ensure the relay is working before B+ comes up?
Regards
M. Gregg
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And what if resistors fails??? I saw thousands of open resistors, even high value and high power rate.
The tubes will continue to conduct plate current for a number of seconds after the amplifier is powered down. Have you measured the B+ after turning the unit off? How far does the B+ drop?
After all these years, why does this amplifier need additional bleeder resistors?
After all these years, why does this amplifier need additional bleeder resistors?
I've wondered about using a DPDT switch to short the B+ at power off (no resistor). Any issues doing that?
You are joking I assume?
You are shorting B+ that could be high under fault conditions.
Think about the energy you are dissipating in the switch contacts and possible damage to the B+ caps. What stops you switching the switch with the amp on?
The switch will probably weld closed.
If the heaters fail to come on you will have high B+ after power off.
If the switch closes with the power TX on and puts a short on the power Tx HT winding it will burn out quite quickly.
If you have a tube rectifier it will damage that as well.
Regards
M. Gregg
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No no you misunderstand and I didn't explain well. We short B+ to drain caps before working on it, right? I have a test board going and I've thought about using some sort of DPDT switch that opens the mains connection while closing the B+ short, and vise versa, so that the same action of powering down also drains the caps.
You should not short the caps.
You should use a discharge lead with a resistor in it.
And never rely on shorting it out use a meter to test for dead.
When you short caps you create a short path that can stress the insulation inside the capacitors.
Putting a temp short on after discharge is risky if you forget to remove it.
Caps have a habit of recharging after a short and can come up again after its removed.
If you have bleeders fitted the caps should discharge anyway after a while!
Bleeders should always be fitted even if its a larger value, the caps must have a discharge path.
It stops recharge and accidental contact after a time away from an amp.
Regards
M. Gregg
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Absolutely. I do, every single time.
Was just thinking of trying to protect against forgetting to remove the discharge lead when powering back up😱.
I've never forgotten to, but it was just something that crossed my mind
If you use full time rated bleeders (always connected in circuit) then the discharge lead is just to be sure.
If you want to use the idea to clamp the B+ after power off its an addition to existing bleeders. I would still fit a resistor or make sure you have close protection with fusing on the HT.🙂
You should not have a problem unless its B+ with high caps in the rail that also means current under fault or shock conditions. Then I would have special precautions.
Regards
M. Gregg
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Regardless, you should never add a short circuit across the supply cap. The reasons for this were explained earlier. Use a discharge resistor.
~Tom
~Tom
I'm now remembering a shop tech telling me that he even puts two resistors in parallel with each other and in series with the discharge lead in case one fails.
What size resistor? Also would you use the same value for a discharge lead as you would for permanent bleeders?
What size resistor? Also would you use the same value for a discharge lead as you would for permanent bleeders?
I do mine with the discharge resistor set to run at around 3ma through an LED. The LED gives a positive indication that the bleed circuit is complete...
Is there a consensus then that using bleed down resistors without a relay are the way to go? In my previous post asking a couple questions about bleed downs, I was given the advice of a relay, and believed it to be the best option. Here is that thread: Two questions concerning bleed resistors. In fact 3 forum members agreed it was a good idea.
There is only one best way to do anything. Which is best?
There is only one best way to do anything. Which is best?
You need more chefs in kitchen 🙂. It looks like the issue was if the bleed was too great then a relay to switch it off would be nice but then DF96 suggested a permanent bleeder for safety. I would suggest going for a permanent small bleeder like Tomchr recommended at first and see if you like it. If you want it to bleed even faster after you switch it off you can try the relay to switch in an out an even smaller (smaller resistance right?) bleeder in addition to the permanent bleeder. I personally don't like relays for switching DC, but that's just me. -Fred
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Best depends on what you are trying to achieve. There is no BEST way, but there are a few good maxims for safety stuff - it should be simple, reliable, and unavoidable. Outside of that, do what you like.
You have to remember that,
The bleed resistor is also a load and draws current. So you have to make sure you don't over load the rectifier if its tube.
I always fit permanent bleed resistors to PSU caps.
So the value is what you are happy with trade off with heat dissipation/discharge time and load.
Even if you don't have much spare current with a rectifier a 470K will eventually pull it down. ie its better than nothing. So use Ohms law and work out what current and heat you are going to dissipate. Its also OK to use two in parallel then if one goes open you will still have the other all be it now at a higher value.
Just remember that the resistor has to have correct voltage rating.
You should always test for dead and ignore the fact that you have a bleeder. Just in case it has failed.
It can be that distraction can cause you to answer the phone and forget that you have just switched off, then come back and put your hands on the supply.
The bleeder will pull it down even if it gets you at a lower level because you forget.<<that's the discharge time how long you make it!
Regards
M. Gregg
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Yes. In addition, don't rely on it: what I mean is always check with a meter that the voltage really has gone before fiddling inside. A safety feature you rely on must always have a backup in case it fails.aardvarkash10 said: