Valve/Tube Safety & Bleeder resistors

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robomatique

Well-known member
Joined
Jun 6, 2004
Messages
145
Location
London / UK
Hello,

I am just about to start my first tube project, the G9, and I am a bit worried because I have never done any tube stuff before.

As I understand it, I should use a resistor to bleed off the capacitors to be sure there is no voltage left in the system. Could someone explain this in detail,? I really want to get this absolutely right and to be as safe as possible.

Somewhere I have seen a small 'bleeder board' that you can build to make everything easy to use when working. Anyone knows what I am talking about?

Robert
 
It is much better to install permanent bleeder resistors.
I use 220 k 1 wat resistors on the first B+ cap.
Takes a while to discharge, check with your voltmeter to watch the resistor in action after pwr down to give you a realtime figure for discharge time.
cj
 
All you need is a resistor with long leads (I usually use a 1K 10W because I have a bunch lying around). Or if you prefer you can attach insulated alligator clips to the leads. Then connect one end to the positive lug of the cap and one end to the negative end. Give it a minute to discharge (or longer depending on the value of the cap). Your best bet is to always measure it with a voltmeter to make sure it still isn't holding any juice.

-E
 
On my desk, I keep an extra voltmeter with the sole purpose of monitoring HT voltage on the current project. That allows for a quick overview of danger. A cheap digi-voltmeter is fine for this, I just mounted alligator-clips on it's wires.

When discharging, it's always a good idea to watch the voltage fall slowly. If you just measure when you think it should be discharged, you won't be really sure that your voltmeter behaves correctly..

Take care,

Jakob E.
 
[quote author="gyraf"]
When discharging, it's always a good idea to watch the voltage fall slowly. If you just measure when you think it should be discharged, you won't be really sure that your voltmeter behaves correctly..

Take care,

Jakob E.[/quote]

Don´t understand why... Can you tell me more, Jakob?
 
..because this will also show you if your voltmeter is connected correctly, set and working right, and has proper battery power. I don't trust a single measurement that says "0V".

Watching the meter slowly go towards zero is a simple way of fail-safing all this.

Jakob E.
 
I soldered a big 'ol 100R 2W resistor to a test lead with an alligator clip on the end and shrinkwrapped the lot. I attach the clip to ground and bleed the cap through the resistor tip. I (almost :? ) always check with a meter to see if it's drained nicely.

cheers,
kent
 
[quote author="Admin"] Or if you prefer you can attach insulated alligator clips to the leads. Then connect one end to the positive lug of the cap and one end to the negative end. Give it a minute to discharge (or longer depending on the value of the cap). Your best bet is to always measure it with a voltmeter to make sure it still isn't holding any juice.

-E[/quote]
Yes, and leave it connected until you're finished --caps have a nasty habit of recharging themselves (another good reason for the permanent-bleeder).

-Eric
 
[quote author="soundguy"]can anyone provide a napkin drawing of how to correctly install a permanent bleeder resistor?

thanks

dave[/quote]

Bumping this one, considering a permanent bleeder resistor on my G9
 
bleeder.jpg
 
I still use a screwdriver. Although I might finaly switch to a bleeder. Had an amp chassis sitting in my lab and my bare foot was touching it when shorted the cap to ground and my foot. That was a nice surprise.

adam
 
I know this is an old topic, but generally;

How do I select the right value for bleeder resistor?
are there any downsides of using something big ohmic values(besides long discharge time)?
 
adamasd said:
I still use a screwdriver. Although I might finaly switch to a bleeder. Had an amp chassis sitting in my lab and my bare foot was touching it when shorted the cap to ground and my foot. That was a nice surprise.

adam

A screwdriver? ??? ??? ???

I found out about this not-so-fun-trick when working on a tube guitar amp a number of years ago. The screwdriver shaft literally vaporized in a huge blue corona as I threw what was left across the room in an extreme fit of "WTF" way before wtf was in the popular vernacular.

In addition to vaporizing the tool it also burned this lesson permanently into my organic ram sticks which I hope can be passed along and save another life via preventative procedures.

For safety's sake please don't use a screwdriver to bleed the caps.... Use a high value bleeder resistor as recommended here. ie; 100k, 200k... And monitor the voltage in the cap with a meter like Jacob described.

Cheers!
-jonathan
 
kooma said:
I know this is an old topic, but generally;

How do I select the right value for bleeder resistor?
are there any downsides of using something big ohmic values(besides long discharge time)?

It is a trade off between speed of discharge and and wasting power. The bleed resistor is always there so it is like an additional load on your HT supply. If you design it for a quick discharge time it will consume more power than for  longer discharge time. Also, if you have some big electrolytics in the HT supply to get the ripple right down for a preamp for example, then these will take longer.

I used to use 270K on my 300V supplies so it wasted only 1mA when in use but it can take several minutes to discharge. I now use a heater elevation pot divider across the HT supply that adds 88K across in and so discharge much faster but it does waste  nearly 4mA.

Cheers

Ian
 
ruffrecords said:
kooma said:
I know this is an old topic, but generally;

How do I select the right value for bleeder resistor?
are there any downsides of using something big ohmic values(besides long discharge time)?

It is a trade off between speed of discharge and and wasting power. The bleed resistor is always there so it is like an additional load on your HT supply. If you design it for a quick discharge time it will consume more power than for  longer discharge time. Also, if you have some big electrolytics in the HT supply to get the ripple right down for a preamp for example, then these will take longer.

I used to use 270K on my 300V supplies so it wasted only 1mA when in use but it can take several minutes to discharge. I now use a heater elevation pot divider across the HT supply that adds 88K across in and so discharge much faster but it does waste  nearly 4mA.

Cheers

Ian

Ok, thanks!
So capacitance  doesnt matter much?
I've got some big-ass-caps for my tube heaters that got me searching for info:)
maybe I just juse something like 100K for HT and 5-10K for heaters.
 
a bleeder does three things that i know of,

1-bleeds the caps for safety
2-improves regulation of the power supply
3-keeps excessive voltage off the caps during power up

tubes will not conduct til they warm up, if you use solid state rectifiers then there will be no delay of the B+ voltage due to a rectifier tube warming up, this means the caps will see excess voltage until the warm tubes drop this voltage by way of the resistance in the xfmr sec..

modern caps do not seem to mind a little over voltage for a few seconds,
but resistors are so cheap, so WTF, over?  ;D

 

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