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Discharging Capacitors In Vacuum Tube Amps

ZeusMC

Member
As in the attached screenshot, the same 100 ohm 5 watt resistors are available on Ebay. I was wondering about leads though, the ones I see on Ebay, their voltage rating is 30V ac and 60V dc. Would these leads be suitable for discharging tube capacitors?
Thanks.
 

Attachments

JimB

Super Moderator
Most Helpful Member
Would these leads be suitable for discharging tube capacitors?
Yes, is the quick answer.

The longer answer is that it may be cheaper to buy a packet of small croc clips and a reel of wire and make your own leads.

JimB
 

JimB

Super Moderator
Most Helpful Member

ZeusMC

Member
Yes, is the quick answer.

The longer answer is that it may be cheaper to buy a packet of small croc clips and a reel of wire and make your own leads.

JimB
I saw these on Ebay, 10 x Insulated Crocodile Clips Electrical Battery Connectors 5A Alligator Clamps £2.29 item number 203075604303.
I then had a look at this YouTube, Making your own high quality custom test lead wires and jumper cables. As as I was watching I thought it could be cost effective to get a suitable reel of wire, that I could also use for wiring up my own vacuum tube projects...... when I'm experienced and knowledgable enough to start :) Can someone advise me, so what gauge of wire could I use for capacitor discharging leads and also wiring up tube amp projects?
Cheers.
 

unclejed613

Well-Known Member
Most Helpful Member
any well designed tube amp has bleeder resistors in the power supply, and as long as there's nothing wrong with them they usually discharge the caps to 10V or less in about a minute or so... a 100 ohm resistor for discharging a 350 to 500V supply is still going to spark, and i don't think a 100 ohm 5W resistor would last very long( 3.5A through the resistor, at a voltage of 350V gives in excess of a 1kW power spike)... a 100K 5 watt would work better, no sparking, and well within the thermal limits of the resistor.
 

schmitt trigger

Well-Known Member
The sparking noise while discharging a 400 volt, 100 uF capacitor is very uncomfortable.
However, it is far more uncomfortable discharging it through your finger.

Ask me how I know.:(
 

ZeusMC

Member
any well designed tube amp has bleeder resistors in the power supply, and as long as there's nothing wrong with them they usually discharge the caps to 10V or less in about a minute or so... a 100 ohm resistor for discharging a 350 to 500V supply is still going to spark, and i don't think a 100 ohm 5W resistor would last very long( 3.5A through the resistor, at a voltage of 350V gives in excess of a 1kW power spike)... a 100K 5 watt would work better, no sparking, and well within the thermal limits of the resistor.
" any well designed tube amp has bleeder resistors in the power supply, " OK thanks I'll make a note of that.
 

Nigel Goodwin

Super Moderator
Most Helpful Member
" any well designed tube amp has bleeder resistors in the power supply, " OK thanks I'll make a note of that.
I wouldn't bother :D

'Some' amplifiers 'may' have a discharge resistor - and in 'some' of those models that do, that resistor 'may' not have gone O/C :D

It's not something you would want to rely on, so best to ignore the fact it 'might' be there.
 

ZeusMC

Member
Yes I'm with you on that. I did a Google on bleeder resistors and read the Wikipedia entry, said the same thing.

" Despite the presence of a bleeder, it is wise to prove that any potentially dangerous capacitors are discharged, perhaps by shorting their terminals (or through a suitable low resistance for high energy capacitors), before working on any circuit. "

Says suitable low resistance for high energy capacitors.

I've ordered, Vacuum Tube Amplifier Basics by EJ Jurich, the author has over 60 years of practical hands-on vacuum tube amplifier experience repairing and building custom audio gear including studio and recording equipment, professionally worked in radio broadcast engineering for 40 years.

In the screenshot from his book that I posted above. He says, " It is best to use a 100 ohm 5 watt wire wound resistor, with a clip lead to short capacitors. The 100 ohm resistor limits current enough, to prevent a damaging spark but will quickly discharge the capacitor. "

But unclejed613 says, " a 100K 5 watt would work better."
Please clarify?
Cheers.
 

JimB

Super Moderator
Most Helpful Member
He says, " It is best to use a 100 ohm 5 watt wire wound resistor, with a clip lead to short capacitors. The 100 ohm resistor limits current enough, to prevent a damaging spark but will quickly discharge the capacitor. "
But unclejed613 says, " a 100K 5 watt would work better."
Please clarify?
As stated earlier, a screwdriver across the capacitor terminals is a quick, easy and sometimes spectacular method of discharging a capacitor.
I remember one instance where I shorted a capacitor, accidentally I think, it was 50 years ago, the equipment was powered at the time but the capacitor was internally damaged, it appeared to go open circuit.
The equipment, a Marconi CR100 receiver instantly developed a very loud mains hum which was cured by replacing the capacitor.

If I wanted to be very precise about it, I would:
1 switch off.
2 unplug from the mains.
3 measure with a multimeter.
4 short the capacitor with a 1k Ohm resistor for 10 seconds.
5 measure with a multimeter.
6 get on and do the job.

Consider, the time constant of an RC circuit is given by
t= CR
t in seconds
C in Farads
R in Ohms

A typical HT supply in a valve amplifier may be 100uF
Our discharge resistor is 1k Ohm
time constant = 100 x 10^-6 x 1 x 10^3 = 100 x 10^3 = 100mS
So if we discharge our capacitor for 1 second, that is 10 time constants, so all the charge will be gone.

But beware, discharged capacitors can recharge themselves due to a property called dielectric absorption.
When the discharge path is removed, energy "comes back out of the dielectric" and the voltage across the capacitor terminals rises a bit.

JimB
 

ZeusMC

Member
OK thanks for that. " But beware, discharged capacitors can recharge themselves due to a property called dielectric absorption. " Yes I've read about that.
 

Nigel Goodwin

Super Moderator
Most Helpful Member
As stated earlier, a screwdriver across the capacitor terminals is a quick, easy and sometimes spectacular method of discharging a capacitor.
Reminds me of a story :D

I used to do the PA for a local band called Avalanche, and initially they had two Carlsboro 200W valve mixer/amps for the PA, and I fed them from a home made mixer I'd built.

Now these amps were HUGE, extremely heavy, used 4xKT88's in the outputs - and used to intermittently make 200W 'farting noises' for no apparent reason. Needless to say, you could never actually catch them doing it, so I was never able to cure them, or even find where the problem might lie - but they both did it.

Anyway, one day I was having another look at one of them - in a flat above an old stables, where the bass player (who ran the band) lived - so I'd got it in pieces on the floor, kneeling down working on it. I then noticed that everyone was crowded round me looking down at what I was doing :D

Now like many valve guitar/PA amps it had a standby switch - this simply disconnects the mains HT rail, leaving the heaters running.

Now this amplifier had four huge reservoir capacitors in series parallel - so I let them charge up, then switched it to standby - I then positioned my screwdriver on the electrolytic's, closed my eyes, and shorted them out :D

There was the biggest bang and flash, everyone in the room screamed and ran for cover - two even dived over the back of the sofa. Meanwhile I was convulsing with laughter on the floor - and all but wet myself laughing. The rest of the band were less amused though!.
 

ZeusMC

Member
To discharge capacitors I ordered a 100 ohm 5 watt resistor, as shown in attached screenshot. But unclejed613 said, " 100K 5 watt would work better, no sparking, and well within the thermal limits of the resistor. " I contacted the seller to cancel my order, but already posted.
I received it yesterday, it looks different to the one in the screenshot. The markings on it say, 212-8 100R 10% r.
Have I been sent the correct 100 ohm 5 watt resistor ?
Cheers.
 

Attachments

ZeusMC

Member
Thanks Nigel, would these test leads be OK to use to discharge the, power filter caps on tube amps? Ebay UK item 133439966283 I was only after one set but a 10 set was cheapest priced, at £2.99 :)
Cheers.
 

ZeusMC

Member
OK thanks I'll order them now. The valve amp construction books are due Saturday. As a newbie, more I read about theory and watch some YouTubes, by those who know what they are doing. Some things fall into place, then again...... some don't. Took a while to get a grip, what voltage drop was all about:)
This business of playing with the electrons is fun..... as long as things don't go flash/bang.
 

Nigel Goodwin

Super Moderator
Most Helpful Member
Valve amps are really simple, nothing much to them at all. I've even got a couple of old valve amp books, which I found in second hand book shops.

The main problem is lack of parts, and their high cost - most valve stuff disappeared decades ago, along with decent valves. A lot of modern designs use weird valves, because that's all people can find.

I only ever made one valve amp, back about 1968?, a guy I knew who built a LOT of valve amps drew me the circuit based on parts I could salvage from old TV's thrown down a local tip. It used a 6BW6 output tetrode, and an EF80 preamp pentode - he gave me a suitable mains transformer, as you didn't find those in TV's. Everything else I got from the tip.
 

KeepItSimpleStupid

Well-Known Member
Most Helpful Member
Brings back memories. Discharging an oil filled cap was quite spectacular and it made a mess out of your screwdrvier tip. Having a cap charge up juts by sitting is an eye opener. A friend had a cap rated at 10 kV. You put "keepers" on them.

This regulated 15 kV power supply I worked on at work, I think had a 100W, 3 Meg-ohm resistor for a bleeder. (V^2)/R is like 75W. One day, I tired of it failing all of the time, so I fixed it for good. My predecessors just kept replacing the same parts every year.

I tightened and thread-locked all of the resistors in the voltage divider, replaced a few of the resistors including the bleeder/
 

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