Discharging filter caps in a Classic 30

[tubeswell]

Ok Mr Ahola has this to say

"...using a well insulated test probe connected to the chassis ground through a 10K resistor. With the chassis removed from the cabinet and the FX jacks to your left you will notice several large 5 and 1 watt resistors mounted on stand-offs away from the board. Directly above from the Bass control is R58; with the insulated 10k resistor/probe short first the left lead and then the right lead. Directly above from the Pre gain pot is R59 mounted horizontally and R60 mounted vertically. With your special resistor probe, short first the left and then the right leads of R59, and then fisrt the back and the front leads of R60. Generally any charge will be drained through the OT (output transformer) after a minute or two, but its best not to take any chances."

I presume the resistor is there to slow down the discharge process, - so the bigger value the resistor, the longer the caps take to drain.

What I would like confirmation about is what you do with the other end of the resistor lead:

is he saying 'connect one end of the lead to the chassis and then alternately connect the other end to each end of the resistor (in each case)'; or

is he saying 'connect one end to one side of the resistor (in each case) and then connect the other end to the other side of the resistor (in each case)'?

Also which is the 'front' and which is the 'back' of R60.

And is he saying to leave the lead in place for a minute or two (in each case)?

Cheers

[tubeswell]

What's more...

And I presume this procedure works with the tubes taken out (since you really shoudl take them out to take the chassis out - or should you have them plugged in?) and the amp switched 'off'... (please correct me if I'm wrong)

[stokes]

To make it simpler,just connect one end of the 10k (any value down to about 1k is okay to use) to the chassis or ground and the other end gets connected to the + side of the cap.I use a 1k 3watt resistor with an alligator clip and a piece of wire to each end of the resistor.Makes no difference tubes in or out,switch doesnt matter,but it is safer to have in off position in case you forget to unplug the amp.A couple of seconds is all it takes and be sure to remove the wire/resistor before turning the amp on or you will get a startling pop when full power is applied and the resistor fries.

[tubeswell]

What about a 1 ohm 5w resistor? Would that do?

[Enzo]

1 ohm? You might as well just use a wire then. The point of the resistor is that the caps are not discharged instantly. That can burn out the connection inside the cap.

I'd prefer a 1k, I don't want to waut around for a 10k to work.

Steve did say to conect the 10k resistor to ground, so there is only one free end of it left for probing with. And it doesn't really matter which order you ground the caps off in. By the time you have touched everything, they will all be discharged.

But why not learn what you are working with. Connect your volt meter to the main filter cap so you can monitor the high voltage. That would be the point where C39, C40, R58, two rectifiers, and the OT CT come together. The drawing says that is nominally 332VDC. Now fire up the amp for a minute. Now turn it off. How long does it take to discharge to levels you would touch? I have no idea. I will say that when I work on these, by the time I have enough screws out to where I might be touching the innards, the caps are long discharged.

[Amp Kat]

Another thing about the C-30 is it doesn't have a standby switch so the discharge resistance is less than a open stanby switch resistance so it discharges pretty rapidly anyway as Enzo was referring to. It used to be anything over 200 I wouldn't touch but now it's about anything under 100. I'd be more worried about leaving the amp plugged in and going across the mains than those quick discharging capacitors in the C-30

[R.G.]

In my opinion, it is slow to the point of stupid to not put 100K or so bleeder resistors across the power filter caps in any amp you open up.

The bleeder resistors will eat maybe 1/4 to 1/2W (so use 1W metal oxide resistors) but when the power switch goes off, they will bleed the DC power supply down to zero in a minute or so. Automagically. No operator intervention needed. Ever. Auto-safe. Self regenerating.

[tubeswell]

In my opinion, it is slow to the point of stupid to not put 100K or so bleeder resistors across the power filter caps in any amp you open up. .

Yup slow and stupid, tha's me.

I have a old 5W resistor that measures 1.2 k (although it has 68Ω stamped on it) . Here is a picture. Will this work?

[stokes]

Yup slow and stupid, tha's me.

I have a old 5W resistor that measures 1.2 k (although it has 68Ω stamped on it) . Here is a picture. Will this work?

Thats it,but I would be concerned with using a resistor that is that far off its stated value.The only point I would add is that I use a piece of heat shrink wrap to cover the resistor and wire so there is no exposed part.As for slow and stupid,sure if you build the amp yourself,it's a good idea to have a bleeder circuit built in,but most amps that come across my bench dont,so I keep a couple of these around and use it anytime I open a chassis.

[tubeswell]

Yup slow and stupid, tha's me.

I have a old 5W resistor that measures 1.2 k (although it has 68Ω stamped on it) . Here is a picture. Will this work?

Actually scratch that, I looked at the resistor again and it has 5W 0.68Ω J stamped on in - is that saying less than 1 ohm or does the "J" mean I have to shift the decimal place some. On my meter it reads "1" on the "2000" setting and 1.2 on the "200" setting - so thats 1.2k right?

The filter caps showed a big fat zero on any meter setting. Is it my meter?

[Enzo]

The 200 setting means that 200 is the highest it will measure before going over range. 1,2 on that range means 1.2 ohms. The 2000 range means the highest resistance it can measure is 2000 ohms. 1 on that range means 1 ohm. Trying to measure .68 ohms on that scale - 1 is close as the meter can read.

On old analog meters with a common scale to all ranges, the selector switch indicated "multipliers." That meant you multiplied the reading by the scale. SO 1.2 on the 1k scale did mean 1.2k. When you have reading ranges on the display though, it works as above.


Your part is not 68 ohms, it is 0.68 ohms, like it says. DOn't ignore those zeroes and decimal points.

Again, using a 0.68 or 1 ohm or 1.2 ohm resistor is about the same to the cap as a plain old wire. You really ought to use a 1000 ohm or even a 100 ohm. The point is to let the cap down gently, not smash it with a shovel.

WHat RG means is that it takes a few seconds and a 2 cent resistor to slap a 100k across the main filter cap in any amp you service right at the start, and then you don't have to even grab a clip wire. The thing will discharge itself through the resistor. And no one can forget to grab a wire...

The thing you made looks fine, but the resistor you made it with is inappropriate for the job.

[Mark Black]

My personal choice is a 10K 5W metal oxide with alligator clip leads attached. Small enough to discharge in a reasonable amount of time, and big enough (wattage and resistance wise) to be able to survive the occasional "forget it's hooked up and turn the amp on" episode. This is important to me since I usually do leave the thing hooked up while working on an amp due to fear of "dielectric absorption" re-energizing a cap.

The smell usually alerts me to my stupidity...

Been shocked by a few amplifiers, even more by strobe lights (NASTY little bastards). Now fairly "gun-shy". I somehow survived the 80's (nudge-nudge wink-wink) and now am fairly aware of the concept of mortality...

[devnull]

In my opinion, it is slow to the point of stupid to not put 100K or so bleeder resistors across the power filter caps in any amp you open up.

The bleeder resistors will eat maybe 1/4 to 1/2W (so use 1W metal oxide resistors) but when the power switch goes off, they will bleed the DC power supply down to zero in a minute or so. Automagically. No operator intervention needed. Ever. Auto-safe. Self regenerating.

Is 100K a good rule of thumb value for permanently installed bleed resistors, or is there a (gasp) calculation involved?

Thanks.

[tubeswell]

Is 100K a good rule of thumb value for permanently installed bleed resistors, or is there a (gasp) calculation involved?

Thanks.

Man these old posts are embarrassing

Check out http://www.welwyn-tt.co.uk/CalcTools.asp

under 'Capacitor Discharge calculator'

[devnull]

Perfect, thanks.

And relative to the old posts, I also consider my own antiquities to be embarrassing and sometimes highly incriminating, but, they are invaluable to us newbies.

Thanks again.