Draining the Filter caps.

Tatosh · Post by **Tatosh** » Wed Nov 30, 2005 12:31 pm

Well, I am getting ready to start fixing my amps but I need to address this issue first. Is the info on the ptpcircuits site the best option for doing this? Here it is for those of you that haven´t checked it yet:

DO NOT touch anything inside your amp chassis before doing this!
In your amplifier the filter capacitors (the large cans that stick on top of the chassis, sometimes the preamp one is board mounted) store voltages long after use, so this voltage needs to be drained, if not it can KILL.

This diagram shows the first preamp tube in a marshall plexi, in a normal marshall plexi it's the one closest to the side edge of the chassis, and furthest from the power tubes.

Start by connecting an insulated crocodile clip to pin 1 of the tube socket, denoted by the grey line on the diagram. Connect the other end to a suitable grounding point, which in this case will be the chassis. The voltage should drain in around a minute, but most people prefer to leave the crocodile clip connected to ground, as capacitors can regrow voltage (like batteries)! Make sure you check that there's no/negligible voltage left before you get to work on the amp.

Never short out the filter capacitors directly.

Any opinion? Also, are those alligator clips like the ones they sell at Radio Shack? And about the end that goes to a grounding part, I see that George put his multimeter in one of the holes of the chassis. Does this sounds doable?

Thanks.

Santiago.

Bainzy · Post by **Bainzy** » Wed Nov 30, 2005 12:38 pm

When I bought the crocodile clips for doing this, they came in a pack of 10. While 1 pair of clips drains the filter caps, I use another pair for the multimeter. Clip one end to the negative multimeter terminal, and the other clip goes to the chassis.

You don't have to use pin 1 of V1 to drain off the voltage with, you can also use the terminal on the board that joins the 100k resistor and the wire that goes to V1 as an alternate draining point.

If you're worried about voltages when doing it for the first time, stick on some rubber gloves that you'd use for doing the washing up (make sure they're dry first though!).

Tatosh · Post by **Tatosh** » Wed Nov 30, 2005 12:47 pm

Bainzy wrote:When I bought the crocodile clips for doing this, they came in a pack of 10. While 1 pair of clips drains the filter caps, I use another pair for the multimeter. Clip one end to the negative multimeter terminal, and the other clip goes to the chassis.

Why should I do this, to check if there is still voltage on the caps?? also, which is the negative in a multimeter, the red or the black? I haven´t bought one yet.

Bainzy wrote: You don't have to use pin 1 of V1 to drain off the voltage with, you can also use the terminal on the board that joins the 100k resistor and the wire that goes to V1 as an alternate draining point.

Well, the pin 1 of V1 just seems as quite easy to do, so I will probably use that one. Is there any reason for using the other one?

Thanks.

Santiago.

Bainzy · Post by **Bainzy** » Wed Nov 30, 2005 1:09 pm

Tatosh wrote:Why should I do this, to check if there is still voltage on the caps?? also, which is the negative in a multimeter, the red or the black? I haven´t bought one yet.

It's just useful to check when you're doing it as it helps put your mind at rest and confirms you've done it right. The black is the negative.

Well, the pin 1 of V1 just seems as quite easy to do, so I will probably use that one. Is there any reason for using the other one?

The only reason I use the other one is that I find it's easier to clip onto the turret there than onto the first pin of V1, and it's less likely to disturb the wire at V1. Other than that, no reason.

Billy Batz · Post by **Billy Batz** » Wed Nov 30, 2005 2:11 pm

Shit guys I just take a long screwdriver and touch it to the chassis near the handle and touch the end to the V1 plate at the turret. Only takes a few moments to drain the filters so I just stand there for a few seconds and watch my meter.

Flames1950 · Post by **Flames1950** » Wed Nov 30, 2005 4:07 pm

Depending on the exact circuit you're using you may have to have the standby switch in the play position for this to work. Or just flip it to the play position anyway and don't worry about it -- draining at V1 will always work if the switch is on play......

Tatosh · Post by **Tatosh** » Wed Nov 30, 2005 5:38 pm

Billy Batz wrote:Shit guys I just take a long screwdriver and touch it to the chassis near the handle and touch the end to the V1 plate at the turret. Only takes a few moments to drain the filters so I just stand there for a few seconds and watch my meter.

Where do you put your multimeter while you are doing this? Also, you mean putting the plastic part of the screwdriver touching the chasis, and the metalic part touching the turret at V1?

I just want to be sure.

Santiago.

Tatosh · Post by **Tatosh** » Wed Nov 30, 2005 5:41 pm

Flames1950 wrote:Depending on the exact circuit you're using you may have to have the standby switch in the play position for this to work. Or just flip it to the play position anyway and don't worry about it -- draining at V1 will always work if the switch is on play......

I am talking about the standard plexi circuit. You mean you need to have the standby on for any method of draining filter caps? or yous for the screwdriver thing?

Thanks.

Santiago.

Billy Batz · Post by **Billy Batz** » Wed Nov 30, 2005 10:45 pm

No. That trick is only for amps whose standby is after the main filter section which wouldnt be most marshalls or any past 67. Actually didnt the JTM's have the standby after the diodes? That would mean they wouldnt need to be off SB either?

When I say I use a screwdriver I touch both parts of the metal rod itself. Near the handle to the chassis and the tip to V1s plate turret. An insulated screwdriver obviously. Course you could go faster by touching the tip to the junction of those 2 resistors (watch the sparks). Actually I just do taht strait to the filter cans in lower powered amps like Bandmasters. Actually my bandmasters voltages are close to 480. Never hurt them but most techs say not to do it especially on marshalls. Youve been warned.

Flames1950 · Post by **Flames1950** » Wed Nov 30, 2005 11:58 pm

I am talking about the standard plexi circuit. You mean you need to have the standby on for any method of draining filter caps? or yous for the screwdriver thing?

There were a number of "standard" plexi circuits.
The '67-'68 era schematic I have shows the standby after the filters, so it would need to have the standby in play position to drain the main filters. (Dan's layout looks to be wired with the filters after the standby though?) The JTM45 and JTM45/100 have the standby before the main filters, so you would not need to worry about the standby. The '69-era circuit has the standby before the filters as well, so no problems there.
Just be sure which you're dealing with before making a gross assumption and frying.......

For fun though, make sure your screwdriver is insulated, and try the screwdriver-to-chassis trick on a fully loaded filter

and see if you ever try it again. The only thing better is draining off flyback transformer charges on CRT's.

rockstah · Post by **rockstah** » Thu Dec 01, 2005 12:21 am

so when woirking on my 67/68, dan layout, when i touch pin 1 of V1 to ground with aligator clips do i need to have the standby switch engaged or not?

i just have been doing that - turn off unplug - power off - standby on - aligator to chassis other end to pin 1 - does it matter if my standby is off or on?

maybe standby is always on just to be safe with either layout?

Flames1950 · Post by **Flames1950** » Thu Dec 01, 2005 8:04 am

The layouts from Dan that I have show the B+ winding from the PT going straight to the standby, then off to the diodes/filters, so if that's how you wired it having the standby in either position will work fine.

NitroLiq · Post by **NitroLiq** » Thu Dec 01, 2005 10:18 am

I've heard the screwdriver method isn't the most ideal way to do it because it's harsher on the amp as it isn't as gradual as other methods. I know with the 18 watters, alot of people solder in a 220k/2W resistor across the capcan or piggy-backed to the first or second filter cap in the power supply if there's no capcan. Some go from the first filter cap to the 2nd contact on the on/off switch to ground so it automatically bleeds the b+ when the amp is off....a nice safety precaution without having to worry about gator clips slipping...hey...accidents do happen. Other folks prefer to not have the resistor soldered in so will get a couple of insulated gator clips and solder the bleeder resistor between them (with heatshrink around the resistor leads), the just leave it connected while working. Some just leave the standyby switch on which will bleed them as well.

I'm guessing you could use any of these methods with the plexi clones but maybe someone more familiar can verify the specifics for your layout (i.e, as has been discussed whether the standby method will work or not, if bleeder resistor's value needs to be changed, etc.)

Billy Batz · Post by **Billy Batz** » Thu Dec 01, 2005 11:12 am

When people are talking about the screwdriver trick theior talking about touching it to the filter cans directly which I only do on amps I dont care about. Using it on the V1 plate is no more dangerouse then using a jumper which is what everyone does. Its not a direct short. You have all the decoupling resistors in the power supply acting as the resistor your refering to as well as the plate resistor on V1. If you have the 56k resistors on the screens they would take care of you in this regard.

79 Transam · Post by **79 Transam** » Sun Dec 04, 2005 3:54 am

For daining filter caps, I turn the amp power switch off and leave the standby switch on. I was told by a guy who used to work at THD in Seattle that this was a good method. Also its supposed to be good for youe filtercaps to shut down the amp in that they dont hold power and will last longer.