earth the chassis when draining filter caps? - Page 2

HOT TUBES 70 · 10-21-2009, 07:47 PM

its great that you guys talk safety to the forum , its nothing to take lightly !!
my dad was a electrician all his working career , i used to work with him when i was
a kid , and from the ages of 13 to 19 i'd been zapped a few times , even though i have
felt a 220 vac shock !! , i still get a little nervous when i pull my chassis out to do any service work .

and again THANKS for sharing all your combined knowledge with the rest of us !!!! this has been a great thread so far !!!

andyman95023 · 10-22-2009, 12:54 PM

Quote:

Originally Posted by Wilder Amplification

Yeah the internal resistance of a MM is way too high to use it as a current drain, which is why it would take years to drain them by itself.

As far as the insulated alligator clips, yes that would be a good idea. You can buy them at any electronics store and they're pretty inexpensive. They usually sell them in multiple amounts.

You shouldn't get much of a spark if any at all by clipping to pin 1 as not much current can flow all at once due to the value of the dropping resistors in the power supply.

Awesome info do you have a recommended value for the resistor?
Also does it matter which end is connected first pin side or ground side

Thanks again
Andyman

Wilder Amplification · 10-22-2009, 04:05 PM

Quote:

Originally Posted by andyman95023

Awesome info do you have a recommended value for the resistor?
Also does it matter which end is connected first pin side or ground side

Thanks again
Andyman

No resistor needed. It's an alligator clip jumper (a wire with alligator clips on both ends). It shorts pin 1 of the first preamp tube directly to ground so that the dropping resistors that are part of the amp's circuitry along with V1's 100K plate resistor act as bleeder resistors. With the 100K plate resistor in addition to the few 10K resistors in the power supply, this gives you a high resistance path to ground across all of the filter caps so that there won't be hardly enough current flowing to fry anything yet still be able to discharge the filter caps within a couple of minutes.

danmahoney · 11-03-2009, 09:04 PM

[QUOTE=is a piece of wire with alligator clips on the ends of it. With the amp off and unplugged, you clip one end to the chassis and the other end to pin #1 of the first preamp tube. This uses the power supply dropping resistors in the amp as a discharge path. You then connect the meter to the HT fuse and watch the voltage there with the amp off...the voltage there will tell you how much of a charge is left on the caps.QUOTE]

Hi Guys,
Just joined the forum but have had the opportunity to get a lot of good info from reading up until now.
One quick question regarding draining caps:
I`ve used Jon`s method of clipping pin1 of V1 to ground and now I`d like to confirm that I`ve actually successfully bled the filter caps in my JCM 900
(4101), but I`m not sure how to access the contacts on the HT fuse in order to do that.
The fuse holder is closed plastic and mounted right onto the board on the 4101.
Can anyone suggest a SAFE way for me to test for voltage using my multi meter?
Just to back-fill a bit here:
I`ve got a 1990 model of the 4101 that shipped with EL34s and I really am a
6L6 kinda guy, so the game plan is to pull the rear PCB (so I can access the power tube sockets, everything is really tight in this particular model) and install a 1 ohm resistor, and then see if I can get a quad of Tung Sol 6L6 STRs to bias within spec.
If not then I`ll have to mod out the resistors and go from there...
Any input or advice would be much appreciated. Thanks!
Dan

Wilder Amplification · 11-04-2009, 12:49 AM

[quote=danmahoney;71346]

Quote:

Originally Posted by is a piece of wire with alligator clips on the ends of it. With the amp off and unplugged, you clip one end to the chassis and the other end to pin #1 of the first preamp tube. This uses the power supply dropping resistors in the amp as a discharge path. You then connect the meter to the HT fuse and watch the voltage there with the amp off...the voltage there will tell you how much of a charge is left on the caps.QUOTE

Hi Guys,
Just joined the forum but have had the opportunity to get a lot of good info from reading up until now.
One quick question regarding draining caps:
I`ve used Jon`s method of clipping pin1 of V1 to ground and now I`d like to confirm that I`ve actually successfully bled the filter caps in my JCM 900
(4101), but I`m not sure how to access the contacts on the HT fuse in order to do that.
The fuse holder is closed plastic and mounted right onto the board on the 4101.
Can anyone suggest a SAFE way for me to test for voltage using my multi meter?
Just to back-fill a bit here:
I`ve got a 1990 model of the 4101 that shipped with EL34s and I really am a
6L6 kinda guy, so the game plan is to pull the rear PCB (so I can access the power tube sockets, everything is really tight in this particular model) and install a 1 ohm resistor, and then see if I can get a quad of Tung Sol 6L6 STRs to bias within spec.
If not then I`ll have to mod out the resistors and go from there...
Any input or advice would be much appreciated. Thanks!
Dan

From the top side of the chassis -

1) Remove the power valves

2) Connect the black lead of your multimeter to the chassis

3) Stick the red lead of your multimeter into pin 3 of one of the power valves (doesn't matter which on). If no voltage here, you've drained the filter caps.

It's always a great idea to leave the jumper in place while working on the amp so as to keep the filter caps from rebuilding some of their charge.

danmahoney · 11-04-2009, 01:16 AM

Thanks Jon, meter`s showing all zeros so it looks really good.
Going to pull the board and install the resistor. I may have to leave the PCB out (up a couple of inches) to have access to the lugs on one of the power tube sockets while I`m biasing the 6L6`s.
I`ll probably foam tape a narrow (2"x2") piece of wood under it to keep it from moving. Any suggestions on that approach? Thanks again.
Dan

Apostle Tone · 11-04-2009, 10:33 AM

Jon,

Since you were in the military would you mind restating that long (but well written and I agree with you) post into bullet format? I spent 10 years in the Air Force and it just makes it easier to read. Thanks!

andyman95023 · 11-05-2009, 11:54 AM

[quote=Wilder Amplification;71377]

Quote:

Originally Posted by danmahoney

From the top side of the chassis -

1) Remove the power valves

2) Connect the black lead of your multimeter to the chassis

3) Stick the red lead of your multimeter into pin 3 of one of the power valves (doesn't matter which on). If no voltage here, you've drained the filter caps.

It's always a great idea to leave the jumper in place while working on the amp so as to keep the filter caps from rebuilding some of their charge.

Can the caps be drained using pin 1 from the top side meaning before any disassembly just removing the tube and contacting 1 and then monitoring the voltage on the other tube 3 pin?
Thanks in advanced
Andyman95023

Wilder Amplification · 11-05-2009, 12:06 PM

Quote:

Originally Posted by andyman95023

Can the caps be drained using pin 1 from the top side meaning before any disassembly just removing the tube and contacting 1 and then monitoring the voltage on the other tube 3 pin?
Thanks in advanced
Andyman95023

Yeah but why bother? You'd have to go through the trouble of sticking something into pin 1 for the alligator clip jumper to clip to, and not only do you risk damaging the socket pin by doing it that way, but more importantly you're taking a huge shock risk by sticking something into the pin socket if you don't know what you're doing.

As long as you don't come into physical contact with the internal circuitry while you're taking it apart (which is pretty hard to do considering you have to hold the chassis with both hands) there's no need to go through all this trouble.

10-21-2009, 07:47 PM	#31 (permalink)
HOT TUBES 70 Senior Member Join Date: Oct 2008 Location: SASKATOON Posts: 1,675	Re: earth the chassis when draining filter caps? its great that you guys talk safety to the forum , its nothing to take lightly !! my dad was a electrician all his working career , i used to work with him when i was a kid , and from the ages of 13 to 19 i'd been zapped a few times , even though i have felt a 220 vac shock !! , i still get a little nervous when i pull my chassis out to do any service work . and again THANKS for sharing all your combined knowledge with the rest of us !!!! this has been a great thread so far !!! __________________ MARSHALL 6100LM MARSHALL 1960A W/-G12H 30's CUSTOM BUILT 2X12 W/-V30's "MARSHALL" - only the best will do !

11-03-2009, 09:04 PM	#34 (permalink)
danmahoney Junior Member Join Date: Nov 2009 Location: Montreal, Canada Posts: 6	Re: earth the chassis when draining filter caps? [QUOTE=is a piece of wire with alligator clips on the ends of it. With the amp off and unplugged, you clip one end to the chassis and the other end to pin #1 of the first preamp tube. This uses the power supply dropping resistors in the amp as a discharge path. You then connect the meter to the HT fuse and watch the voltage there with the amp off...the voltage there will tell you how much of a charge is left on the caps.QUOTE] Hi Guys, Just joined the forum but have had the opportunity to get a lot of good info from reading up until now. One quick question regarding draining caps: I`ve used Jon`s method of clipping pin1 of V1 to ground and now I`d like to confirm that I`ve actually successfully bled the filter caps in my JCM 900 (4101), but I`m not sure how to access the contacts on the HT fuse in order to do that. The fuse holder is closed plastic and mounted right onto the board on the 4101. Can anyone suggest a SAFE way for me to test for voltage using my multi meter? Just to back-fill a bit here: I`ve got a 1990 model of the 4101 that shipped with EL34s and I really am a 6L6 kinda guy, so the game plan is to pull the rear PCB (so I can access the power tube sockets, everything is really tight in this particular model) and install a 1 ohm resistor, and then see if I can get a quad of Tung Sol 6L6 STRs to bias within spec. If not then I`ll have to mod out the resistors and go from there... Any input or advice would be much appreciated. Thanks! Dan __________________ JCM 900 (4502) (Tung Sol 6L6 STRs) (1999) JCM 900 (4101) (1990) JTM 30 (Vintage Tung Sol 5881s) (1995) JTM series (1x12") ext cab (1995) 1912 ext cab (1997)

11-04-2009, 01:16 AM	#36 (permalink)
danmahoney Junior Member Join Date: Nov 2009 Location: Montreal, Canada Posts: 6	Re: earth the chassis when draining filter caps? Thanks Jon, meter`s showing all zeros so it looks really good. Going to pull the board and install the resistor. I may have to leave the PCB out (up a couple of inches) to have access to the lugs on one of the power tube sockets while I`m biasing the 6L6`s. I`ll probably foam tape a narrow (2"x2") piece of wood under it to keep it from moving. Any suggestions on that approach? Thanks again. Dan __________________ JCM 900 (4502) (Tung Sol 6L6 STRs) (1999) JCM 900 (4101) (1990) JTM 30 (Vintage Tung Sol 5881s) (1995) JTM series (1x12") ext cab (1995) 1912 ext cab (1997) Last edited by danmahoney; 11-04-2009 at 02:09 PM.

11-04-2009, 10:33 AM	#37 (permalink)
Apostle Tone Member Join Date: Oct 2009 Location: Colorado Posts: 35	Re: earth the chassis when draining filter caps? Jon, Since you were in the military would you mind restating that long (but well written and I agree with you) post into bullet format? I spent 10 years in the Air Force and it just makes it easier to read. Thanks! __________________ Ignorance+Ego+Insecurity+Technology=funny

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