Ready, Set, STOP! Filter caps discharged?

[pickupcentral]

Hi y'all. Sorry if this has been covered, but I couldn't find it in all the many posts.

I have a Valve Jr, and a crapload of new parts to start doing all the cool mods available. I opened the amp up pulled out my multimeter and started checking to see if there were any deadly volts lurking. I measured from both sides of R10, R12 and R13 to the chassis ground and got absolutely nothing. I know my meter is working.

Are the caps discharged?

I know this is a dumb newbie post, but I would rather look stupid than dead. (Others may disagree!)

Thanks!
-Phil
(Sitting here, waiting for a reply...)

[justonwo]

Set your meter to volts DC (make sure you know the difference between DC and AC). Attach the negative lead on your meter to the chassis using an alligator clip. Now find the caps. They will usually be larger components and will have a "uF" label on them to indicate the capacitance. Put the positive lead of your meter on the positive side of the cap (or on both sides if you don't know which side is positive). Do you see a high voltage (i.e. greater than a few volts)? If not, you are safe.

[pickupcentral]

Hi justonwo!
I have my meter set to DC volts. The leads from the caps are not accessible from the top of the circuitboard, so that's why I was measuring at the resistors (they are the ones between the positive sides of the caps). I was just surprised that I didn't get any reading at all. I haven't turned the amp on for several months - I guess that was enough time for them to drain.

Thanks for your help.
-Phil

[justonwo]

Yep, those resistors are all connected to the positive side of the caps, so you should be okay. After a few months, it's not at all surprising that the caps don't have a charge. Good luck with the mod.

[pickupcentral]

I am thrilled! The amp not only works, but sounds great. Thanks for your encouragement. Next up: standby switch and output transformer swap.

Dang. Now I have to discharge the caps. <grin>

Thanks!
-Phil

[pickupcentral]

Strange, but I played the amp last night, and today when I went to work on it, there was again no voltage from those spots to ground. Maybe they discharge quickly, but I'm still checking several times before doing anything.

[nateG.]

I know what you mean. When I first got it I plugged and played, then set out to remove the chassis. There's no standby switch, so I couldn't use that old trick. I measured the dropping resistors, then the plates, then even the rectifiers (I know, just being cautious). On mine, I can wait as little as 20 minutes and the caps are in the single millivolt range. Other's might be different, so never assume. I always check both the dropping resistors and the plates, but it's always been discharged.

[pickupcentral]

Thanks Nate! Glad to know I'm not doing something wrong.

I did a bunch of mods, and the distortion is much smoother, but it is still muddy on neck pickups and motorboats some, so I have more stuff to do. Hopefully the new output transformer gets here soon (Hammond 125ESE). Also going to lower the cap values in C3 and C4.

I'm having mucho fun.
-Phil

[pickupcentral]

I found this on page 86 (!!!) of the VJ thread on 18watt.com:

"If you have a newer version of the circuit, there will be a resistor labled R11 which should be 220K. This is a bleeder resistor and will drain the caps when the amp is turned off. "

Gotta look into this. Might be a nice "first mod" for any amp.

-Phil

[nateG.]

You know, with all the mods i did, I didn't want to alter the actual signal path. So far the only one that has stuck is a 3-way switch to alter the bias of the preamp stage. That, and changed some filter caps with Sprague, and select resistors with carbon comps. But with all those, it just got less and less headroom. I went a bit overkill and put a 12AT7 in there, and I actually like the rounded mellow tone alot. Plus I can put some spike back in with the bias switch.