Are these the filter caps? (Picture included)

[Dr. Lo]

Hi all,

I'm a complete newbie when it comes modifying amps. I want (need!) to drain the filter caps before I muck around in there. I'm not sure which components are actually the filter caps. I've taken a guess (see the attached picture). Can you please tell me whether what I've pointed out are indeed the filter caps that need tobe drained of lethal voltages?

If I am correct, do I need to remove the PCB from the chassis to properly drain the filter caps?

Thanks in advance

TT_gutshot.pdf

[Steve Conner]

Yes.

The traditional advice doesn't really apply in this case though. The capacitor terminals are hidden under the PCB, so you can't discharge them by hand without removing the PCB. And it isn't safe to remove the PCB without first discharging the caps.

A modern amp design should have bleeder resistors that fully discharge all the capacitors within about 5 minutes of removing power. If you want to verify this, apply your multimeter between chassis ground and the plate pins on the tube sockets. (or alternatively, the spade lug marked "CT Brown" would be a suitable test point)

[Dr. Lo]

Thanks Steve! This has helped boost my self-confidence.

Since the cap terminals are hidden beneath the PCB, would the safest approach be to take pair of alligator clips (with a 1 meg resistor between them to prevent arcing) and then clip one end to the chassis and the other end to exposed wire connected to pin 3 of either power tube?

[Steve Conner]

With 1 meg, you'd be waiting a long time. 1k would be better.

When working on an unfamiliar piece of tube equipment, I test the voltage after power off, as described above, to see whether bleeder resistors are fitted already.

If they weren't, and the equipment belonged to me, I'd fit them. If they were, then there's no need for the cap-discharging gadget.

[Dr. Lo]

Thanks again

[MikeH]

Thanks Steve! This has helped boost my self-confidence.

Since the cap terminals are hidden beneath the PCB, would the safest approach be to take pair of alligator clips (with a 1 meg resistor between them to prevent arcing) and then clip one end to the chassis and the other end to exposed wire connected to pin 3 of either power tube?

You could also just connect pin 1 of v1 to ground. This will drain the filter caps through the v1 plate resistor (100k in most amps). This works for any amp with a plate resistor on v1; the OTT is no exception. I can't think of any amp that doesn't off the top of my head.

Edit: As always, double check with a meter before you dig in.

[Dr. Lo]

Thanks Mike!

[GRIMESPACE]

Hi - this is great info - I'm working on a Jet City 50H, and have a similar situation with PCB-mounted filter caps that don't have exposed leads. When checking voltage on pin 1 of V1, is it DC or AC? Another thing I haven't been able to determine is do you do the discharge / testing with the tubes in or removed? I ask because I like to pull all the tubes before removing the chassis so that I don't break something.

Thanks!

[Steve Conner]

DC, and it doesn't matter whether the tubes are in or out.

[MikeH]

Hi - this is great info - I'm working on a Jet City 50H, and have a similar situation with PCB-mounted filter caps that don't have exposed leads. When checking voltage on pin 1 of V1, is it DC or AC? Another thing I haven't been able to determine is do you do the discharge / testing with the tubes in or removed? I ask because I like to pull all the tubes before removing the chassis so that I don't break something.

Thanks!

Don't take offense - but this is dangerous stuff. You have to know some things; start here: www.geofex.com/tubeampfaq/TUBEFAQ.htm#safety

In fact, that whole section is full of great amp tips, I recommend reading it all to anyone.

[GRIMESPACE]

DC, and it doesn't matter whether the tubes are in or out.

Thanks!

Don't take offense - but this is dangerous stuff. You have to know some things; start here: www.geofex.com/tubeampfaq/TUBEFAQ.htm#safety

In fact, that whole section is full of great amp tips, I recommend reading it all to anyone.

I appreciate your concern, and certainly appreciate the potential danger. I've seen that site, a few videos, etc. Biggest problem is that the assumptions are pretty high, and it's really hard to know for certain that what I'm doing is right. I actually have a background in electronics, and while I'm not an expert, I do understand that capacitors store a charge and can kick your @#$ into next week if you don't respect them.

That being said, I tried discharging and testing with a meter this past weekend, but there was so little voltage on pin1 that I figured I was doing something wrong. According to the first reply, whether or not the tubes are in won't matter. I'm new to this forum and I don't mean to offend anyone, but can someone confirm whether or not this is true?

Thanks! Glad to be here!

[52 Bill]

Most amps will drain off the charge on the filter caps in a few seconds if they have bleeder resistors on the main filters. For that matter most amps will drain off any residual charge as long as the circuitry is not cut off from the power supply circuit by a standby switch.

To be safe, at least until you gain some level of experience you should probably still check with a meter before doing anything inside the amp.

[MikeH]

That being said, I tried discharging and testing with a meter this past weekend, but there was so little voltage on pin1 that I figured I was doing something wrong.

As 52 Bill said, most modern amps will drain themselves. If you're not sure whether the amp is draining itself or if you meter is wrong, etc, try this: Using alligator clips, hook up your meter to measure the B+. Turn the amp on, notice the reading. If your not getting a high voltage reading, something is amiss, Then turn the amp off and notice if the B+ drains itself.

[GRIMESPACE]

Most amps will drain off the charge on the filter caps in a few seconds if they have bleeder resistors on the main filters. For that matter most amps will drain off any residual charge as long as the circuitry is not cut off from the power supply circuit by a standby switch.

To be safe, at least until you gain some level of experience you should probably still check with a meter before doing anything inside the amp.

Good advice that I plan to take
Here's a clip from the schematic - Would R1 and R2 be considered bleeder resistors in this design, since they're running current to ground?

As 52 Bill said, most modern amps will drain themselves. If you're not sure whether the amp is draining itself or if you meter is wrong, etc, try this: Using alligator clips, hook up your meter to measure the B+. Turn the amp on, notice the reading. If your not getting a high voltage reading, something is amiss, Then turn the amp off and notice if the B+ drains itself.

I'm going to check this before digging in - am I correct that B+ is the high-voltage line coming off the rectifier and the positive side of C2 in the schematic above?

Thanks again!

[52 Bill]

- Would R1 and R2 be considered bleeder resistors in this design, since they're running current to ground?

I'm going to check this before digging in - am I correct that B+ is the high-voltage line coming off the rectifier and the positive side of C2 in the schematic above?

Yes and yes.

R1 and R2 will drain off the residual voltage left on the filter caps. And yes, the B+ is the high voltage supply coming from the rectifier diodes.