resrvoir cap not draining

[Structo]

The old trick of putting a jumper wire between the V1 plate and ground, using the plate resistor to bleed the voltage, is a good one but you must be sure of the amp design before you try it.

I just made a jumper cable with a 100K resistor in series with two alligator clips. Simple and easy.

I would recommend anybody not familiar with a certain amp, to always double check the charge on a power filter capacitor with a volt meter before diving into the chassis.

Also there is a phenomenon called dielectric absorption that can generate charge to the cap in question, so it is advisable to leave the discharge jumper in place while servicing the amp.

Not sure I have ever observed that in real life but it is a good safety precaution to leave a discharging lead in place while servicing an amp.
Just don't forget to remove said jumper when you power up the amp.

[NickC]

Here's my combination DIY Cap Drainer and Comic Relief Tool:


That's a 1x12 cabinet it's resting on.

[tubeswell]

Let there be Blood!

[Leo_Gnardo]

Not sure I have ever observed that in real life but it is a good safety precaution to leave a discharging lead in place while servicing an amp.
Just don't forget to remove said jumper when you power up the amp.

Oh my I've done that, and my bleeder is a 10W 1k WW. When I see the smoke come up I know I goofed... resistor has survived all my attempts at wrecking it.

Yeh that "bounce back" voltage is a hassle.

[Structo]

Hehehehe, why do you think I said that about forgetting to remove the bleeder thingy? <poof>

[tubeswell]

which is why a ~220k 2W permanent bleeder is the best solution

[Structo]

In several designs where a resistor (typically 220K-330K) is placed in parallel with filter capacitors, the object is to balance the voltage between two capacitors connected in series.

The side benefit is that they act as bleeder resistors, that will quickly bleed the voltage down to a safe level within about a minute or two.

My favorite example is the Dumble line of amps where the power supply shows two resistors paralleling the first capacitors in the supply filtering.

[Smokebreak]

Well this brings up another related topic...seems we all use different size and rated power resistors for our bench safety bleeders. What do you all use, and why? I still use a 5W 10K, the first one I ever put together before I started my 1st amp. I must've read it on the internet somewhere.

I've also left the bleeder on after working, and powering up, but never had an issue, luckily. Which makes me wonder why not have a 50K 5W, and just leave it on while you power up/down(strictly for discussion, as obviously leaving HV hanging out of the amp is never a good idea). Sure it would take a little longer to bleed...

[tubeswell]

... why not have a 50K 5W, and just leave it on while you power up/down...

Its a compromise between how long it takes to discharge and how much of a load a low resistance would put on the B+ supply (and therefore on the rectifier and PT).

220k doesn't put that much of a load on (hardly noticeable) and only takes about 8 seconds to discharge 400V or so.

[Reeltarded]

I use a >2w 100k resistor in my bleed rig, or in an amp that works as stated above I clip a wire with nothing but clips from the plate on V1 to the chassis. 270k to 100k, depending on the plate.