Draining A Silverface Champ

[KYChucky]

OK want to put a 3 prong plug in and have a look at my new '68 silver face champ. SO, dont want to die, Thus want to make sure how to drain the power out of it:

Wile playing with amp on and turned up, unplug and keep on until sound is gone, then turn it off and:

1-make wire with gater clips on both ends, 10 watt 25 ohm resister someplace between the clips (insulated).
2-hook one end to amp chassis as ground
3-hook the other end to A) pin 1 on preamp tube, or B) positive out on cap, or C) jump both ends of the cap
4-check cap with meter to make sure juice is out.

Correct? Thoughts?

I also know: one hand only other in pocket, safety glasses, shoes on, no distractions or beer, leave jumper wire one wile performing any work,

My experience level; built my 5E3 and 10 stomp boxes and a 'strat', plus years of working on cars.

Thanks,

[Offshore Angler]

I use a lot bigger resistor. I think I have a 1M Cermet. Put your Voltmeter in series with the resistor so you are measuring the voltage across it. I like the longer time constant so I can watch the voltage drain. I just go across the cap(s). Nothing guarantees your amp is properly grounded. So, meausre the voltage on the caps.

[Don]

I think I used a 10k.

[Dangerine49]

Check out the Billm video:

http://billmaudio.com/wp/?page_id=1150

[cdw2000]

Yes unplug the amp for safety while working on it. But leaving the power switch on while unplugging it does nothing. The power switch is on the primary side of the power transformer and is in series with one side of the now-open power cord.

If it had a standby switch then you would want that in the on position while draining the caps to make sure caps on both sides of the standby switch get drained.

I think I used a 22K 2-watt resistor for my drain cable. Since I made a cable with the resistor in the middle, I double insulated it with heatshrink tubing, so I can't see it now to know for sure. I think 25 ohms is way to low a value. You want to drain at a good rate but not hit the capacitors with that much of a current surge.

Yes, use the one hand rule. And if probing voltages, I connect my voltmeter's black wire to a ground point with a clip and probe only with the red probe.

I also put heatshrink tubing over most of the probe tip, leaving just a little metal (1/16" - 1/8") exposed at the point. This makes it less likely to accidently short across two test points with the probe.

[KYChucky]

wow - Billmaudio video - what the heck, never seen that kind of description. So glad I have a good meter. Thanks guys. Will post pics.

[KYChucky]

Im still sort of confused...since the silverface champ has a cap can can I just meter / drain from #1 pin on my preamp tube? Or can I meter from any one of the other caps?

[cdw2000]

If you can't get to the cap can terminals easily, you would be better off draining/metering from pin 8 of the rectifier socket. This is directly connected to the power supply caps.

Using Pin 1 of the preamp socket is not a good idea because it has a 100K plate resistor between the pin and the caps. So you wouldn't be reading the voltage at the caps, you would be reading a voltage through the voltage divider formed by the plate resistor and your drain resistor.

[Offshore Angler]

...you would be reading a voltage through the voltage divider formed by the plate resistor and your drain resistor.

Yeah, but zero divided by anything except zero is still zero.

[cdw2000]

Yes, but if he's using a 10K drain resistor and he see's 10V and thinks good enough, the caps are still at 100V!

[KYChucky]

Im going to use a hot dog.

[cdw2000]

Well obviously when the hotdog is cooked, the caps are drained...



Really nice looking amp, BTW. Saw the pictures in the other thread.

[Offshore Angler]

Hence I wait till zero and then short the caps just to be sure.

But one of the things I do with the big resistor is calculate the time constant for the RC circuit and then use that theoretical decay curve to check the condition of the caps. But then again, I'm more inclined towards the geek side of the population. It's fun to take the initial voltage and then plot the curve and then plot the results over it.

[Direstraits]

I'm more inclined towards the geek side of the population.

It's fun to take the initial voltage and then plot the curve and then plot the results over it.

Q.E.D.

[KYChucky]

well... i got it warmed up and then switched it off. the caps read 35 volts. easy enough to drain that off - 3 prong success with just the cost of the resistor. Thanks yall.