Vibro Champ filters self-discharging

Groover · 03-17-2009, 08:53 AM

I am a newbie to amp hacking, so bear with me.

I am working on a Vibro Champ, and as I should, I have been careful to discharge the filter caps after the amp has been on for testing etc. However they don't seem to hold a charge - they make no spark, and if I check the first filter node with a VOM after the power is off I only get a reading of like 9 volts.

When the amp is powered on, there is plenty of voltage at all the filter nodes and plates. It actually exceeds the voltage on the Fender schematic I have seen, could be because I have a replacement Mercury power trans.

So does it sound like the filter caps are bad? - it seems to be the original cap can if I am reading the code #s correctly.

Thanks!

FWIW the amp seems to work right, but it does sound mushy. I assumed that this was due to it being a 6 watt amp.

Enzo · 03-17-2009, 09:53 AM

SO connect yiour meter while it is powered and flip off hte switch. You can watch how quickly it discharges on your mater.

Caps can carry a charge a long time by themselves, but in a circuit, they often discharge fairly quickly.

Don Moose · 03-17-2009, 10:58 PM

Yeah - there's no standby on these little guys, so they get sucked down pretty quickly.

If you want to try it, remove the 6V6 and 12AX7(s) (leave the rectifier) and watch the meter as you turn the thing off.

BTW - a screwdriver across the caps is a good way to:
1) blow the tip right off the driver
2) blow up what had been a perfectly good cap

Use a 220k or so resistor with insulated leads and insulated alligator clips as a bleeder ... until you wire one in permanently.

Hope this helps!

Groover · 03-18-2009, 04:22 AM

Should I add a 220K or thereabouts resistor in parallel to the first filter cap as a bleeder then on my amps?

What sort of wattage would be indicated?

400v / 220,000 ohms = 1ma x 400v = 0.7 watts, so a 2 watt looks like it would be adequate.

Don Moose · 03-21-2009, 02:43 AM

A bleeder sure wouldn't hurt and, 2W should be plenty.

(you can do the power calculation as (V^2)/R if you want to skip a step)

Higher bleeder values are just as safe - they just take a proportionally longer time to drain off the charge, with the very slight advantage of a lower static current drain/power dissipation.

Just check that the filter is under 24VDC before reaching in.

Hope this helps!

Groover · 03-21-2009, 05:46 AM

Well then what about a 500k 1 or 2 watt?

Don Moose · 03-22-2009, 12:00 AM

Should do fine.

(400V^2)/500000ohm = 320mW

03-17-2009, 08:53 AM	#1
Groover Senior Member Join Date: Jan 2009 Location: Los Angeles Posts: 123	Vibro Champ filters self-discharging - why? I am a newbie to amp hacking, so bear with me. I am working on a Vibro Champ, and as I should, I have been careful to discharge the filter caps after the amp has been on for testing etc. However they don't seem to hold a charge - they make no spark, and if I check the first filter node with a VOM after the power is off I only get a reading of like 9 volts. When the amp is powered on, there is plenty of voltage at all the filter nodes and plates. It actually exceeds the voltage on the Fender schematic I have seen, could be because I have a replacement Mercury power trans. So does it sound like the filter caps are bad? - it seems to be the original cap can if I am reading the code #s correctly. Thanks! FWIW the amp seems to work right, but it does sound mushy. I assumed that this was due to it being a 6 watt amp.

03-17-2009, 09:53 AM	#2
Enzo Senior Member Join Date: May 2006 Location: Lansing, Michigan, USA Posts: 10,366	SO connect yiour meter while it is powered and flip off hte switch. You can watch how quickly it discharges on your mater. Caps can carry a charge a long time by themselves, but in a circuit, they often discharge fairly quickly. __________________ Education is what you're left with after you have forgotten what you have learned.

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03-17-2009, 10:58 PM	#3
Don Moose Senior Member Join Date: Jul 2007 Location: Indianapolis Posts: 676	Yeah - there's no standby on these little guys, so they get sucked down pretty quickly. If you want to try it, remove the 6V6 and 12AX7(s) (leave the rectifier) and watch the meter as you turn the thing off. BTW - a screwdriver across the caps is a good way to: 1) blow the tip right off the driver 2) blow up what had been a perfectly good cap Use a 220k or so resistor with insulated leads and insulated alligator clips as a bleeder ... until you wire one in permanently. Hope this helps!

03-18-2009, 04:22 AM	#4
Groover Senior Member Join Date: Jan 2009 Location: Los Angeles Posts: 123	Should I add a 220K or thereabouts resistor in parallel to the first filter cap as a bleeder then on my amps? What sort of wattage would be indicated? 400v / 220,000 ohms = 1ma x 400v = 0.7 watts, so a 2 watt looks like it would be adequate.

03-21-2009, 02:43 AM	#5
Don Moose Senior Member Join Date: Jul 2007 Location: Indianapolis Posts: 676	A bleeder sure wouldn't hurt and, 2W should be plenty. (you can do the power calculation as (V^2)/R if you want to skip a step) Higher bleeder values are just as safe - they just take a proportionally longer time to drain off the charge, with the very slight advantage of a lower static current drain/power dissipation. Just check that the filter is under 24VDC before reaching in. Hope this helps!

03-21-2009, 05:46 AM	#6
Groover Senior Member Join Date: Jan 2009 Location: Los Angeles Posts: 123	Well then what about a 500k 1 or 2 watt?

03-22-2009, 12:00 AM	#7
Don Moose Senior Member Join Date: Jul 2007 Location: Indianapolis Posts: 676	Should do fine. (400V^2)/500000ohm = 320mW

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