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Old 7th December 2012, 09:27 PM   #1
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Default Auto capacitor "drain" fixture

I've been reading a "ton" of books on tube amps over the last year and "messing around" with making changes to a little Fender Champ I have. I'm getting ready to have a go at rolling my own. Being a "5 volt" guy (microcontrollers and such) sticking my fingers and scope probes in this thing has at least on one occasion been "shocking". I (of course) pay very close attention to safety procedures (but did have that one "slip up"). I found it a bit tedious discharging the caps in the amp each time I wanted to change something and got to thinking. I've drawn a schematic of what I have in mind.

Basically, it's a fixture to plug the amp into with some "test leads". The test leads are resistors in series with the contacts of a relay; the other side of the switch to ground. The fixture is setup so that when the main "power switch" is "on" (or "off" - up to you) the amp is powered. When the switch is flipped the other way the amp is not powered; AC is switched to the coil of a relay, closing the resistor/contacts discharging the caps. It's an "exclusive OR (XOR)" function so you can't power the amp and discharge the caps at the same time. I've drawn the "basis" for the idea; one can think of all kinds of variations; adding indicators showing that the voltage on the cap has dropped below some threshold, etc. I "drew in" a GFCI for the amp side; maybe not "needed" but couldn't hurt.

I wanted to throw this out there for discussion. I'm planning on building one up this weekend from my junk box. Seems to me that other than the relays one can get everything at Home Depot/Lowes/Hardware store.

The obvious caveats apply; high voltage, don't use your tongue for testing, etc.
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Old 7th December 2012, 09:33 PM   #2
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I do not really see a use for that - it is well-known and good practice to fit high-voltage filter caps with bleeder resistors to ensure they discharge within minutes after switching off.

A resistor is easier to fit and more reliable than a relay circuit, isn't it?

Greetings,
Andreas

Last edited by Rundmaus; 7th December 2012 at 09:36 PM.
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Old 7th December 2012, 09:38 PM   #3
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Quote:
Originally Posted by Rundmaus View Post
I do not really see a use for that - it is well-known and good practice to fit high-voltage filter caps with bleeder resistors to ensure they discharge within minutes after switching off.

A resistor is easier to fit and more reliable than a relay circuit, isn't it?

Greetings,
Andreas
Maybe in some amps, but you are not going to find them (bleeder resistors) in the majority of "guitar" amps. Have a look at YouTube for "draining capacitors"; tons of people posting all kinds of variations on the theme.
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Old 7th December 2012, 11:12 PM   #4
JMFahey is offline JMFahey  Argentina
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Just fit 470K 1 or 2W from +HV to ground and be happy.
Maybe the original amp didn't have it, but you may correct that easily.
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Old 7th December 2012, 11:19 PM   #5
Simon B is offline Simon B  United Kingdom
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I can't think of a guitar amp I've worked on that didn't have bleeder resistors on the reservoir caps, and I have fixed quite a few. If I did get one without them, I'd fit them, straight away.

If the main reservoir is composed of two caps in series, each taking half the voltage, fit one across each, say 220k and be generous rather than stingy with the power rating. This will help ensure that the ht voltage is split evenly across the two caps - leakage will otherwise tend to make that not so.
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Old 10th December 2012, 06:24 PM   #6
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I noticed my big Fender Super Twin has two filter caps in series. Was there some cost advantage to using two lower-voltage caps in series, or perhaps physically smaller caps were easier to mount or package??? Though the diodes are a full bridge, they used the center-tap of the secondary to connect to the center between the series caps, nicely forcing the mid-point to always be half of B+.

And there are seperate resistors across each seperate cap...39K 2-watt across each 220mfd 265v cap. I first thought they were to dampen inductance and capactance in the supply from ringing or slow pulses, but now that you mention it they are probably mostly safety drains. I'll have to watch that voltage drop on a meter so I learn how long to wait.
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Old 10th December 2012, 07:02 PM   #7
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Quote:
Originally Posted by cyclecamper View Post
I noticed my big Fender Super Twin has two filter caps in series. Was there some cost advantage to using two lower-voltage caps in series, or perhaps physically smaller caps were easier to mount or package???
Could be cost advantage, or availability of high voltage, high capacity caps. In my current project, I also use filter caps in series, as the 410V design voltage of the choke-input supply may easily rise to roughly 600V with no load present. Difficult to find caps with that voltage rating.

Greetings,
Andreas
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Old 10th December 2012, 11:10 PM   #8
Enzo is offline Enzo  United States
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Two 350v caps in series was cheaper than finding one 600v cap.

You don't need a fancy circuit. Add the bleeders as suggested above by others, and you ought to just make a discharge lead. That is nothing more than a resistor with clip wires at each end.

Such a resistor is not connected while the circuit is powered, so it only needs eneough dissipation to last through discharge. If I do this, I grab a 5 watter just because it is eailer to hold, a smaller resistor will be fine, you could sleeve it in heat shring. The value is not critical at all. It needs to be large enough not to strss the caps by discharging them too fast (A screwdriver across the leads as an example), but also a low enough resistance that it doesn't take an hour to discharge. I use a 1k myself, but whatever you got.

CLip it to ground and to a filter cap hot lead. Wait a few moments.


And the old cheap way? Take a clip wire, gfround to chassis, then ground pin 1 or pin 6 of any 12AX7 socket in the amp. The plate resistor of that tube acts as the discharge resistor.
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Old 11th December 2012, 02:38 AM   #9
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So what's the point if you know the amp and know the drain resistors are good? Is this to avoid death in the rare case one just blew, or for any caps downstream of the standby switch or just good work habits or something? I was so sure mine were already drained that I just used a piece of wire...if I was wrong it would have arced but of course there was nothing there.
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Old 11th December 2012, 02:46 AM   #10
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I do understand weighting your actions with the seriousness of the consequences though. Like in motorcycling, I try to mentor newbies. They do dangerous things and get away with it 999 times out of a thou. Until I point out that if they do that just a few times a day it will probably kill them inside of a year. No easily-avoided chance of death is worth risking. But I learn best from understanding rather than from experience. Is there any reason I can't make all my amps so they are safe after 5 minutes? Or would that just forster bad habits that kill me working on someone else's amp?
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