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.
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.
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
A resistor is easier to fit and more reliable than a relay circuit, isn't it?
Greetings,
Andreas
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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.
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.
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.
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.
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.
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
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.
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.
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.
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?
It is good practice to ensure that any HV cap is sufficiently drained before working on a circuit - a bleeder, a series resistor or a choke might have gone open circuit, a cap downstream of some regulator circuit might still be charged, whatever. Especially if the amp hasn't been constructed by you.
If I am working on a circuit that has been powered on before, I usually leave the DMM connected to the HV rail I am working on, so the '0.xxx V' reading is always in sight.
Greetings,
Andreas
Hi Guys
If you encounter an amp without bleeder resistors, add them in.
You do not need a "fixture" to drain caps. At most just a loose resistor and an alligator clip. Clip the lead onto one end of the R and the other to ground, then touch the free end of R to the cap.
DO NOT use less than 1k. Make it a 5W to have plenty of distance between your fingers and the leads.
The TUT-series always states to add bleeders, and the projects in TUT3 and TUT5 all have them.
I did not read this entire thread but the second post gave the only answer required.
If servicing old Marshall amps, some disconnected the caps from the circuit where bleeders were so the caps stay charged up if you mistakenly use the standby switch - a completely unnecessary device on a guitar amp.
Also, anything below 300V or so is considered 'safe".
Have fun
Kevin O'Connor
If you encounter an amp without bleeder resistors, add them in.
You do not need a "fixture" to drain caps. At most just a loose resistor and an alligator clip. Clip the lead onto one end of the R and the other to ground, then touch the free end of R to the cap.
DO NOT use less than 1k. Make it a 5W to have plenty of distance between your fingers and the leads.
The TUT-series always states to add bleeders, and the projects in TUT3 and TUT5 all have them.
I did not read this entire thread but the second post gave the only answer required.
If servicing old Marshall amps, some disconnected the caps from the circuit where bleeders were so the caps stay charged up if you mistakenly use the standby switch - a completely unnecessary device on a guitar amp.
Also, anything below 300V or so is considered 'safe".
Have fun
Kevin O'Connor
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