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#1
I have measured my plate voltage, where i got 440 when stanby was on and 450 when standby was off,

now ive put on a bias probe connected to my multimeter, for one of the two powertubes,

but it says its 20,0 mV when its set on DC volt , and when i put on the mA thing on the multimeter it says 06.83

so what does this mean?
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#2
o_O Makes little sense to me. It should read around 40mA no? Did you turn it off standby for the reading and did you let it warm up a bit? Also, make sure the mA is set to a 200 scale
WTLTL 2011
#3
yeah , warm and ready, thats why i think its weird

200 scale is something i should be able to set on the multimeter?
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#4
yeah

let me forward you an email I got about biasing from eurotubes
WTLTL 2011
#5
I sent it as a PM lol. Howd you get a kaboom with that adapter???
WTLTL 2011
#6
I'm going to assume that you're measuring across a cathode resistor? The reason why your current reading is off is because your multimeter is in parallel across the cathode resistor. To measure current you need to have the multimeter in SERIES. The point of having the 1 ohm cathode resistor there at all is so you can read off the current as the V/R. You don't actually directly measure the current. Your idle current is 20 mA.

Another note: placing an ammeter in parallel with a resistor will usually short circuit and either blow a fuse or fry the instrument.
Last edited by al112987 at Dec 15, 2008,
#7
Quote by Mark G
I sent it as a PM lol. Howd you get a kaboom with that adapter???


i tried to do it like the guy on the eurotubes video with only multimeter, for it to not kaboom the red and black wire need to be in place before the power is on, which i have learned now :P
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#8
Quote by al112987
I'm going to assume that you're measuring across a cathode resistor? The reason why your current reading is off is because your multimeter is in parallel across the cathode resistor. To measure current you need to have the multimeter in SERIES. The point of having the 1 ohm cathode resistor there at all is so you can read off the current as the V/R. You don't actually directly measure the current. Your idle current is 20 mA.

Another note: placing an ammeter in parallel with a resistor will usually short circuit and either blow a fuse or fry the instrument.


could you tell me how to set the multimeter up if i show you a picture of it?

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Last edited by Jaekae at Dec 15, 2008,
#9
I don't think that is necessary. You've said everything you need, all you need to do is

1) have the plate voltage (with your tubes installed)
2) have a reading for your idle current which is going to be equal to the mV.

Again, you don't need to measure the current directly. You're measuring the voltage across a 1 ohm resistor, so the current is just going to be that voltage/1 ohm which gives your your idle current. What you need to do is to just get that idle current into correct range so you get somewhere between 60-70% of the tube's maximum plate dissipation when you multiply your idle current by your plate voltage.

I'm just curious as to whether or not your multimeter still works.
#10
^ so you say that what i get as 20 mV, should be 45 mV ?

and how would i do this :

"Another note: placing an ammeter in parallel with a resistor will usually short circuit and either blow a fuse or fry the instrument."

?

You mean that if i set the multimeter on mA it could damage the multimeter? or damage the amp?
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Last edited by Jaekae at Dec 15, 2008,
#11
40-45 mA should be about right. I think, at least that was the way I was taught to bias my own amp (Marshall), but it's kind of a generic way of biasing amps ASSUMING that your amp has that 1 ohm (I think 5 watt) resistor that goes from the cathode to ground.

and yes, that can damage your multimeter.

Also a disclaimer, don't go prodding around in there, use one hand at a time when going under the amp's chassis if that is where the bias pot trimmer is, and wear rubber soled shoes. There are lethal voltages and everything is grounded to the chassis.

Edit: if you're doing this with your ENGL, here is a guide that I found by quick google search...

http://forum.lordriffenstein.com/viewtopic.php?t=79&postdays=0&postorder=asc&start=20
Last edited by al112987 at Dec 15, 2008,
#12
i bought a electric classed screwdriver, if i only touch the screwdriver ill be fine right?
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#17
Ok, did the probe come with a guide? because different probes work different ways, some ask you to measure the bias current, others use a 1 ohm resistor and ask you to measure a voltage (which is what I was telling you to do)
#18
nope i got nothing with the probe

when i measure the stock tubes, they say 38 mV, with the bias pot at stock setting,

and with these JJ,s tubes it says only 20 mV with stock bias pot setting,

and it only went up 2-3 mV when bias pot was set on maximum ( i did increase very slow to see if anything happened)
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Last edited by Jaekae at Dec 15, 2008,
#19
Ok.

I think it's a tube thing then. Are these 6L6gc's? Or el-34s? I've heard of cases where people could not bias their tubes to 70%.
#20
its JJ 6L6GC s

think its a specific failure with these tubes, or will it be same if i buy new JJ,s ?
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#22
Biasing scares the crap out of me

why are you measuring mV when you should be measuring mA anyway?
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#23
Quote by Mark G
Biasing scares the crap out of me

why are you measuring mV when you should be measuring mA anyway?


because of that 1 ohm resistor thing that al112987 talked about, i think
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#24
a resistor doesn't change volts to amps... You should get a friggen mA reading between prolly 30 and 50 mA with that probe of yours set to read mA with a 400 scale. I checked that thing of yours and you can set it to 400mA...not 200. 400mA will be ok.

Dial it in to the mA scale and Im guessing that yellow button sets the scale? Not sure if you are measuring AC or DC...
WTLTL 2011
#25
Quote by Mark G
a resistor doesn't change volts to amps... You should get a friggen mA reading between prolly 30 and 50 mA with that probe of yours set to read mA with a 400 scale. I checked that thing of yours and you can set it to 400mA...not 200. 400mA will be ok.

Dial it in to the mA scale and Im guessing that yellow button sets the scale? Not sure if you are measuring AC or DC...


No, there are different bias probes, there is one type that places your meter between the cathode and ground so you read a current that way. The second one puts a 1 ohm resistor between cathode and ground and measures the voltage across the 1 ohm resistor. Which by V=IR will tell you the cathode current.


Edit: Do not forget to drain your caps before placing the amp back into the chassis. Do not forget to do this. Those caps will hold charge for weeks if not drained. Turning your amp off and unplugging it does not do ****. These are the most dangerous parts of the amp, one terminal is grounded, while the other is connected to other components in the amp which is why must only use one hand at a time when working inside the amp (I always put my other hand in my pocket)
Last edited by al112987 at Dec 15, 2008,
#26
i know almost nothing about how electric stuff works :P

When i press the yellow button the display says AC and Trms instead of DC

I dont know whaty Trms mean..
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#27
You're measuring DC...

AC stands for alternating current and you'd be measuring a true RMS (Trms) value for it.


Also, read what I just wrote about draining the filter caps. They will kill you if you don't.
Last edited by al112987 at Dec 15, 2008,
#28
Quote by al112987


Edit: Do not forget to drain your caps before placing the amp back into the chassis. Do not forget to do this. Those caps will hold charge for weeks if not drained. Turning your amp off and unplugging it does not do ****. These are the most dangerous parts of the amp, one terminal is grounded, while the other is connected to other components in the amp which is why must only use one hand at a time when working inside the amp (I always put my other hand in my pocket)


How would i drain a the caps? It sounds like it dangerous to do so. Does not doing it make the amp dangerous when even when its back at the chassi?
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Last edited by Jaekae at Dec 15, 2008,
#29
Quote by al112987
You're measuring DC...

AC stands for alternating current and you'd be measuring a true RMS (Trms) value for it.


Also, read what I just wrote about draining the filter caps. They will kill you if you don't.


So putting it on mA and pressing the sel button to switch to where it says AC and Trms wont make it work?
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Last edited by Jaekae at Dec 15, 2008,
#30
Quote by Jaekae
How would i drain a the caps? It sounds like it dangerous to do so. Does not doing it make the amp dangerous when even when its back at the chassi?


I think you mean cabinet, the chassis is the big metal box everything is housed in.

It won't be dangerous when it's back in the cabinet, but it's dangerous to work on the amp when it is not in the cabinet. I always drain my caps anytime I'm about to go near the inside of my amp and I always drain them before putting the amp back into the cabinet. There is a reason why they warn you about having an authorized tech do these things if you don't know what you're doing (though I think something like biasing amps does not really require a tech, just some basic know how) Regardless, I feel nervous biasing my own amp with my somewhat limited knowledge.

How you drain the caps depends on how your amp is laid out. Whether the filter caps are accessible or not.

First. Turn the amp off with the amp on standby (ie. standby is on). Wait for maybe 10 minutes or so.

If the filter caps are exposed in your amp, measure the residual voltage stored in the cap, the filter cap's (-) is grounded to the chassis so (I'm assuming you have probes here...) measure the the (DC) voltage from the (+) terminal to the chassis.

If your filter caps are exposed, what I usually use a 20k ohm 10 watt power resistor with insulated leads and alligator clips (really I just cut a jumper cable in half and solder the resistor to each end and shrink wrap the exposed leads), and jumper between the (+) teriminal and the chassis. That will drain the caps.

If your filter caps are not accessible, then you can jumper between the chassis and the pin on your tube sockets that lead to the plates (the same pin you used to measure your plate voltage). This will cause the charge to drain through your tubes. Be sure that you are using the correct pin, otherwise prepare for a nice little spark. and remember to only use one hand, and wear rubber soled shoes.

Always be careful, first time I drained my caps was done by accident when I accidently caused a short with my multimeter probe leads, got a HUGE spark, which freaked me the hell out.
Last edited by al112987 at Dec 15, 2008,
#31
I've electrocuted myself biasing my amp, I was holding my guitar in my other amp, plugged in I know, I'm an idiot. 330 volts through my whole body, not good. I've also just electrocuted my hand and that isn't nearly as bad.
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#32
okey, thanks for all help,

im trying to do this bias myself because i want to learn and it was fun, got to expereince one of those accident short also :P got me a adrenaline rush

so to conclude its the biggest chance that its the tubes that are damaged? And that i sohuld buy new tubes and try same thing with those?
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Last edited by Jaekae at Dec 15, 2008,
#34
Quote by al112987
Wow you're lucky to be alive!


You mean kevin right?
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#35
Yeah!

The answer to your question. I'd email Eurotubes about it first, see what he recommends.

Anyway, props to you for putting forth the effort to try to learn how to do some basic amp maintenance on your own. It requires some basic know how and requires a bit of work to know what's going on, but it's really worth it in the long run and saves you the money of having a tech do it for you every time. Just know the safety precautions and be careful. You do learn a lot of stuff by doing it hands on, everything I picked up about amps came from building my own.
Last edited by al112987 at Dec 15, 2008,
#36
I realize that, I'm surprised it didn't hurt more. Infact, it didn't really hurt at all. I yelled pretty loud though and I felt pretty shaky for a while.
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#37
It might have been a more minor shock, maybe not the several hundred volts that you'd get from putting your body across a filter capacitor (that'd be bad), but still enough to really shake you up a little.
#38
Yeah, probably not hundreds. All I know is that it felt like someone was shaking me very hard and very fast, definitely won't let that happen again.
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#39
asked the eurotubes guys, and they said it was normal because my amp have a very low bias sweep, so i need a very hot grade of tubes for it to be able to get the bias up where it should be
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#40
Yeah sounds about right. If you REALLY wanted, you could change a resistor in the bias circuit to a lower value but that involves dicking around with the main board which I don't think you really want to do.
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