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Post subject: Biasing my BDRI (first timer!)
Posted: Thu Oct 28, 2010 10:34 pm
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Hey guys, so I got a pair of matched TAD 6L6WGC-STR tubes to put in my amp, and I decided I would rather put them in and bias them myself than pay a tech to do it. Plus I have been toying with tube amps for a while, and I feel like it's time to take the next step. So, I found this video on how to do ithttp://www.youtube.com/user/TubeDepot ... 9IUadZJf-c

It seems simple enough. The only thing is, this guy just says be careful what you touch, but I've heard other people say to drain the capacitors? Is this not necessary? If so, how do you drain the capacitors?

Thanks!


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Posted: Thu Oct 28, 2010 11:12 pm
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To drain caps, I use 1 resistor 100K 2W with 20cm wire with Crocodile isolated to fingers. One is connected to cap minus, one is connected to cap plus. Note that I try to never use the chassis ground for that, and I control with my multimeter directly on -&+ before maintenance.
I suppose that it's obvious to remind that the "beast" has to be unpluged and the voltage can be very dangerous even fatal.

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Posted: Thu Oct 28, 2010 11:37 pm
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Hey thanks for the reply but you're gonna have to dumb that down a bit for me haha. I don't know much about electronics, but I'm smart and can follow a step by step guide.

Thanks


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Posted: Fri Oct 29, 2010 4:14 am
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What he means is to take the resistor, bend the wires on each end 90 degs and touch each end of the resistor wires to the wires coming out of the capacitor. Hold the resistor with an insulated pair of pliers. Hold it there for 30 seconds.

Test the voltage across the capacitor with a volt meter after to make sure there is minimal voltage (as close to 1 as possible). If you still get a reading do it again. Practice using one hand for all of this and keep the other hand in you pocket. Do this with a couple of the capacitors to make sure they're all dead.

Don't trust draining the capacitors with the off by stand-by trick. I've tested the voltage after doing so and still read a significant amount of voltage.

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Posted: Fri Oct 29, 2010 8:18 am
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Ok thanks guys but I don't know what a resistor or a capacitor even looks like, much less where to find it on my amp. This is why the video I found was so great. It showed you exactly how to do everything, and you didn't need to have electrical knowledge.

From what I gather, I can electrocute myself just as easily trying to drain the capacitors. So I might as well just bias the tubes and be careful with that, instead of trying to drain the capacitors and then bias the tubes, which seems more complicated but is just as dangerous....


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Posted: Fri Oct 29, 2010 8:31 am
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dgcobb wrote:
Ok thanks guys but I don't know what a resistor or a capacitor even looks like, much less where to find it on my amp. This is why the video I found was so great. It showed you exactly how to do everything, and you didn't need to have electrical knowledge.

From what I gather, I can electrocute myself just as easily trying to drain the capacitors. So I might as well just bias the tubes and be careful with that, instead of trying to drain the capacitors and then bias the tubes, which seems more complicated but is just as dangerous....


Do yourself a favor and take it to a tech. There are potentially lethal voltages in there. If you don't know what basic electronic components look like, It's tech time. It won't cost much to get it properly biased.

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Post subject: BDRI biasing
Posted: Fri Oct 29, 2010 8:36 am
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Hi dgcobb,

No need to drain the caps for a bias adjustment. There is no need to touch anything inside the chassis with your hands anyway. You should really use a plastic screwdriver for adjustment in case it accidentally touches something besides the adjustment pot. I like to encourage people to learn new things, but opening up the back of an amp might not be the best place to start, especially since you know nothing at all about the dangers of doing so. In order to do a bias adjustment, measuring the test point, the amp will be on and have some lethal voltages inside. I would strongly suggest learning about the discharging the caps, resistors and safety issues involved a bit more before you subject yourself to the dangers involved in opening the amp. :idea:

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Posted: Fri Oct 29, 2010 8:45 am
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+1 You don't want to be a first timer and a last timer. :wink:
When you bias an amp it needs to be on, so draining the caps will do nothing to keep you safe. The high voltages are no joke.

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Posted: Fri Oct 29, 2010 10:01 am
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I agree with all the others. If your knowledge of electronics is minimal at best, don't attempt to bias the amp youself. I earned my degree in EE back when tubes were still in general use. I know what to touch and what not to touch. I have still been zapped a couple of times, but I lived through it. You may not be so lucky.

If you are really interested in poking around inside a tube amp, see if any adult education courses are available in your area. If not, check out Amazon and do a search on "tube guitar amplifier" books. There are a lot of good books available. Study them hard. Then maybe play around with a small, cheap solid state amp to become familiar with the components. If you do decide to try biasing your amp yourself, get the proper tools, such as a bias probe (a Google search will come up with many types and prices). Using the proper tools will minimize or even eliminate, the amount of time you spend sticking your hand (never both at the same time) into the amp.

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Posted: Fri Oct 29, 2010 10:06 am
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bluesky636 wrote:
I agree with all the others. If your knowledge of electronics is minimal at best, don't attempt to bias the amp youself. I earned my degree in EE back when tubes were still in general use. I know what to touch and what not to touch. I have still been zapped a couple of times, but I lived through it. You may not be so lucky.

If you are really interested in poking around inside a tube amp, see if any adult education courses are available in your area. If not, check out Amazon and do a search on "tube guitar amplifier" books. There are a lot of good books available. Study them hard. Then maybe play around with a small, cheap solid state amp to become familiar with the components. If you do decide to try biasing your amp yourself, get the proper tools, such as a bias probe (a Google search will come up with many types and prices). Using the proper tools will minimize or even eliminate, the amount of time you spend sticking your hand (never both at the same time) into the amp.


+1000!

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Posted: Fri Oct 29, 2010 11:54 am
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Don't worry guys, I just followed the instructions on that great video I posted and it was EASY! I was very careful to use the one hand rule. I grounded the black end, and tested the bias point until I got it to where I wanted it. Really wasn't hard at all, and I feel very accomplished now. Plus it was actually FREE, b/c I had a multimeter in a physics lab kit. I live in an apartment so cranking it at different settings to find my "sweet spot" wasn't an option. So I just biased it to ~65 and called it a day. I wanted more clean headroom so I figured I'd bias it a little below the recommended 68 mv. I know fender biases them stock at 60, but I read this was really cold, so I figured ~65 would be a good medium. Plugged it in and it sounds great. I'll be biasing my own amps from now on! :D


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Posted: Fri Oct 29, 2010 12:05 pm
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That's great dgcobb! 65 mV is still OK, some might say a little cold, my HRDlx is at 68 mV now, I like the headroom also. ;) Stock it was about 63 mV. It sounds like you are being smart/safe about it, that's very important. :)

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Posted: Fri Oct 29, 2010 12:24 pm
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shimmilou wrote:
That's great dgcobb! 65 mV is still OK, some might say a little cold, my HRDlx is at 68 mV now, I like the headroom also. ;) Stock it was about 63 mV. It sounds like you are being smart/safe about it, that's very important. :)


Without knowing the actual plate voltage the mv values mean nothing.
And yes, many including myself feel 68mv is pretty cold. But all that will vary with tube manufacturers specs. I just like a warmer grittier more complex tone. The thing that matters most is the tubes are biased in a safe range and most of all you're happy with the tone.
I ran my JJ's in my HRDlx around 85mv with no ill effects at all except that my amp was a piece of crap before I even changed the tubes. It was great when I sold it. I just couldn't trust it to gig with. JJ's are probably one of the best current production tubes around. Still, they don't make em like they used to.

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Posted: Fri Oct 29, 2010 12:39 pm
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What the others said.

Also, keep in mind that the Fender test point is a 1 ohm resistor with a +/-5% tolerance and is measuring both tubes. The bias probe I use has 1% tolerance 1-ohm resistors for each tube.

As 63 Supro said, you also have to measure the plate voltage for the bias value to have any real meaning. I have found the BDRI power supply to be not very well regulated. Unloaded, B+ is 430 VDC. At my bias setting, plate voltage drops to about 410 VDC. As measured, the cathode current is about 38 mA per tube. Using the Weber bias tables, my actual plate current is about 36-37 mA per tube. At the Fender test point, the measurement is about 85 mVDC for the two tubes. According to the Weber tables, I am right at the 50% mark.

Do a search on my posts in this subforum and you will find several detailed posts by me about biasing a BDRI.

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Post subject: BDRI biasing
Posted: Fri Oct 29, 2010 12:50 pm
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63supro wrote:
...Without knowing the actual plate voltage the mv values mean nothing....


Excellent point 63supro! I presumed that his amp was close 440 plate volts like mine, and 65 mV at the test point would be about 28 mA of Plate current per tube, about 12.5 watts idle Plate dissipation (about 50% for a 5881), still on the cold side lol. 85 mV then would be about 17 watts idle, or 70% of a 5881 25 watt tube. I guess that the JJs were 30 watt tubes so 85 mV sounds pretty good. For me it depends on the amp for the bias setting. I like the HRDlx at about 50%, but I like the BJr closer to 70%. To me the 50%-70% range is ideal, and within most everyone's specs. :)

I started to post about the Plate current/voltage/wattage, but I thought it might be too much for the OP. :lol:

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