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KDAWG1966
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Joined: 15 Dec 2010
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PostPosted: Sat Apr 09, 2016 1:35 pm   Reply with quoteBack to top

I'd call Eurotubes but it's the weekend and I'd like to get this done.

I've ordered new tubes for my X100b reissue and was going to bias my amp before I did anything (I know I'll need to do it again after adding new tubes but wanted to A/B it).

I've watched the video several times, have my multimeter set to 200MA, have the amp warmed-up with all tubes glowing, and then when I touch the opposite ends of the standby and on/off wires (with the amp turned on in the standby mode) I get a reading of -1. I know this isn't correct but have no idea why I get this reading.

Any suggestions?

Thanks
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Brian D
Too Much Free Time


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Joined: 19 Feb 2005
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Location: Oceanside, Ca.

PostPosted: Sun Apr 10, 2016 10:54 pm   Reply with quoteBack to top

From what I am reading, you are trying to measure the current between the on/off switch and the stand-by switch. That is probably why you are getting 1.1 or there abouts.

You want to measure the current across the stand-by switch. As you look at the stand-by switch you will see two spade plugs. There should be one on the bottom of the switch and one on the top of the switch. [Note - the could be on the left and right, or at diagonals].

With the power on and the stand-by switch in STANDBY, you are going to place the RED probe on the bottom spade plug. Then place the BLACK probe on the top spade plug. Word of warning... there may be a spark and a very loud pop from the speakers. Be ready for that and do not jerk the probes.

ANOTHER WORD OF WARNING - STUFF IN THE AMP CAN KILL YOU SO BE AWARE AND BE SAFE!

Look this over a few times before doing anything else.

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X100BNut
Too Much Free Time


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Location: Hayward, CA

PostPosted: Mon Apr 11, 2016 9:14 pm   Reply with quoteBack to top

Also be aware that Brian's method can blow the fuse in your meter as the big caps on B+ charge just as fast as the power transformer will allow. Some fuses blow, and some don't. It's much better to connect your meter leads to the switch, take the amp off standby, power up, then flip the switch to standby. Of course, this implies three hands or probes with clips (and probes with clips are an excellent idea). Eurotubes has a probe for $25 that hooks up to your meter and lets you measure the current to each tube individually, allowing you to check for matching. It's all hands-free. Shoot for 36 to 38 mA per tube. Using the standby method, figure that the preamp draws about 10mA. You can measure the actual preamp current, but if you can't figure out how by looking at the schematic, you probably shouldn't do it.

And even Hasserl admits that 70% of max rated plate dissipation is a hot bias. Don't go over 65% unless you want to re-check weekly, and you get your amps and tubes for free. Kaboom.pdf is well-named.
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X100BNut
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PostPosted: Mon Apr 11, 2016 9:16 pm   Reply with quoteBack to top

[color=white] And understand that if a probe slips off the standby switch you're pushing it onto, and touches the chassis that's about a half inch away, it will be very exciting.
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KDAWG1966
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PostPosted: Thu Apr 14, 2016 11:22 pm   Reply with quoteBack to top

Thanks for the replies, fellas. I'm waiting to get a bias probe (and extension) from Eurotubes and will go that route.
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hankwillow
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Posts: 484
Location: Denton, TX

PostPosted: Sun Apr 17, 2016 10:47 pm   Reply with quoteBack to top

Good choice. Be aware that the Eurotubes probes measure plate current directly. That means there is high voltage on both banana plugs. Be very careful handling this setup. Other brand probes mostly measure cathode current. One plug is at or very near ground and the other is only a few mV above it. Much safer for a noob but then you have to subtract the screen grid current. And some of them incorporate a 1ohm resistor into the probe so you actually read mV not mA. No problem since Ohms law states I=E/R. With R being 1, I will = E.

BTW, my favorite probes are the Eurotubes. Acurate and well made. I built a little adapter box to be able to connect a meter for both I and V simultaneously and switch between up to 4 tubes in succession.

Rick

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