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Post subject: Shocking Experience
Posted: Wed Sep 08, 2010 10:21 pm
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I was checking the bias on my Blues Deluxe Reissue tonight when I had a bit of an unpleasant surprise. I've been playing with the bias settings, listening to the amp and comparing the sound at settings of 70 mV and 80 mV (JJ 6L6GCs). While checking the plate voltage on V5, the DVM probe tip slipped and touched both pin 3 and pin 2 of the tube. There was a brief arc that went out as soon as I pulled the probe away (Which was damn fast!). :shock: I shut the amp down, unplugged it and left it on standby for a few minutes to (hopefully) drain the caps some. I took a Q-Tip soaked in some alcohol and cleaned off some slight charring of the PCB between the two pins. After things dried, I plugged it back in and turned the amp back on. Both bias and plate voltages were where I had set them before the mishap (70 mV and 411/412 Volts). Both power tubes looked fine with no unusual brightness or colors in either tube. B+ was good. I closed the amp up and plugged in my guitar. Everything sounded just great and a half hour of playing showed no unusual heat coming off the tubes. :D

Other than some leftover black charring of the PCB between pins 2 & 3 that I could not remove, nothing seems to be amiss. Is there anything else I should look for or check out to be sure?

Oh, I decided I prefer the sound of the amp at 70 mV vice 80 mV. I think I'll quit playing with the bias for awhile. :wink:

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Post subject:
Posted: Wed Sep 08, 2010 10:30 pm
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Doubtful that there's any damage other than the cosmetic charring on the PCB. Any serious issues would've become apparent when you switched the amp back on -- likely it would've failed the "smoke test" and fried the fuse instantly.

Arjay

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Post subject: BDlx bias
Posted: Thu Sep 09, 2010 12:18 am
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Those pins are awfully close aren't they? You might consider checking your bias using a different method. With the amp unplugged and caps discharged, measure the resistance of each half of the primary of the output transformer with an ohm meter, CP1 to CP3, and CP2 to CP3, record the resistances (mine is about 82 ohms on each side). Then turn the amp on and let it warm up, both switches on, make sure your volumes are turned down, nothing plugged into the input. If you check the DC voltage from ground to CP1, and from ground to CP2, that is the plate voltage for each tube. Then check the DC voltage drop across both halves of the transformer, CP1 to CP3, and CP2 to CP3, record the voltages. Now divide the voltage by the resistance to get the actual plate current at idle. So, the voltage measured across CP1 to CP3, divided by the resistance measured from CP1 to CP3, equals the plate current for that tube. Multiply the plate current by the plate voltage to get the plate idle dissipation in watts. It doesn't matter which tube is which, but check both to see if there is much of an imbalance between the two output tubes, a little imbalance is OK. It seems that many other methods of checking the bias, use the cathode current measurement, which is going to be a higher current than the plate current. The plate current is what's important anyway, not the plate current plus the grid currents, which is what the cathode current is. :)

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Posted: Thu Sep 09, 2010 3:08 am
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Millivolts is not the good way to do a proper bias


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Posted: Thu Sep 09, 2010 7:06 am
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stratele52 wrote:
Millivolts is not the good way to do a proper bias


Indeed!

Idle bias is a measurement of current draw, not voltage drop.

Arjay

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Post subject:
Posted: Thu Sep 09, 2010 7:17 am
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Retroverbial wrote:
Doubtful that there's any damage other than the cosmetic charring on the PCB. Any serious issues would've become apparent when you switched the amp back on -- likely it would've failed the "smoke test" and fried the fuse instantly.

Arjay


That's what I had hoped.

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Post subject: Re: BDlx bias
Posted: Thu Sep 09, 2010 7:18 am
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shimmilou wrote:
Those pins are awfully close aren't they? You might consider checking your bias using a different method. With the amp unplugged and caps discharged, measure the resistance of each half of the primary of the output transformer with an ohm meter, CP1 to CP3, and CP2 to CP3, record the resistances (mine is about 82 ohms on each side). Then turn the amp on and let it warm up, both switches on, make sure your volumes are turned down, nothing plugged into the input. If you check the DC voltage from ground to CP1, and from ground to CP2, that is the plate voltage for each tube. Then check the DC voltage drop across both halves of the transformer, CP1 to CP3, and CP2 to CP3, record the voltages. Now divide the voltage by the resistance to get the actual plate current at idle. So, the voltage measured across CP1 to CP3, divided by the resistance measured from CP1 to CP3, equals the plate current for that tube. Multiply the plate current by the plate voltage to get the plate idle dissipation in watts. It doesn't matter which tube is which, but check both to see if there is much of an imbalance between the two output tubes, a little imbalance is OK. It seems that many other methods of checking the bias, use the cathode current measurement, which is going to be a higher current than the plate current. The plate current is what's important anyway, not the plate current plus the grid currents, which is what the cathode current is. :)


Thanks. I will keep this in mind when I next work up the nerve to open the back of the amp. :shock:

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Post subject:
Posted: Thu Sep 09, 2010 7:21 am
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Retroverbial wrote:
stratele52 wrote:
Millivolts is not the good way to do a proper bias


Indeed!

Idle bias is a measurement of current draw, not voltage drop.

Arjay


Not a discussion I am interested in getting into. I use the tools that I have available (bias test point, plate voltage, and various bias calculator tools)and they work fine for my purposes.

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Post subject: Bias check
Posted: Thu Sep 09, 2010 9:56 am
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stratele52 wrote:
Millivolts is not the good way to do a proper bias


You are right again! I think that in many amps there is a 1 ohm resistor between the cathode and ground, and Fender among others, uses the voltage drop across that resistor to calculate the cathode current. Since the voltage drop across the 1 ohm resistor is in millivolts, it's easy to just test one point and use the millivolt number as a reference. However, that millivolt reading is at the Cathode, so it isn't a true representation of Plate current, and the current splits to however many tubes there are, so you can't get accuracy for each tube that way either. It seems that the Cathode current is usually 5 to 10 milliamps higher than the plate current. The Fender schematics use the millivolt reading as a reference, and it has become popular because of the simplicity of reading one point and done. I prefer accuracy over ease, but guess what most people do? Right, take the easy way out. :)

Edit: I should be more clear about the millivolts vs milliamps readings, (current is represented in "amps", short for Ampere).The mV is millivolts, the mA is milliamps. 60 mV (0.060V) across a 1 ohm resistor, is 60 mA of current (0.060A). So the mV reading is going to be the same as mA number when using a 1 0hm resistor. That's why a 1 ohm resistor is used.

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Post subject:
Posted: Thu Sep 09, 2010 10:30 am
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Folks, I am not stupid. I have a degree in electrical engineering. I was in school when tubes were still in common use in radios and tvs and were part of the EE curriculim. I have read many of the treatises published in multiple forums and web sites, including the recent, very lengthy one on TGP and the Aiken amp page. I am familiar with the various methods and tools used when working with tubes. I came here asking a question and expressing a concern. I did not ask for, nor do I need a lecture on the proper way to bias an amp. Nor do I appreciate the implication that I am "taking the easy way out." I do not play professionally. I play for myself, by myself. I have no need for tools that I may use once every several years. In my opinion, the tools I use are of sufficient accuracy to achieve an excellent sound from a properly operating amp. That's all I have to say on this subject.

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Post subject: Amp bias
Posted: Thu Sep 09, 2010 11:23 am
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I am sorry Bill, I certainly didn't mean anything negative, especially about you or what method that you choose (you didn't even say what method that you used). I posted my response to you with the confidence that you know what you were doing. I appreciate your input. :) Most people don't have the knowledge that you do, so for the layman (most people) "the easy way out" is their only choice. Cool? :)

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Post subject:
Posted: Thu Sep 09, 2010 11:34 am
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Since Pin 2 is heater and Pin 3 is the anode, there shouldn't be continuity between the two, with the tube pulled. If there is, I'd check for an arc tracing across the two, caused by the short. Since your amp bias is ok & it plays well, you are prolly ok.


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Post subject: Re: Amp bias
Posted: Thu Sep 09, 2010 11:53 am
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shimmilou wrote:
I am sorry Bill, I certainly didn't mean anything negative, especially about you or what method that you choose (you didn't even say what method that you used). I posted my response to you with the confidence that you know what you were doing. I appreciate your input. :) Most people don't have the knowledge that you do, so for the layman (most people) "the easy way out" is their only choice. Cool? :)


I thought the method I used was obvious since it is discussed often.

I appreciated your original response as an alternate method that can be done with the tools I have (DVM).

Guess it was a combination of three posts saying my method is not the best/correct method. No argument there, but it works for my purposes.

I'm calm now. I just want to go home and play my guitar. 8)

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Last edited by bluesky636 on Thu Sep 09, 2010 11:55 am, edited 1 time in total.

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Post subject:
Posted: Thu Sep 09, 2010 11:54 am
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BMW2002Ti wrote:
Since Pin 2 is heater and Pin 3 is the anode, there shouldn't be continuity between the two, with the tube pulled. If there is, I'd check for an arc tracing across the two, caused by the short. Since your amp bias is ok & it plays well, you are prolly ok.


I'm going to try scrubbing the area a little more with alcohol and an old toothbrush when I get home tonight to get any remaining residue off the PCB.

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Post subject:
Posted: Thu Sep 09, 2010 12:48 pm
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The air arc is about 1K to 3KVolt per mm really depending on humidity, may be with dust or electrostatic charge or 450V DC with half a mm could be not abnormal condition. If you remember TV with 8KV transformer with the screw driver you can generate long arc 8)

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