# Bias Help



## blam (Feb 18, 2011)

well, another 2 pairs of tubes red plated on my Phaez amp. at this rate I'll be broke buying tubes!!! unfortunately I have my first gig coming up in a week and I reaaaallly need this amp working


all the solder joints look pretty solid to me.


could you kind gents show me where I read the voltages and currents?


these are the numbers I got from Randy the builder, but I'm not quite sure how/where to do the measurement.


"you can measure the bias voltage at the grid of the output tubes. Should be about -13.5vdc.


current across the 1 ohm cathode resistors should be 25 to 30 mv which equates to 25-30ma."


I know i'd have the amp off standby with a speaker load and have my multimeter set to DC on the "20" range


I know the 1 ohm resistor is the blue one attached to black wire


here are a couple gut shots of my amp: 


http://files.onthelam.ca/guitar/board.jpg

http://files.onthelam.ca/guitar/sockets.jpg

thanks guys!


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## GTmaker (Apr 24, 2006)

I think if you bought one of these it would make life easier...

http://www.amp-head.com/product_info.php?cPath=21&products_id=31


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## blam (Feb 18, 2011)

yeah, I think i might have to....but it wont arrive before the show unfortunately.


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## ampaholic (Sep 19, 2006)

I'll defer to others more knowledgeable than me so take this with a grain of salt!
I'm assuming those are EL84 power tubes.
Pin 2 is usually the grid and you would look for that -13.5 VDC directly at the tube pin. This method of checking bias is not the most accurate but should at least quickly indicate if things are in the ballpark. Keep in mind here that if the tubes are biased hot this voltage will read "more negative", i.e. adjusting the voltage closer to 0 VDC will bias the tubes colder.

That does look like the 1 ohm resistor connected to pin 3 which is the cathode of the tube. You can check the voltage across this resistor and equate it to ma of current.

I see that there is a bias pot on the board which is very helpful if things need to be tweaked.

I must say though that I would be questioning why all of a sudden these problems burning out tubes. Did someone play around with the bias and then you started having problems?

It also goes without saying to be careful in there. You might have more experience than me but I've stung myself a couple of times by being careless and it does not feel good. One hand in the pocket at all times!


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## blam (Feb 18, 2011)

yes, that blue box is the bias adjustment.

the first time it redplated, it was after I swapped the tubes for el84s and had a 3 hour jam. packed her up and took her home.
turned it on when i got home and played a bit when I heard some static so i peeked inside and sure enough red plated.
shut her down and swapped the tubes. was fine for a couple jams and then it redplated again. 
swapped the tubes and off to my jam. worked fine all night. red plated when I got home again (friday)
my initial thoughts was the tubes didn't like being moved after a 3 hour session into the harsh cold here (-25 the last night we jammed)
so friday, I swapped the tubes again and although it didn't redplate (yet) i heard the static/scratchyness that has accompanied my redplating. I'm sure if i blasted a few chords i would get the redplating.

when checking the voltage at the pins, do I just ground one probe to the chassis? and when you say check the voltage across the resistor, just each probe on either side of the resistor?

thanks

I ordered one of the bias meters GTmaker linked and even paid the express shipping. hopefully it arrives in time but in the mean time I have to get my hands dirty.


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## keto (May 23, 2006)

Yeah, meter set for mV, hot probe on the wire side of the 1 ohm resistor, ground probe to the chassis (that bolt sticking up right beside the tube socket should be fine) or other ground point.

Amp on in standby for a couple minutes, volume on 0 (I like to also turn the gain pots to 0 but dont think that matters), then flip it off standby and measure. Like you said, speaker (or load anyways) connected.

Corrected to mV, SEE POST 17 BELOW


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## blam (Feb 18, 2011)

thanks kent!

will give it a try tonight.


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## keto (May 23, 2006)

ALSO! I forgot to mention - sometimes builders/techs wire their bias pots in backwards (it's just a matter of swapping 2 wires to different spots), so don't run it all the way to one end before you start. You might end up with it maxed instead of min! Check where it's at then just turn it *a little bit* and re-measure.


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## blam (Feb 18, 2011)

great advice. thanks.

ive been told clockwise will turn the idle down.


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## ampaholic (Sep 19, 2006)

blam said:


> the first time it redplated, it was after I swapped the tubes for el84s and had a 3 hour jam. packed her up and took her home.
> turned it on when i got home and played a bit when I heard some static so i peeked inside and sure enough red plated.


Sounds like you were swapping out tubes without checking the bias. If you used tubes that were not rated at a similar current draw and didn't check/adjust the bias that could be your problem.

If that is the case I bet if you put in the original tubes you would be back in business.


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## ampaholic (Sep 19, 2006)

keto said:


> Yeah, meter set for mA


I think you meant to say mV


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## blam (Feb 18, 2011)

ampaholic said:


> Sounds like you were swapping out tubes without checking the bias. If you used tubes that were not rated at a similar current draw and didn't check/adjust the bias that could be your problem.
> 
> If that is the case I bet if you put in the original tubes you would be back in business.



yes. I am thinking this is what it is now. I was always under the impression this amp was cathode bias, but clearly it is not?


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## keto (May 23, 2006)

see post 17 below


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## blam (Feb 18, 2011)

both are correct.


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## ampaholic (Sep 19, 2006)

blam said:


> I was always under the impression this amp was cathode bias, but clearly it is not?


No, it is a fixed bias amp, which requires checking bias when changing tubes. The cathode connected to ground (your 1 ohm resistor doesn't count) is the giveaway. Plus the bias adjustment pot!


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## blam (Feb 18, 2011)

D'oh.....i guess that solves my problem....

live and learn. c'est la vie.

thanks again all.

I shall get this bad boy up and running when I get home.


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## jb welder (Sep 14, 2010)

There were a couple of errors in this thread that should be corrected as they could be dangerous.


ampaholic said:


> Keep in mind here that if the tubes are biased hot this voltage will read "more negative", i.e. adjusting the voltage closer to 0 VDC will bias the tubes colder.


This is backwards, the more negative the grid voltage is, the colder the tube will run. When dealing with unknown or new tubes, it is best to set the grid voltage to max. negative before installing the tubes. Then after they warm up you can adjust the bias.

You did not need a bias probe, using the cathode resistors provided in your amp does the same thing. The only advantage of the bias probe is that you don't have to remove the chassis to access those resistors.

As far as the mA versus mV, do NOT use mA. Measuring mV across a 1 ohm resistor tells us exactly what the mA (current) is through that resistor. Never use any current range for measurement unless you break into the circuit and place the meter in series.
In this case, if you measured across the resistor with mA range (parallel), you would still have part of the current going through the resistor. This would mean the idle current would actually be higher than what your meter indicates. This could lead to overheating or premature tube wear.

One more thing, in this case the builder has given you a number for the idle current 25-30mA, which is measured by reading mV across the 1 ohm resistors. 25mV is .025V, so set your meter to it's lowest DC range (often 200mV).
Because he has told you the setting exactly, there is no need to worry about plate voltage. But normally, if no exact current is specified, the plate voltage is also measured. Then a math calculation is done to figure out the idle dissipation in Watts, and set to be a certain percentage of the max. wattage that particular model of power tube can handle. Not sure what the plate voltage is for the EL84's in this amp, but let's use 350V as an example. With 350V on the plate and 25mV idle current, the idle dissipation wattage is .025 x 350 = 8.75watts. EL84's are rated for max 12watts, so 8.75/12 translates to 73% idle dissipation.


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## blam (Feb 18, 2011)

jb welder said:


> There were a couple of errors in this thread that should be corrected as they could be dangerous.
> 
> This is backwards, the more negative the grid voltage is, the colder the tube will run. When dealing with unknown or new tubes, it is best to set the grid voltage to max. negative before installing the tubes. Then after they warm up you can adjust the bias.
> 
> ...


wow, lots of info. thanks.

I guess I'll add, the builder says clockwise will drop the idle. if i turn it all the way clockwise, that would be its coldest state?


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## jb welder (Sep 14, 2010)

blam said:


> wow, lots of info. thanks.


Yes, hopefully not too much all at once. :smile-new:

But,after all that, forgot to address the actual problem. Red plating tubes. It could be that the tubes themselves are faulty which would not surprise me with the quality of modern tubes being what it is. But it also could be an intermittent failure of the bias circuit. If this were the case, the negative voltage on the grids of the power tubes (approx. -13.5VDC) would be occasionally dropping out (going less negative or even to zero) resulting in extreme overheating (red plating) of the power tubes.


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## blam (Feb 18, 2011)

I sure hope that isn't the problem!

i've used many JJ el84s in the past in all my amps and this is the first time I've ever had an issue.


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## ampaholic (Sep 19, 2006)

jb welder said:


> This is backwards, the more negative the grid voltage is, the colder the tube will run.


Ooops! You're right of course.


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## blam (Feb 18, 2011)

Alright then. I adjusted the voltage across the grid to - 13.5 (it was reading about 10.1) I took a measure across the blue resistor and got a reading of about 16.8, not 25mV. Suggestions?


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## jb welder (Sep 14, 2010)

I hope you meant it was -10.1V (negative). Adjust it back till you read 25mV across the resistor. I'm guessing that will be somewhere between -10 and -13V at the grids. Then check the other resistor for the other tube, you want the average value around 25mV.
Any gross difference indicates a very mismatched or defective tube.


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## blam (Feb 18, 2011)

Yes. -10.1 originally. I'll give that a try now. Thanks


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## blam (Feb 18, 2011)

The difference between resistors was only 2-3
I averaged it out to a little over 25mV and got a grid voltage of -11.8 give or take 0.2


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## blam (Feb 18, 2011)

Played the amp a while last night. All seemed well so far. Will play some more today and see what happens. Thanks everyone for your help!


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