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mxpxismyrx

Biasing with a multimeter

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hey whats up, heres the deal. i have a framus cobra and it has the bias test points on it to use with a multimeter. i was wondering if anyone knew which setting youre supposed to set the multimeter to to test the tubes. and any other guidelines for biasing the tubes. thanks

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I believe that the bias test points will put your meter accross a 1ohm resistor in parallel measuring the voltage drop across it in Milli Volts which due to ohms law will translate 1 for 1 to milli Amps.

 

To answer your question put it on the low volt setting, what ever your meter shows is your bias in mA.

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You will need to know (try Framus website, manuals) the correct bias setting for your amp.

 

The usual procedure is to bias the amp after it has been allowed to warm for a minute or two. Anything longer then the heat produced can alter the resistance of the resistor used in the bias measuring circuit - depends if the manufacturer used a high quality ohmic resistor or not.

 

After that it's usually just a matter of turning a trimpot until the reading is correct.

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My XXX has a place to read with a meter on it too and several have told me it was useless for biasing my amp. I do not own a Cobra so I cannot comment on it. I hope yours are more useful than mine!

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It'll depend on what kind of tubes are in there. You dont have to worry about heat affecting the accuracy of the resistor as there is current flowing through it but virtually no voltage, so its not dissipating any power, thus no heat.

 

If you're retubing set the bias trim pot to the side that gives you the maximum negative voltage @ Pin 5 of the power tubes (control grid...should be anywhere from -35V to -55V for EL34's or 6L6's). Thats DC voltage btw. Then you'll need to know roughly what plate voltage you're looking at so, be very very careful and set your multimeter to the HIGHEST DC voltage setting and measure from ground to Pin 3 of the power tubes (plate), the voltage will be high...usually above 400V. Set the multimeter to its lowest DC voltage scale, and if in fact its using the 1 ohm resistor method then you can read directly from the test point to ground. You can read the voltage as current. Jot that number down, now you need to find a safe operating range given plate voltage.

 

I treat all EL34's as having a max plate dissipation of 24W. And All 6L6GC's as 30W tubes. You'll want to be usually between 60 and 70% of that. Try not to go above 70% unless you cant LIVE without that tone.

 

So:

 

(Low figure)

EL34 -> 24W * 0.6 = 14.4W

6L6 -> 30W * 0.6 = 18W

 

(high figure)

EL34 -> 24W * 0.7 = 16.8W

6L6 -> 30W * 0.7 = 21W

 

Now, we'll assume your plate voltage is 450VDC. You measure plate dissipation by multiplying voltage * current, so we can find out what our high and low points for current are by dividing by the figures above. So for EL34's

 

Bias for:

14.4W / 450V = 0.032A (32mA) LOW

16.8W / 450V = 0.037A (37mA) HIGH

 

So adjust the bias pot until you get a reading from the bias test points in the .032V to .037V range. (For EL34's)

 

Same deal with the 6L6's.

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ok, i just tried that and i got a low reading from the ubes, V1 doesnt read at all, i dunno if the test point is bad or if the tube is bad. V2 was 29ma V3 was 26ma and V4 was 27. so i just turn it up till it reads about 35ma? also i started playing it a few days ago, and it sounds super fizzy on the crunch and lead channels and extra bright on the clean, i had to turn the treble and presence all the way down on all channels to make em not rip my face off. is that the power tubes fault or the preamp tubes? also when the amp is fully on and my guitar is turned down all the way it makes this very low like crackling sound kinda like a bad pot. f anyone can shed some light on all that it wold be greatly appreciated thanks!

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Depends on what tubes you have, look above. I wouldnt attribute an overly trebly sound to the power section unless something else is jacked up. Have a look at the tube, aside from that im not gonna have you poking about the power section. Just be sure the meter is making good contact. You may also have a tube socket thats shot.

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