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Post subject: Tube Bias Question for 74' Bassman
Posted: Sat Jul 26, 2014 12:59 pm
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Hi Everyone, I just bought a 74 Bassman 50 watt. One of the 6L6 tubes is starting to turn blue, I know that means it's reaching the end of its life. The brand of tubes used are Electro Harmonix. I'm on a tight budget so I really don't want to buy a matched set. Also, I don't want the tube to die during a gig. If I bought the same brand Tube that is starting to go, do I have to Bias the amp? If so, how difficult is it to learn to bias?


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Post subject: Re: Tube Bias Question for 74' Bassman
Posted: Sat Jul 26, 2014 2:47 pm
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Are you speaking of a blue glow within the tube with the amp powered up?

If so, that's not necessarily an indication that the tube is failing or about to fail.

Arjay

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Post subject: Re: Tube Bias Question for 74' Bassman
Posted: Sat Jul 26, 2014 3:59 pm
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mikeph0210 wrote:
If I bought the same brand Tube that is starting to go, do I have to Bias the amp? If so, how difficult is it to learn to bias?



You must check bias , you never know if bias will be same .

Easy to learn , need some skill a meter and to know there is a lethal voltage in tubes amps that kill you .

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Arjay is right , tube might be good


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Post subject: Re: Tube Bias Question for 74' Bassman
Posted: Sat Jul 26, 2014 9:50 pm
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Just got a '68 Twin Reverb in last week( trannies- 606735). So it's a straight AB763. It came in for noise. It had EH6L6GC power tubes with plenty of blue glow. Retentioned and cleaned all amp sockets(tube,input&output). Checked the bias and sure enough one tube was pulling too much current(causing a 10ma disparity on which ever side it was on). Since the owner isn't pro, he decided to go with new EH6L6GC's. I opted for a NOS Phillips matched 12AT7 phase inverter. Biased the new tubes to draw about 34ma @ tube at 480VDC plate volts. The amp is really quiet for an oldie!!! But those new tubes have a pretty good blue glow to them. And yeah, it sounds and plays great!!! Art

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Post subject: Re: Tube Bias Question for 74' Bassman
Posted: Mon Jul 28, 2014 3:41 pm
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Some shots of normal tube operation. Some tubes have the blue, some do not.

A pair of EH 6V6 in a HRDlx, nice blue glow:
Image

A pair of 6L6, some blue glow:
Image

A quad of 6L6, very little blue glow:
Image

A Quad of EL34, just barely a little blue glow:
Image

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Post subject: Re: Tube Bias Question for 74' Bassman
Posted: Mon Jul 28, 2014 3:48 pm
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If one tube of a pair has blue glow and the other does not, likely that one tube (with the blue glow) is pulling more current than the other, or the viewing angle does not allow the glow to be seen on the other tube.

As stated, check the bias, and replacing one tube is probably not the best idea, as one tube will have more miles on it, and likely an imbalanced output tube current will be the result.

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Post subject: Re: Tube Bias Question for 74' Bassman
Posted: Thu Jul 31, 2014 12:11 pm
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shimmilou wrote:
If one tube of a pair has blue glow and the other does not, likely that one tube (with the blue glow) is pulling more current than the other, or the viewing angle does not allow the glow to be seen on the other tube.

As stated, check the bias, and replacing one tube is probably not the best idea, as one tube will have more miles on it, and likely an imbalanced output tube current will be the result.


Is it okay to replace matching powertubes but leave the pre amp tubes?


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Post subject: Re: Tube Bias Question for 74' Bassman
Posted: Thu Jul 31, 2014 2:17 pm
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Yes.

Any preamp tube can be replaced singly, or replace a couple, or all, to your taste, no worry about bias.

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Post subject: Re: Tube Bias Question for 74' Bassman
Posted: Thu Jul 31, 2014 2:31 pm
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So I found the potentiometer when I took the amp out. Does anyone know where the bias test point is on this amp?


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Post subject: Re: Tube Bias Question for 74' Bassman
Posted: Thu Jul 31, 2014 7:32 pm
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I think that you don't have one on that amp. You could double check to be sure though. Look for a one ohm resistor connected to both output tubes' pins 8.

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Post subject: Re: Tube Bias Question for 74' Bassman
Posted: Thu Jul 31, 2014 9:57 pm
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shimmilou wrote:
I think that you don't have one on that amp. You could double check to be sure though. Look for a one ohm resistor connected to both output tubes' pins 8.


Correct.

As well, you'll need to ascertain the plate voltage to compute the desired bias level by measuring pin 3 of each output tube socket. You'll be looking for something in the neighborhood of 445 VDC.

Arjay

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Post subject: Re: Tube Bias Question for 74' Bassman
Posted: Fri Aug 01, 2014 7:32 am
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I called fender and found out my bassman is actually a 77! Anyway, they sent me the proper schematic for it, however I don't know how to upload it to the forum. I searched google images and found this one that is 99% identical.

Image


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Post subject: Re: Tube Bias Question for 74' Bassman
Posted: Fri Aug 01, 2014 10:33 am
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Your Bassman has a fixed bias with an adjustable balance control. Thus, you're really just matching one tube against the other.

Arjay

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Post subject: Re: Tube Bias Question for 74' Bassman
Posted: Fri Aug 01, 2014 10:56 am
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Retroverbial wrote:
Your Bassman has a fixed bias with an adjustable balance control. Thus, you're really just matching one tube against the other.

Arjay


I am new to biasing, however I really would like to learn how to do it on this amplifier. Most importantly I want to make sure that the two power tubes are running evenly (As one is glowing blue and the other is not). I have electroharmonix 6l6EH in there now. I am wondering how I go about learning how to bias. What is plate current, and how do I find it? Also, I couldn't find the exact ratings for the Electro Harmonix 6L6EH, but my hunch is between 30-35mA


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Post subject: Re: Tube Bias Question for 74' Bassman
Posted: Fri Aug 01, 2014 3:42 pm
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The easiest way to check current is with bias probes.

An alternative for many amps with only two output tubes, is to use Ohm's Law. With amp off, unplugged from electricity and caps discharged, measure the resistance of each half of the OT primary, note the resistances. The OT primary halves are red wire to blue wire for one half, red wire to brown wire for the other half. Then power up the amp, standby in play, let warm up, check the DC voltage drop across the same places that you measured the resistances before (around 4VDC per half). Be careful as the voltage applied to the tubes is actually 400+ volts, you're only measuring what is dropped across the OT primary halves. Then, divide the voltage reading by the resistance reading corresponding to each half of the OT primary, and the result is the Plate current for each tube. (I = E/R)

Then, the Plate current times the Plate voltage gives you the wattage dissipated at idle. (W = I x E)

Tubes should be in the range of 50% to 70% of max wattage rating (30 watt max rating for each 6L6). Adjust your balance pot to equalize the current between the two tubes. For higher or lower current, you can either change a resistor in the bias circuit, or get a set of tubes rated higher or lower.

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