# Amp Bias



## knight_yyz (Mar 14, 2015)

Hey guys, I have a question about biasing an amp. I've tried searching but I'm probably not entering the right words to get the answer I'm satisfied with.

When I moved my 63VVRI (vibroverb reissue) the other day I accidentally moved the bias knob that Chris Church installed and I would like to put it back to where it should be. But I have no idea what I should set the bias to.
IIRC , bias is measured in milliamps. So assuming you have an adjustable bias, and you buy some tubes that are rated say 60ma. Do you set the bias at 60ma? Is the bias number dependent on the amp model? Where can i find what my fixed vias measurement is?

What happens if you put tubes in at the wrong bias?


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## dtsaudio (Apr 15, 2009)

Bias set the amount of current flowing through the tube at idle (no guitar playing). While it can be measured directly in milliamps, typically it is measured in millivolts taken across a 1 ohm or 10 ohm resistor. Then, a little thing called Ohms Law is used to calculate the current. Chris probably installed 1 ohm cathode resistors to measure off of, so the math is basically whatever you read in millivolts, it's the same as milliamps. 
The question is, did Chris install 1 ohm at both tubes or each tube. Also, did he give you one test point (common to both tubes) or two test points (each tube). 
The bias setting is determined by the tube in question, and its maximum plate power dissipation based on the voltage on the tube. In your case, from the schematic, 60ma *per tube* would be right near the maximum but still safe. However if it's common to both tubes, 60ma would be pretty cold.
Too much current and the tubes would have a short life, too little current and distortion rises.
So, how many test points do you have to measure off of. It might be best to ask Chris.


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## loudtubeamps (Feb 2, 2012)

The bias control should not have a knob on it that could easily be moved by accident.
If the control is panel mounted,it should be a short shaft with locking nut or.... a control pot typically used in early Fender amps.








As a starting point....your vibro with a plate voltage of around 435v.d.c. and a neg. bias supply delivering -50 to -60 volt d.c , when adjusted, should yield a recommended current draw of around 35 to 40 m.a. + - or 35 to 40 or m.v. + - per tube...total 70 /80 m.v. + - f0r a pair of 6L6/5881's
Milli-Amp reading ...(for those more experienced) using the current shunt method.
Milli-Volt reading... ( much safer) Having a low tolerance (5%) 1 watt, 1 ohm resistor connected to pins 1/8 on the o/p tube socket. Voltage measurment is taken between the resistor-pin(cathode) junction and chassis gound. The 1 ohm resistor(s) should always be measured before installation to ensure they fall within specified resistance assuring an accurate reading for your o/p tubes.


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## knight_yyz (Mar 14, 2015)

Ok, so for now, and until I can get in touch with Chris, the voltage required, no matter the brand/strength of tube, I should always aim for 35-40 ma/mv? The tubes that are in there have a rating of 48 and 48.5 So I thought I was supposed to aim for that number on the matched pair. I said 60ma as an example.

There was no knob on the pot. I just wasn't looking where I put my hand to move the unit and I touched the shaft and it did move. The pot is where they put the fake ground switch


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## knight_yyz (Mar 14, 2015)

I think I found what I was looking for


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## knight_yyz (Mar 14, 2015)

The fender recomended tube is a 6L6WGC, but I'm actually running the 6L6GC. There is a difference in wattage (4). I guess i should recalculate for the higher voltage?


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## knight_yyz (Mar 14, 2015)

On another note I should have paid attention to what Chris took out of the amp. Seller told me all JJ or Groove tubes, turns out there were a few gems in there. A Jan Philips 12at7 which is going back in. And a NOS Mullard 12ax7 made in GB, SO that's going into V2


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## dtsaudio (Apr 15, 2009)

knight_yyz said:


> There is a difference in wattage (4). I guess i should recalculate for the higher voltage?


 Your sweating the details too much here. While you can run the GC a little hotter than WGC, the difference is negligible. The calculator is basing the figures on a percentage of maximum dissipation, the GC and WGC are going to work equally well at the WGC bias point. The only differennce being you can push the GC further. Unless you are trying for a less distorted "class A" sound then I wouldn't worry about it.


knight_yyz said:


> Ok, so for now, and until I can get in touch with Chris, the voltage required, no matter the brand/strength of tube, I should always aim for 35-40 ma/mv? The tubes that are in there have a rating of 48 and 48.5 So I thought I was supposed to aim for that number on the matched pair. I said 60ma as an example.


35-40ma (per tube) is a good bias point regardless of the tubes used. 
The rating on the tube is the amount of current that will flow in the *test circuit *for a given bias and plate voltage. It will not be real world, and is a number to use only as a matching guideline. This number will vary from supplier to supplier as well since test procedures may differ. As an example, if you bought tubes from Tubestore with a 40 rating, and one died the Tubestore would replace it with one of a similar rating. However if you tried to replace it with one from Tube Depot, that rating would be meaningless to them.


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## knight_yyz (Mar 14, 2015)

ok, I think I have figured it out. There is one pot which has ten full rotations. And there is a black and a red test port. Took a few tries because I forgot the pedal was unplugged, so turning reverb and tremolo off didn't work. I realized my mistake when I was putting it all together. Plugged the pedal in turned off reverb and dialed in at 80ma.
I was at work, and I couldn't remember which tubes were the power tubes, so I looked it up. But when I got home and looked at the tube label there inside the amp there was no W . Just 6L6GC


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## loudtubeamps (Feb 2, 2012)

Once you have contacted Chris on how to monitor your current draw on the o/p tubes with your multi-meter ....(m.a.) or most likely m.v. and u get them into that 35-45 range, I would suggest a fine tweak by ear.With the amp warmed up, running and tone/volume controls set to mid points, plug your guitar in , let a low note sustain and do a fine clockwise/counter clockwise adjust on your bias control, you should hear a sweet spot where the bottom end opens up. After the tweak, check you bias reading again with the amp at idle /no guitar ,to be sure you are not exceeding the high side of current draw.
I use separate bias controls...(one for each side of the o/p transformer) on the amps that I build and the audible 'sweet spot' tweak is very noticable.
Even with a set of fresh 'matched' tubes, measurements can show a spread of 0 to as much as 5 m.a. between tube draw.
As Dan suggests, don't sweat the numbers too much, in the end, it's all about how it sounds.
The advantage with dual biasing...U don't necessarily need a set of matched tubes and as the tubes age, you can always keep them in the audible sweet spot zone.
FWIW..if your bias control is prone to being bumped, I have an easy fix which I have used when the correct pots are not available.
Tape up any openings in the pot case, the pot can then be placed in a vise, held at the shaft tip and cut down with 1/8" of the shaft protruding. 
A needle file or fine metal cutting blade can be used to cut a slot in the shaft for screwdriver adjustment.


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## WCGill (Mar 27, 2009)

Yup, bad news putting a bias pot with a shaft anywhere it can be manipulated or bumped. The location Fender commonly uses on the underside of the chassis would be cool.


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## loudtubeamps (Feb 2, 2012)

WCGill said:


> Yup, bad news putting a bias pot with a shaft anywhere it can be manipulated or bumped. The location Fender commonly uses on the underside of the chassis would be cool.


You bet Bill...except for the cool part......I wish Leo had mounted the bias adjust closer to the back of the chassis. 
It can get a wee bit warm on the wrist making an adjustment, especially with a hot quartet fired up.
I learned from that and now........
'♪I don't burn myself anymore♫'. Sounds like the makin' of a good 'hurtin' tune'


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## knight_yyz (Mar 14, 2015)

I don't think it is possible to dual bias a Vibroverb because the bias is somehow affecting the tremolo, or the other way around. I wouldn't say it is prone to being bumped. the amp sat in one spot for 2 years and was played regularly, but I moved it over a few feet to make room for the 2x12 cab


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## nonreverb (Sep 19, 2006)

It is possible... the circuit is a little more involved


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## knight_yyz (Mar 14, 2015)

Well, this is really embarrassing, but it wasn't Chris Church who worked on my amp. I showed him the pictures and he said it wasn't him. So I searched through all my old bank records, and on this forum and realized that I dropped the amp off with Dan. I remember taking it to his house, but I have no idea as to his contact information. So I am guessing it is Dan at DTS Audio.

@dtsaudio does this picture look familiar? Are you the one near Lawrence and Albion Hills?


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## dtsaudio (Apr 15, 2009)

It took a while, but I do remember doing that. I didn't recognize you from your forum name.I must confess I'm a bit surprised you could turn that pot so easily. 
I think if that's going to be a problem, I can change that for you pretty quick. I have some short shaft pots that I use now that will work quite nicely. You need a screwdriver to adjust them
I also agree with loudtubeamps, that dual bias is better, but if I recall, we went the simple route and used the existing hole for the ground switch to place the pot.
IIRC bias was set on the low side 70ma (35ma/tube).



Dan


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## knight_yyz (Mar 14, 2015)

it spins fairly easy, but I'm not going to worry about it. I don't move the amp around much. So I am supposed to measure in milliamps dc?


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## dtsaudio (Apr 15, 2009)

No, millivolts. 70 millivolts translates to 70 milliamps.


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## knight_yyz (Mar 14, 2015)

ok Thanks Dan.


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## StevieMac (Mar 4, 2006)

I generally install a Fender/CTS _recessed_ bias pot in that unused Ground switch location. Easy to access and virtually no chance of accidentally re-setting the bias...


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## knight_yyz (Mar 14, 2015)

edited


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