Colonial Biasing Questions

[andyjames]

Alright, so not long ago, I received a used heritage colonial in the mail. One thing I wanted to do upon getting it was bias it, so I looked in the manual and it said to set multimeter to dc MilliVolts and run the amp for a while. Which is what I did, I adjusted it between the 25-35 mv range and let it sit for about 20 minutes, LIKE THE MANUAL SUGGESTS. Anyway, I've recently come across a lot of info saying that it is to be biased using MilliAmps. Switched the meter over to MilliAmps and the tubes were running at about 11 MilliAmps.

 

Am I correct in biasing using mV's? If what I did was wrong, what kind of harm did I do to my amp by letting it run where it was set for 15-20 minutes. This being my new primary amp, i am somewhat worried that I may have screwed something up and repairs may be in my future.

 

Please let me know if I am in the wrong here....

 

Andy

 

[H]

Which manual are you referring to - a 'Heritage Colonial manual' or a generic tube amp manual?

 

It sounds as if you've biased the amp quite 'cold' so don't worry about repairs.

[andyjames]

 

Which manual are you referring to - a 'Heritage Colonial manual' or a generic tube amp manual?

 

It sounds as if you've biased the amp quite 'cold' so don't worry about repairs.

 

 

I'm looking in the Heritage owners manual that lists descriptions of all of their amps as well as biasing instructions.

[andyjames]

Maybe i am actually making a big fuss over nothing, contacted the owner and he said he always used dc volts, I guess there is just some controversy over which setting to use for biasing. Either way, I'd still like to hear anyone's feedback if you know anything about biasing these amps.

[H]

If it states to do it that way in the manual then I'd say go ahead and do it that way.

 

Is the Colonial a 6L6 amp? If so, you might want to bias them a little hotter. The 'cold' bias they're set to now will make the tubes last a long time but the amp will sound overly bright and thin in my experience.

[H]

I found this on a Marshall forum: http://www.marshallforum.com/workbench/32259-bias-question-ma-mv.html

 

Hope it clarifies things. Basically, both measurement types (mA and mV) are fine - it depends on the test points supplied.

[Gitfiddler]

First off, I know NOTHING about biasing. I just had my Heritage Liberty re-tubed and the 6L6's biased. The amp tech said no real harm will come to the amp if not correctly biased, as long as they are not WAY off. He matched them perfectly so there would be less break-up. The amp sounded great before biasing but now sounds fantastic. For the lay person, it's a matter of degrees of goodness. Technically I have no clue...but I just bought a meter to learn some of this cool stuff.

[H]

Biasing tubes is a little like setting the idle speed on your car: there's an optimum level that keeps things ticking over without wasting fuel.

 

Colder bias can produce a brittle, thin sound and hotter bias produces a fuller sound which will break up sooner but will also burn out your tubes faster. There's an ideal mid-point which varies by tube type and amp type.

[kbp810]

Bias is measured in mA... but, in the case of the Colonial, you are determining the bias by measuring the voltage drop across a resistor in between the tubes cathode and ground. You then divide that number by the value of the resistor used to determine the respective mA value. In this case, a 1ohm resistor is being used, so in a nutshell, the value you read in mV equates to the same value in mA.

 

This is the most common method of measuring Bias because it is the safest and easiest for the end user, but it is not actually the most accurate. The tolerance of the 1 ohm resistor can alway be off by a little either way, and it should be noted that the value read will also include screen current (which is just a few mA's - so the reading you get is generally "cold" by a couple of mA's).

 

Either way, it gets you into ballpark, and just like horse shoes and hand grenades, close is good enough

[TalismanRich]

I would go with the manual if there is a question. If you set for 30 mV and got 11 mA then it seems that it might not be going through a 1 ohm resistor. That's a drop of almost 3x. Even at 20% tolerance it seems out. I used the mV readings on my Patriot. The tubes were matched to about 3mV of each other.

4. For amps using 6L6 power tubes, adjust the bias pot to give a

reading between 25 to 35 mv for each tube. For amps using 6V6’s

adjust the bias between 18-27mv. For amps using EL34’s adjust the

bias between 30-40mv. Lower settings will sound cleaner with longer

tube life. Higher settings will have an earlier break-up and shorter

tube life. A properly matched set of tubes will generally be less than

5mv off from each other.

[kbp810]

To clarify, the test points on the back of the Colonial are parallel with a 1 ohm resistor; you are measuring for the voltage drop across that resistor. You are not reading current. (read that as - do not read current here)

 

To measure cathode current, you would need to remove that 1 ohm resistor on the inside and measure directly from the cathode (pin 8) to ground (no need to do this, just saying that to read mA the meter would need to be in series between cathode and ground)

 

So your reading in mV is what you should base your result on.

 

If you wanted to confirm the value of that 1 ohm resistor - power off the amp, pull the power tubes, and using the same test points set your meter to read ohms.