Proper bias setting for a 6L6

xxmartinxx · Aug 22, 2001

I was just wondering if anyone here knew the proper bias setting for 6L6's? I've read that it's 60, 47, and 45 VDC, but I don't know what is correct.

Thanks.

Martin

Mojojojo · Aug 22, 2001

Well, you need to set them between 60% and 70% of maximum power. So, to give you a correct answer, we need to know what the plate voltage is to calculate the idle current. The formula for 6L6's @ 70% is:

(30 (max watts for the 6L6) / Plate Voltage) * .7 (70%)

For example:

(30 / 500) * .7 = .042A (42mA) (for 70%)
(30 / 500) * .6 = .036A (36mA) (for 60%)

Note: You CAN'T SET THEM ACCURATELY USING NEGATIVE BIAS VOLTAGES!!!

If you are unsure of what I'm talking about, please see a tech or refer to Randall Aiken's excellent biasing article here:
http://www.aikenamps.com/Biasing.html

xxmartinxx · Aug 22, 2001

Could I find that info on the schematic? If so, where? It's a 100 watt head, does that make a difference in calculation?

Thanks.

TedB · Aug 22, 2001

Actually, that 70% number should be based upon the rated dissipation of 25w for a 6L6.

Anyway, you won't find this info on a schematic, and what little info I've seen on the Marshall schematics indicates that they do not bias their amps, but rather set the grid voltage to a 'safe' number (translate: cold setting) so they can crank these things out in a cookie cutter and get them out the door.

If you want to maximize the sound from your amp, you need to have it properly biased. This is like setting the idle speed in a car. With a 100w amp, there is only one such 'idle adjustment' to control the setting for all four tubes. This is why it is important to buy matched tubes (so they all idle at the same 'speed').

Typically, a cold setting will make the amp sound a bit sterile. A warm setting (e.g. 70% of rated dissipation) gets the most out of the tubes. Setting warmer does not improve the sound (makes it worse in some cases), and always shortens tube life. So, the 70% figure represents a happy medium. In order to find what 70% translates to in your amp, we need to know the plate voltage. This should be measured at the tube socket.

xxmartinxx · Aug 22, 2001

I thought that 6L6's were 30 watt tubes?

How exactly do I measure the plate voltage from the tube socket? I know I have to used a trusty Fluke voltmeter, but at what points?

Thanks for all the great help I'm getting.

TedB · Aug 22, 2001

No, you don't use the absolute maximum values for biasing. You use the design center values. This value is 25w for an EL34, and actually about 19w for a 6L6, so as you can see, even using 25w for a 6L6 is ambitious. In fact, I bias 6L6s using 22w as a design center value. Forget the 30w figure.

The plate voltage should be measured from pin 3. Looking at the bottom of the socket, pin 3 is the third pin counting clockwise from the tube pin keyway. If you measured the voltage from all pins, the plate voltage will be the highest measured reading.

xxmartinxx · Aug 22, 2001

Everything you ever wanted to know about tube biasing but were afraid to ask

So I put one probe on the 3 pin of one power tube and the other on the same circuit (inner or outer)? Should it be on, or in standby mode, or does it matter? Does the settings on the amp matter?

Thanks for answering my stupid questions.

TedB · Aug 22, 2001

The amp should be fully on with the volume controls turned to zero. Naturally, the amp should be plugged into a speaker cabinet. One meter lead should be grounded (chassis), and touch the other to pin 3. Be prepared for a little pop. Get the voltage reading.

BTW, what exactly is the amp in question?

xxmartinxx · Aug 22, 2001

It's a Sunn Model T reissue. It's got a built in bias adjustment that is rather easy to use, however they suggest to use 60 mVDC which just doesn't seem right. The amp sounds a little cold and tubes look a little faint, but 60 mVDC would be a pretty hot tube setting. I kind of think it's 30 mVDC per pair, for a total of 60 which would make more sense.

Any suggestions, as always, are greatly appreciated.

TedB · Aug 22, 2001

No, none of that is correct. Biasing is measured in ma of current flow at idle. The voltage measured at the grid *after* you have achieved the proper bias is measured in -V DC. If you are talking about them recommending -60V DC, this is meaningless, and will likely give you a cold bias setting every time. There is only one source of bias voltage, so the voltage at every tube's grid will be the same (it better be!).

If you want to bias the amp properly, you will need either a bias probe, or a very good 1 ohm resistor and an equally good meter.

xxmartinxx · Aug 22, 2001

*scratches head* I guess this is why I normally only play amps and not fix them. haha...

Sean · Aug 23, 2001

Bias is *definitely* worth making the effort to understand.

I don't mean all the jargon, I still don't get all that stuff (yet), but using the simple v*mA = w formula is dead easy, and I for one find that the extra control over the sound of my amp is totally worth it.

Screw the Biasprobe, install a 1ohm, 1w resistor on each tube for measurung mA, as Ted suggests. It costs all of one dollar, and you can tailor your amp's sound any way you want...

Proper bias setting for a 6L6

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