6L6 Biasing help

fretzburner · Post by **fretzburner** » 03 Nov 2011, 13:42

Hello i am a newbie on tube amps and wanted to learn how to bias my old amp because the output is much much lower compared to my valveking 112 which uses same output tubes 6L6.Upon reading from the net i am confused how to bias each tube using a single bias pot(cathode bias method).Example i already biased one tube using the bias pot,how to bias the 2nd tube without changing the pot position used from the first tube?(only one bias pot for the two tubes)Here are some pics of my amp

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DrNomis · Post by **DrNomis** » 03 Nov 2011, 14:25

That bias pot basically sets the biasing of both the 6L6 power valves in your amp, so you only really need to set it for one valve because if your amp is using a set of two matched 6L6 valves, the second valve should bias up close to the other, ideally when biasing the power valves in an amp, you need three pieces of test equipment, a Sine/Squarewave Signal Generator, an Oscilloscope, and a Dummy Load of the correct impedance and power rating.

When you set the biasing of power valves, you are setting the operating conditions so that when the amp isn't amplifying a signal, the power valves aren't conducting much current, when the amp is amplifying a signal, the biasing needs to be set so that the amp produces the minimum amount of crossover distortion which shows up as a kink in the waveform, EL34 valves are typically biased to about 30mA, and 6L6 valves are typically biased to about 40mA...

fretzburner · Post by **fretzburner** » 03 Nov 2011, 22:40

Thanks DrNomis for the helpful tips.I am just doing it for my personal use and don't have those great equipments.Maybe I could do it with just a multimeter.I will try the cathode bias method and measure only one tube or maybe try to measure the other tube just for reference if they have almost same reading.Is it necessary to install a second bias pot ?If i will,is it that simple just to separate the two 220k resistor and duplicate the resistor/pot,cap from the rectifier and connect the second in parallel with the original bias circuit? please check my schematic.

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mojah63 · Post by **mojah63** » 04 Nov 2011, 00:38

Cathode biased amps usually use a high wattage resistor and a cap to ground to set up the bias. Look at any 1950's fender schematic. Grid biasing is what PV would do. What's the B+ voltage on the anode? Is the PV higher than your other amp... That may explain it being louder as it may be pushing more watts. Biasing is part science part art. As long as your plates aren't cherry red and it sounds good I wouldn't get too worked up about the current. That said check it to make sure. 32 - 40 ma per 6l6 is normal but the current would be lower at a higher B+ voltages for the same power output. I use the transformer shunt method to check current. There are other ways look here http://www.duncanamps.com/technical/lvbias.html. I've included an xl spreadsheet for calculating cathode resistors.

Power Tube Dissipation Cathode.xls: (10.5 KiB) Downloaded 61 times

fretzburner · Post by **fretzburner** » 04 Nov 2011, 12:36

Thanks mojah63 for the explanation,I will try to measure the anode voltage tomorrow.As in my picture pins 8 of the two 6L6's were connected to ground meaning the amp is not originally cathode biased? I will try your mentioned transformer shunt method because i can't find here a 1percent tolerance resistor.I bought two 1 ohm today but it reads 1.2ohm by my fluke multitester.So my second option is the transformer shunt method.

fretzburner · Post by **fretzburner** » 06 Nov 2011, 03:05

these are my biased readings,please advise if this is correct.All readings done with tubes in socket
plate voltage(center tap of OT to ground):422V
OT impedance (center to pin3 tube1):70.9ohms
OT impedance (center to pin3 tube2 ):77ohms
Voltage OT center to pin3 tube1:2.7V by ohms law 38mA
Voltage OT center to pin3 tube2:3.77V by ohms law 48.9mA
I noticed the plate voltage slightly increase as i lower the plate current thru the bias pot.My initial readings before i tweaked the bias pot were:
Plate voltage 416V
Voltage OT center to pin3 tube1:5.04V by ohms law 71mA
Voltage OT center to pin3 tube2:6.7V by ohms law 87mA
And with these readings the output tubes plates are red but louder sound output.

DrNomis · Post by **DrNomis** » 06 Nov 2011, 06:20

fretzburner wrote:Thanks DrNomis for the helpful tips.I am just doing it for my personal use and don't have those great equipments.Maybe I could do it with just a multimeter.I will try the cathode bias method and measure only one tube or maybe try to measure the other tube just for reference if they have almost same reading.Is it necessary to install a second bias pot ?If i will,is it that simple just to separate the two 220k resistor and duplicate the resistor/pot,cap from the rectifier and connect the second in parallel with the original bias circuit? please check my schematic.

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Actually, no you don't need to install a second bias pot, although I have seen schematics of some High-End Valve/Tube HiFi amps with separate bias pots for each power tube, the biasing system you see in your schematic is pretty standard and unless you have mis-matched power tubes installed, it should work fine, for best performance the power tubes should be closely matched and if you buy matched tubes they'll be matched close enough...

If the plates of the power tubes are glowing a bit red then it means that the tubes are biased a bit too hot, or maybe the coupling caps on the grids are a bit leaky, personally I like using the signal-generator, dummy-load, and scope method of setting the power tube biasing.....

fretzburner · Post by **fretzburner** » 07 Nov 2011, 10:45

I tried to exchange the tubes and the result is much more unmatched current readings of the two tubes this time the smaller side of OT is now bigger and the other side is smaller(current).The result is reversed but wider difference than before so i switched back the two tubes.Current readings are:
Tube1 37.1mA
Tube2 48.7mA
And noticed tube2 is more hotter(red) than tube1 but still playable.Are these acceptable readings?Should i still lower the bias to make tube2 maybe around 40mA which result tube1 to be much lower maybe somewhere 29mA.

wyatt · Post by **wyatt** » 07 Nov 2011, 16:39

The tubes aren't matched. Matched 6L6GC's would be ≤5mA apart (≤3mA for 6V6's and EL84).

Right now, by simple estimates, you are running one tube at near 70% dissipation and the other closer to 50% dissipation. Both should be running safely, but I wouldn't call it optimal.

On a side note, this is a red herring as far as output is concerned. You're Valveking may be louder because it may have higher voltages, a larger output transformer, a more efficient speaker or any combination of the above. Speaker sensitivity alone can make up the output difference between a 100-watt and 50-watt amp. Just because they both use the same tubes does not make them equal, it's how the tubes are being run.

wyatt · Post by **wyatt** » 07 Nov 2011, 16:41

DrNomis wrote:
If the plates of the power tubes are glowing a bit red then it means that the tubes are biased a bit too hot, or maybe the coupling caps on the grids are a bit leaky, personally I like using the signal-generator, dummy-load, and scope method of setting the power tube biasing.....

A bit is a understatement.

If the plates are red, then the tube is closing in on 200% dissipation.

fretzburner · Post by **fretzburner** » 08 Nov 2011, 22:38

Thanks,yes looks i have a mismatched tubes.they are the stock tubes when i bought this amp used.My question now is it safe for the amp if i will use it until the tubes will totally breakdown?The sound is still okay to my ears.Just the low output thing because aside from my valveking,i have a single ended EL34 with one 12AX7 and very very much louder than this amp too.Full volume on the dual 6L6 amp(10inches speaker) equal to 4 volume on the EL34(12inches speaker).Maybe something to do with the speaker size too aside from sensitivity.I understand speaker is one component that affects volume as what our good fellow members here said,but very big difference in SPL.If it will not endanger my amp to continue using the unmatched tubes that will be fine so i can save a new matched pair tubes.

Zipslack · Post by **Zipslack** » 09 Nov 2011, 15:55

I may be completely wrong here, forgive/correct me if I am. The way I understand it is that the bias pot is mis-named and should be called a "balance" pot. It does not necessarily change the bias of both tubes, but just balances the bias to help eliminate cross-over distortion. This makes it easier to use tubes that are not "matched". Adjust it so both tubes pull the same current --> 1. Does it sound good? 2. Are the tubes red-plating? If it sounds good and isn't red-plating, play it.

wyatt · Post by **wyatt** » 17 Nov 2011, 08:48

Zipslack wrote:I may be completely wrong here, forgive/correct me if I am. The way I understand it is that the bias pot is mis-named and should be called a "balance" pot. It does not necessarily change the bias of both tubes, but just balances the bias to help eliminate cross-over distortion. This makes it easier to use tubes that are not "matched". Adjust it so both tubes pull the same current --> 1. Does it sound good? 2. Are the tubes red-plating? If it sounds good and isn't red-plating, play it.

Depends on the amp (which i believe has never been established). Balance pot setups are rarer, Fender started phasing them in in '65 (on the Bassman). It was a horrid idea that resulted in more hum than ever and not being able to adjust the bias in one half at all. One of the most popular mods to to scrap the balance pot and install a bias pot setup.

I believe the problem with the OP may be that his tubes are old and shot. They don't match (which can drift over time) and don't have any "omph!" anymore.

fretzburner · Post by **fretzburner** » 20 Nov 2011, 12:42

Here comes my problem.using the amp it produces random cracking sound when i play about 10 minutes and sometimes weaker sound and randomly huge crack sound in seconds.Sometime weak and a little distorted sound on clean channel.Is this a sign of bad tube?Initial on the amp is okay but after some time of playing it starts to randomly crack.Is phase inverter tube easily get damaged?Or some bad capacitor maybe?Or the output tubes? Don't have experience yet where to start,need your inputs.