follow up biasing Laney aor 50

[seaker]

After following the excellent advice I got here I found that I had blown the fuse in one of my meters and the other one I had hooked up wrong. Duuuuhhhh. Amazing that you can read over and over directions and look and look at the schematic and still pull a bone head thing like that.

Anyway, got everthing straightend out and measured the cathode idle current. adjusted till it settled down at about 38ma - with plate voltage at 406v I'm dissapating ~15.5watts at idle. I let it burn for a couple hours and checked again. It was stable.

Question: There seems to be lots of controversy about the 'correct' way to bias a tube amp. It seems to come down to various ways of measuring idle current versus using a scope to measure Xover distortion. Everything thing I read has its very strongly stated reasons why this way or that way is the best/safest/acurate/soulfull/mojo producing way. The question is what method(s) do the manufacturers use and recomend? And why isn't this good enough for the rest of us?

sorry if this a newbie question but it's really bugging me

[Enzo]

Aha, well you have tapped into the religion of the tubes here. Bias is one of those things that everyone can weigh in on, and really it is not all that critical. People will get red faced angry arguing about which way to put the toilet paper roll on the holder - loose end out or loose end along the wall. And each proponent has a rationale for his position.

Bias is like that. Everyone has his idea of how it ought to be done, and more importantly why every other way is WRRROOONNNGGG.

Why doesn't everyone adopt a standard method? Then no one could put his own little fingerprint on the process.

What do the manufacturers do? First, I will exclude the little boutique makers, since they have time to sit there and diddle with them to their hearts content. And then you pay several thousand dollars for the amp. Let's look at the big boys.

None of them get out a scope or a distortion analyzer and hunt for crossover distortion. And most of them don't use matched tubes, or if they are they are just crudely matched - for example the Fender Groove Tube 6L6s come in three colors, not ten grades.

Many amps don't even HAVE a bias adjust. Lots of PV and Mesa amps are like that and certainly smaller amps from anybody. ANything with cathode bias has no adjust.

Fender Hot Rod DeVille has a little test point on the corner of the tube socket board. Says adjust for 60mv. Doesn't matter what tubes are in it, it wants you to set it for 60mv. WHich in that case is 30ma current per tube. Nice round number, not hot enough to trouble any tube and not so cold as to pinch the amp off.

Marshall amps tend to have little bias test point these days. SOme stick out the rear of the deck while others are inside. The factory notes I have mention specific settings there. XX millivolts, I forget what. SO again, I am sure they slap the tubes in and set them all to the same numbers.

The thing is all these different methods and everyone manages to get an amp to sound good. Pick a method you like, and you are bound to be happy with it. If you want to then learn all the reasons why the other ways are total crap and should never be used, well go ahead and join the crowd.

The scope and crossover distortion method is an offshoot of hi fi amp-dom. The point is to get the cleanest reproduction of signal possible. Well that is swell for hifi. But guitar amps are not for REproduction of sound , they are part of the instrument, they are part of making the sound in the furst place. That is why a guy plays a MArshall instead of a Fender or whatever. As a rule, the guitarist is not going for ultra clean. But if it sounds good to you, then that is the end of it. You please you, what any one else thinks is irrelevant. To me a little crossover distortion is no big deal, and most people won't hear a small amount of it. But I wouldn't tell a crossover guy he was wrong.

All the current measure methods do the same thing. COnsider how tall I might be. You could measure from the floor to the top of my head. You could measure from my head to the floor. You could measure floor to ceiling, and ceiling to the top of my head and calculate the difference. Or wait until I walk out the door of the 7/11 store and watch as I walk by the heighth bar on the door frame. Ultimately you find out how tall I am.

You can shunt method, you can add a 1 ohm resistor somewhere, you can bias probe. And all of those directly or indirectly determine current through the tube. Adjust things until the number you want comes up on the meter. Oh dear, we need to have a target number. SOme folks go by the 70% rule - that is adjust the current for plate dissipation of 70% of the rated dissipation of the tube. 70% is an interesting number, but so is 75 or 80 or 65. Pick one.

More than one guy has reported that the best tone they ever heard from their amp was when the bias was WAY off and the tubes were running red hot on the verge of melting. SOunded terrific... until the tubes gave out shortly thereafter.

Now some guys will tell you that the 1 ohm resistor method is flawed because it includes the screen current. Yes it does. That means your setting will be a whole ma or two off from the ideal. OK fine. Measure voltage across the screen resistor and calculate the tiny screen current and subtract it from the total. Or just tell yourself the plate current is 2ma lower than your reading. Or just ignore it.

The shunt method is dangerous. Well of course it is. Working on live tube amps is dangerous in general. Point is that making a shunt method reading is no more nor less dangerous than reading the plate or screen voltages at the sockets. If you slip you could ground something off and blow a fuse or worse. Yup, you could, and it doesn't matter if you slip while shunt methoding or while voltage readening. Probes shorting things to ground or elsewhere can cause trouble.

I do believe in safety, don't get me wrong, but if I am not able to operate my meter well enough to take a shunt reading, I ought not to be in there taking any readings at all.

The bottom line here is take the great bias debate with a grain of salt. If your current is a couple ma off from someone else's nothing bad will happen, and you likely won't hear a difference. Of your amp is not running the tubes with cherry red plates and the sound is not biased into oblivion, you will likely have a good sounding amp. Between those extremes, try a variety of settings and see what differences YOU can hear, and what differences you do NOT hear. Often as not "set it in the middle" doesn't sound bad.

[seaker]

Thank you for the comprehensive and straightforward reply. It is as I feared a bit of a religious topic, it was not intended to be a troll. While I wasn't expecting a 'standard' method - the method would change with the output topology - it just seemed that it shouldn't be that complicated. I mainly put myself through this as a learning experience.

my solidstate training tells me that setting the bias on a tube amp should be a straightforward engineering problem. There is a note on the Laney schematic that indicates the bias voltage at a test point should be -36v. I figured that was what they did at the factory and that that figure should put the tubes on a reasonable spot on the load line.

Since this an audio amp and not a motor control, I guess that the art to all this comes in finding where the sweetspot is in terms of getting the guitar sounds we want as - opposed to getting as linear a response as we can get.

Thanks again
Justin Harrison

[Enzo]

Factories also tend to set things cooler than many of us might simply for reliability reasons. Tubes last longer cooler, and the given bias setting even considering the range of tube performance out there will still leave 99% of all the random tubes you might install within a reasonable range of operation.

[Mars Amp Repair]

Excellent dissertation. I tend to agree that there is much less mystique to bias that some would have you believe. The proof as always is in the pudding.
Personally I use the % max wattage calculation for idle wattage purely for some semblance of consistency & then use the ammeter insertion to the OT centertap method. I use the OT shunt method only when I have to. Something about 'shorting out' the primary of a transformer just doesn't sit well with me. glen

[Enzo]

I tend to be anal careful anyway, but I am more worried I might cause an arc on a tube socket which I would then have to replace. ANd of course I worry about my meter. Those little fuses are inconveniet and can be expensive. When I short something at the OT, I am really shorting the B+ to ground.

[Satamax]

Hi everybody!

Enzo, to avoid shunting anything to ground here's my take on my personal amps! Screw connector.



I ust unscrew one side, leave one of the wires in, and get another screw connector on the other end of the wire with the wire screwed in, then screw the probes in, as foolproof as can be!

[Enzo]

Adding test points such as that is a fine idea. I would keep the wires as short as possible to keep the very high signal levels on the plate leads from radiating into other areas.

But if you run a shop, you can't really add test points to all the amps that come through the shop. Then you are back to being careful. It is like anything else that is dangerous, know what you are dealing with, respect it's danger, and handle it with proper care.