Biasing of Tubes, part 1

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Roger A. Modjeski

Biasing of Tubes, part 1
« on: 1 Jul 2007, 10:15 pm »
Biasing of Tubes: Part 1.

If you and I went back in the “Way Back Machine” * and visited an engineer or a radio repairman (called “radioman” in the 1920's-40) and started a conversation on bias we might not get past the first sentence with this competent fellow. If we said “I adjust the bias on my amplifier to be 50 mV or 300 mV or any number under 20 Volts he would look at us very strangely. For him “bias” is typically –20 to –100 volts. No millivolt bias in 1929. He would call your bias zero and wonder how you kept your tubes and amp from burning up.

Positive reading low voltage bias came into the amplifier world in the 1955 with the birth of Dynaco. See:  http://en.wikipedia.org/wiki/Dynaco for more information on that company.  My dear friend, Bruce DePalma (brother of Brian) worked for David Hafler in the early years. We had many interesting conversations over gin and tonics. He told me some humorous stories about David which I will relate if there is interest.

I had worked on many a Stereo 70 by then and I knew why 1.56 Volts was the chosen numbers because I had built several meters by that time. Anyone who has built and calibrated a Heathkit VTVM or most any other meter, will recognize that number, it’s the voltage of a “fresh flashlight cell”.  In the age before accurate digital meters that was the best “voltage standard” one could get.  In that day the most sensitive scale on common meters was 1.5V with just enough swing at the top to allow 1.56. Some meters even had a little mark up there.

DePlama told me an interesting thing I would never have known otherwise. It turns out that a common cathode resistor of around that value also reduced the distortion appreciably. DePalma was 16 when he started working for Hafler. Another early starter.

I had better warn you now. I go off the subject sometimes to get a few details in, my intent is to fill in the gaps and history for the younger readers who may have never seen an analog meter. You know, the kind with the needle that swings up and down and all those numbers on the scales. I will say here that analog meters are still the best for many audio applications and the good ones are quite accurate. One thing to know about them is the 1/3 scale rule. The rule is you should not read below 1/3 scale. Most meters had a 1,3,10,30 series so you could range down if below 1/3 scale. If the needle barely moves, forget it. Analog meter accuracy was specified as a percentage of full range. So if you have a meter that is 5% accuracy and you are on the 5 volt scale you can be .25 volts off. Add .25 volts to 1.56 and you will be 16% high, or low if the meter is off in the other direction. Bias does not have to be dead on, but 16% is a bit much, especially if on the high side. The ST-70 tubes are already running at full dissipation, and 16% over would shorten their life considerably.

David Hafler undoubtedly knew all this. Not all meters had a 1.5 volt scale. Some had 3 or 5. The manual told you to get a fresh flashlight cell, see where it read on your meter and just adjust the bias to the same spot on the meter scale.

Now what does all this have to do with our visit back to the old radioman and why don’t you find numbers like this on a tube data sheet that he would be looking at? Look up an EL-34 data sheet and you will find typical bias is around –35 volts (note that’s a negative voltage). What’s even more confusing, when you BIASET the Dynaco you read a positive 1.56 V (all with respect to the chassis which is typically ground). By now there is great confusion in the conversation with the radioman. BTW, I have found that many students have trouble getting a feel for voltages, both magnitude and sign. I play this little game: 1 volt = $1 and you know that $1.56 is not the same as $35. You also know that if you balance your check book and you have -$35 you are going to get a note from your bank. If you have +$35 you can at least get dinner. To a technical person voltages are as real as money and reporting them should get the same care as your bank takes reporting the status of your account.

There is one other reason the radioman thought in terms of bias voltage and not plate current. The tubes he was dealing with, types 71 and 45, were all low Gm tubes (1.6 to 2.5 mA per volt) so variations in the tube parameters or the setting of the bias voltage made little difference in the plate current. When the 2A3 was introduced in RCA manual RC-11 (1933) there was no mention of bias matching though they did mention, rather proudly, the high Gm of 5.25 ma/volt. By the publication of RC-12 (1934) they had inserted a very important paragraph, undoubtedly based on things learned from applying the tube. It’s a long paragraph but the most telling sentence (paraphrased) is this: “The very high value of transconductance makes the 2A3 somewhat critical to grid-bias voltage since a very small bias change produces a very large change in plate current” In push-pull they strongly urged that the tubes had individual bias pots and the ability to read the plate current and make it the same in both tubes.  Please note that both plate current and cathode current are the same in triodes and both terms are used interchangeably. In pentodes the difference is usually no more than 5% due to the screen grid current, but that’s another story. Note also that today’s common pentodes; EL-34, KT-88 etc. have Gm’s of 6 to 10 mA per volt.

So what are we measuring if we are not measuring bias voltage? We are measuring the current in the output tubes as a voltage drop across a resistor. We don’t really care what value of negative voltage is applied to the grid via the bias pot that you are adjusting. As long as it is enough to dial the tube in. One maker, whose initials are ARC, purposely limits the range on bias adjustment thus requiring output tubes to fall in a rather narrow range (less than 10 volts) of grid bias. Because we get all the bias and Gm data from our tube tester, Ram Tubes can select tubes in the required range for these amps. (We sell the same tubes at lower prices).  When I test 6550’s only half of the tubes will fall within a 10 volt range. I designed the RM-9 to give the user a very wide bias adjustment range from –32 to –64 volts (32 volt range). This range also allows the user to use EL-34’s, KT-77’s, KT-88’s, 6550’s and any other tube that can be biased within that range of negative grid voltage.

The ST-70 had a 15.6 ohm 1% resistor in series with the cathodes of each output pair. That’s a rather odd value. Why not 15, that’s a standard value. Well David wanted to have exactly 100 mA through the two tubes and 15.6 ohms yields (by Ohm’s Law) 1.56 volts in that situation. He even bothered to get a precision resistor. When its all biased up you can take your meter and read the negative grid voltage that you adjusted on the bias pot and it will be somewhere between negative 30 to negative 40 volts with average tubes. Now if you said to the radioman. “I bias my tubes at –35 on the grid for 50 mA of plate current per tube.” He would say… gotcha.

End of Part 1.

Stay tuned for Part 2. Where we will look at the terms: autobias, fixed bias, cathode bias, self bias. These are terms I feel need clarification. It makes discussing things easier.

* The “WayBackMachine was a creation of the cartoon show “Bullwinkle”.






JohnR

Re: Biasing of Tubes, part 1
« Reply #1 on: 3 Jul 2007, 09:47 am »
Good stuff, Roger! :thumb: (It did take me a while to remember what this is all about, thank you for the refresher!)

Roger A. Modjeski

Re: Biasing of Tubes, part 1
« Reply #2 on: 3 Jul 2007, 03:18 pm »
John,

Thanks for being the first reply. I fear I went too deep for most readers.

I would appreciate hearing from others. I'm testing the waters here.

DeadFish

Re: Biasing of Tubes, part 1
« Reply #3 on: 3 Jul 2007, 03:58 pm »
Hey Roger, if the water didn't get deep sometimes, nobody would learn how to swim....
I appreciate your information.  Since the last amp I picked up happened to be a Stereo 70, you piqued my interest. 
Since I started messing with tube amps (again) I've had a lot of 'brain freeze' in learning, and the more exposure I get to functional facts (as opposed to technical manuals) the more sinks in, hopefully. 
Thanks for the efforts!

Regards,
DeadFish

Jampot

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Re: Biasing of Tubes, part 1
« Reply #4 on: 3 Jul 2007, 04:53 pm »
It could be complete waffle for as much as I know about it, but Roger's style kept me reading to the end.

Love the asides and anecdotes - keep it up and we can learn as we are entertained :thumb:

Jim

Gordy

Re: Biasing of Tubes, part 1
« Reply #5 on: 3 Jul 2007, 06:33 pm »
Excellent Roger, thank you!  I'm with Jampot, I love to learn about all the background info, asides and anecdotes.

Roger A. Modjeski

Re: Biasing of Tubes, part 1
« Reply #6 on: 4 Jul 2007, 05:16 am »
Thanks to all for reading and writing.

Gordy, what's the name of the cartoon character?

lowrider

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  • Posts: 75
    • DIY
Re: Biasing of Tubes, part 1
« Reply #7 on: 4 Jul 2007, 07:00 am »
I got my first tubes 1 year ago, thanks for the very nicely put usefull information...  :thumb:

Gordy

Re: Biasing of Tubes, part 1
« Reply #8 on: 4 Jul 2007, 12:38 pm »
Hi Roger,

He's Marvin The Martian, from Chuck Jones' Loonytoons... http://en.wikipedia.org/wiki/Marvin_the_Martian



I noticed yours as well, Plato from Raphael's The School, very nice!

Carl V

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Re: Biasing of Tubes, part 1
« Reply #9 on: 4 Jul 2007, 01:42 pm »
Quote
I fear I went too deep for most readers.

I would appreciate hearing from others. I'm testing the waters here.

Deep but informative. 

I apologize for simply reading your posts & not responding. 

Your posts have that Trifecta Blend of the 3-E's
Entertaining, educating & enlightening.  Please continue.

Your notion of a weekend Santa Barbara 'primer' sounds appealing.
However, since most of us are at many different levels of knowledge
I'm curious how you'd balance that out....remedial for some of us, while
in depth for others.

richidoo

Re: Biasing of Tubes, part 1
« Reply #10 on: 5 Jul 2007, 03:51 am »
Yeah, that was fun. Nice work! My Cary biases in mA, my Manleys bias in mV across a 10ohm resistor, same result. I have lots of bias questions, but I will wait patiently for the sequel.
Thanks Roger!
Rich

flatmap

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Re: Biasing of Tubes, part 1
« Reply #11 on: 6 Jul 2007, 11:02 pm »
Roger, this is great.    I come to the table with not much more than V=IR, but I have an appetite to learn more.  Please keep going.

Thanks,
Steve

Roger A. Modjeski

Re: Biasing of Tubes, part 1
« Reply #12 on: 7 Jul 2007, 04:22 am »
Thanks to all for the encouragement. For people fooling with tubes it's great to know Ohm's law. I had a student that wanted to understand electronics without Ohm's law. I told her... Good Luck!

One of the things I encourage my students to do is think about the voltage, current, resistance.... wattage (if you care to go that far) of simple things, like turning on a lamp. If you close the switch on a 100 watt incandescent bulb, what  happens? What is the current flowing through the switch, wire, circuit breaker... all the way back to Hoover Dam if you care to.

Here's something to consider. If you have an ohm meter, measure the resistance of a 100 watt lamp. What current will flow when you close the switch?

Roger

flatmap

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Re: Biasing of Tubes, part 1
« Reply #13 on: 7 Jul 2007, 06:00 pm »
Well, I have a 30 Watt bulb on hand.  Measuring this with my multimeter -- it takes a beat to settle down -- I get 37 Ohms.  With around 120 Volts AC from the socket, V/R would be more than 3 Amps.  I find that pretty surprising as I would have expected much less.

Well then, isn't the power in Watts given by IV?  If so that would say the power dissipated is about 390 Watts -- which must be way off for a 30 Watt bulb.  To get a 30 Watt result for 120 Volts gives 30/120 = 1/4 amp.  That sounds more like it.

So I'm not sure what's going on with this.  Maybe the bulb is more than just simple resistance?  Or maybe the resistance varies with the applied voltage?  What am I missing?

Gordy

Re: Biasing of Tubes, part 1
« Reply #14 on: 7 Jul 2007, 06:05 pm »
heat?

flatmap

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Re: Biasing of Tubes, part 1
« Reply #15 on: 8 Jul 2007, 05:33 am »
Maybe so.  Taking your lead, that means the filament becomes more resistive as it heats up.  And so for this 30 Watt bulb it would then start out cold at 37 Ohms and increase to something like 480 Ohms.

Roger A. Modjeski

Re: Biasing of Tubes, part 1
« Reply #16 on: 9 Jul 2007, 05:15 am »
Flatmap got this one. The resistance does go up, about 10 times when the filament is hot. If you hold a cold bulb in your hands you can actually see the resistance go up a tad from the heat of your hands. So every time you turn on a 100 watt lamp there is close to a 10 amp inrush. The heaters in your tubes behave the same way but the ration is not quite 10 to 1 because they never get up to the temp of a lightbulb. Once a glowing body gets in the visible range it goes from dull red to red to orange to yellow then white. Studio photographers rate the color of light in degrees Kelvin. Fluorescent lamps are also rated on this temperature scale.

Here is good time to comment on the yellow-white flash you see near the base of many 12AX7's and other small tubes. This flash is perfectly normal and does not affect the life of the tube. What is happening is the small uncoated section of the filament gets hot faster because the assembler has to scrape away the white filament insulation in order to spot-weld the filament wire to the pin.

richidoo

Re: Biasing of Tubes, part 1
« Reply #17 on: 9 Jul 2007, 12:48 pm »
By same token, things get more conductive when cooled. I think that even a strawberry will become a electromagnet and levitate over a permanent magnet if chilled cold enough. Is this correct?

JoshK

Re: Biasing of Tubes, part 1
« Reply #18 on: 9 Jul 2007, 01:45 pm »
I've been messing about with tube schematics and design for the better part of the last two years, so I knew the message you were telling but I still found the historical context really interesting to read.

Ohm's law, Kirchoff's law (law of preservation of currents), some elementary algebra and problem solving get you most of the way in playing with tubes.


Roger A. Modjeski

Re: Biasing of Tubes, part 1
« Reply #19 on: 9 Jul 2007, 04:01 pm »
Thanks everyone for your participation. A few notes: There are materials whose resistance stays constant with temperature, nichrome wire is one. It's the wire you see laced back and forth that glows when it makes your toast. It's virtually the same resistance hot (glowing) and cold. Here's an interesting tip, from Bruce DePalma again. If you are ever in need of a high power roughly 8 ohm load. A 1200-1500 watt portable heater, toaster or any nichrome wire heating element  is very close to 8 ohms cold and the same hot. If it has a fan and you have a big enough amp to get above 60 volts you might even get the fan to run. How much power would your amp be putting out at that point?

As far as law an order: I hardly ever (maybe never) use Kirchoff, but Thevenin is a very good thing to know. The trouble with Kirchoff's law is it gets very cumbersome and you have to solve a matrix, ugggg. Thevenin on the other hand is precisely what we need to understand the effects of loads on amps and preamps. It's one I use every day. It's quick, nothing more than  adding, subtracting, multiplying, dividing. Throw me a bone and I'll get into that one.

Here's a little history:  http://en.wikipedia.org/wiki/Thevenin. Here's a quote from that article.
 
"Appointed as a teaching inspector at the École Superieure in 1882, he became increasingly interested in the problems of measurement in electrical circuits. As a result of studying Kirchhoff's circuit laws and Ohm's law, he developed his famous theorem, Thévenin's theorem, which made it possible to calculate currents in more complex electrical circuits".