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Old 16th April 2003, 03:18 AM   #1
PassFan is offline PassFan  United States
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Default Amplifier Design

From the different sites Like Gabevees and Boozhound I'm coming to an understanding of determining operating values for different tubes and how they relate to the loadlines. In choosing one tube to drive another what do you look for? Is it just to drive enough voltage for the next stage? If this is so is this why they say tube amplifiers are designed from the output back? Then if I want this much output I select a tube that is capable and then match it to a load that will give me what I want. Then I will see what voltage is needed to drive this tube and go and find a tube than can do it...and so on and so forth until I can get to a point where .7 to maybe 1.7 volts can drive it, correct?
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Old 16th April 2003, 09:31 AM   #2
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Hey Passfan,

For me, I dunno. I have read a couple of philosophies. One guy constructed and modified an old (early 30's) Western Electric theater amp. The amp was designed to clip at the drivers, not allowing the output to clip. This way a clean clip was made, and it made for a more stable output, since the inductors were not involved with the clipping.

Most designs include the output stage's amplification to get that overall 0.7-1.7 volt sensitivity. Me included. I hadn't thought much about making the drivers such that they clip first.

I design each stage to do its best. If I want enough gain to be sensitive to 0.5 volts (standard line level), then I think about that for the first stage, amplifying it to a certain level. Then the next stage I think of as adding some current as well as voltage gain to drive the final stage. So it usually is not as much voltage gain. Then I design the output for the optimum parameters for linearity, gain notwithstanding.

So let's say I want the first stage to amplify the voltage about 20 times. Then I get 10 volts out for 0.5 volts in. The next stage then needs to amplify another 10 for 100 volts.

Here is the tricky part, though, because your last, or output stage, may be a triode or a beam power tube. So its gain may be 3 or 30. So the second stage can either be a common cathode or a cathode follower. For the triode, we need as much input as possible to get it to B+. So the above example will do it, because for 0.5 volts in the driver stage has 100 volts. For a B+ of 300 volts, the triode will amplify the signal to 300 volts, it being a gain of 3. Comprende?

But for a pentode, or beam power tube, since its gain is high, all that would be needed is the signal of the first stage. So one could just feed from the anode of the first stage, or put it through the cathode follower so the first stage isn't loaded down for optimum gain. Then we have a signal of 10 volts being amplified another 30 for the 300 volts.

Now, all that said, the actual output voltage at the secondary is whatever the ratio of the transformer is. So actual voltage to the speaker may be 15 volts. But that is at full amplification, so that's OK.

Does this help any?

Gabe
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Old 16th April 2003, 02:11 PM   #3
PassFan is offline PassFan  United States
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So, it's dependant on the MU of each tube and what you can get out of it, versus what you need to drive the final power tube. You surely wouldn't need to drive a 6L6 with 100 volts, right? I will have to go back to your site and check out the calculations for determining gain. This and the MU info of a tube would then seem to be the key. Thanks
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Old 16th April 2003, 02:12 PM   #4
Joel is offline Joel  United States
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Yes, always work from the output backwards - that way you know what is required of the front end.
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Old 16th April 2003, 07:21 PM   #5
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Lesse...

Usually I grab some operating points for some tubes that cought my fancy, figure how much and what kind of drive I'll need for them, then find some tubes which can do a good bit better than that (check my quad 6146 design, eheh). The idea is the output stage is the only one which goes anywhere near full power. The driver stages contribute far less distortion if the output maxes out when they're at half power (or less). If you're driving a 6L6 SE, about 25V, the driver stage should be good for 50V or more.
Generally the driver stage has little current and voltage requirements, so the choice of an input tube (if necessary) is based more on remaining gain you need.

Tim
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Old 16th April 2003, 11:45 PM   #6
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Default Figuring Out What's Needed.

Hi,

Quote:
Yes, always work from the output backwards - that way you know what is required of the front end.
Indeed, you figure out what power output you fancy, pick your output tubes, loadlines, etc. and than work your way to the front.

Simple as that...on the surface at least.
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