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Old 3rd November 2013, 10:54 PM   #1
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Default Stage gain and cascading

I see that many amp designs employ multiple gain stages. What is the inherent result of trying to obtain too much gain from a single stage?

How much SIGNAL input can say, a 12ax7 tube tolerate. I am sure there is a spec or calculation, I just haven't run across it. Are the OTHER advantages to multiple gain stages?
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Old 4th November 2013, 11:19 AM   #2
DF96 is online now DF96  England
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Valves are limited in the voltage gain they can give. For a triode the upper limit is mu: a number typically in the range 15 to 100. A pentode can go higher bit it has a very high output impedance so that can limit gain due to loading from the next stage.

The grid signal input is limited by two bounds: valve cutoff (grid too negative) and grid current (grid insufficiently negative). Valve cutoff can be seen from the graphs in the datasheet. It could be a few volts negative for a 12AX7 or a few tens of voltage negative for a 12AU7 - in either case it depends on the anode/plate voltage. The start of grid current is usually not specified in the datasheet as it varies with age and from sample to sample, but it is often somewhere in the region of -1.5V to -0.5V.

Low gain valves generally give good results in simple circuits without too much thought. High gain valves can create distortion unless carefully used (this is why people often complain that the 12AX7 is non-linear - it isn't but it does need the right circuit).

If you need a voltage gain of 100 it may require less careful design to sling together two stages with gains of 10 each, than a single stage with gain 100.
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Old 4th November 2013, 12:30 PM   #3
45 is offline 45  Italy
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Quote:
Originally Posted by DF96 View Post

Low gain valves generally give good results in simple circuits without too much thought. High gain valves can create distortion unless carefully used (this is why people often complain that the 12AX7 is non-linear - it isn't but it does need the right circuit).
However there are exceptions (i.e. more linear high mu valves) like the 6SF5GT and the 6SQ7GT. The drawback is they are single triodes and expecially the latter is getting really expensive.....
If employed with plate CSS they are incredibly linear just like the best DHT's....and one can get really high mu (with between 75 and 90 depending on the application) and plenty of output signal.
There are Russian and Chinese variants of the 6SQ7 (metal type) but I don't know how good they are in comparison to the original American.
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Old 4th November 2013, 12:43 PM   #4
DF96 is online now DF96  England
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Originally Posted by 45
However there are exceptions (i.e. more linear high mu valves) like the 6SF5GT and the 6SQ7GT.
Yes, as I said, you need careful design to get the best out of linear high mu valves like the 12AX7.
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Old 4th November 2013, 01:29 PM   #5
Merlinb is offline Merlinb  United Kingdom
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Quote:
Originally Posted by johnbalich View Post
How much SIGNAL input can say, a 12ax7 tube tolerate. I am sure there is a spec or calculation,
A rough approximation for the maximum peak-to-peak input headroom is:

Supply voltage / mu.

For a perfect triode this equation would be exact, but for practical triodes you may have to reduce this expectation somewhat if you want to avoid the very non-linear parts of the transfer characteristic. But it's a good rule of thumb to keep in your head, all the same.
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Old 4th November 2013, 01:56 PM   #6
DF96 is online now DF96  England
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Yes. You are unlikely to do better than this, but with careful design you might not do much worse. The snag is that grid current can be a problem with higher mu valves, as their grid is nearer the cathode space charge, so the forbidden region becomes a greater proportion of the 'supply/mu' estimate.
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Old 4th November 2013, 03:14 PM   #7
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Quote:
Originally Posted by 45 View Post
However there are exceptions (i.e. more linear high mu valves) like the 6SF5GT and the 6SQ7GT.
What do you do with the diode connections on the 6SQ7? Connect them to the cathode or ground them?
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Old 4th November 2013, 04:41 PM   #8
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Default Re: Cascading stages

Thanks all. Merlinb's answer is very helpful as it gives me a rough 'baseline' to work from. I am not about designing the ultimate amp, but about learning general principles and guidelines.

A question to which the answer might be obvious.....but if I exceed the supply voltage/mu equation, does it lead to clipping?

DF 96 looks like good info in your answer as well. It will take a little longer to digest. Thanks for every answer guys. I couldn't get through this learning curve without ya.

Last edited by johnbalich; 4th November 2013 at 04:46 PM.
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Old 4th November 2013, 04:53 PM   #9
DF96 is online now DF96  England
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As a general rule unused electrodes should be connected to the cathode. If two cathodes either the relevant one, or the most negative one.
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Old 4th November 2013, 06:16 PM   #10
Merlinb is offline Merlinb  United Kingdom
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Quote:
Originally Posted by johnbalich View Post
A question to which the answer might be obvious.....but if I exceed the supply voltage/mu equation, does it lead to clipping?
More or less, yes. It will certainly put you very close to clipping, although exactly how close depends on the chosen bias and how linear the valve is (an unbypassed cathode will also increase the input headroom, but that is yet another variable). If you bias dead-centre then you get the most headroom, although you still have to contend with grid current at one end and 'bunching' at the other. Because valves clip rather softly it can be tricky to define exactly where clipping begins. Easiest thing is to draw a load line and see what it tells you.

Last edited by Merlinb; 4th November 2013 at 06:19 PM.
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