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Old 18th April 2011, 08:33 PM   #1
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Default HV on high voltage amps

Quick question. Why is that modern high gain amps use higher voltage feeding the preamp (>300V)? Could I use a lower voltage to the same job? how would that affect gain and bias?

Thanks a lot
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Old 18th April 2011, 09:20 PM   #2
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*) Increased "headroom", thus ensuring that the final, and only the final, clips on overdrive. Especially when gNFB is employed, multiple stages clipping at the same time makes for a real mess.

*) Lower distortion: the smaller the output swing relative to the maximum possible output swing, the lower the distortion.

(Although I've seen this done a lot more often with solid state designs.)
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Old 18th April 2011, 09:51 PM   #3
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Depends on what you are driving.
Valves will work down to very low voltages.

I prefer to keep the voltage as low as possible if just for safetys sake.
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Old 18th April 2011, 11:36 PM   #4
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Depending on the tube the higher B+ may be needed to get the optimum load resistor without dropping to an excessively low idle current. BTW, I would consider 300V a pretty normal B+ not all that high really. Table radios often used lower B+ so that they could go direct wire PS without the added expense of transformer or doublers but this is to save money not to improve performance. Some tubes were made specifically for such low B+ but most general purpose tubes really do better with a bit more voltage.
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Old 18th April 2011, 11:47 PM   #5
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thanks guys....i was thinking on the lines of the soldano SLO for examplo that uses a 360V for the first stage and 350 for the second and so on....
Where can i get some good information on designing high gain amps? Maybe 4 or 5 stages...

Thanks a lot
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Old 19th April 2011, 12:16 AM   #6
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High gain amps? Look at guitar amp sites (AX84, 18watt.com etc) and also phono preamp designs.
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Old 19th April 2011, 01:45 AM   #7
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He is reading the voltages backwards from output to input.

The highest voltage is at the output stage. RC filters are used to progressively better filtering as one progresses to the input stage which is most sensitive to power supply noise. Each RC filter stage drops the voltage relative to the previous stage.
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