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Old 21st May 2012, 02:37 AM   #1
jkorten is offline jkorten  United States
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Question Bypass capacitors in Tube amps

I have often used capacitors in parallel with electrolytics in solid state designs. As most solid state driver stages are current sourcing - bypassing makes sense to me.

Tube amp designs are mostly voltage amplifiers except for the pentode/triode stage were current is being pulled through the transformer.

But I rarely see any schematics showing a parallel film cap along with the B+ capacitor, nor in the voltage gain and splitter stage do I see any bypassing of the power supply capacitors.

Why is this? Is there really no benefit? Or do people do this but they just don't speak about it? (like listening to Lady Gaga).

Any advice would be appreciated.
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Old 21st May 2012, 03:34 AM   #2
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Conrad Johnson bypassed the power supply caps on all but the earliest amps and preamps.
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Old 21st May 2012, 03:55 AM   #3
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There's a lot of sloppiness that's held onto in the name of tradition. It's up to folks like you to move things forward out of the 1950's.

All good fortune,
Chris
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Old 21st May 2012, 04:05 AM   #4
fredy2 is offline fredy2  United States
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For tube amps, the impedance is pretty high and the frequency response for feedback via the B+ is not that great compared to the transistor type circuits. Any sag in the voltage is taken care of by the feedback used for linearity.
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Old 21st May 2012, 10:21 AM   #5
DF96 is offline DF96  England
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Bypass caps can help or hinder, or do nothing. It all depends. Some people use them routinely. Others avoid them. Some use them when necessary in the right place.
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Old 21st May 2012, 10:42 AM   #6
SY is offline SY  United States
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As they say in real estate, "location, location, location." If the power supply cap is 15 cm from the circuit it's bypassing, putting a second bypass cap across it may make you feel good, but does little to nothing electrically. If you can place the bypass cap right at the circuit, it may be useful in some circumstances.
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Old 21st May 2012, 11:26 AM   #7
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Quote:
Originally Posted by SY View Post
As they say in real estate, "location, location, location." If the power supply cap is 15 cm from the circuit it's bypassing, putting a second bypass cap across it may make you feel good, but does little to nothing electrically. If you can place the bypass cap right at the circuit, it may be useful in some circumstances.
I disagree. It helps in some situations in my experience.

Shoog
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Old 21st May 2012, 11:52 AM   #8
SY is offline SY  United States
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Given the added resistance and inductance of the wires, nullifying any advantage of "superior" cap performance, that is unlikely. This is discussed in some detail in both "Valve Amplifiers" and "Building Valve Amplifiers."
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Old 21st May 2012, 03:27 PM   #9
jkorten is offline jkorten  United States
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Sy - not sure what you are implying. If a small value bypass capacitor is in parallel with a power supply smoothing capacitor, you are decreasing resistance because you have twice as much wire. Also given the lead length of a capacitor - is it that much inductance you are adding? (typically you put the capacitor right across the terminals of the electrolytic). Are "Valve Amplifiers" and "Building Valve Amplifers" books that are still sold?

Thanks for all the comments.

Jerry
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Old 21st May 2012, 03:40 PM   #10
chrish is offline chrish  Australia
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I think the point SY is making, is that there is no point putting the bypass cap across the electrolytic if that whole part of the circuit is 15 cm from the load. If that is the case, you are better off putting the bypass cap right up close to where it is needed - the load, not the reservoir cap. Hence the real estate term 'location location location'. Both caps are 'in parallel', but only one location will do what you want it to. Yes, this, and many other useful tips, are in the two books.
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