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 Post subject: Re: How to add overdrive to a guitar amp circuit
PostPosted: Jan Fri 19, 2018 9:45 pm 
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Location: Vancouver, WA
dberman51 wrote:
No, you size the resistors to achieve the operating voltages the stage needs to operate according to the objectives you have, then you choose a capacitor value to eliminate hum.

-David


Yes, that it technically the correct way to approach it. I suspect very few guitar amps were actually built that way.

Looking through old schematics you will see the same size capacitors used on every stage spanning many different models. That may be because they were using available big can multi-caps or because they didn't want to stock a wide variety of individual caps. The resistors strung between the caps do vary quite a bit from stage-to-stage and amp-to-amp.

Just because an old amp used a particular voltage doesn't necessarily mean that it was the ideal voltage for the task.

How much voltage is needed for the first pre-amp tube? Let's say a 12AX7 driving a tone stack has a gain of 40. It is being fed a peak 10 mV signal from the guitar pickup. So the output is swinging less than a volt. Do we really need to supply it with 400 volts? Will we lose headroom if we drop it to 200 volts? Practically speaking, it won't make any difference. Put it at the end of the power supply chain since it needs less voltage than each of the other stages and will benefit most from the compounded filter stages. Whatever voltage is left [after adequately filtering each stage] is what you write on the schematic when it's all said and done. That way, someone reading it 50 years from now will be able to painstakingly hand pick components to perfectly match that "ideal" voltage.


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 Post subject: Re: How to add overdrive to a guitar amp circuit
PostPosted: Jan Fri 19, 2018 11:49 pm 
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No again. Guitar amplifiers were designed by electrical engineers who knew what they were doing. Not hard to do -- audio amplifiers were reduced to practice in the 1920s.

-David


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 Post subject: Re: How to add overdrive to a guitar amp circuit
PostPosted: Jan Sat 20, 2018 12:37 am 
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I'd love to see the look on the face of those 1920's electrical engineers if they heard a Marshall stack cranked to "eleven". :shock:


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 Post subject: Re: How to add overdrive to a guitar amp circuit
PostPosted: Jan Sat 20, 2018 3:23 am 
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Location: Boston, MA USA
They would say "apply some negative feedback to get rid of that terrible distortion!" :-)

-David


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 Post subject: Re: How to add overdrive to a guitar amp circuit
PostPosted: Jan Sat 27, 2018 8:21 pm 
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Joined: Jan Thu 01, 1970 1:00 am
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Location: Gainesville, Florida
try swapping a 12AX7 for one of the 12AY7 tubes. you would have one input with 12AX7 for changing effect. also eliminate the negative feedback. you could add a switch to cut neg feedback in/out if you like the sound and want to change the effect whenever


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