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Old 6th August 2012, 12:12 AM   #1
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Default Dynaco low Power Amp

Anyone?

Can anyone tell me how the cathodes of the 12AX7'S are grounded? It seems the only path to ground is through the second tube grid? Or does the ground float? If I knew the name of the circuit I might be able to find a description somewhere.
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Old 6th August 2012, 12:29 AM   #2
20to20 is offline 20to20  United States
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Put your pencil on the cathode and trace the line backward through the resistors. This is a K biased tube so it is not directly grounded like a fixed biased tube would be.

HTH

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Old 6th August 2012, 12:33 AM   #3
kevinkr is offline kevinkr  United States
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Through the feedback connection to the secondary.
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Old 6th August 2012, 01:03 AM   #4
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Kevin,
if it is grounded through the secondary how does that not put DC into the outputs? Is it just that the DC would rather go to ground and bypass the voice coils?

AH! I don't have the ground from the speaker common to the chassis! That's why the preamp tubes aren't working. Should that connection go to the star ground?

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Old 6th August 2012, 01:11 AM   #5
kevinkr is offline kevinkr  United States
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The cathode current is miniscule, (a couple of mA max) so the voltage developed across the dcr of the secondary winding is no more than a couple of mV at most.
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Old 6th August 2012, 02:32 AM   #6
rmyauck is offline rmyauck  Canada
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I believe that's a Floating Paraphase circuit.

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Old 6th August 2012, 03:41 AM   #7
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AC is grounded through the very large electrolytic... fwiw.

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Old 6th August 2012, 01:00 PM   #8
DF96 is offline DF96  England
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Yes, the input stage looks like a floating paraphase. The OPT secondary should be grounded to the same point as the input ground, so the input signal and the negative feedback are referenced to the same potential.
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Old 6th August 2012, 05:02 PM   #9
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Old 7th August 2012, 12:58 AM   #10
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This is a portion of the dynaco SCA35 circuit which Greg Dunn covers on his website.
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