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Old 2nd May 2010, 06:48 PM   #1
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Default 12AT7 plate voltage

Hi!

I am building a tube guitar amp, and I want to copy the reverb section. I found on the schematic http://www.schematicheaven.com/fende...b763_schem.pdf
that the plate voltage is 440V, but at the 12at7 datasheet, the max plate voltage is 300 V. I have 420V on power supply and the plate is connected to a transformer, so it will swing even further positive.. Do you guys think it is a good idea to connect it that way or not? I have a cheap chinese tube.

Thanx!

Miha
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Old 2nd May 2010, 06:54 PM   #2
SY is offline SY  United States
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According to that schematic, the plate to cathode voltage of the 12AT7 is 145V. So... no problem with that circuit other than the (all too common) abuse of the heater-to-cathode max voltage rating.

I'm not sure what you mean when you describe your circuit. Could you do a sketch, with voltages?
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Old 2nd May 2010, 11:10 PM   #3
Merlinb is offline Merlinb  United Kingdom
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Quote:
Originally Posted by overdrajv View Post
at the 12at7 datasheet, the max plate voltage is 300 V. I have 420V on power supply and the plate is connected to a transformer, so it will swing even further positive..
Max peak plate voltage for a 12AT7 is 550V, but you are correct, some 12AT7s do die when used as reverb drivers. One approach you sometimes see in amps is to connect a resistance in parallel with the transformer primary (100k? say) to limit the AC load somewhat.
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Old 3rd May 2010, 04:18 AM   #4
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Quote:
According to that schematic, the plate to cathode voltage of the 12AT7 is 145V.
It is? Ohh,I see..I think Sy was looking at the phase splitter.

Down in the left corner I see +440V plate,and 8.6V at the cathode (I thought it said 86V at first,but with a 25V rated cathode bypass cap,it must be 8.6V.) That leaves 431.4V across the tube.A bit high.
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