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7591 G-2 volts

1/29/1997 2:08 PM
James Hurley
7591 G-2 volts
I'm working on a "Universal" amp that uses two 7591s for power tubes, the plate  
voltage is 450, the screen grid voltage is 50. The tube manual says 440 max.  
My question is this; Is the radical disparity a design factor resulting from  
using 7591s in a guitar amp or is this thing messed up?
1/30/1997 1:03 PM
John Martin

This is a complex question. First different manufacturers had different methods of rating tubes. Ther were "design center" specs. "design maximum" specs, etc. The 7591 seems to have been used at a wide range of screen grid voltages unlike many of the more common tubes. The 7591 had some nice features like bringing out the screen to two pins to keep the heat down and add some stability. Another tube like this was the 6973, which has the same form factor as the 6BQ5. These were designed from what I can tell, for high end audio applications.  
You need to find some designs that are close to what you want to achieve and use them for reference. This will give an indication if you are at least "within the ballpark" with your own design.
1/30/1997 6:54 PM

i was working on a sovtek mig 60 when i noticed the plates were running near 700 volts. this is way out of range for 6ca7s but the screens were running at 350 volts. usually ratings in the manuals are for typical designs and the parameters relate to each other. incidentally one of the tubes in the mig 60 had gotten so hot in a small area of the plate that the glass had softened and was sucked in by the vacuum as if you had simply pressed your thumbtip and indented the glass. and the fuse held. it was cool. i gave it to r.g.keen. he thought it was neat too. good luck.
2/3/1997 12:03 AM
John Martin

We have also seen the melted tubes in the Mig 60. Musicman also ran their tubes at upwards of 700 volts. Most likely running towards the class B end of the class AB1 range. Like the sound of the lower voltage amps better anyway (unless you are using transmitter triodes!)  
JM out

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