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In Reply to: u're right! posted by Jalla on May 8, 2006 at 13:07:26:
So- are you getting 16 peak to peak then? Its not clear from the statement in your post.At any rate, it is possible to drive the 6AS7 grids positive, if you have the current available in the driver to do so. When the tube saturates, no extra amount of current will help. The grid current window actually begins at *about* -15V with respect to the cathode, and goes *about* 15V positive. This varies with B+ voltage!
Follow Ups:
I get 16volts peak both pos and neg into 8ohms, 32Vpp...thats 32watts peak, but this is at hard clipping.
I actually never measured the grid-cathode voltage at clip, only assumed this is at very close to 0V. The signal clips at the same time both at the driver and at the 6AS7s output, so the clipping is at the driver due to grid current. Both the driver and the 6AS7 has plenty of voltage to spare. My driver can at best source 1mA so I dont consider this a true driver, just a gain stage.
Shouldn't the tube start to glow redish at this dissipation? I only see blue lights when running them at clipping, but no other colors. Guess I should try for a little longer, but need a beefier load resistor. Somehow I am going to break these tubes!
-for four tubes.32Vp-p x .5= 16V peak.
16V x .707=11.312V RMS
11.312V/8ohms= 1.414Amps1.414Amps x 11.312V= 15.99 Watts
Sounds like you are on track right now- your power output is on target.
Thanks for answers. Yeah, 16watts rms is what I am claiming too. My motivation for pressing more out of the tubes is that the low current drive gives a rather abrupt clipping behaviour, kinda more SS like than the typical soft clipping of most high impedance tube transformer coupled amps. I want better compression on the onset of clip. I think if the grids get a little juice, the tubes will clip more due to saturation than the grid flat lining. Any ideas?
It sounds to me like your driver is clipping before the output tubes saturate. You'll need to have a driver circuit that is running considerably more current through it- that's why we run 6SN7s for driver tubes as they can handle a bit more current than 1mA.
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