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Anyone know the max gridcurrent I can allow on 6AS7s? I get about 600mA out of each triode section in my OTL with 120volt supply. The driver I have now cannot source any current and I want to try to use a beefier driver that will drive the grids positive. But without shortening the tube's life.
If anyone does not know, I will probably set up a test rig with one tube running hard with grid current to find out how much is too much. But the question is then, how do I figure out what they can handle? For instance if I run the tube with some 50mA grid current and it lasts for one week, how can I estimate what the max current is if I want the normal life of several years? I assume accelerated fatigue tests like this is a standard way to find a lifespan under certain conditons. But that means I need a factor to find out how much I must back off from the test grid current. Am I making any sense? Any ideas?
Follow Ups:
I understand your question, but I have no answers except to refer you to Atma-sphere's long experience with this tube. However, I must say that 600mA per triode section (I presume that's what you meant; there are 2 sections per tube) sounds like a hellacious amount of current for a 6AS7 (max 1.2A per tube with both sections driven). Even the 6C33C is barely rated for that much current, but then again I don't know what the 6C33C can handle on peaks.
I just checked my notes, and I get +/-16volt peak into 8ohms, +/-2amps, with a total of 4 tubes, so thats 500mA per section. That's with seriously flat-top clipping.
I took these measurements a few weeks ago and I think I said to myself I can probably get 600mA with positive grid drive, and somehow mixed the numbers. Too many hours in a cloud of solder...
But with 500mA out the tubes have approx 110volts on the anodes which gives a peak dissipation of 55watts! I should probably not push it any further with an addition grid dissipation? I idle the tubes at roughly 65mA and use a 120-125volt supply so the idle dissipation is well within limits.
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!
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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