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Old 23rd November 2006, 11:00 PM   #11
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This is the actual amplifier.
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Old 24th November 2006, 03:45 AM   #12
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that looks cool!
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Old 24th November 2006, 02:17 PM   #13
Jaap is offline Jaap  Netherlands
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Default fixed bias in IT coupled designs

What is better in an amp that is transformer coupled to the power tube. Fixed bias into the secondairy of the transformer and a grid resistor to ground. Or fixed bias into the grid resistor and the center tap of the transformer attached to ground ?
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Old 27th May 2013, 01:21 AM   #14
jerrys is offline jerrys  United States
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Originally Posted by HollowState View Post
Large tubes like these require higher voltages to operate correctly. In order to use self (cathode) bias, a large value resistor would be needed which would seriously hurt efficiency. So it's a matter of practicality. It also gived better control.

not exactly. Cathode bias works fine with the 6L6 family. IF you want to get crazy with the plate voltage, then you have an incentive to use fixed bias. It also bears nothing that the screen grid (and thus the voltage applied to the screen) actually exerts more control over the tube than the plate. Normally, in pentode mode, 250 ohms is good on a 6L6 (or member of the 6L6 familyi) and you waste very little power in cathode-bias mode UNLESS you push the plates beyond 400 volts AND raise the screen voltage as well. Then the difference in power output between cathode and fixed bias will become relevant. Look at the tube data sheets, study them a bit. The whole story is right there. But, it really depends on the TUBE, and not necessarily how powerful the tube is. The 7591 is normally run in fixed bias for a reason and it's LESS powerful than the 6L6GC. Bottom line, in a 6L6 with plate voltage of 400 or maybe a bit more and a reasonably sane screen voltage (say, less than 350), you have no incentive to use fixed bias over cathode bias.
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Old 27th May 2013, 03:58 AM   #15
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Yes exactly. We were not talking about 6L6s or 7591s. These are not "large tubes". The original poster was refering to 211s and 845s. These tubes work at 1000 plate volts (or more) and negative 80 and 150 grid volts respectively because of their low . Under these conditions fixed bias is far more pragmatic.

With smaller tubes like the 6L6 etc, I tend to agree with you. Self bias can be more easily implemented because the losses are smaller. Both bias systems have their pros and cons with these smaller tubes. However, I am of the opinion that fixed bias is generally superior for all but the lowest power output tubes. Self bias is popular with some DIY's because it is simpler, and it raises the bottom line for manufactures. So here, it comes down to designer's choice.
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