Fixed Bias v. Cathode Bias [message #9062] |
Sat, 16 July 2005 13:43 ![Go to next message Go to next message](/forum/theme/AudioRoundTable/images/down.png) |
Manualblock
Messages: 4973 Registered: May 2009
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Illuminati (13th Degree) |
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I have tried both on the EL 84 amp. I like cathode bias sound better, but I am not sure why that is. Can we discuss the differences between the two?
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Re: Fixed Bias v. Cathode Bias [message #9063 is a reply to message #9062] |
Sat, 16 July 2005 19:46 ![Go to previous message Go to previous message](/forum/theme/AudioRoundTable/images/up.png) ![Go to next message Go to next message](/forum/theme/AudioRoundTable/images/down.png) |
Forty2wo
Messages: 163 Registered: May 2009
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Master |
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Hi M, (sorry I forget your real name) The main claim to fame of fixed bias, is efficiency, you get more power out of a set of tubes if you are not heating up a cathode resistor. But with FB you are applying a voltage, aka (signal) to the grid. If this is not deep black it will be heard. With CB you will have a certain amount of degenerative feedback , which is not a bad thing and my guess is that is what you ( and me ) are hearing With CB often you are running you tubes deep into "class A". You can test this. Measure current across the cathode resistor . then when you switch to FB set the current to the same or more. Try this and tell me what you think…John
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Re: Fixed Bias v. Cathode Bias [message #9064 is a reply to message #9063] |
Sat, 16 July 2005 21:25 ![Go to previous message Go to previous message](/forum/theme/AudioRoundTable/images/up.png) ![Go to next message Go to next message](/forum/theme/AudioRoundTable/images/down.png) |
Thermionic
Messages: 208 Registered: May 2009
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Master |
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Some personal musings on bias: Grid bias exhibits lower distortion (and more power, as Forty2wo noted) than cathode bias does, and the orders of distortion artifacts present with each differ. Output Z and therefore damping factor as well also vary with the bias method, and whether or not the cathode resistor is bypassed. With a cathode biased stage there is indeed degenerative feedback, but the parallel bypass cap provides a low impedance AC path to ground and breaks it. Adding the bypass cap increases gain, distortion, and stabilizes the bias effect. A grid biased stage can be either Class A or Class AB operating class. A cathode biased stage found in a hi-fi amp is typically gonna be Class A, while a cathode biased PP guitar amp may be either class. The reason is that with Class AB operation the plate current increases significantly with output, and an increase in current drawn through the cathode resistor creates a higher bias voltage, according to Ohm's Law. If an amp is (cathode) biased into Class AB operation far enough away from Class A, it will increase it's bias voltage on the fly enough with increased output wattage to the point where crossover notch distortion becomes horrendous. As you increase the volume further, it'll sound more and more distorted until it actually starts losing volume from the power tubes being cut off! That's of course worthless for hi-fi. But, there are a few guitar amps like the legendary Vox AC30 (and boutique clones) that use a cathode biased Class AB output stage to produce a unique set of harmonics as the amp is turned up. The AC30 is biased close to Class A operation, so the power tubes' current draw doesn't increase really sharply with output, which keeps it out of trouble. Another difference with cathode and grid bias is that with cathode bias, the plate voltage you measure is NOT the real plate voltage. The real plate voltage is the plate to cathode voltage potential. Subtract the cathode voltage from the measured plate voltage, and you have the real plate voltage. This must be taken into account when designing a cathode biased stage on the plate characteristic curves. Thermionic
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Re: Fixed Bias v. Cathode Bias [message #9067 is a reply to message #9065] |
Sun, 17 July 2005 11:31 ![Go to previous message Go to previous message](/forum/theme/AudioRoundTable/images/up.png) ![Go to next message Go to next message](/forum/theme/AudioRoundTable/images/down.png) |
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Wayne Parham
Messages: 18756 Registered: January 2001
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Illuminati (33rd Degree) |
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I think he probably means the tube is biased so that it conducts a lot at idle. If you set idle current past 50% of saturation, you're kind of shooting yourself in the foot but up to that point there is some room to play with biasing.Class A circuits have the active component always conducting, never cutoff even at idle. Class B is push-pull where each device is set for zero idle current, so that it only begins to conduct on its half cycle. Class AB is a push-pull configuration that sets each active device quiescent current above zero, so each device conducts through the crossover region. It prevents switching spikes and reduces distortion. So Class A is conductive all the time, Class B is cutoff at opposite half cycles and Class AB is somewhere in between. The whole idea of single-ended circuits is to set the active element in the middle of its linear range, basically right at its 50% current point at idle. Then the input will swing current above and below that, with maximum output being near the point where the device is completely saturated on the positive side and completely cutoff on the negative. If the idle current is too far off 50% either way, you'll hit one of the limits considerably before the other one, wasting some of the dynamic range potential.
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Re: Fixed Bias v. Cathode Bias [message #9070 is a reply to message #9066] |
Sun, 17 July 2005 16:08 ![Go to previous message Go to previous message](/forum/theme/AudioRoundTable/images/up.png) ![Go to next message Go to next message](/forum/theme/AudioRoundTable/images/down.png) |
Forty2wo
Messages: 163 Registered: May 2009
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Master |
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OK, first, off how are you running your el84’s. As pentodes with a screen supply, ultralinear, or triode connected. The output transformer (+speaker) is the load and for modeling, is the plate resistor and they are fixed. So, the Z-out is determined by the tubes dynamic plate resistance. If you add a unbypassed cathode resistor the DPR increases. For a triode it is Rk(u+1) "cathode resister x mue(amp factor) + 1. This is the effect of degenerative feedback.For a pentode, I don’t know ask someone else.;) Now if you bypass the cathode resistor with a big cap. From a AC (impedance) point of view it’s not there. So you are more or less back were you started, from a Z-out point of view. So is any of this to worry about. No. there is not much you can change, so go with what you like and don’t worry…John
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