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PostPosted: February 20th, 2019, 3:01 am 
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Cogito wrote:
dberning wrote:
Traditional self bias requires the amp to draw more or less constant power regardless of the signal level. This implies class A operation. If you try to violate this by driving the tubes harder, the bias voltage will be unstable. Fixed bias allows Class A-B or B, and much greater power output for the same operating conditions for the tubes.

David


David, Can you explain the operational details of Fixed and Cathode bias? My question is specifically centered around the bias topology and how it effects the operational characteristics of the tubes.

For example, the grid voltage can be set to -5V and Cathode 0V which gives -5V grid bias
or
the grid voltage can be held at 0V and the cathode at +5V which again gives -5V grid bias voltage.

What exactly happens in the tube which makes the first one operate in AB mode and later in A mode?

Thx.


Whether the grid is kept at -5V or the cathode at +5V makes no difference. The difference comes in how that voltage is achieved. In fixed bias, the difference between the cathode & grid is fixed (at DC), hence the name. It makes no difference how hard the stage is driven, the DC difference between the grid and cathode remains constant. In a self-biased circuit, the bias is achieved via a voltage drop in a (usually bypassed) cathode resistor. During class A operation, the average current through the cathode resistor remains constant, so the DC bias remains constant. When a PP stage is pushed into class AB, the average net current in both tubes increases since the tube that's drive with an increasingly positive grid voltage continues to increase it's current flow, but the tube that is cutoff by the negative grid voltage can't have the current go less than zero. When this happens, the bias voltage is increased.

Roscoe


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PostPosted: February 20th, 2019, 9:18 am 
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Roscoe Primrose wrote:


Whether the grid is kept at -5V or the cathode at +5V makes no difference. The difference comes in how that voltage is achieved. In fixed bias, the difference between the cathode & grid is fixed (at DC), hence the name. It makes no difference how hard the stage is driven, the DC difference between the grid and cathode remains constant. In a self-biased circuit, the bias is achieved via a voltage drop in a (usually bypassed) cathode resistor. During class A operation, the average current through the cathode resistor remains constant, so the DC bias remains constant. When a PP stage is pushed into class AB, the average net current in both tubes increases since the tube that's drive with an increasingly positive grid voltage continues to increase it's current flow, but the tube that is cutoff by the negative grid voltage can't have the current go less than zero. When this happens, the bias voltage is increased.

Roscoe


The fixed bias amplifier, with capacitor coupling to the control grid is more prone to coupling capacitor blocking than the cathode bias (self-bias) amp. This occurs when the grid becomes positive with respect to the cathode and the grid draws current. When this occurs, the cap discharges and the amp distorts badly until the grid/cathode voltage drops to a negative value again. The cap then recharges, but the distortion is present for up to seconds. A single large transient can cause this. It happens all the time when listening to music.

In a self-biased amp, as Roscoe pointed out, once in the Class B portion of the operating range, the cathode voltage rises along with the grid voltage, making blocking practically a non issue.

Many a reviewer has marveled that a cathode biased amp sounds more powerful than a fixed bias amp which actually produces more MEASURED power. Blocking distortion is the reason why. We've now heard several fixed bias amps without output tube cap coupling and they do sound like they are way more powerful than the numbers indicate.

On the flip side, a self-biased amp is more prone to crossover notch distortion, at least in the amps I've had experience with.

If one builds a fixed bias amp, avoid cap coupling to the grid. Use a follower or transformer coupling.

Stuart


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PostPosted: February 25th, 2019, 10:22 am 
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The fixed bias amplifier, with capacitor coupling to the control grid is more prone to coupling capacitor blocking than the cathode bias (self-bias) amp. This occurs when the grid becomes positive with respect to the cathode and the grid draws current. When this occurs, the cap discharges and the amp distorts badly until the grid/cathode voltage drops to a negative value again. The cap then recharges, but the distortion is present for up to seconds. A single large transient can cause this. It happens all the time when listening to music.

In a self-biased amp, as Roscoe pointed out, once in the Class B portion of the operating range, the cathode voltage rises along with the grid voltage, making blocking practically a non issue.

Many a reviewer has marveled that a cathode biased amp sounds more powerful than a fixed bias amp which actually produces more MEASURED power. Blocking distortion is the reason why. We've now heard several fixed bias amps without output tube cap coupling and they do sound like they are way more powerful than the numbers indicate.

On the flip side, a self-biased amp is more prone to crossover notch distortion, at least in the amps I've had experience with.

If one builds a fixed bias amp, avoid cap coupling to the grid. Use a follower or transformer coupling.

Stuart


To add additional detail here, when the tube is driven sufficiently hard the grid draws current (acting like a rectifier) and this forces the average dc grid voltage more negative than it should be and you get severe crossover distortion that, as Stuart says, can last for a second depending on the time constant of the coupling capacitor and the resistor connected to the bias source. As Stuart says, direct coupling from the driver stage eliminates this. Transformer coupling may not eliminate it depending how dc grid bias is established. If the configuration includes a high-impedance bias source supplying the center tap of the coupling transformer, for example, the problem will still be there. But if that bias source is itself bypassed with a very large capacitor, musical transients would not be sufficient to upset the bias. Prolonged overdriving would.

David


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PostPosted: February 27th, 2019, 1:04 pm 
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Sorry I got away from this thread since I started it. The history here is I had these two Antique Sound Labs Hurricane monoblocks rebuilds on the back burner way too long. I practically gutted them and replaced many of the passive components. I got one working and had problems with the other. I got busy with other things I deemed more important so on the shelf they went. So now I decided to revisit them and finish the project. The biasing circuit is very troublesome. I want to get rid of the circuit board and simplify. things.
I thought about self bias but after seeing your responses I decided not to go with self bias. I like the way this amp sounds and do not want to do much to change its character.
The biasing circuit seems much to complex. I have other KT88 PP amps that just use a pot to adjust each tubes bias. Walt says the transistor circuit is used to isolate the tubes. I do not really understand this. Walt also said I could use a diode to do the same thing. If someone understands this can you show me what to do.


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PostPosted: February 27th, 2019, 3:53 pm 
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Pelliott321 wrote:
Sorry I got away from this thread since I started it. The history here is I had these two Antique Sound Labs Hurricane monoblocks rebuilds on the back burner way too long. I practically gutted them and replaced many of the passive components. I got one working and had problems with the other. I got busy with other things I deemed more important so on the shelf they went. So now I decided to revisit them and finish the project. The biasing circuit is very troublesome. I want to get rid of the circuit board and simplify. things.
I thought about self bias but after seeing your responses I decided not to go with self bias. I like the way this amp sounds and do not want to do much to change its character.
The biasing circuit seems much to complex. I have other KT88 PP amps that just use a pot to adjust each tubes bias. Walt says the transistor circuit is used to isolate the tubes. I do not really understand this. Walt also said I could use a diode to do the same thing. If someone understands this can you show me what to do.


I'll jump in. The circuit for each tube provides the D.C. potential to each tube's grid by means of an emitter follower with the D.C. potential applied to the base of the transistor with the isolated output fed direct to each individual tube grid. The idea was that biasing one tube would not influence or impact in some way a neighboring tube. I get that.

Other multi-tube amplifiers that I've looked at simply apply the bias voltage to a pair of push-pull tubes through the summing point of a pair of 220K-ohm resistors -- one 220K-ohm resistor to a grid. Tubes and the resistors in matched pairs are especially required here (especially the resistors should be matched to at least +/- .5%). Although I would used matched pairs regardless. Repeat for the other pairs. Or -- to maintain the individual isolation one can use diodes in lieu of the transistors in a more simplistic manner.

Since the amplifier power stage is tied to the board through Molex connectors and you have the feature of switching the meter to display each tube's cathode current I would -- if I were you -- stick to the designer's scheme. If the transistors do not have a currently available equivalent that can work then I see no reason why diodes could not accomplish the same thing. Biasing each tube separately is desired because I found that even matched pairs, as established with a tube tester, will require a different setting to get the same cathode current. Why? Manufacturing tolerances. The bias pots, the resistors in the circuit, and the transistors are not (NOT!) exact equals. The individual settings to get the same cathode current through each tube compensates for those tolerances.

And finally -- just because other manufacturers design other approaches to get to the same end does not make them RIGHT -- it only makes them different.

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PostPosted: February 27th, 2019, 5:54 pm 
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As a method for making fine bias adjustments, has anyone ever added a trimming pot to the center of those two 220k resistors? That might even get you out of needing a matched pair of output tubes.

Mark


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PostPosted: February 27th, 2019, 8:07 pm 
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natandmark2 wrote:
As a method for making fine bias adjustments, has anyone ever added a trimming pot to the center of those two 220k resistors? That might even get you out of needing a matched pair of output tubes.

Mark


Not so much a trimming pot as some amplifiers have balance pots for bias. Some even have AC balance pots between the phase split-er and the power tubes.

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