Hey Everyone,
I had an idea and would like to know what you all might think? In a fixed bias amp you have the problem of the 'wiper' or some other component failing resulting in zero bias and setting free the smoke. In a cathode bias amp you have the problem of lost voltage across the cathode resistor and higher output impedance.
What if I were to use a cathode resistor small enough that on its own would result in maximum safe current flow. Then using a battery provide extra adjustable negative bias voltage. Or for more voltage a pair of batteries in series.
The point would be a safer, simpler bias scheme that combines both fixed and cathode bias.
I had an idea and would like to know what you all might think? In a fixed bias amp you have the problem of the 'wiper' or some other component failing resulting in zero bias and setting free the smoke. In a cathode bias amp you have the problem of lost voltage across the cathode resistor and higher output impedance.
What if I were to use a cathode resistor small enough that on its own would result in maximum safe current flow. Then using a battery provide extra adjustable negative bias voltage. Or for more voltage a pair of batteries in series.
The point would be a safer, simpler bias scheme that combines both fixed and cathode bias.
In a amplifier, the bias supply is about the simplest part. A rectifier and filter. Then a voltage divider to adjust what the tubes get. If you are worried your wiper might fail, then also wouldn't you worry that the battery would fail? I have replaced more "keep alive" batteries in my career than defective bias adjust pots. A battery either needs to have a charging circuit, or you need to replace them regularly.
Yes, you could make an amp using both cathode bias and fixed bias, but if you want fixed bias, I see no reason to add the cathode resistor.
It would be a poorly designed circuit that takes the bias to the tubes through a wiper. Instead we wire the adjust pot as a variable resistor (wiper tied to one end), and in combination with another fixed resistor form an adjustable voltage divider. That way a failed pot wiper results in maximum bias voltage instead of loss of voltage. No battery required.
Yes, you could make an amp using both cathode bias and fixed bias, but if you want fixed bias, I see no reason to add the cathode resistor.
It would be a poorly designed circuit that takes the bias to the tubes through a wiper. Instead we wire the adjust pot as a variable resistor (wiper tied to one end), and in combination with another fixed resistor form an adjustable voltage divider. That way a failed pot wiper results in maximum bias voltage instead of loss of voltage. No battery required.
In the design stage you provide enough B+ to the plate to get the power you want, recognizing the cathode bias voltage as part of the equation.
Do you ...really... want a battery in your amp?
Yes it would work (as long as the battery works). Hybrid bias schemes are occasionally spotted in amp designs, although here you seem to be creating a solution in need of a problem. A pull-down resistor protects against a failed wiper perfectly well. A mains bias supply will track any changes in mains voltage and B+, which is a useful bonus that batteries don't have. Fast fuses or polyfuses in the cathode circuit can further protect the tubes.
Thanks everyone,
I can build a bias supply when there is a bias tap and when there isn't. I've used small second transformers to build bias supplies.
There are two situations where this scheme might prove useful. When using a vintage set of transformers. As an example I'm currently building an amp using Harmon Kardon A500 iron. The power transformer doesn't have a bias tap. And even worse the amp used a voltage doubler so the B+ is a bit low to begin with. Not wanting to use the rare 7355 tube I switched to 6L6GC's because I had them. The amp has 270 ohm cathode resistors and each tube draws 74ma of current with 303 volts B+. The design called for 40ma of current and 380 volts of B+. The amp is running in triode in part to keep the power output around thirty watts. With the amp built there isn't room for another transformer and power supply. I want to lower the current and raise the B+ so using a larger cathode resistor would lower the current but cost B+ across the resistor. Hence the idea.
The second situation is when the end user is not electrically capable to set the bias in the traditional manner. With two pots on the back I usually use three flush mount banana jacks for a two tube amp. A black one in the center and a red one on either side. The current sense 'cathode' resistors are wired from each red jack to the black which is grounded. But this requires a DVOM and the ability to use it. I do this because I don't want the user to adjust the bias with the amp open and the power on.
With a 9 volt battery attached to the bottom of the amp chassis on the outside anyone can periodically change the battery without having to open the amp. So the only maintenance necessary for an inexperienced end user is to yearly change the battery and periodically buy a new matched set of tubes.
Am I making more sense now?
I can build a bias supply when there is a bias tap and when there isn't. I've used small second transformers to build bias supplies.
There are two situations where this scheme might prove useful. When using a vintage set of transformers. As an example I'm currently building an amp using Harmon Kardon A500 iron. The power transformer doesn't have a bias tap. And even worse the amp used a voltage doubler so the B+ is a bit low to begin with. Not wanting to use the rare 7355 tube I switched to 6L6GC's because I had them. The amp has 270 ohm cathode resistors and each tube draws 74ma of current with 303 volts B+. The design called for 40ma of current and 380 volts of B+. The amp is running in triode in part to keep the power output around thirty watts. With the amp built there isn't room for another transformer and power supply. I want to lower the current and raise the B+ so using a larger cathode resistor would lower the current but cost B+ across the resistor. Hence the idea.
The second situation is when the end user is not electrically capable to set the bias in the traditional manner. With two pots on the back I usually use three flush mount banana jacks for a two tube amp. A black one in the center and a red one on either side. The current sense 'cathode' resistors are wired from each red jack to the black which is grounded. But this requires a DVOM and the ability to use it. I do this because I don't want the user to adjust the bias with the amp open and the power on.
With a 9 volt battery attached to the bottom of the amp chassis on the outside anyone can periodically change the battery without having to open the amp. So the only maintenance necessary for an inexperienced end user is to yearly change the battery and periodically buy a new matched set of tubes.
Am I making more sense now?
Tadiran makes batteries with ~25 year lifetime. As long as there's no significant load, just solder that sucker into the circuit and call it done.
Nope. In the first case you are trying to force a PS tranny not meant for the job to be used with a hack in the circuit. The solution there is to get the right tranny or rethink the original project down to the right power level. Second case, you are again building a failure mode into the amp for "someone who isn't inclined.... or is an inexperienced end user....?" by using a battery to supply the most important voltage in the amp. If the battery fails the the tubes go into meltdown. Cathode biased amps don't need adjustment, just good tubes.
You don't need another transformer; you can always build a bias supply using the main high-voltage winding. It's particularly easy if it has a centre tap, but if it doesn't then you can always use a cap-coupled bias supply.
I can understand why you might use hybrid bias in this case, but why resort to a battery? A cathode resistor and a fixed voltage supply gives you same result, with no need to replace anything, ever!
Not really. That's always been the big dilemma when doing designs. Fixed bias is superior in almost every way to cathode bias, but for one reservation: it's not "set it and forget it", and requires the end user to take some responsibility for managing his equipment. The most user friendly thing to do is include a meter on the front panel, with an on-off-on switch for each final, and then ther end user need not bother with anything beyond twiddling a couple of knobs to make certain the current balance remains.
Some end users aren't capable of even that: either because they're that atechnological, or they get just plain lazy. They expect to just turn it on and have it play, and they couldn't care less even if you paid them about the internals. Then something either poofs, or the sonic performance deteriorates, and guess who gets all the blame for that. If they can't manage that, then don't expect them to remember to replace a bias battery, because they won't. Your average, atechnological "idgit" doesn't know what "bias" is, what it does, where it comes from, or why it's important. Go that route, and poofed finals (
) are guaranteed.Hell, they can't even manage to replace the CMOS back-up batts for their "Pooter Muh-sheens"
Friends, neighbors, relatives are always calling on me for that stupid simple "fix" and think I'm the next best thing since Turing for saving them $$$$ the computer fixit shop would have charged for two minutes' worth of work.
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I solid state the negative rail in my projects. that way, the bias comes up first, and drops last. I've also seen designs where they use a small signal hollow state diode like the 6AL5 or 6H6 to derive bias voltages. I wouldn't expect to see any sort of catastrophic failures there since the current demand is do low.
Since these are fixed bias designs, you need cathode current sense resistors anyway, so make these small. A 0.25W/10R resistor maxes out at 160mA, and so acts like a fuse of last resort if there's a problem that leads to red plate currents before anything gets ruined.
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