As an academic exercise, I wanted to investigate if it was possible to improve upon the bias supply of the Ampeg B15. In the new heritage release, the B15 comes equipped with a switch to toggle between cathode and fixed bias. After gleaning some information from searches, I've drawn a fixed bias supply for each power tube which can be switched out to a cathode bias topology.
The old B15 schematics show heater elevation at the respective bias points, so I thought it might be an improvement to the heritage to switch between these elevation points respective to the bias setting.
What I've drawn is a DPDT switch, one half controlling the bias and the other half controlling the heater elevation.
I would like to hear your feedback on this implementation. Thanks for your time.
The old B15 schematics show heater elevation at the respective bias points, so I thought it might be an improvement to the heritage to switch between these elevation points respective to the bias setting.
What I've drawn is a DPDT switch, one half controlling the bias and the other half controlling the heater elevation.
I would like to hear your feedback on this implementation. Thanks for your time.
Do you understand the point of heater elevation? In preamp tubes where the cathodes can be sitting at only a volt or two positive, those cathodes are more positive than the 6.3vAC heaters right next to them for part of each cycle. SOme electrons boil off the heater, and are thus attracted to the cathode. This adds a small but audible AC component to the cathode, which we hear as hum.
By elevating the heaters by some positive voltage, instead of grounding them, we make sure the entire AC cycle of the heater current is always more positive than those cathodes, and so none of any loose electrons from the heater will be drawn there, and that source of hum is prevented.
It doesn't matter so much just what the elevating voltage is, just that it be enough to stay more positive than the cathodes. Oh there is a limit to how high it can be, look up the data sheet for your 12AX7s or whatever you have and check the rating for maximum cathode to heater voltage. SO +100v might be too much elevation for the tube.
Power tube bias is the relation between cathode and grid. The grid wants to be more negative than the cathode to control current through the tube. But making the cathode more positive than the grid does the same thing. SO in cathode bias, they do just that. In an amp like this, that cathode voltage is relatively stable with its bypass cap there. SO it is easy to steal that voltage for this additional use. It draws no current. This elevation is really there for the low level preamp stages, not so much the power tubes.
The problem I see with your proposal is that it works in cathode bias, but for some reason you have connected your elevation line to a NEGATIVE supply when in fixed bias mode, in the bias supply. That is the exact opposite of the idea.
If you are feeding a preamp along with this power amp from that 6.3vAC winding or if you are just sold on elevating your power amp heaters, you need a positive voltage source for it. The cathode works fine for itself for preamps, but in fixed bias mode, you need a new supply. Now there is a diode coming from 55vAC to make the negative bias supply. So at the 55vAC add a second diode pointing the other way, add a filter et voila, you now have a positive supply suited to the task. And you wouldn't need to switch it, just leave it is the permanent source of your elevation.
If you are doing this for only the power amp we see, the elevation is not sufficient. Remember the point is to get the heaters more positive than the cathodes. If you are using the very cathode voltage we need to exceed, then you can't achieve that this way. A voltage cannot exceed itself. Frankly, I'd just ground that center of the heater string.
By elevating the heaters by some positive voltage, instead of grounding them, we make sure the entire AC cycle of the heater current is always more positive than those cathodes, and so none of any loose electrons from the heater will be drawn there, and that source of hum is prevented.
It doesn't matter so much just what the elevating voltage is, just that it be enough to stay more positive than the cathodes. Oh there is a limit to how high it can be, look up the data sheet for your 12AX7s or whatever you have and check the rating for maximum cathode to heater voltage. SO +100v might be too much elevation for the tube.
Power tube bias is the relation between cathode and grid. The grid wants to be more negative than the cathode to control current through the tube. But making the cathode more positive than the grid does the same thing. SO in cathode bias, they do just that. In an amp like this, that cathode voltage is relatively stable with its bypass cap there. SO it is easy to steal that voltage for this additional use. It draws no current. This elevation is really there for the low level preamp stages, not so much the power tubes.
The problem I see with your proposal is that it works in cathode bias, but for some reason you have connected your elevation line to a NEGATIVE supply when in fixed bias mode, in the bias supply. That is the exact opposite of the idea.
If you are feeding a preamp along with this power amp from that 6.3vAC winding or if you are just sold on elevating your power amp heaters, you need a positive voltage source for it. The cathode works fine for itself for preamps, but in fixed bias mode, you need a new supply. Now there is a diode coming from 55vAC to make the negative bias supply. So at the 55vAC add a second diode pointing the other way, add a filter et voila, you now have a positive supply suited to the task. And you wouldn't need to switch it, just leave it is the permanent source of your elevation.
If you are doing this for only the power amp we see, the elevation is not sufficient. Remember the point is to get the heaters more positive than the cathodes. If you are using the very cathode voltage we need to exceed, then you can't achieve that this way. A voltage cannot exceed itself. Frankly, I'd just ground that center of the heater string.
"Elevating" to the negative voltage is done by Ampeg in the fixed bias version of the B15.
I have also read that negative elevation is an option in Designing Tube Preamps for Guitar and Bass:
"In a fixed-bias amp another possibility is to use the bias supply as the elevation...using a negative elevation voltage is sometimes more effective at reducing hum than a positive one."
I have also read that negative elevation is an option in Designing Tube Preamps for Guitar and Bass:
"In a fixed-bias amp another possibility is to use the bias supply as the elevation...using a negative elevation voltage is sometimes more effective at reducing hum than a positive one."
OK, then go for it, you will have positive with one bias arrangement and negative with the other.
One problem with hum abatement is that hum comes from MANY sources, and is not generic. All the extra filter caps in the world will do nothing about hum from a lack of shielding, and all the shielding in the world will do nothing about hum from insufficient filtration. ANd you can have hum from several sources that add together. Sometimes hum sources are out of phase and cancel. Perhaps you have had the experience where an amp hums a little at zero volume, but as you turn up the volume the hum DECREASES until you hot maybe 3 on the dial, and then after that minimum it then increases after that. That is an example of cancellation. One source is present all the time, and another source out of phase with the first is coming through the control. The magical minimum point is where the hum from the one source is exactly balanced by the other, so they cancel.
That is how I see negative elevation working, it works when you already have a second source of hum present out of phase with the heater hum. SO making it negative actually increases the hum, but it cancels a different hum source.
But you will have the perfect opportunity to find out. Build your bias supplies, leave the amp in cathode bias mode, have the heater center tap line free, then clip it to the cathodes, then move it to the negative supply. Make no other changes but that one clip. What differences if any do you hear in hum? And my question remains, is this power amp the whole project at this point or is there a preamp too powered off this heater bus?
One problem with hum abatement is that hum comes from MANY sources, and is not generic. All the extra filter caps in the world will do nothing about hum from a lack of shielding, and all the shielding in the world will do nothing about hum from insufficient filtration. ANd you can have hum from several sources that add together. Sometimes hum sources are out of phase and cancel. Perhaps you have had the experience where an amp hums a little at zero volume, but as you turn up the volume the hum DECREASES until you hot maybe 3 on the dial, and then after that minimum it then increases after that. That is an example of cancellation. One source is present all the time, and another source out of phase with the first is coming through the control. The magical minimum point is where the hum from the one source is exactly balanced by the other, so they cancel.
That is how I see negative elevation working, it works when you already have a second source of hum present out of phase with the heater hum. SO making it negative actually increases the hum, but it cancels a different hum source.
But you will have the perfect opportunity to find out. Build your bias supplies, leave the amp in cathode bias mode, have the heater center tap line free, then clip it to the cathodes, then move it to the negative supply. Make no other changes but that one clip. What differences if any do you hear in hum? And my question remains, is this power amp the whole project at this point or is there a preamp too powered off this heater bus?
"Elevating" to the negative voltage is done by Ampeg in the fixed bias version of the B15.
I have also read that negative elevation is an option in Designing Tube Preamps for Guitar and Bass:
"In a fixed-bias amp another possibility is to use the bias supply as the elevation...using a negative elevation voltage is sometimes more effective at reducing hum than a positive one."
Yeah, but why? It doesn't make sense to me. Elevating into the positive is good for not exceeding Vhk and to make use of the 'diode' characteristic between heater and cathode to eliminate hum...
But please educate me, because I am curious why Ampeg did this
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