shielding with plate voltage
Moderators: pompeiisneaks, Colossal
Re: shielding with plate voltage
the trick is very very old, it is there for a reason, and reason is? lets hear ..
Re: shielding with plate voltage
Long distance wiring, think transatlantic cable. Did I win?
Tube junkie that aspires to become a tri-state bidirectional buss driver.
Re: shielding with plate voltage
The essence of design and its nerd-brother engineering is control.
Even back when all humans had was "magic", there was the concept of apprenticeship to learn the hidden knowledge and books of spells to tell the magicians how to do [whatever]. I kind of fall this way - learn how stuff works to be able to apply it the way I want it.
That's what's so very interesting about this concept. I can't see what the concept is doing. I can see that it might, maybe, be doing something, depending on where the shield is connected - to the plate of a tube stage, or to its local plate voltage supply, or to a B+ somewhere else. But it seems like there are variants of the trick, both to plate voltage supply and to the plate.
I know of a somewhat similar technique in high impedance PCB work, that being guard shields and rings. That involves driving a conductor surrounding a high impedance trace or pin with a buffered voltage of what's on the trace or pin. That way, stray capacitance to the high impedance conductor doesn't matter, as the low impedance guard/shield has the same(ish) voltage and no current transfers, and the guard ring eats any stray leakages from outside the ring. But that involves driving the shield with the same voltage as the high impedance conductor.
Shielding a grid wire with shielded cable adds a certain capacitance to the shield itself; good quality shielded cables have their capacitance per unit length specified. RF coax, for instance, is tightly specified. K-mart RCA cables cannibalized late at night are less well specified, but still have some capacitance per length. So shielding involves adding a capacitor to ... somewhere.
Connect the shield to ground, and you have added a capacitor to ground. Add a resistor or ferrite bead on the far end and you have an RC or LC low pass filter. The capacitance from plate to grid is unchanged by this, so you still have the local feedback capacitance inside the tube that limits the high frequency response.
Connect the shield to the plate, and you have added the shield capacitance in parallel with the plate-to-grid capacitance, and that shield capacitance is multiplied by the voltage gain by the Miller effect. It reduces high frequency response even more; if this is what the shield-to-plate is intending, fine. I personally would use a real capacitor so I could set the value of feedback/Miller capacitance with some known tolerance instead of relying on the length of cable shield, but maybe it's a useful hack sometimes. Late at night, out of parts, sure...
Connect the shield to the local tube's plate supply voltage and you've added a capacitor to feed the grid a version of the voltage change on the plate voltage supply. Since the plate supply is supposed to be quiet and well filtered, this amounts to using a grounded shield to a somewhat noisy ground, since the plate supply inevitably wiggles around some with whatever is happening elsewhere in the amp, with current loop pickup, and with local tube current changes and so on. There's some noise/signal injection, especially as the electros in the dropping chain get older and more inductive/resistive and less effective at filtering. This could amount to certain peaks and valleys in frequency response in the amp that tone mavens would hear as more or less character. My concern there is that the response changes would change as the filtering caps aged. It would change the amp's response around in ways I (if I was the designer/builder) hadn't necessarily wanted it to.
Connect the shield to some other point in the plate supply chain and you've added capacitor feedback to some other place in the amp, and if this is an input stage, a larger, less well filtered point on the plate supply chain. Connect it to the first filter cap and you're feeding in the tens-of-volts of ripple voltage and any ripple caused by output tube current flow, just as a horror-house example.
This is the line of thinking that ran through my mind when I was asking where it was connected.
Even back when all humans had was "magic", there was the concept of apprenticeship to learn the hidden knowledge and books of spells to tell the magicians how to do [whatever]. I kind of fall this way - learn how stuff works to be able to apply it the way I want it.
That's what's so very interesting about this concept. I can't see what the concept is doing. I can see that it might, maybe, be doing something, depending on where the shield is connected - to the plate of a tube stage, or to its local plate voltage supply, or to a B+ somewhere else. But it seems like there are variants of the trick, both to plate voltage supply and to the plate.
I know of a somewhat similar technique in high impedance PCB work, that being guard shields and rings. That involves driving a conductor surrounding a high impedance trace or pin with a buffered voltage of what's on the trace or pin. That way, stray capacitance to the high impedance conductor doesn't matter, as the low impedance guard/shield has the same(ish) voltage and no current transfers, and the guard ring eats any stray leakages from outside the ring. But that involves driving the shield with the same voltage as the high impedance conductor.
Shielding a grid wire with shielded cable adds a certain capacitance to the shield itself; good quality shielded cables have their capacitance per unit length specified. RF coax, for instance, is tightly specified. K-mart RCA cables cannibalized late at night are less well specified, but still have some capacitance per length. So shielding involves adding a capacitor to ... somewhere.
Connect the shield to ground, and you have added a capacitor to ground. Add a resistor or ferrite bead on the far end and you have an RC or LC low pass filter. The capacitance from plate to grid is unchanged by this, so you still have the local feedback capacitance inside the tube that limits the high frequency response.
Connect the shield to the plate, and you have added the shield capacitance in parallel with the plate-to-grid capacitance, and that shield capacitance is multiplied by the voltage gain by the Miller effect. It reduces high frequency response even more; if this is what the shield-to-plate is intending, fine. I personally would use a real capacitor so I could set the value of feedback/Miller capacitance with some known tolerance instead of relying on the length of cable shield, but maybe it's a useful hack sometimes. Late at night, out of parts, sure...
Connect the shield to the local tube's plate supply voltage and you've added a capacitor to feed the grid a version of the voltage change on the plate voltage supply. Since the plate supply is supposed to be quiet and well filtered, this amounts to using a grounded shield to a somewhat noisy ground, since the plate supply inevitably wiggles around some with whatever is happening elsewhere in the amp, with current loop pickup, and with local tube current changes and so on. There's some noise/signal injection, especially as the electros in the dropping chain get older and more inductive/resistive and less effective at filtering. This could amount to certain peaks and valleys in frequency response in the amp that tone mavens would hear as more or less character. My concern there is that the response changes would change as the filtering caps aged. It would change the amp's response around in ways I (if I was the designer/builder) hadn't necessarily wanted it to.
Connect the shield to some other point in the plate supply chain and you've added capacitor feedback to some other place in the amp, and if this is an input stage, a larger, less well filtered point on the plate supply chain. Connect it to the first filter cap and you're feeding in the tens-of-volts of ripple voltage and any ripple caused by output tube current flow, just as a horror-house example.
This is the line of thinking that ran through my mind when I was asking where it was connected.
Re: shielding with plate voltage
in short it is to remove hiss from hi gain amp, actually it is obvius...now i'm expecting for this tip 100 usd /will be enough/ from every big manufacturer lurker here to my private account
Re: shielding with plate voltage
It could have been born from necessity, fathered by someone modifying an amp and wanting to run a shielded input wire straight to the tube socket and realizing that the shortest wire that could get the job done was just one pin over from the grid pin.
The extra capacitance being beneficial could have been pure luck.
Then the telephone game happened, and now here we are.
The extra capacitance being beneficial could have been pure luck.
Then the telephone game happened, and now here we are.