Hey guys, I have been doing quite a bit of reading over the past year regarding SE design, tube biasing, etc but I'm having trouble wrapping my head around the concept of plate loading with a choke/OPT.
From what I understand, a resistive plate load simply limits the current to the tube via resistance. Since a typical resistor is purely resistance and has no reactive component, it's impedance is always constant.
However, when it comes to an inductive/reactive load(OPT/choke), it's impedance varies with the frequency; so how is it expected to maintain a certain impedance in the absence of a signal/when the amp is idling? Aside from the DCR, it's really no more than a wire (at idle); so how does this keep the plate loaded?
Since we are on the topic of plate loading, I thought I'd also ask about CCS loads. For the purposes of discussion, say a CCS allows 10mA on the plate of a tube; how does this allow for current to swing when a signal is applied?
Sorry in advance if these questions sound noobish.
From what I understand, a resistive plate load simply limits the current to the tube via resistance. Since a typical resistor is purely resistance and has no reactive component, it's impedance is always constant.
However, when it comes to an inductive/reactive load(OPT/choke), it's impedance varies with the frequency; so how is it expected to maintain a certain impedance in the absence of a signal/when the amp is idling? Aside from the DCR, it's really no more than a wire (at idle); so how does this keep the plate loaded?
Since we are on the topic of plate loading, I thought I'd also ask about CCS loads. For the purposes of discussion, say a CCS allows 10mA on the plate of a tube; how does this allow for current to swing when a signal is applied?
Sorry in advance if these questions sound noobish.
Think of the inherent impedance due to reactance as being a ceiling. The reflected impedance then sets the actual impedance. So, imagine you have a transformer with 40H of inductance and a 25:1 winding ratio. This would translate to a 625:1 impedance ratio which is 5K:8 and 5K of impedance at 20Hz (impedance = 2*pi*Frequency*Inductance). If you load it with an 8 ohm load, at less than 20Hz the reactance is lower than the reflected impedance, so you won't be able to actually reflect 5K which means you'll lose low bass. But, at 20Hz and above, there is enough inductance that you can reflect to 5K, so the tube sees a 5K load. In the case of a plate loading choke, it is only the inductance that matters, so above a certain frequency, the load is very high.
The CCS sets the DC operating point. As signal raises and lowers the grid, the tube wants to draw more or less current. This adjustment is made through the load (the next stage, or the parafeed transformer.)
This might be helpful: http://www.ecpaudio.com/pdf/parafeed_basics.pdf
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The CCS impedance doesn't change. The internal resistance of the device connected to the CCS is doing the changing. Think of the tube (triode usually) as a voltage controlled resistor (or more accurately transconductance) with a constant current through it. Change the resistance and the voltage dropped across it changes.
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It's true that the impedance of a choke varies with frequency, but chokes are rarely employed as loads in broadband amplifiers, they are more likely to be found in a narrowband, RF, amplifier.
At idle (DC) the current in the active device is controlled by the biassing, not the load. The load impedance is of significance only at AC (when there is a signal), the resistive component of a loudspeaker driver is multiplied back to the plate through the transformer turns ratio.
As regards the CCS, this is easiest thought of in terms of the limiting conditions. When the valve is saturated (fully ON) all the current flows from the CCS through the valve. When the valve is cut off (fully OFF) all the current flows from the CCS into the load (which is connected at the junction of the CCS and the plate).
w
It's important to realise that there's a certain amount of operator overloading taking place. Sometimes load means the device that gets the power in the end, and sometimes it means whatever arrangement of components is in the anode circuit.
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No. The DC resistance stays as it is for 0Hz 'signals'. The inductive reactance is set by frequency, not signal amplitude. A 10H choke has an impedance of 6283ohms at 100Hz, whether or not there is actually any 100Hz signal present.
A choke and OPT are not really the same thing, although there are some similarities. If you think they are the same you will get confused. If you think they are completely different you will also get confused!
I think your problem is that you associate impedance with signal. Impedance is unaffected by signal. It tells you what would happen if a signal is present, but with the signal gone the impedance is still there.
Thanks for the replies!
The plate choke idea is for parafeed, I am still exploring my options for a SE amp.
I have to admit, I'm still a bit confused; maybe I'm looking at it from the wrong way... I tend to think of biasing in terms of a load line. You choose a plate load, apply a negative voltage to the grid(that corresponds to one of the tube curves) and that should set the idle current. If at idle, the DCR of an OPT was the load, would this not cause the current to shoot up?
The plate choke idea is for parafeed, I am still exploring my options for a SE amp.
I have to admit, I'm still a bit confused; maybe I'm looking at it from the wrong way... I tend to think of biasing in terms of a load line. You choose a plate load, apply a negative voltage to the grid(that corresponds to one of the tube curves) and that should set the idle current. If at idle, the DCR of an OPT was the load, would this not cause the current to shoot up?
with a resistor load, AC and DC load lines are the same. With a choke load, the load line is the choke in parallel with the actual load but this does not correspond to DC conditions which are set by B+ and bias. So yes, if you would use say a 300V supply with a resistor load to get 150V on the plate, then with a choke load you are going to want a ~150V supply. This is explained in the link I posted above.
No. That is the procedure for designing a resistive loaded voltage amplifier. In that case an upper limit to the current is imposed by the anode resistor. The procedure for designing a power amplifier is different. The biassing sets the quiescent current in the case of an anode-transformer power stage (as it actually does in the voltage amplifier, when an operating point is chosen)
w
An ideal current source has infinite output impedance. Hence, the output current does not change as the voltage across the current source changes. So if used as a plate load for a tube, the current through the tube (and CCS) will remain constant as the anode voltage swings with the signal.
In reality, a CCS will have a finite output impedance (usually 100 kOhm or above for a good semiconductor-based CCS). A real CCS can be modelled as an ideal CCS with a resistor equal to the output impedance in parallel. The output impedance will cause the current of the CCS to vary slightly with the voltage across the CCS. For most circuit design, the output impedance of the CCS is neglected and the current assumed to be constant.
~Tom
To bias a choke- or opt-loaded stage, you can draw a load line using the DCR. You'll see that it is almost indistinguishable from a vertical line through your B+, so you might as well draw that, too. Then, you can freely choose the quiescent current to set the operating point, and from that you can derive the necessary grid bias voltage.
At dc it doesn't, it acts, as you say, just like its DCR. The dc load line and the ac load line are therefore very different. The dc load line is nearly vertical (effectively constant voltage). The ac load line is effectively the secondary load reflected into the primary plus the transformer leakage inductances.
Cheers
Ian
No it just means the dc load line is nearly vertical. You don't use the (ac) load to set dc conditions in a choke/tranny amp any more than you do in one with a plate resistor. It's just a very small dc resistance is all. Very large resistances make nearly horizontal dc load lines and very small ones (like the few ohms of a tranny) make near vertical ones. So just draw a vertical line at your plate supply, choose an idle current and read off the required bias. Its just the same as always.
Cheers
Ian
It is called horizontal. Vertical is when load is shorted, like for lower tube in cascode.
Ah, sorry, my bad: I thought the question was about AC with no load.
Of course, for chokes and gyrators on DC it will be vertical!
Excuse, please, again for confusion.
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