Tyimo said:
Hi,
I use fet 2SK30 with IDSS current (gate is connected to source) witch is ca 2mA. In posted schematic current is set to some higher value, IMHO and can be measure indirectly by measuring voltage on known value of R3 (ohm law). It is possible to lower this current with trimmer in FET source for some degree.
Regards
ECC88 cascode
I'm not familiar with tubes/ECC88.
What is the bias current for ECC88 to get best audio result?
Transistor can be cascoded, like the example below. Mr. Borbely even cascode his fets down as 2V.
If I run the cct at about 3mA, how low can ECC88 be cascoded (Vplate-cathode) and still working properly?
I'm not familiar with tubes/ECC88.
What is the bias current for ECC88 to get best audio result?
Transistor can be cascoded, like the example below. Mr. Borbely even cascode his fets down as 2V.
If I run the cct at about 3mA, how low can ECC88 be cascoded (Vplate-cathode) and still working properly?
Hi, Fdegroove,
Waw, thats big. I first tought only about 2-3mA.
Yes, the the tube will be a follower only (like pic in the first message), but the 10k resistor will be replaced with CCS, and plate will be cascoded with transistor. In transistor I hear better performance with cascode. Is it applicable also in tubes?
Waw, thats big. I first tought only about 2-3mA.
Yes, the the tube will be a follower only (like pic in the first message), but the 10k resistor will be replaced with CCS, and plate will be cascoded with transistor. In transistor I hear better performance with cascode. Is it applicable also in tubes?
Hi,
Sure.
However, if I understand you correctly you intend to replace the 10K cathode resistor from post # 1, the IGC with a tube buffered input, with a constant current source as in Moamps circuit a bit higher up.
So far so good.
However, I don't understand what you want to do with a fet on top of the triode.
Won't you end up with a stack of CCSs in the tail of the transistor?
Instead of doing something like that, if it would work at all, wouldn't it be better to improve the working conditions of the CCS in the tail of the triode by adding a regulator_ provided you have spare voltage to drop_in front of it?
Making a cascode out of a triode and a transistor could probably be done but I just don't see what you could possibly do with it here.
The buffer circuit is not supposed to have any gain, it's there to drive the chip with current, not voltage.
Maybe I just misunderstood,
Sure.
However, if I understand you correctly you intend to replace the 10K cathode resistor from post # 1, the IGC with a tube buffered input, with a constant current source as in Moamps circuit a bit higher up.
So far so good.
However, I don't understand what you want to do with a fet on top of the triode.
Won't you end up with a stack of CCSs in the tail of the transistor?
Instead of doing something like that, if it would work at all, wouldn't it be better to improve the working conditions of the CCS in the tail of the triode by adding a regulator_ provided you have spare voltage to drop_in front of it?
Making a cascode out of a triode and a transistor could probably be done but I just don't see what you could possibly do with it here.
The buffer circuit is not supposed to have any gain, it's there to drive the chip with current, not voltage.
Maybe I just misunderstood,
Hi, Fdegrove,
I will explain why I want to put cascode in tubes.
I learn in transistor, that distortions comes from fluctuating current and fluctuating voltage. The remedy for fluctuating current is to make bias current as big as possible, so the signal current is small compared to bias current. This will make the current induced distortion smaller.
It is the same as fluctuation voltage problem. They way to eliminate distortion from fluctuating voltage is to fix the voltage of the transistor to a certain point, with cascoding. The picture of cascode that I submit is not right, the 100N and 3V9 zener should be tied to emitors of BC560, instead to rails. This will reduce the fluctuating-voltage distortions, since the BC560 will always works in (3V9-0,6V=3V3). So no matter what signal there is, the BC560 will always have steady 3V3 Vce voltage. I tried this and hear the result more detailed, so I want to put steady Vplate-cathode in tubes, maybe they will result the same as transistor.
Also, in transistor, by putting low Vce by cascoding, we can give bigger bias current, since the dissipation will be lower due to low Vce.
My problem is how low can this ECC88 be cascoded? I'm afraid if too low it will ruin the performance. Will ECC88 cascoded to 10V plate-cathode with 10mA bias?
I will explain why I want to put cascode in tubes.
I learn in transistor, that distortions comes from fluctuating current and fluctuating voltage. The remedy for fluctuating current is to make bias current as big as possible, so the signal current is small compared to bias current. This will make the current induced distortion smaller.
It is the same as fluctuation voltage problem. They way to eliminate distortion from fluctuating voltage is to fix the voltage of the transistor to a certain point, with cascoding. The picture of cascode that I submit is not right, the 100N and 3V9 zener should be tied to emitors of BC560, instead to rails. This will reduce the fluctuating-voltage distortions, since the BC560 will always works in (3V9-0,6V=3V3). So no matter what signal there is, the BC560 will always have steady 3V3 Vce voltage. I tried this and hear the result more detailed, so I want to put steady Vplate-cathode in tubes, maybe they will result the same as transistor.
Also, in transistor, by putting low Vce by cascoding, we can give bigger bias current, since the dissipation will be lower due to low Vce.
My problem is how low can this ECC88 be cascoded? I'm afraid if too low it will ruin the performance. Will ECC88 cascoded to 10V plate-cathode with 10mA bias?
Hi,
I understand what you want to do but if you look at the circuit Moamps posted you'll see that the tube is current regulated already.
The reason it sits at the cathode end of the tube is that if you'd put it on the plate side it would replace a plate load resistor.
In a cathode follower you can't use one or it wouldn't be a cathode follower anymore but an anode follower.
By doing that the circuit would perform in a totally different way, it would amplify the voltage at the input and it would be taken from the plate (anode), something we don't want here as we strive to have a low output impedance for the buffer circuit.
If you look at Moamps circuit you will see a MosFet configured as a constant current source set by varying VR 2.
The picture is in post # 16.
Imagine it would operate form a regulated -24 VDC supply and it would be as close as you could come to an infinite load impedance supply for the tube.
This is exactly what you want to achieve with what you describe, only you should look at the circuit sort of upside down if you like as there are no P-channel tubes.
The cathode follower shown here is a little unusual as it uses a bipolar supply of +/- 24 V.
To the tube and the AC current flow in the circuit it is as if the supply is a 48 V B+, it would work exactly the same way, the only reason that it is used here is that the chip is using a low voltage (compared to what is common for a tube circuit) taken from a bi-polar supply already.
So, no, from the above that won't work as the cascode as you call it is already in place as per Moamps design.
Cheers,
I understand what you want to do but if you look at the circuit Moamps posted you'll see that the tube is current regulated already.
The reason it sits at the cathode end of the tube is that if you'd put it on the plate side it would replace a plate load resistor.
In a cathode follower you can't use one or it wouldn't be a cathode follower anymore but an anode follower.
By doing that the circuit would perform in a totally different way, it would amplify the voltage at the input and it would be taken from the plate (anode), something we don't want here as we strive to have a low output impedance for the buffer circuit.
If you look at Moamps circuit you will see a MosFet configured as a constant current source set by varying VR 2.
The picture is in post # 16.
Imagine it would operate form a regulated -24 VDC supply and it would be as close as you could come to an infinite load impedance supply for the tube.
This is exactly what you want to achieve with what you describe, only you should look at the circuit sort of upside down if you like as there are no P-channel tubes.
The cathode follower shown here is a little unusual as it uses a bipolar supply of +/- 24 V.
To the tube and the AC current flow in the circuit it is as if the supply is a 48 V B+, it would work exactly the same way, the only reason that it is used here is that the chip is using a low voltage (compared to what is common for a tube circuit) taken from a bi-polar supply already.
So, no, from the above that won't work as the cascode as you call it is already in place as per Moamps design.
Cheers,
Hi, Fdegrove,
Thanks for the explenation. You right, it has no sense to cascode the plate if the tube is cathode follower. My mistake.
But your explenation gives me another question.
What makes a tube do not function properly in low voltages? Is it the voltage or the current that is not sufficient?
If it is the current, maybe tubes can work in low voltages, with big current forced. (Again, this is another maybe).
Take example of post#16 cct. It is cathode follower with +/-24VDC rails and CCS in the bottom.
What happens if this circuit is operated in +/-15V, but the CCS is forced tobe 10mA? Will this CCT still works?
Another question. If the current through the tube is quite big (like 10mA compared to 1-2mA) will the tube have shorter life?
Thanks for the explenation. You right, it has no sense to cascode the plate if the tube is cathode follower. My mistake.
But your explenation gives me another question.
What makes a tube do not function properly in low voltages? Is it the voltage or the current that is not sufficient?
If it is the current, maybe tubes can work in low voltages, with big current forced. (Again, this is another maybe).
Take example of post#16 cct. It is cathode follower with +/-24VDC rails and CCS in the bottom.
What happens if this circuit is operated in +/-15V, but the CCS is forced tobe 10mA? Will this CCT still works?
Another question. If the current through the tube is quite big (like 10mA compared to 1-2mA) will the tube have shorter life?
Hi,
It's just that most tubes were not designed with low operating voltages in mind, you'll go outside their linear operating point which will increase distortion levels.
The ECC88 can work with lowish voltages but don't expect large voltage swings under those conditions.
Contrary to most transistors, tubes are made for voltage amplification, starve them on voltage and they'll compress the dynamic range...something they're used for in studios BTW.
Look at a cathodefollower as an impedance transformation device like a buffer (which it is), it doesn't need to swing volts so B+ can be rather low depending on the choice of tube.
+/- 24 V still is seen by the device as a total of 48 VDC.
It will still work but probably also distort too much.
Operation points are chosen from the loadlines on the curves of the tube.
The curves are often, but not systematically, published together with other characteristics of the tube such as heater voltage, transconductance and amplification factor among others.
Quite often the usable range is relatively large but there are limits.
No. Provided the maximum dissipation is not surpassed life will not be shortened.
Mostly too high a plate voltage kills the tube first so lower voltage is sometimes advisable.
Naturally you need to consider what you expect the circuit to do for you when you need to decide on operating parameters such as class of operation, desired output power, etc.
Cheers,
It's just that most tubes were not designed with low operating voltages in mind, you'll go outside their linear operating point which will increase distortion levels.
The ECC88 can work with lowish voltages but don't expect large voltage swings under those conditions.
Contrary to most transistors, tubes are made for voltage amplification, starve them on voltage and they'll compress the dynamic range...something they're used for in studios BTW.
Look at a cathodefollower as an impedance transformation device like a buffer (which it is), it doesn't need to swing volts so B+ can be rather low depending on the choice of tube.
+/- 24 V still is seen by the device as a total of 48 VDC.
It will still work but probably also distort too much.
Operation points are chosen from the loadlines on the curves of the tube.
The curves are often, but not systematically, published together with other characteristics of the tube such as heater voltage, transconductance and amplification factor among others.
Quite often the usable range is relatively large but there are limits.
No. Provided the maximum dissipation is not surpassed life will not be shortened.
Mostly too high a plate voltage kills the tube first so lower voltage is sometimes advisable.
Naturally you need to consider what you expect the circuit to do for you when you need to decide on operating parameters such as class of operation, desired output power, etc.
Cheers,
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