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In Reply to: RE: Please don't start this nonsense again posted by sony6060 on August 24, 2023 at 16:21:08
A cathode resistor provides bias for a tube that is auto biased (also known as self biased or cathode biased).A tube that is fixed biased will have a small cathode resistor and the purpose of that resistor is so the owner can read a voltage to know what the idle current is. The bias voltage (almost all) is provided by a negative voltage supply feeding the grid.
In the dyna 70 the total idle current is 100ma (50ma for each 6ca7) so there will be 1.56 volts across the 15.6 ohm resistor. Yes, 1.56 volts is added to the bias voltage but is a very small percentage of it.
If the dyna 70 was cathode biased (self biased), all 33 bias volts would be provided by the cathode resistor. So all I was saying is that not all tubes have a cathode resistor and not all tubes with a cathode resistor use that resistor for the main bias voltage source. In guitar amps they often leave off the small resistor in the cathode circuit and there is no way to know (without disconnecting a plate wire and inserting a current meter) how much idle current is flowing. So there is your "tube without a cathode resistor". I can find you a schematic is you want. I just did.
"stop attacking me"
I am not attacking you. I am attacking the things that you say that are wrong. Big difference.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Edits: 08/24/23 08/24/23 08/24/23 08/24/23 08/24/23 08/24/23Follow Ups:
Nonsense on a zero bias tube using a cathode resistor supplied by the factory to read cathode current. Second, so what if a cathode resistor supplies 1/2 the bias and other 1/2 is fixed bias at the grid. The cathode resistor still supplies cathode bias. Stop your nonsense Tre and calling me full of BS twice now. Where is my apology for the 5U4GB tube capacitor that you stated I was full of BS and proved you wrong? Man up Tre. Tre man up, of course not. Not much of a man.
Edits: 08/24/23
so what if a cathode resistor supplies 1/2 the bias and other 1/2 is fixed bias at the grid."
That is called "mixed bias" The dyna st70 is not an example of mixed bias. The dyna st70 is fixed bias and the sensing resistor in the cathode (while it does ad to the bias voltage a tiny bit) is not there to help provide the bias voltage for the tube. So the dyna st70 is just fixed bias, not mixed bias.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
"All tubes have cathode resistors that provide bias.'
This is what you said. It isn't true. Now you're making it personal to avoid having to admit the error. Something is wrong with you.
99% of tubes use cathode resistors for bias Tre. You are hanging your Hat on 1%.
I believe you are also mistaken about the fraction of circuits that have no need for cathode resistance other than providing a current measuring shunt.
I don't think it is going to be difficult to provide signal tube circuits with hard grounds to their cathodes either. Examine 'grid leak'... :)
Douglas
Friend, I would not hurt thee for the world...but thou art standing where I am about to shoot.
Like the dyna st70 it is fixed bias. It also uses 6ca7s running 50 ma each.
There is .68 volts across the 6.8ohm sense resistor. The -36 volts bias is supplied by the negative supply and applied to the grid by the bias controls.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
This explains how to use the math after reading the voltage across the small sensing resistor and adjusting the bias pot to change the negative voltage at the grid until you get the voltage that corresponds with the idle current you are trying to set.
Again, the VTL amps are fixed bias with a sense resistor in the cathode circuit so there is a place to measure.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
with 10 ohm sense resistors. One for each of the four output tubes. The output tube have 40ma. of idle current each so that is .4 volts across 10 ohm. The 4 bias pots can be seen on the schematic being feed by the "-VL" from the -50vdc bias power supply.
Again the 10 ohm resistors in the cathode circuits are not cathode bias resistors but sensing resistors. The output tubes in this Velleman amplifier are fixed bias, not mixed bias or cathode biased.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
The Eico HF-87 uses cathode bias. The HF-87 is designed to run the output tubes at almost 75ma. per tube so there is 35 volts across 235 ohms (148.94 ma. of current total for both tubes). Notice there is zero voltage at the grid and no negative supply. The tube gets 100% of it's bias voltage from the cathode resistor.So some tube amplifiers use cathode bias and some tube amplifiers use fixed bias for their output tubes.
It is true that most small signal tubes use cathode bias but you said "All tubes have cathode resistors that provide bias." and that simply is not true. I have clearly shown that.
If you decide to stick around and pay attention, you just might learn something.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Edits: 08/25/23
You should have apologized.
"You're a very bad man." - Babu Bhatt
:)
nt
Have Fun and Enjoy the Music
"Still Working the Problem"
Tre, stating I was full of BS on the rectifier tube and you almost got away with it. I posted the specs and proved your BS statement was wrong. I do not like your nasty demeaner, you just ran me off of AA. I will not be back.
This is exactly what I tried to tell you. It is the max rated peak plate current value that should not be exceeded and if you increase the "impedance per plate" series resistance you can use a bigger input cap. That is exactly what the datasheet above says. Furthermore, if you decrease the AC voltage and/or the current draw that the supply has to deliver then you can increase the cap value without increasing the series resistance. If you don't understand that I can not help you.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
"where is my apology for the 5U4GB tube capacitor" Rod ended that discussion. You remember Rod, don't you? He owns this website. I don't think it is smart of you to bring that back up.
The dyna gets 32 of 33.56 volts of it's bias from the negative supply and only 1.56 from the resistor in the cathode circuit. The resistor in the cathode circuit of the dyna 70 should not be thought of as a cathode bias resistor at all. It is just there so we can measure the voltage and calculate the idle current.
What about the Fender amp with no resistor at all in the cathode circuit?
I really think that proves that your statement is false.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
I take the apology here Tre as you gave none before the posts was closed.Now here this- cathode resistors are used just measuring cathode current. Nonsense Tre.
Edits: 08/24/23
"Now here this- cathode resistors are used just measuring cathode current."
A large value resistor in the cathode circuit intended to provide the bias voltage for the tube is there for that purpose and is called a cathode bias resistor. In a circuit using cathode biasing (self bias, auto bias) there is no negative bias power supply as it is not needed.
The small value resistor, found in the cathode circuit of a tube that is using fixed bias (a negative voltage provided by a negative voltage power supply dedicated to biasing the tube), is there for the purpose of measurements.......yes. That is correct. It IS a resistor in the cathode circuit but it is NOT called a cathode bias resistor because that is not it's job. It is sometimes called a "sensing resistor".
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
We use resistors to measure current all the time. Shunts they're called. Usually measured in the tens of mV for hundreds of amps. With tube circuits it is more like 1V/100 mA... :)
Douglas
Friend, I would not hurt thee for the world...but thou art standing where I am about to shoot.
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