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I wonder how the lower first triode gets biased? The grid resistor is too low for grid leak bias.
The circuit recommends B+ between 250 to 300v, assume 6SL7 is in place, what voltage is more adequate?
Follow Ups:
I wonder how the lower first triode gets biased? The grid resistor is too low for grid leak bias.
The circuit recommends B+ between 250 to 300v, assume 6SL7 is in place, what voltage is more adequate?
Some valves work OK with 0 bias. In this, the top valve sets the current and the bottom will find it's anode voltage for 0V grid bias.I've seen this circuit for ages but thought it not ideal to drive a low Z LCR. LCR EQ is not easy and expensive so you want to be sure it will work. Try something that someone else you know has got working properly not just "some circuit of the interweb"
And as for the others, they're arguing at cross purposes referencing different circuits to support their wanting to be right. It ain't pretty :)
Edits: 08/25/23
Yes, I agree. I see two problems with the circuit as posted: 1) the input impedance of a typical small-signal triode at zero bias is pretty low due to grid current; quite possibly less than the desired 47K resistor; 2) the output of the first stage is around 500mV (with 12AX7 and a 5mV cartridge); the 600-ohm LC module will draw abut 0.8mA, which is equal to the quiescent current. Since cartridges can output up to 50mV on peaks the stage will ve well into clipping on such peaks.
The tube types listed at each location need differing operating points. The 5755 in particular is very different from the others. Surely there's a legitimate commercial LCR schematic somewhere online?
Edits: 08/23/23
All tubes have cathode resistors that provide bias.
That is not true. Fixed biased tubes don't need and often don't have cathode resistors.
The dyna 70, for instance, doesn't have a cathode resistor for bias for the output tubes. The 15.6 ohm cathode resistor is just there so the user can measure the voltage across it determine the idle current. The bias is the negative voltage at the grid set by the 10k pots feeding the bottom of the grid resistors. I idle current is set by the negative voltage at the grid of the tube (fixed bias) not the positive voltage at the cathode of the tube developed across a cathode resistor (cathode bias).
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
"All tubes have cathode resistors that provide bias."The 5755 can operate at zero bias with no cathode resistor at typical anode voltages. The curves are not like the other types listed in the OP's schematic. Also see the link below showing an example where it is used in exactly this way.
Edits: 08/24/23
Errr, the schematic shows cathode resistors.
.
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"Still Working the Problem"
Poster did not post that schematic. My reply is to the poster only.
Your statement about cathode resistors is incorrect. Just admit it and move on.
First Tre, you owe me an apology regarding the 5U4GC 450uF capacitor that you stated never existed (I was full of BS) and I proved it on a Cary amplifier that I postd the specs including the large capacitor in those specs.
Next, do not tell me a cathode resistor does not provide bias in a tube. You are completely wrong. What do you think a cathode resistor does? You should be embarrassed and stop attacking me.
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 08/24/23
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'
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"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
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"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.
He posted the wrong schematic at first, then he corrected it.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
What you have posted is not an LCR phono stage!
The absence of L in the schematic is a clue...
Alex
This is the original Shishisdo LCR
And there is the Rk on lower tube
Walter
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