U-47 Bias with another tube.
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RuudNL
Well-known member
Of course it depends upon the tube if 1.1 volt on the grid is the most ideal voltage.
Take care that the 6.3 volt should be extremely well filtered, because any hum and/or noise will be amplified by the tube!
Take care that the 6.3 volt should be extremely well filtered, because any hum and/or noise will be amplified by the tube!
Thanks for answers.
I already did the offset as on U67, everything works fine. But I wanted to try a different bias, like in the U47, only on a tube with standard 6.3 V heating.
If you look at the U47 circuit even easier, as in the image, is it possible to make a similar bias, for example, on EF86?
I already did the offset as on U67, everything works fine. But I wanted to try a different bias, like in the U47, only on a tube with standard 6.3 V heating.
If you look at the U47 circuit even easier, as in the image, is it possible to make a similar bias, for example, on EF86?
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RuudNL
Well-known member
But in this case the filament will only get 4.8 V...
(That's what you call: "underheating"...
)
(That's what you call: "underheating"...
)
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RuudNL
Well-known member
That could work, but why not simply use two resistors?
It's like the picture at the beginning of the post ?
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Of course, this is most likely the case.
I made the offset as in U-67 (only adjustable by potentiometer) and I liked it.
To do this, I modified my CAD TRION 8000.
Left only his transformer.
I made adapters for different lamps - from 7-pin EF95 (stock) to 9-pin EF86 / 12AX .. / E88CC.
Now I want to make another version of the fixed bias, as in U47 .. just an experiment for me.
Monte McGuire
Well-known member
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- Jul 26, 2013
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Tungsten filament life goes down very rapidly as voltage is increased. In typical tube mikes with under-voltage filaments, tubes can last many decades - this is not a bad thing.
Yes, but we are talking about the fact that the filament was no more than 6.3V.
Matt Nolan
Well-known member
Haha! This is, of course, true. However it does give you a pretty rock-solid operating point which other means of biasing do not, so maybe there is something in it.
And the fact that again in the U67 they used fixed bias ,although this time with a negative voltage tapped from the heater and applied to the lower end of the grid resistor , cathode grounded .
That eliminates the traditional RC in parrallel from cathode to ground and with it the phase shift and falling gain at low frequencies , maybe it has something to do with the way the negative feedback is applied and stability . Cathode directly connected to ground maximises the gain and minimises output impedence compared to a simple resistor where theres degeneration , adding the bypass cap helps but only down to a certain frequency .
Depending on which way we wire the DC heater supply we can easily derive either a positive or negative voltage , usually negative at the grid or positive at the cathode , although some valves may benefit from a slightly positive grid .
In the early examples of tube audio equipment the heaters were quite fragile so the LT battery was often fed via a rheostat with an off position at the lower end of travel , as you increase the LT voltage across the heater the tube wakes up , it acts a bit like a volume control , in the old days the gain of the tube was low they usually avoided attenuating the signal to the grid under any circumstances . You do also see times when a battery tap is used to supply a grid resistor or to sit a cathode at the appropriate voltage .
Battery grid or cathode bias has its attendant disadavantges ,
I have a one tube preamp with a Nimh cell sat between the cathode and ground , the anode current of around 10mA keeps the battery topped up ,but I generally precharge the battery to the 1.2 volts I need and it takes days of continual usage to rise to 1.25V in circuit . I did forget to check the state of the battery once , the W/W anode load resistor instantly went o/c , transformer was Ac coupled in that case so a graceful enough fail .
With a modern regulated DC heater supply adding the extra volt or two required to sit a cathode or grid at the wanted place on the curve is easy to do , if we want to undervolt the heater a bit thats also easy to arrange .
In return for a couple of extra resistors across the heater supply we loose the RC network at the cathode of the tube , thats one definate benefit , we also get very stable voltages for heater and what ever other electrode we supply , assuming a stable /well filtered HT supply the only thing that will change over the longer term is the emission of the tube itself , if we run the tube conservatively that reduction in emission might take years .
That eliminates the traditional RC in parrallel from cathode to ground and with it the phase shift and falling gain at low frequencies , maybe it has something to do with the way the negative feedback is applied and stability . Cathode directly connected to ground maximises the gain and minimises output impedence compared to a simple resistor where theres degeneration , adding the bypass cap helps but only down to a certain frequency .
Depending on which way we wire the DC heater supply we can easily derive either a positive or negative voltage , usually negative at the grid or positive at the cathode , although some valves may benefit from a slightly positive grid .
In the early examples of tube audio equipment the heaters were quite fragile so the LT battery was often fed via a rheostat with an off position at the lower end of travel , as you increase the LT voltage across the heater the tube wakes up , it acts a bit like a volume control , in the old days the gain of the tube was low they usually avoided attenuating the signal to the grid under any circumstances . You do also see times when a battery tap is used to supply a grid resistor or to sit a cathode at the appropriate voltage .
Battery grid or cathode bias has its attendant disadavantges ,
I have a one tube preamp with a Nimh cell sat between the cathode and ground , the anode current of around 10mA keeps the battery topped up ,but I generally precharge the battery to the 1.2 volts I need and it takes days of continual usage to rise to 1.25V in circuit . I did forget to check the state of the battery once , the W/W anode load resistor instantly went o/c , transformer was Ac coupled in that case so a graceful enough fail .
With a modern regulated DC heater supply adding the extra volt or two required to sit a cathode or grid at the wanted place on the curve is easy to do , if we want to undervolt the heater a bit thats also easy to arrange .
In return for a couple of extra resistors across the heater supply we loose the RC network at the cathode of the tube , thats one definate benefit , we also get very stable voltages for heater and what ever other electrode we supply , assuming a stable /well filtered HT supply the only thing that will change over the longer term is the emission of the tube itself , if we run the tube conservatively that reduction in emission might take years .
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We know how much heater current the EF86 uses and the voltage , calculates out to 31 ohms resistance .
Lets say we want 6.3volts @ 200mA on the heater plus 1.5 volts for the cathode , lets redo the calculation using 7.8 volts supply and add in extra resistance roughly equal to a quarter of 31 ohms , say 7.5 ohms ,
Heres the link to the calculators I used ,
https://www.rapidtables.com/calc/electric/watt-volt-amp-calculator.htmlhttps://www.rapidtables.com/calc/electric/voltage-divider-calculator.html
As you see the second calc confirms the dropped voltage across R2 as 1.5volts
A tube heater has a much lower resistance at the moment of switch on than it does after warming up , so you will see a larger positive voltage at the cathode initially , as the tube comes up to temperature the electrodes will settle at their proper voltages .
The extra 7.5 ohms we added in series with the heater supply acts as current limiter at the moment of switch on , l measured cold resistance of a Mullard and a Sovtek EF86 , 5 and 7.5 ohms respectively .
Lets assume a total of 15 ohms cold , at the moment of switch on our resistor takes roughly half the voltage and dissipates 2w , lets look at the peak current in the heater without any current limiting/dropping resistor with 6.3 volts applied , double the peak current at switch on . Limiting the heater inrush current is likely to prolong tube life , even if we dont need the subsidiary supply for other electrodes .In any case were only wasting a small amount of power in the resistor once the tube has warmed to opperating temp, around 0.3W . Probably a 1W resistor will easily withstand the momentary overload at switch on .
https://www.ebay.ie/itm/294527258206
Of course I didnt bother adding in the cathode current of the tube itself into the equation for simplicity , in the case of an Ef86 ,with a fraction of a mA its makes no odds eitherway.
Lets say we want 6.3volts @ 200mA on the heater plus 1.5 volts for the cathode , lets redo the calculation using 7.8 volts supply and add in extra resistance roughly equal to a quarter of 31 ohms , say 7.5 ohms ,
Heres the link to the calculators I used ,
https://www.rapidtables.com/calc/electric/watt-volt-amp-calculator.htmlhttps://www.rapidtables.com/calc/electric/voltage-divider-calculator.html
As you see the second calc confirms the dropped voltage across R2 as 1.5volts
A tube heater has a much lower resistance at the moment of switch on than it does after warming up , so you will see a larger positive voltage at the cathode initially , as the tube comes up to temperature the electrodes will settle at their proper voltages .
The extra 7.5 ohms we added in series with the heater supply acts as current limiter at the moment of switch on , l measured cold resistance of a Mullard and a Sovtek EF86 , 5 and 7.5 ohms respectively .
Lets assume a total of 15 ohms cold , at the moment of switch on our resistor takes roughly half the voltage and dissipates 2w , lets look at the peak current in the heater without any current limiting/dropping resistor with 6.3 volts applied , double the peak current at switch on . Limiting the heater inrush current is likely to prolong tube life , even if we dont need the subsidiary supply for other electrodes .In any case were only wasting a small amount of power in the resistor once the tube has warmed to opperating temp, around 0.3W . Probably a 1W resistor will easily withstand the momentary overload at switch on .
https://www.ebay.ie/itm/294527258206
Of course I didnt bother adding in the cathode current of the tube itself into the equation for simplicity , in the case of an Ef86 ,with a fraction of a mA its makes no odds eitherway.
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