DHT filament bias

[vetmed]

Anyone tried this with the 3S4? That is, returning ground side of the cathode through a resistor to ground to establish bias. Negative of D cell would go to ground. For 2 volt bias and 100 ma filament current you would need a 20 ohm resistor. Would dissipate ~ .2W so you could get away with a 1/2 watt resistor but a 1 watt would probably be better. Would eliminate the electrolytic cap in the cathode circuit as well. Thoughts and opinions greatly appreciated

Robert Lees

 

[Paul Birkeland]

Negative of D cell would go to ground. For 2 volt bias

How do you plan to get 2V of bias from a 1.5V battery?

 

[Doc B]

FWIW there are D cell sized 2V SLA batteries.

 

[vetmed]

Clearly I have not made myself clear : With the Quickie you have one 1.5 volt battery. Positive side of this battery goes to pins 1 and 7 of the 3S4. Terminal 5, the negative end of the filament/cathode, connects to ground via a 20 ohm resistor. The negative end of the battery is also grounded. Of course you have to remove the 1K resistor and its accompanying capacitor. 2volts/100ma gives 2 volt bias and no cap in the cathode circuit. Only down side I can see is that such a setup will approximately halve battery life, but paralleling another D cell is easy enough ;D

Robert Lees

 

[Paul Birkeland]

Clearly I have not made myself clear : With the Quickie you have one 1.5 volt battery. Positive side of this battery goes to pins 1 and 7 of the 3S4. Terminal 5, the negative end of the filament/cathode, connects to ground via a 20 ohm resistor. The negative end of the battery is also grounded. Of course you have to remove the 1K resistor and its accompanying capacitor. 2volts/100ma gives 2 volt bias and no cap in the cathode circuit. Only down side I can see is that such a setup will approximately halve battery life, but paralleling another D cell is easy enough ;D

To get 2V of bias, you'd need to start with 3.5V, feed this in to pins 1/7, then use a 20 Ohm resistor to ground from there.  This would give you 1.5V of filament drop at 100mA, and 2V of bias across the resistor.

 

[vetmed]

PB you say "...feed this in to pins1/7, then use a 20 ohm resistor to ground from there." Not sure I understand, if there is already 1.5 volts there then why not just ground the negative end and call it biased at 1.5 volts? Forgive me if I am being thick about this :

Robert Lees

 

[Paul Birkeland]

You need 1.5V across the filament, and 2V from filament to ground, this is 3.5V.

If you put a 20 Ohm resistor between the filament and ground, then put 1.5V across the filament and resistor in series, there will not be enough voltage across the tube to heat it. 

Perhaps if you draw a schematic of what you're looking to do, then fill in the voltages and currents, things will shape up.

 

[vetmed]

Well, I did one better, 20 ohm resistor in series does indeed not work :'( So I connected the D cells in series and get 3.08 volts at the + end of the filament, and 1.88 volts at the top of the 20 ohm resistor ;D So I hooked it up and it works Its late and I will try and make sense of this tomorrow Thanks Paul

Robert Lees

 

[Paul Birkeland]

It's all about learning here!

With four D-batteries, this should be fun to try out.

-PB

 

[Paul Joppa]

Well, I did one better, 20 ohm resistor in series does indeed not work :'( So I connected the D cells in series and get 3.08 volts at the + end of the filament, and 1.88 volts at the top of the 20 ohm resistor ;D So I hooked it up and it works Its late and I will try and make sense of this tomorrow Thanks Paul

Robert Lees

Cool - I look forward to your report on the sound difference!

Notice that this leaves only 1.20 volts on the tube filaments, which are probably good down to 1.00 volts. As the D cells drain and voltage drops, the filament will run cooler, giving them less resistance, so the filament voltage will drop more rapidly than the D cell voltage.  Running 3 volts power, I would suggest a 15 ohm resistor for bias in order to get reasonable use from the cells.

 

[vetmed]

Actually the thought occurred to me as well, but it was more of an issue of not flattening the batteries quite as quickly ;D Understand that the setup is not optimal right now with the 2 batteries powering both channels, will be making a trip to the local electronics surplus store for battery holder(s). Still and all sounds very promising and will report back when everything is running more to my liking

Robert Lees

 

[vetmed]

For me this is an unqualified success. Installed 2 single D cell holders directly underneath the original ones, had to use slightly longer bolts and the CCS now just dangles in space. Put some velcro strap on the holders to keep the batteries from falling out ;D Pretty straightforward and getting the electrolytic cap out of the way has eliminated a slight coarseness that frankly prevented me from using this preamp on a regular basis. Now have VPI TT, Eros, Quickie, 2A3 Paramount and this pre is not out of place with much pricier gear Very pleased with this mod and since it is easy to do I would recommend it. And easily reversed if you don't like the results.

Robert Lees

 

[johnsonad]

Nice work Robert! Hopefully others will give this a try!

 

[Hierfi]

My friends all have quickies and I must make amends. I have heard these units and like the simplicity and performance. This inspired some thought that resulted in some experiments on directly heated cathodes connected to ground. This was done mainly to eliminate the electrolytic and simplify the circuit.   

After some testing on the 3S4 I am currently experimenting on a single dual triode tube for stereo. The tube is the directly heated 3a5. I have no idea what reputation this tube has for audio, however it seems that the tube has some merit in just having fewer grids to contribute to the microphonics. After several experiments the power supply voltage used was set to 20 volts with the plate voltage at 10 volts. The plate load resistor was fixed at 470K, requiring a plate current of 20uA. Arguably these parameters are not at all useful as a stand alone preamplifier.

Nevertheless, to obtain such a result the center tap of the heater was attached to ground and the ends connected together (pin 1 and 7) and then connected to an adjustable positive current source. To set the quiescent tube voltage to 10 volts and tube current to about 20uA required the heaters to be set to around 130 ma (65 ma each tube). This resulted in the voltage being around 0.67 volts. Ultimately the heaters are running about 1/2 voltage and 1/2 current, or 1/4 power. Given that electron emissions are a function of temperature this drastically reduces the electrons boiling from the heater.   

Nevertheless, the result was that the output AC gain was about 10X and the output swing was remarkably close to 1 volt off ground and 1 volt from the 20volt supply. In other words the 20 volt supply was adequate to produce almost a 20 volt peak to peak swing. The limitations near the B+ supply and ground was also gradual, resulting predominantly in low order harmonic distortion components.

In further testing the output impedance was estimated to be around 100Kohm, appearing dominated by the characteristic impedance of the output tube, not the plate load resistance. This suggests that the 470Kohm plate load resistor was behaving more reminiscent of a constant current source from the B+ supply. Like a constant current source this offers increased isolation from the noise of the B+ supply getting into the output.

Having no aversion to bipolar devices I would like to add a discrete bipolar push pull output stage and keep the total quiescent B+ current to about 50uA per channel As a result I should be able to get away with 3 x 9volt batteries lasting perhaps more than 10x longer than the quickie and a single D cell or Lipo for the heaters.

I don't know what to expect though it looks good on the scope.

cheers 

 

[Paul Birkeland]

20uA on a 3A5 is nearly 0 current.  You could also reduce the plate current in the Quickie to 20uA to increase battery life, but the circuit's performance would go to hell.

I've used the 3A5 on several occasions, I usually use 2.5V bias, 10mA and ~125V on the plate.

As you can see, with no plate current and no plate voltage, the circuit's output impedance has become so high that it's not able to really act as a preamp.  I would go as far as to say that this qualifies as a class B preamp. 

With one end of the filament grounded and the other end being supplied by a current source, one end of the filament is at zero bias, which will further degrade the performance. (Especially depending on what's feeding the grid of the 3A5)  You actually have a preamp that requires a source with low impedance to drive it (100 Ohms), yet the gain stage itself has incredibly high output impedance. 

I don't mean to sound overly negative, just trying to highlight all the trades you have made to get rid of one capacitor. 

 

[Hierfi]

[size=8pt][size=10pt]

20uA on a 3A5 is nearly 0 current. As you can see, with no plate current and no plate voltage, the circuit's output impedance has become so high that it's not able to really act as a preamp.  I would go as far as to say that this qualifies as a class B preamp.

With one end of the filament grounded and the other end being supplied by a current source, one end of the filament is at zero bias, which will further degrade the performance. (Especially depending on what's feeding the grid of the 3A5)  You actually have a preamp that requires a source with low impedance to drive it (100 Ohms), yet the gain stage itself has incredibly high output impedance. 

I don't mean to sound overly negative, just trying to highlight all the trades you have made to get rid of one capacitor.

[size=8pt][size=8pt][size=10pt]
Thanks. No negativity is considered. Class B is exceeding generous considering it is not deserving of any class as a preamplifier   

Decreasing the plate voltage decreases the attraction of electrons. The literature indicates that starving the plate causes the input impedance to drop drastically on the grid, as you have mentioned. However, this is considered to occur as a result of the cloud of electronics ending up surrounding the grid and  overwhelming the effectiveness of input signal currents. By drastically reducing the heating of the cathode/filament the cloud is drastically reduced. To confirm this, and despite any reality to this form of thinking, I added resistance in series with the input signal to determine the source impedance. The output dropped in 1/2 with 100Kohm connected in series with the input, hence a 100Kohm input impedance. The conclusion is that the magnitude of electron emissions from the heater/cathode drastically affects the input impedance. 100Kohm appears more than adequate for a preamplifier.     

It is not clear what you meant by a zero bias point in relation to attaching the center of the heater to ground. A voltage gradient is created by heater current over the length of the tubes heater and goes from ground (as physically attached) to 0.6 volts set by the current source. With the grid being at ground potential the cathode is on average 0.3 volts positive with respect to the grid.  Despite this voltage drop the tube can be considered at 0 volts on the cathode and on the grid in examining the characteristic curves of the tube. It is only under higher plate voltage with the cathode and grid set equally that the plate dissipation can destroy the tube.

Grid voltages that swing positively and negatively in relation to the heater/cathode voltage is not indicated of any unnatural nonlinearity in the characteristic curves. Neither was this observed in the reality even when using a 100Kohm signal source. It should be noted however, that the measurement was done using a X10 probe that minimized both resistance and capacitance on the output. Hence to make this network usable it requires some form of impedance converter on the output.

It should be noted that this network has more than twice the peak to peak swing capability of the quickie in using only a 20 volt supply. This is not to suggest that any implementation can remotely compete or challenge the quickie. It is simply an interesting comparison.

By the way, how did the 3A5 tube work out at the 90 volts?        [/size][/size]

 

[Paul Birkeland]

Try driving the grid with a 10K source, then measure distortion. When you drive the grid positive the impedance drops like crazy and grid current is drawn. This will pop up immediately as high distortion that has poor symmetry.

 

[Paul Joppa]

Before I get into technical stuff - thanks for posting this! I always hope that our products will inspire others to experiment and play with tubes, and the Quickie is especially inviting because it's so cheap ...uhhh... inexpensive, yeah that's it inexpensive. It's fun, and cheaper that taking the family to a movie! OK, now some tech notes:

Max rated plate current (presumably at normal filament voltage of 1.4 volts) is listed as 5mA. Cathode emission goes as the 13th power of filament voltage*, so at 0.7 volts or half the normal filament voltage the max current would be 0.61 microamps (uA). So based on this, 20uA would greatly exceed the current necessary to maintain the protective space charge, exposing the cathode to possible poisoning from ion bombardment. On the other hand, at such a low plate voltage the ion energy is very small, and much less likely to do any damage. Hard to say if that's meaningful.  :^)

* This rule is only known for pure tungsten filaments, I've never seen any comparison with thoriated tungsten or oxide cathodes. So it may be quite inaccurate for them!

Assuming the filament is being starved, there should be a saturation current which is the maximum the cool cathode can deliver. You can short the 470K plate resistor with a current meter to see what the maximum is. If it's less than twice the 20uA you are using then the output voltage capability should be adequate. This saturation effect is what causes reduced distortion with starved filaments or heaters - when it's just right you get all third harmonic with no second. As above, at normal voltages this will kill the cathode fairly fast, but all bets are off at these very low voltages.

Grid current begins at a bias voltage that depends on the metallurgy of the cathode and grid as well as the size of the space charge. A few tubes can take up to a half volt or so of positive bias before grid current becomes measurable, while most start at around -0.6 volts. I've measured anywhere from -1.0 volt to -0.2 volts on various 6DJ8s and 6922s, operated at normal heater and plate voltages and currents. Some of the battery tubes are made so that they can operate at zero bias and maintain high grid impedance. For example, the Tung-Sol 3A5 curves show zero plate current at zero bias for grid voltages below about 1.2 volts, which the 12AX7 curves show a tiny current at -0.5v grid bias - all these at normal filament voltage of course.

 

[Hierfi]

Try driving the grid with a 10K source, then measure distortion. When you drive the grid positive the impedance drops like crazy and grid current is drawn. This will pop up immediately as high distortion that has poor symmetry.

[size=8pt]
There are several things to consider. The magnitude of grid current is a function of free electrons boiled from the heater/cathode. This is drastically reduced. Secondly the circuit has a natural gain of about 10x. Given a preamplifier with some realistic value of output, lets say a 2 volt peak to peak swing requires 0.2 volt peak to peak to cause that output. This means that the gradient across the heater, being on average 0.3 volts remains more positive than the grid under all circumstances.

The network is effectively a DC amplifier. In the case of positive grid currents the negative half of the output will start to limit in amplitude relative to the positive half. In other words there would be some form of square law distortion function that decreases amplitude on the negative side. 

In the testing there is no observable variance in amplitude of the positive and the negative half relative to the mean, even for a +/- 5 volt peak to peak output swing. This was using a 10Kohm or 100Kohm series input resistance. In other words no square law function was observable to indicate any grid current issues. In the case of the 100Kohm, as more seriously affected by grid current, was only observable starting to limit at around 7volts or 8volts negative relative to positive half. 

The testing suggests that grid current artifacts would not be the dominant source of distortion under any normal circumstances. By comparison, in the case of the quickie there is only 4 volts across the 4Kohm plate load resistance for a 1mA plate current. This means that in applying a negative grid voltage there is only a 4 volt swing to the positive rail before hard clipping occurs and a series of high order harmonic components results. IMO there seems no comparison to suggest of any necessary disadvantage as a result of grid currents in comparison to the quality of the quickie.           

 

[Hierfi]

Before I get into technical stuff - thanks for posting this! I always hope that our products will inspire others to experiment and play with tubes, and the Quickie is especially inviting because it's so cheap ...uhhh... inexpensive, yeah that's it inexpensive. It's fun, and cheaper that taking the family to a movie! OK, now some tech notes:

Max rated plate current (presumably at normal filament voltage of 1.4 volts) is listed as 5mA. Cathode emission goes as the 13th power of filament voltage*, so at 0.7 volts or half the normal filament voltage the max current would be 0.61 microamps (uA). So based on this, 20uA would greatly exceed the current necessary to maintain the protective space charge, exposing the cathode to possible poisoning from ion bombardment. On the other hand, at such a low plate voltage the ion energy is very small, and much less likely to do any damage. Hard to say if that's meaningful.  :^)

* This rule is only known for pure tungsten filaments, I've never seen any comparison with thoriated tungsten or oxide cathodes. So it may be quite inaccurate for them!

Assuming the filament is being starved, there should be a saturation current which is the maximum the cool cathode can deliver. You can short the 470K plate resistor with a current meter to see what the maximum is. If it's less than twice the 20uA you are using then the output voltage capability should be adequate. This saturation effect is what causes reduced distortion with starved filaments or heaters - when it's just right you get all third harmonic with no second. As above, at normal voltages this will kill the cathode fairly fast, but all bets are off at these very low voltages.

Grid current begins at a bias voltage that depends on the metallurgy of the cathode and grid as well as the size of the space charge. A few tubes can take up to a half volt or so of positive bias before grid current becomes measurable, while most start at around -0.6 volts. I've measured anywhere from -1.0 volt to -0.2 volts on various 6DJ8s and 6922s, operated at normal heater and plate voltages and currents. Some of the battery tubes are made so that they can operate at zero bias and maintain high grid impedance. For example, the Tung-Sol 3A5 curves show zero plate current at zero bias for grid voltages below about 1.2 volts, which the 12AX7 curves show a tiny current at -0.5v grid bias - all these at normal filament voltage of course.

[size=8pt]
The substance is interesting Paul. Thanks.

I still recall that in any circuit with a "device" connected through a fixed resistor to a power supply that drops half the voltage across the resistor means that the "device" is operating at it's maximum power dissipation. Hence the tube dissipation would drop if the voltage goes either more positive or negative from 10 volts. This means that power dissipation of the tube peaks at the quiescent operating point or 20uA x 10volts or 200uW. It seems more likelihood the tube will expire by losing its vacuum in the millennium than for any other cause.

The cathode isn't apparently starved. Shorting the 470Kohm resistor to the 20 volt supply resulted in 0.3mA, or 0.3mA x 20volts or 6mW dissipation. I suspect the current would still go up with a higher B+ supply. Of note is that the output goes into more symmetric clipping at the extremes, hence producing more odd order harmonics. 

As you suggest the 3A5 appears as a special breed. There were no substantial negative issues observed in the experimentation. It is all very interesting stuff. Thanks again.