Hierfi said:
Man, you're either really good, or you're really lucky!
Slap a fet follower on it and you'd have an interesting preamp!
DHT filament bias
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Man, you're either really good, or you're really lucky!
Slap a fet follower on it and you'd have an interesting preamp!
Paul Joppa
Moderator
The easy check for saturation current is to connect the grid to the plate :^)
[size=9pt]Caucasian Blackplate said:
There seems no end to what you can try with grounded heaters, or otherwise. To experiment, all that is necessary is to use an adjustable current source or a voltage source that can operate at low voltages. It seems overtly limiting for experimenters like us to restrict heater currents based upon criteria of normal operation that doesn't apply, particularly in the hunting in the bushes for the last bit of resolution we strive to obtain.
It is a simple matter to try the same thing with the 3S4 tube. Just select the plate load resistor and then adjust the heater current to cause a 10volt drop for the 20volt supply. Done. Either you exceed the heater current limits, end up with ion bombardment (sounds appealingly heavy and ominous), excessive grid current or low input impedance. What can be most critical is having low inherent voltage gain that requires higher input amplitudes to produce a desired output amplitude.
In some ways it seems silly not to remove the resistor/capacitor pair in the cathode of the current quickie, short the heater to ground and diminish heater current to cause the same voltage drop across the 4Kohm plate load resistor. You end up using less heater current, no components in the capacitor to ground and the same output signal parameters. In other words, no fet follower or other buffer required. With that said I am not remotely suggesting that the removal of the capacitor/resistor pair in the cathode would improve and not seriously deteriorate the reproduction of music by the quickie as it stands. The reality rules.
Logic is the systematic method of coming to the wrong conclusion with confidence. It seems that feeling confident, smart or lucky is a diversion to emotions from reasoned thought. It seems for another discussion on which is more appearling, the chicken or the egg.
Paul Joppa said:
Sure Paul. Saturation voltage occurs at 700mV from ground or 19.3Volts below the 20Volt supply. Hence saturation current is equal to 19.3 volts across the plate load resistance of 500 Kohm or 38.6uAmps. The plate saturation current cannot exceed 40uA as restricted by having all voltage drop across the 500Kohm plate resistor and none remaining on the plate. Under saturation conditions the plate dissipation was 27uW.
In an act of exceeding trepidation and brevity I connected the grid and plate together and attached it directly to the 20volt supply. The resulting saturation current was about 38mA or more than 7 times the 5mA maximum design limit. Plate dissipation was only about 760mW although more than 30,000 times that with a 500Kohm plate load resistor. I suspect some awkwardness developed by various elements in the tube and some consideration by the ions.
[size=9pt]Paul Joppa said:
I hope everyone doesn't mind my input. A couple of errors to report. First, the saturation current with the grid and plate attached to 20volts appeared unbelievable high. In rechecking it was around 3mA not 38mA. As Gilda Radner would say "Oh, never mind."
Secondly, the input resistance of the circuit was far higher than reported. I inadvertently left a 100Kohm resistor to ground under testing, hence the input impedance was found to be 100Kohm. Wow, I have made this mistake before. Anyway, after removing the offending resistor the input is around 5Mohm.
To determine further what was happening I left a 1Mohm resistor in series with the input to observe grid currents. The findings were that at around positive 0.5 or 0.6 volts the input voltage began visibly limiting on the grid. This suggests that for an observable error of 0.1 volt using a 1Mohm resistor implies grid currents where in the 0.1uA region. This is not necessary low. However, under conditions of a circuit with 10x gain this would become observable at -5 or -6volts on the plate. It appears proper to consider that the output is a reflection of what is happening on the grid and judged accordingly.
The findings appear correlating in a reasonable way to expectations in the discussions. This raises the question as to the interaction of the input potentiometer having a variant source impedance to the input. For a zero source impedance to the pot the maximum input impedance to the amplifier is 1/4 of the value of the pot. Even for 100Kohm this would be 25Kohm. It seems this could work.
Paul Joppa
Moderator
Wow - I had hoped you might try that, and I am stunned. Even 3mA is far, far higher than I expected. Clearly that 13th-power rule is NOT applicable to oxide cathodes! Thanks so much for this contribution to our knowledge.Hierfi said:
Incidentally, the more important purpose of the cathode resistor is to adjust the bias and operating point as the high-voltage battery ages, to maintain reasonably good operation over the 24-36v range. With a fixed high voltage, I would have used an LED or something similar - because I totally agree, the resistor/capacitor combination does limit the ultimate audio performance.
[size=9pt]Paul Joppa said:
It is nice when the preamplifier expires when the batteries expire and a circuit that uses the fewest components to address a multiplicity of concurrent issues. It is a sign of admirable elegance. Anyway, LED's are simple and have a reasonable transfer curve without going unnecessarily blunt as a constant voltage drop. For better or worse it adds a semiconductor in the signal path and that I will need of add of such devices to create a buffer in the circuit I would like to try out.
As an aside, what are your thoughts on using constant current sources in the B+ supply? I have been considered this for awhile and have some thoughts on why this can have benefits.
Paul Joppa
Moderator
Sorry for the late response - the "show unread posts" function has been very unreliable for me lately.
The PJCCS optional kit is a current source as plate load. All of our shunt regulators in the higher-end kits use a current source to feed the shunt regulator. Obviously I - and we - like them!
The PJCCS optional kit is a current source as plate load. All of our shunt regulators in the higher-end kits use a current source to feed the shunt regulator. Obviously I - and we - like them!
[size=10pt]Paul Joppa said:
Current sources seem as a clear advantage respecting noise on the B+ supply. I have a coupe of attachments to explain what might be going on. Given that B+ is in series with the plate load resistor feeding the output this can be considered a voltage source with an output impedance that is the load resistor. This is one of two networks feeding the output, the other is the tube with its inherent plate resistance. Both can be represented as indicated by A in the AC equivalent circuits sketches.
Both sources are connected to feed the same point, the output of the preamplifier for example. This is indicated in figure C. The second page indicates the actions of resistive division. Hence we can simply set 1 volt of noise on the B+ and zero volts on the tube side. We can then find the output as a result of voltage division and determine the dB values of B+ noise attenuation. For the analysis I have used 8 examples numbered on the second page from the top to bottom on the left side of the page. Note that the voltages appearing in the center can be considered as the output of the preamplifier. For a 1 volt output this translates into 0dB of attenuation of B+ noise, the worst case. For zero volts output this translates into infinite attenuation of B+ noise, the best case.
1. With a plate load resistor at 0 ohms and the plate resistance at 10K ohms the output is 1 volt as equal to the B+ This indicates 0dB of attenuation of B+ noise. This is also independent of any plate resistance that is non-zero. In other words no attenuation of B+ noise exists at the output.
2. Again 0dB of attenuation given that the tube resistance is infinite. In other words there is no voltage drop across the 10K ohm given that no current can flow through the infinite plate resistance
3. Only about 1dB of attenuation in cases when plate resistance is considerably higher than the plate load resistor. As an example for the quickie with a 4k ohm load resistor and perhaps 8K ohm plate resistance (don't really know) the attenuation might be around 3dB. Also of note is that pentodes can have inherently higher plate resistance and result in less B+ noise attenuation
4. There is 6dB of attenuation when plate resistance and plate load resistances are equal
5 When the plate load resistance is 9x higher than the plate resistance the attenuation of B+ noise is now about 20dB ( this is around the attenuation I am getting in the manner implementing the 3A5 tube)
6 This is the current source (if perfect) having infinite B+ noise attenuation.
7 This shows there is no dependency on plate resistance, still infinite B+ noise attenuation
8. This shows infinite B+ noise attenuation for plate resistances at zero ohms. For something like the 300B tube with 800 ohms plate resistance and the same plate load resistor of 4K ohm as the quickie the 300B can achieve perhaps 15dB of nose attenuation compared to the quickies at 3dB.
What is important to note, seemingly the most critical point, is that D in the AC equivalent circuits can be viewed the reality. For situations with 0 dB B+ noise attenuation the output can be considered fed by the B+ supply instead of ground and whatever signal happens to be on top. On the tube side the output is being fed from ground and whatever signal happens to be on top. In other words it seems far better to keep to the tube side having proportionately lower resistance.
Clearly constant current sources never reach infinite attenuation yet yours must be more than adequate given the reality of your findings. .
Hopefully my thinking isn't totally flawed. Anyway, back to the dungeon. Since my 3A5 couldn't drive anything useful I am trying to turn it into a little 2-3 watt "quickie integrated" and skipping the preamp or headphone amp, though I fear revealing of bipolars amongst the bottleheads!
