Had this idea for a cathode follower using directly heated triodes (the smaller ones, like those used in battery operated portable radios or hearing aids. Ones with specified + and - filament terminals). Fed by large resistors from high voltage + and - to approx constant current sources and to look like a 1K load. If I wanted 1K follower load, and the filament wants 50ma at 1.4V, I'd need a 2K resistor fed by 100VDC supplies on both sides (+100V to 2K, to filament, to another 2K resistor to -100V). Well, that could be done, but would burn a fair amount of power (10W). Well, if I did that, would I get any "DHT magic" from this cathode follower?
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hi
you don't need to do that, just wire up the heater as normal, then, if say you wanted a 10k cathode resistor, get two 20k ones, connect one to each heater connection, join the other ends together, and hey presto. they are now in parallel to make 20k.
The amount of heater current going through 40k worth of resistance is virtually zilch.
hope that this is of use.
bill
you don't need to do that, just wire up the heater as normal, then, if say you wanted a 10k cathode resistor, get two 20k ones, connect one to each heater connection, join the other ends together, and hey presto. they are now in parallel to make 20k.
The amount of heater current going through 40k worth of resistance is virtually zilch.
hope that this is of use.
bill
billr said:
I agree, just remember you do need a floating filament supply which cannot be shared with anything else, and should not have a lot of capacitance to ground. Don't forget to ground the other end of those two resistors either.. Alternately you can use 3 resistors, two of which get connected in series across the filament, third gets connected to the center point of the first two and ground. Use 2 x 100 ohm and 1 x 10K, again filament supply most float. (I tend to prefer this as it is symmetrical from an ac signal perspective/cathode bias perspective, but either approach should work fine.) 😀
I think that the point wa2ise is trying to make is that he also wants to use the +/- 100 volt supplies to light the filament as well as provide the cathode return path. There are tubes out there that have low current filaments.
The 1U4 can be triode wired and has a 50 mA filament. It could be used in his example, but I have no idea how this would sound since I haven't tried it. I experimented with the 1U4 and the 1R5 since I have a bunch of them. I came to the conclusion that most any conventional tube could kick their butt and these were best used in a low powered portable device, which is their intended application.
The 1U4 can be triode wired and has a 50 mA filament. It could be used in his example, but I have no idea how this would sound since I haven't tried it. I experimented with the 1U4 and the 1R5 since I have a bunch of them. I came to the conclusion that most any conventional tube could kick their butt and these were best used in a low powered portable device, which is their intended application.
tubelab.com said:
I guess if nothing else it is a (really not so) good way to burn almost 10W of power to provide 70mW of heater power.. LOL Frankly I'd use a battery.. (AA)
kevinkr said:
I'd agree, that this is rather impractical to actually build. If I did build this, I'd probably use the battery, or constant current sources to create the illusion of high impedance as seen by this cathode follower. But for this post, I wanted to keep the question as simple as possible, and to see if anyone might be able to say if this concept would merit further work 🙂 , or is just a silly idea and don't waste the time... 😀
Could something like this can work?? correct me if i am wrong, the heater is floating.
An externally hosted image should be here but it was not working when we last tested it.
If this works then one can make a differential DHT stage as well, both halves having separate floating heaters joined together with resistors and ultimately fed by CCS, just like LTP.
No DHT magic at all. As drawn, it's not gonna work. You referenced the grid to ground, and the cathode will be at -- maybe -- +1.0V. That provides no headroom at all. You'd need to elevate the control grid in order to get any sort of useful headroom.
Otherwise, the idea is a variation on filament bias that's sometimes used with DC power supplies and DHT types. The idea being that the filament can be elevated to the necessary cathode bias voltage, but since it's filament current, not plate current, causing the voltage drop, the resistor can be small enough to not require a bypass capacitor. It may or may not be desirable since you can burn off a lot of power in the filament dropping resistor.
As for those battery powered VTs, I don't have any and have no experience with these. However, I haven't heard anything that suggests sonic excellence. As it stands, why bother? You're using a +/-100V power supply; you have more than enough voltage to run a conventional triode as a CF here.
After all, these 1.4V VTs designed to run the filaments from a 1.5V "D" cell, and plate power from 22.5V -- 90V dry cells, because they didn't have SMPS's or any other means than mechanical to develop the higher DC voltages that VT's like to see. Sort of defeats the whole purpose, doesn't it?
A cathode follower needs transconductance, the more the better. That's why I have been using mosfet followers in many of my circuits. Look up the transconductance of the battery tube you are playing with, then you will understand why they sound rather anemic as a follower.
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