Would it be possible to make a solid state vacuum tube? - Page 1
Forum section: electronics / projects

author: SK_Caterpilar_SK
author_rank: Regular Contributor
timestamp: August 13, 2020, 09:45:34 pm
content: I have found a rather more and more common issue in my old radios..most of the ones I can get my hands have most of the tubes nearly shot. The biggest pain however is the power output vacuum tube. The ECL86 is a really problematic tube.. It develops a insane amounth of grid leak and it stresses the output transformer quite a bit and it doesnt even sound useable.I thought..hey lets try to build one from transistors..then I quickly realised my brain isnt really up to that task. I am not trying to fully replicate the vacuum tube in its characteristics and so on. Im just trying to replace a vacuum tube that skyrocketed in price (i wont buy a 30 dollar tube to put it into my 20 dollar radio...unacceptable).The triode section of the ECL86 isnt all that bad to replicate with two Jfets, but the beam tetrode is another story. Reallistically with semiconductors I wouldnt need any other grids so it could act like a triode but it being a 9W (Total power dissapation).I would want this to be a drop in replacement so no mods to the radio done. I just need it close enough so it will work and isnt a crappy solution.Anyone has any ideas how could I tackle this thing?

author: rsjsouza
author_rank: Super Contributor
timestamp: August 13, 2020, 10:10:51 pm
content: There are zillions of solid state replacements for rectifiers - they are quite easy to make.More complex vaccuum tubes, however are a bit more involving but they do exist. I have a few PCL82 solid state "vacuum tubes" in my collection. The circuit is very simple but I didn't extract it - I am pretty sure someone on the internet already has this.Ah, I found something:https://www.antiqueradios.com/forums/viewtopic.php?f=1&t=302235

author: rsjsouza
author_rank: Super Contributor
timestamp: August 13, 2020, 10:19:46 pm
content: Ah, found them. Actually they are PL802 (simpler, single function). The power FET is a BF459.

author: SK_Caterpilar_SK
author_rank: Regular Contributor
timestamp: August 13, 2020, 10:20:58 pm
content: Dangit a PCL82 solid state would be quite close to what I need

author: KE5FX
author_rank: Super Contributor
timestamp: August 13, 2020, 10:28:56 pm
content: Amusingly, the answer to this question is literally"Yes."Not something you'd normally find in atube radio, though.

author: Ian.M
author_rank: Super Contributor
timestamp: August 13, 2020, 10:40:40 pm
content: There's a PL802 'fetron' schematic here:https://el34world.com/Forum/index.php?topic=12710.0

author: David Hess
author_rank: Super Contributor
timestamp: August 13, 2020, 11:26:33 pm
content: They used to use high voltage power JFETs to make vacuum tube replacements but now you can do it using depletion mode MOSFETs.

author: SK_Caterpilar_SK
author_rank: Regular Contributor
timestamp: August 13, 2020, 11:56:09 pm
content: thats not a bad idea. I think ill get some and just for fun try it. All tho I dont think it will work all that great on the output (power part of the tube..attached to a transformer in class A I dont think it would work nicely but I want to see it..)For this I think I will have to modify the cathode bias resistor and cappacitance in the radio.

author: SK_Caterpilar_SK
author_rank: Regular Contributor
timestamp: August 13, 2020, 11:57:13 pm
content: Quote from: KE5FX on August 13, 2020, 10:28:56 pmThat definitely looks cool but not really reallistic for a 20 dollar radio

author: SK_Caterpilar_SK
author_rank: Regular Contributor
timestamp: August 14, 2020, 12:15:38 am
content: https://www.tme.eu/Document/ac9567136014fdabbb1abce538ee5550/IXTA%28P,Y%2908N50D2.pdfI think this could work

author: magic
author_rank: Super Contributor
timestamp: August 14, 2020, 07:37:02 am
content: I'm notveryfamiliar with "toobs" and their characteristics, but IIRC whole point of tetrodes and pentodes was that they were supposed to have high output impedance. So you could cobble together some small signal circuit which generates the desired I/V curve by whatever means and cascode it with any convenient HV FET to get the desired "plate" voltage rating and high output impedance.

author: SK_Caterpilar_SK
author_rank: Regular Contributor
timestamp: August 14, 2020, 11:50:11 am
content: I dont think you really got that right.. Pentodes and tetrodes can both have high impedance outputs or low impedance just as triodes. 12AX7 for example is a high impedance output tube, but it has a even higher impedance input. A 6C33 is a very low impedance triod and so the sam applies to the input side.The pentodes are here so we can controll with a very high input impedance a lower output impedance. EL34 has a high input impedance but the load impedance can be as low as 3kohm while its driven from a 100kohm input.And you have many flavors and colors and shapes of tubes.. the big point of pentodes is their ampliffication factor. Its A LOT.On the secondary thought it would be nice if I could put together something that could generate that I/V curve... I see it more as a V to I converter tho. I just dont really know where to start or what to look for

author: TimFox
author_rank: Super Contributor
timestamp: August 14, 2020, 01:23:41 pm
content: Triodes vs. tetrodes/pentodes:All other things being equal (which they never are), the triode tries to be a constant-voltage device (for AC), with a relatively low plate resistance (output impedance), since both the plate and grid voltages affect the field at the cathode.The added screen grid in a tetrode or pentode greatly reduces the effect of the plate voltage on the cathode field, with a constant voltage on the screen, thus making the plate current less dependent on plate voltage and greatly increasing the plate resistance. It is somewhat similar to a cascode series connection of two triodes, where the plate voltage on the lower triode (emulating a screen grid in the tetrode) is constant and its current flows through the upper triode (without much effect of the upper triode's voltage). Of course, there is current through the screen grid in the tetrode, but not in the grid of the upper triode in the cascode.It is common to convert a tetrode/pentode to a triode by connecting the screen to the plate (preferably through a small resistor), since there are more power pentodes out there than power triodes.The value quoted for an EL34 isnotthe output impedance, which is high, it is the recommendedloadimpedance, which is much lower than the plate resistance for power pentodes. There is a rule of thumb for power triodes that the load impedance should be twice the plate resistance.

author: magic
author_rank: Super Contributor
timestamp: August 14, 2020, 01:32:51 pm
content: By output impedance I mean output impedance of the tube itself, i.e. how much extra anode voltage it takes to force additional current into the tube, i.e. the slope of the anode curve. I found a datasheet of ECL86 and it's pretty flat. The curves look quite similar to those of JFETs/MOSFETs but with less transconductance and higher grid voltages. It's probably a matter of a few resistors to adapt a FET to do the same job.Maybe post the circuit you would like to plug it into?

author: TimFox
author_rank: Super Contributor
timestamp: August 14, 2020, 02:14:57 pm
content: Yes. Triode curves of Ip vs Vp at constant grid voltage slope upwards with a reasonable slope, while pentode curves of Ip vs Vp at constant grid voltages and a specified screen voltage are horizontal (high resistance) at higher voltages. There is a subtle difference between pentode curves and FET curves at the far left, however, where the pentode curves start from a common rising curve, while the FET curves come separately from the zero point (the effective resistance of the individual curves in that region depends on gate voltage).

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 14, 2020, 02:38:27 pm
content: They're the same thing, on a low enough level.In semiconductors, electrons are promoted from a bound (valence) state, to a conduction state. The energy change is the band gap, a few eV.Unique side effect: the vacancies (holes) also move, with somewhat poorer mobility (in silicon, about 2.5x slower -- which is why PMOS have to be bigger than NMOS to make a complementary (CMOS) circuit; most semiconductors have a much greater ratio, making CMOS impractical in anything else).Semiconductors have a bulk charge balance, so the charge carrier density can be extremely high. Downside, the bulk also obstructs flow, causing thermalized diffusion motion. Since the junctions can be made very small (indeed, must be), this relatively low velocity only has a limiting effect at high frequencies.Also, the energy distribution of those electrons, given by Fermi-Dirac statistics, has an exponential cutoff. Which is essentially why semiconductors have an exponential response. (Diodes and BJTs, and MOSFETs in the subthreshold region; FETs above subthreshold are another matter, which I forget why at the moment.)In vacuum tubes, electrons are promoted from a bound (valence) state, to a free (vacuum) state. The energy change is the work function of the cathode, a few to tens of eV.Electron emission can be stimulated by simply heating the cathode, or by irradiating it with light of adequate energy (photoemission) or other particles (secondary emission).Only electrons can be emitted in this way. Probably, with some work, a "P type" tube could be made, using a hydrogen fill gas and a hollow cathode to emit ions (protons). Unfortunately it would have utterly terrible performance (proton mass ~2000 times the electron!).Vacuum tubes do not have a bulk charge balance, so the electron density is low, and this is forced by the space charge of those electrons in the beam. (That is, electrons repel, so a cloud or beam of them tends to push back on itself.) With no obstructions, the beam can reach high velocity -- ballistic transport. Unfortunately, though high voltages can be used, practical reasons limit how closely electrodes can be spaced, so the effective transit time is worse than most semiconductors.Electron energies are given by the Maxwell-Boltzmann distribution, which has an exponential tail. Vacuum tubes are also exponential in the cutoff region; this is a relatively small part of their operating range however, and dynamic range isn't that great to begin with (due to the high temperatures, and construction and some chemistry details, leakage currents are relatively high). In the normal operating range, a 3/2 power law (give or take exact electrode geometry) occurs. (While this is more linear than an exponential transfer function, it's also much lower gain -- on top of the already low gain due to the low charge density in the vacuum.)The peculiar characteristic of the triode (and to a lesser extent, multigrid tubes as well) is a low plate resistance; this is not fundamental to the mechanics, so much as a consequence of the electrode arrangement. Some plate voltage leaks through the grid. The grid acts as an electrostatic shield, but a poor one, so there is a ratio of influence between grid and plate -- the amplification factor, µ. This applies for all electrodes, so that in a pentode, the suppressor grid partially shields the plate, then the screen and grid in turn as well. So the µ from plate to grid can be quite high, but it's just the product of each grid's µ to the next. (This is different from FETs, which have a mechanism that effectively shields the gate and drain. A different, much weaker effect dominates: channel length modulation.)So to a certain extent, vacuum tubes and semiconductors already are the same things.To a more meaningful extent -- if you want a semiconductor that acts like a triode specifically, there's this:https://en.wikipedia.org/wiki/Static_induction_transistorLike the triode, the electric field inside the junction happens to influence current flow, thus giving a low output impedance. (They aren't very available, if they're in production at all?)Others have covered the more traditional interpretation (substituting semiconductors into a vacuum tube circuit) so I thought I would talk about something more fundamental.Tim

author: KE5FX
author_rank: Super Contributor
timestamp: August 14, 2020, 04:10:39 pm
content: Quote from: T3sl4co1l on August 14, 2020, 02:38:27 pmBut itimpresses visitors, which is what really counts.

author: TimFox
author_rank: Super Contributor
timestamp: August 14, 2020, 04:33:40 pm
content: Note that most gas-filled tubes (except for VR tubes) are not gas-discharge devices. In a Hg-vapor rectifier, for example, the very heavy Hg+ions move very slowly, but neutralize the space charge from the faster electrons. This allows high electron current at very low plate-cathode voltages, compared with a vacuum rectifier. (If the peak current is too high, ion bombardment can damage the hot cathode. See data sheet.)

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 14, 2020, 04:46:35 pm
content: Yup, gas serves the purpose of neutralizing charge, just as the semiconductor bulk does; the difference is that the gas atoms have much lower ionization energy, which is easily met with applied voltages. (Whereas in semiconductors, quite a lot of current can flow before ballistic transport is encountered*, and impact ionization -- avalanche breakdown -- usually occurs in the same region.) This means the operating range where controllable, neutralized current flow can occur, is rather narrow, and tricky to use. (Deforest's original tubes were supposed to operate this way; turns out hard vacuum simply works better for amplification. He wasn't actually all that clever when it came to science.) This is fine for uncontrolled devices (rectifiers) and latching devices (thyratrons, etc.), where the increasing ionization simply makes the device more efficient.*This is actually possible in some materials, GaAs for one. A Gunn "diode" is actually a monode, a single lightly doped hunk of semiconductor -- no PN junction. It's able to oscillate at such high frequencies (10s GHz) because the transition from drift to ballistic transport manifests as a negative resistance V(I) characteristic, and has almost no time constant -- it's an intrinsic property of the semiconductor, no recombination or anything involved. The applied voltage must be limited (including reflected power, say if the coupling network is tuned badly), else avalanche breakdown soon ensues and destruction may occur!Tim

author: David Hess
author_rank: Super Contributor
timestamp: August 14, 2020, 04:47:37 pm
content: Quote from: TimFox on August 14, 2020, 01:23:41 pmFETRONS almost always used two JFETs in the cascode configuration to replace triodes, tetrodes, and pentodes but they were application specific. So a different FETRON was used to replace a pentode amplifier and a pentode oscillator because the operating conditions were different.

author: TimFox
author_rank: Super Contributor
timestamp: August 14, 2020, 04:55:04 pm
content: I think that the first FETRON was designed by Teledyne to replace 6AK5/5654 pentodes in specific communications gear. An interesting Teledyne ad:https://www.qsl.net/kh6grt/page4/tubesters/Fsht39.pdf

author: Pawelr98
author_rank: Regular Contributor
timestamp: August 15, 2020, 02:29:27 pm
content: ECL86 is expensive.But the PCL86 which is basically the same tube is not expensive.Just get a 13-14V source for the heater.A voltage doubler from 6.3Vac should be able to reach it.You may also try PCL82 which is even cheaper.Worse than 86 but it should be able to replace it as an output tube.Pinouts are different so it would have to be rewired (if it's point to point wiring, PCB would require an adapter).The PCL tubes were produced in mass numbers as they were used for TV's that didn't have mains transformers and the heaters were powered directly from mains.ECL tubes just ran out because the production numbers were low and this caused the prices to skyrocket.Online is one place to buy tubes.Flea markets is another, you may try to get a decent deal there.Just recently I bought two brand new PCL82, in a box for 0.5USD each.As for solid state, just get a high voltage FET and make it so that there's a positive gate bias.There's a transformer already (no hot chassis) so you could also ditch the output transformer and use the 6.3V heater voltage to power a small TDA chip or whatever is available.