It looks for me as stupid schematic (Oldtimers welcome!) - Page 1
Forum section: electronics / projects

author: 001
author_rank: Super Contributor
timestamp: September 29, 2019, 04:37:36 pm
content: Check ebay toy herehttps://www.ebay.com.au/itm/100-300V-DC-High-Voltage-Power-Supply-PSU-for-6P6-6N6-EL34-Tube-Preamp-Amplifier-/332777814374It looks stupid for meWhy they bias transistor this way?Is it good schematic?

author: Yansi
author_rank: Super Contributor
timestamp: September 29, 2019, 04:58:35 pm
content: Why wouldn't they bias it this way? I do not see anything wrong there. That's how simple regulated linear PSUs've been made last decades. The schematic is not stupid, but could be made certainly way better.There is certainly missing a grid-stopper resistor. (gate series resistor would be likely required even when mosfet used instead of tube). Can oscillate, and probably will without it, due to the stupidly placed capacitor from G pin. (making the power tube/mosfet work as a grounded grid/common-gate RF amplifier/oscillator)The NPN needs to be a high voltage type, but, more intelligent designer would use a cascode configuration with a high gain low voltage one, or stick in there even a TL431 to get very good results.To get better voltage regulation, one could supply the reference (75V zener in this case) from the output of the supply itself. (stick a resistor from the zener cathode to the output of the PSU) - that way it self-regulates the zener current. The more current from the output to the zener compared to the current through the NPN, the better the result.The circuit is lacking any over-current and short protection. Implementing fold-back current limiter may also be handy.

author: 001
author_rank: Super Contributor
timestamp: September 29, 2019, 05:35:52 pm
content: Quote from: Yansi on September 29, 2019, 04:58:35 pmThe problem is about maximum voltage between base and emiter (Typicaly it is lower than 5V). It is 300V PS

author: Yansi
author_rank: Super Contributor
timestamp: September 29, 2019, 05:51:21 pm
content: And why do you think the reverse Vbe is violated in the circuit?Here you have the approximate steady stage voltages when the output will be set for 300V.(plus a note how to ad the grid-stopper resistor)

author: 001
author_rank: Super Contributor
timestamp: September 29, 2019, 06:18:42 pm
content: Quote from: Yansi on September 29, 2019, 05:51:21 pmWellTurn pot to HV bus

author: Yansi
author_rank: Super Contributor
timestamp: September 29, 2019, 06:29:00 pm
content: Then this will happen.So what now?//EDIT: Note the output voltage is a function of the potentiometer tap position. Output voltage will be equal to aboutVout = 75*(1+Ru/Rb)where Ru is the upper part of the resistive divider and Rb is the bottom part.(Also note the range of the pot will be something like 90 to 330V as the output setting - depending on the exact zener voltage, transistor gain, resistor and pot. tolerances, etc.)

author: 001
author_rank: Super Contributor
timestamp: September 29, 2019, 06:34:51 pm
content: Quote from: Yansi on September 29, 2019, 06:29:00 pmWhy Yours pics sign constant pot voltage for different output voltage?

author: Yansi
author_rank: Super Contributor
timestamp: September 29, 2019, 06:38:13 pm
content: Because that's how the circuit worksThe zener diode voltage is (kind-of) constant. So is the voltage drop across the B-E junction. Hence voltage from the base to ground will be constant. But output voltage will change with the change of the potentiometer tap position.You can analyze the circuit knowing the voltage from base to ground is about 75V. That voltage has to be present on the voltage divider tap to ground. That tells something about the current flowing through that divider (you can assume for simplicity current gain of the NPN is high, the base of the transistor therefore consumes negligible current).You know the good ole TL431? Think of this has the same behavior - just the reference voltage will be about 75V (given by the Vzd + Vbe).//EDIT: This is the simplified circuit, same working principle:

author: Yansi
author_rank: Super Contributor
timestamp: September 29, 2019, 07:19:46 pm
content: With TL431 it would look like this:However this would only work up to the voltage limit of the 431, which would be about 25-30V. If further voltage is required, then cascode configuration may be used: you add a common base amplifier on top of it. Such amplifier has none current gain, however large voltage gain.In this case TR2 is what takes most of the voltage. It is a good practice to bypass R5 with a capacitor, and/or replace it with a zener diode. The R4/R5 divider shall be designed so about 10-20V is present at the base of TR2.

author: ArthurDent
author_rank: Super Contributor
timestamp: September 29, 2019, 09:24:41 pm
content: Quote from: 001 on September 29, 2019, 04:37:36 pmThe basics of the supply above are very similar to a supply I made decades ago using all tubes.

author: 001
author_rank: Super Contributor
timestamp: September 30, 2019, 02:56:19 pm
content: Quote from: ArthurDent on September 29, 2019, 09:24:41 pmHave You some links to this project?

author: ArthurDent
author_rank: Super Contributor
timestamp: September 30, 2019, 03:31:43 pm
content: oo1 - "Have You some links to this project?"Here's a post I made in another thread a while ago and that's basically all there is. Keep in mind I made this supply about 40 years ago and had to redraw the schematic recently. Like I said, it is very similar to the transistor version shown above"When I made my 150-250VDC 100ma power supply I was trying to use parts I had in my junk box while still making a good regulated power supply. The case was from a military surplus magnetic amplifier and the case was the only part of that unit that was useful to me. I didn’t have one transformer that would work but I did have 2 identical smaller transformers that would fit perfectly in the back part of the case and give me the voltages and current I was looking for if I wired the HV windings in parallel. I did buy new meters and to give you an idea how old this is, I bought them from Lafayette Radio Electronics (kind of like Radio Shack) who went out of business in 1981! The circuit I used was one I modified from one I saw in an HP manual from about that time period.The voltmeter was modified to have an expanded range by using a 150V Zener and using a series resistor to make the meter really read 0-100 volts and then marking the dial to read 150-250 volts. If you’re wondering why there is a 1N4007 between the output and the meter, that is to prevent meter damage if the output somehow dropped to zero, effectively putting 150 volts on the meter in the reverse direction. The milliamp meter is the same basic movement as the voltmeter with the series resistor removed and a shunt resistor in parallel and the scale marked in Ma.One of the transformers 6.3 volt windings is used for the heater supply for the tubes and not referenced to ground to eliminate any heater to cathode voltage problems. The other 6.3 volt winding is connected to terminals on the back of the case to power tube heaters in devices I might be constructing.The capacitor from the supply output to the pot wiper eliminates the D.C. component but sends the outputs a.c. ripple voltage unattenuated to the error amplifier. Instead of using a 5651 gas reference tube like the original design I used an 82 volt Zener that gets its supply voltage from the regulated output. It feeds the reference voltage to the error amplifier through the triode section of the 6U8A used as a cathode follower. Where I didn’t use the 5651, the triode might not be needed and the Zener could probably supply enough current to be connected between the error amplifier cathode and ground, then I could replace the 6U8A with just a 6AU6 pentode. The 6BX7 has a maximum plate voltage rating of 600VDC so it is used within its specs.The HV rectified output from the two 1N4007 diodes is connected to the two 40 mfd electrolytics through a 10 ohm resistor that lessens the surge current and is also an emergency fuse. The two deck power switch is three position non-shorting and in the off and standby positions there is a 6.8K resistor connected from the two 40 mfd electrolytics to ground to safely discharge them. This capacitor has a 500 VDC rating because with no load there is about 450 volts on this capacitor. When the switch is in the operate position the 6.8K drain resistor is switched off and the HV center tap is connected to ground and the output is present."

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 30, 2019, 03:52:22 pm
content: No current limiting (aside from the impedance of the poor tube itself), and it'll explode if ever you should accidentally short the output. But yeah, that's about all you need for a basic dumb regulator.Tim

author: Audioguru again
author_rank: Regular Contributor
timestamp: September 30, 2019, 04:13:29 pm
content: I am also an oldtimer (74). Why are you playing with antique tooobs that soon wear out? I sold my tooobs tuner and amplifier about 55 years ago and the solid state replacements still work fine today.

author: ArthurDent
author_rank: Super Contributor
timestamp: September 30, 2019, 04:36:25 pm
content: T3sl4co1l - "No current limiting (aside from the impedance of the poor tube itself), and it'll explode if ever you should accidentally short the output."As I said, I made that supply 40 years ago and I'm still waiting for the 6BX7 tube to explode, maybe in 50 years.I've never ever seen a tube explode but I have seen some tube plates glow bright red.There is both a primary fuse and a 10 ohm resistor as protection in case you missed that. I also stated I wanted to build a supply out of parts I had in my junk box and I didn't intend to make a complicated supply that would have all the bells and whistles that my programmable precision bench supplies have today. You should be pleased I even put meters on the supply!

author: 001
author_rank: Super Contributor
timestamp: September 30, 2019, 04:59:12 pm
content: great projectthanx a lot for charing

author: 001
author_rank: Super Contributor
timestamp: September 30, 2019, 05:46:16 pm
content: Quote from: Yansi on September 29, 2019, 07:19:46 pmIs it good or bad example?http://www.next-tube.com/articles/hvs/hvrEn.pdf

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 30, 2019, 05:57:01 pm
content: Quote from: ArthurDent on September 30, 2019, 04:36:25 pmI was referring to the OP schematic, perhaps I should've been more specific.The same hazards do apply in your case, but with much less severity thanks to the low current capacity, huge energy capacity, and high voltage ratings (especially K-G) of the tubes in question.Tim

author: SiliconWizard
author_rank: Super Contributor
timestamp: September 30, 2019, 06:16:17 pm
content: Indeed. The eBay kit should work fine, but is it designed to last or be safe? Not quite.Can be an interesting starting point for those willing to experiment/learn about tubes though. Proof is that this eBay sale triggered a series of interesting points made in this thread.Oh, and while we're at it, just be careful. High voltages can be a hazard. So this is not the kind of kit for your kid's birthday, or if you're currently drunk.

author: 001
author_rank: Super Contributor
timestamp: September 30, 2019, 06:36:30 pm
content: Quote from: SiliconWizard on September 30, 2019, 06:16:17 pmDo You have some ideas about possible improovments? Is hfe critical here?Quote from: SiliconWizard on September 30, 2019, 06:16:17 pmI`m drunk for at least last 56yrs so I can

author: SiliconWizard
author_rank: Super Contributor
timestamp: September 30, 2019, 10:28:33 pm
content: Quote from: 001 on September 30, 2019, 06:36:30 pmWell, re-read the thread. Others have suggested a couple things. Add a series resistor to the grid. Add a current limiter. Add a fuse at the input.You could also add some kind of physical protection (like a plastic shell or something) around the high-voltage part, so there's less chance you touch it when you're experimenting with the board.Quote from: 001 on September 30, 2019, 06:36:30 pmProbably not that much.Quote from: 001 on September 30, 2019, 06:36:30 pmNo comment. But still be careful, I don't think it gets any better with age.

author: 001
author_rank: Super Contributor
timestamp: October 01, 2019, 01:35:32 pm
content: Thanx for Yours commentsBut I still misunderstood why transistor didn`t blow when base-emitter voltage rises to 300-75=225V instead 5V

author: SiliconWizard
author_rank: Super Contributor
timestamp: October 01, 2019, 06:04:40 pm
content: Quote from: 001 on October 01, 2019, 01:35:32 pmBecause it doesn't?

author: 001
author_rank: Super Contributor
timestamp: October 03, 2019, 12:41:26 pm
content: OLALA! I GET SOMETHING SIMILAR!Can U tell me is it good idea to use MOSFETs? Why author use neon bulb instead zeners?Check this:

author: ArthurDent
author_rank: Super Contributor
timestamp: October 03, 2019, 02:18:06 pm
content: 001 - "Why author use neon bulb instead zeners?"The 0A2 is not a neon bulb as you imagine it but a 7-pin miniature gas filled cold cathode tube designed to be a voltage regulator with fairly tight regulation, unlike a neon bulb designed to be a really cheap indicator lamp.http://www.mif.pg.gda.pl/homepages/frank/sheets/035/0/0A2.pdf

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 03, 2019, 03:08:19 pm
content: The capacitors are laughably oversized, more a liability than any electrical benefit. It won't handle a short for very long but I think it will actually survive one momentarily.Yeah, it'll perform better -- not to mention be cheaper to make -- with a zener. Less bias current needed, too.Tim

author: schmitt trigger
author_rank: Super Contributor
timestamp: October 03, 2019, 03:34:52 pm
content: One aesthetic advantage of the 0A2 tube vs a zener diode, is that they have that gorgeous glow that only a gas discharge produces. For those of us which look into the aesthetic looks of tubes, it is beautiful.On the other hand, the breakdown voltage changes with ambient illumination. And parallel capacitance should be limited to less than 0.1uF, otherwise the tube will start motorboating. These characteristics are described in the linked datasheet.Plus they are significantly more expensive.

author: SiliconWizard
author_rank: Super Contributor
timestamp: October 03, 2019, 04:04:24 pm
content: Quote from: schmitt trigger on October 03, 2019, 03:34:52 pmAhah yeah, that's part of their charm. Like they also have a "microphonic" behavior. They are susceptible to vibration and shock.

author: 001
author_rank: Super Contributor
timestamp: October 03, 2019, 05:07:45 pm
content: Quote from: SiliconWizard on October 03, 2019, 04:04:24 pmVery strange design for REGULATOR circuit

author: floobydust
author_rank: Super Contributor
timestamp: October 03, 2019, 06:30:17 pm
content: We don't know the requirements for voltage and especially output current.For audio applications, these are HV regulators to get of line and load variations, and absolute voltage is not critical. 1% regulation at 400V is 4V so 396V or 404V is not a problem. I find low noise is harder to get.Don't tell anyone but I've used0B2,12AX7 and 6LC/6550 and no drama there. About 550VDC input and 350VDC output to over 300mA. I had the parts lying around decades ago and thought it would be fun to learn as a teenager. Otherwise I use a simple BJT and zener with CCS for low current loads, say under 50mA.OP's circuit is missing a dedicated (floating on high side) filament winding for the pass tubes, which is an extra expense.These HV regulator threads always end up in a flame war of solid-state verses vacuum tube (pass device) because people want unrealistic output currents in a $5 circuit with a handful of parts. Solid-state HV regulators in the few hundred mA league need many parts, SOA and current-limiting to be tough and reliable. They are dangerous to develop on the workbench and unforgiving of mistakes. There is no simple circuit out there that is 100% solid-state. Sorry but that fantasy has to end, so we end up with these hybrid designs.

author: 001
author_rank: Super Contributor
timestamp: October 03, 2019, 07:38:07 pm
content: Thank You for detailed answerCan You post schematics You told (and built)?

author: 001
author_rank: Super Contributor
timestamp: October 03, 2019, 07:40:50 pm
content: Quote from: floobydust on October 03, 2019, 06:30:17 pmThat the right way to use pass tube with solidstate driver circuit?

author: Yansi
author_rank: Super Contributor
timestamp: October 03, 2019, 09:25:33 pm
content: Follow here, some nice article about twoob power supplies:http://www.angelfire.com/electronic/funwithtubes/bench_psu.htmlI almost got a thirst to cobble up some of these supplies and compare and test their performances against each other.

author: techman-001
author_rank: Frequent Contributor
timestamp: October 04, 2019, 01:13:00 am
content: Quote from: ArthurDent on September 30, 2019, 04:36:25 pmThat's a really nice schematic, it took me back 40 years ago as well.I've never seen a tube explode either and I've run them with red hot anodes for hours, I've had then bright blue inside and sparks flying everywhere (from massively positive grids) before they burnt out.Tubes are incredibly rugged, far more so than semiconductors.I've seen a few TV CRT's IMPLODE ......... after I threw a brick at them

author: floobydust
author_rank: Super Contributor
timestamp: October 05, 2019, 05:54:35 am
content: Quote from: 001 on October 03, 2019, 07:38:07 pmYou'll have to decide if you want to use vacuum tubes or solid-state, or a mix of both.Here are scans of classic, all-tube voltage regulator circuits from some GE and RCA databooks I have.I used the GE circuit with an extra RC filter after the VR tube to filter noise, as gas tubes are noisier than zeners.The RCA one has 0.1% load and line regulation with 0-225mA. The Compactron circuit is very cheap using a neon lamp as the reference.The architecture is very basic and anything can replaced by solid-state components. It's whatever you like. Add some op-amps if you want better performance and more parts and have lots of money.The hardest part is the pass transistor or mosfet, if you use one. They need a lot of protection for high voltage linear use.If your load has big capacitance 100's of uF like some audio circuits use, it will blow the transistor/mosfet on power up. Too much inrush current for too long.

author: Yansi
author_rank: Super Contributor
timestamp: October 05, 2019, 09:47:09 am
content: R1 in the first schematic above is not placed the best. Shall be going from the cathode of the pass tube, not anode. This way, you can get better regulation.In the second schematic you can get rid of OB3 completely, not necessary as you can reference cathode of 6SJ7 to the output minus terminal and the bottom of the feedback divider to the negative reference voltage.

author: 001
author_rank: Super Contributor
timestamp: October 05, 2019, 10:06:10 am
content: Quote from: floobydust on October 05, 2019, 05:54:35 amThanx You a lot! Great workQuote from: Yansi on October 05, 2019, 09:47:09 amHahah No way because it is variable REGULATOR

author: Yansi
author_rank: Super Contributor
timestamp: October 05, 2019, 01:45:26 pm
content: Hahaha, don't be silly. Couple days ago you did not have even slightest understanding how the circuit works. Now trying to be smart, but still no, you have to learn more and harder.Variable or not, the reference still can be supplied from the output. It all depends on many things.

author: floobydust
author_rank: Super Contributor
timestamp: October 05, 2019, 08:04:58 pm
content: In the second schematic, note the 6SJ7 screen is also regulated by the 0B3. This is favorable for better PSRR and line regulation.In the first (Compactron) schematic, which is bare bones cheap, placing the VR tube at the output can add instability as an NE-2's dynamic impedance is not zero. Have to look if the VR ends up in-phase or out-of-phase with the pass tube, as far unexpected feedback in these circuits. I'm used to the VR tube being at the raw DC side, I thought it was to save 10's mA off the pass tube's load in the case of the big 0B2, 0B3.Testing these circuits is a PITA, I used to use a scrap bin big 814 tetrode connected to a signal generator as a load. The workbench looked like Frankenstein's lab lol.

author: 001
author_rank: Super Contributor
timestamp: October 06, 2019, 03:37:23 pm
content: Quote from: Yansi on October 05, 2019, 01:45:26 pmDon`t be @#^&&% pleaseQuote from: floobydust on October 05, 2019, 08:04:58 pmThanx a lot for Your postingIs it good idea to replace ONLY diver tube (6AU6/6SJ7 etc) with some transistor(s)? What the way to do it right?

author: schmitt trigger
author_rank: Super Contributor
timestamp: October 08, 2019, 01:17:38 am
content: floobydust:Thanks for this particular RCA schematic you attached.I have the exact same RCA receiving valve manual RC-29, but in Spanish. Published by Editorial Arbo from Argentina, @ 1974.Although the very first page indicates that this is an authorized version from RCA Corporation, Electronic Components, Harrison NJ, the book has a serious omission:The project 29-22 Series-type stabilized voltage supply (Fuente de Alimentacion estabilizada tipo serie) does not include a schematic! Seriously!It is the only topic in the whole book which is missing an attached image or schematic.Other than that, the Spanish description and parts list is wholly accurate, word by word.Back in my tube experimenting days of the mid 1970s, I remember reading the circuit description and being enticed to build it, but then becoming frustrated that without a schematic I wouldn't be able to do so.Roughly 45 years later, I finally see what the schematic looks like.Thanks!

author: Yansi
author_rank: Super Contributor
timestamp: October 08, 2019, 12:11:45 pm
content: Why R1 in the anodes? That should have been in the cathodes...... to provide negative feedback for dividing current evenly between the twoobs.

author: 001
author_rank: Super Contributor
timestamp: October 10, 2019, 03:47:55 pm
content: Quote from: schmitt trigger on October 08, 2019, 01:17:38 amCan You tell what minimal and maximal output voltages achieved with this strange potentiometer R9?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 10, 2019, 11:48:17 pm
content: Well, not less than 80V, where it will oscillate. But that would require a larger value than is shown. Adjustable range looks to be around 180V to Vin (unregulated).Tim

author: 001
author_rank: Super Contributor
timestamp: October 11, 2019, 12:26:50 pm
content: Quote from: T3sl4co1l on October 10, 2019, 11:48:17 pmThanx a lot!But is it real Vin (unregulated)? It means zero voltage drop at pass tube, izn`t it?

author: schmitt trigger
author_rank: Super Contributor
timestamp: October 11, 2019, 01:18:44 pm
content: It would be at Vin's valley voltage, plus some saturation plate saturation voltage.However, if Vin is very well filtered thanks to a Pi-filter, the valley voltage should be very close to the average voltage.As I mentioned previously, this is a project that I've wanted to build for 45 years...... I might give it a try.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 11, 2019, 11:17:37 pm
content: Yes, it would be saturated high and not regulating.Somewhat less than Vin, it will be in regulation, at least up to some current. The dropout voltage varies with load, of course.Tim

author: 001
author_rank: Super Contributor
timestamp: October 12, 2019, 08:54:26 am
content: ThanxThe strange thing about tube regulators is tetrode mode.Why 6L6 is used as tetrode if anode voltage (Anode to Catode) may be LOWER than second screen voltage (2nd screen to catode)?Triode mode benefit is lower Ri, isn`t it?

author: Yansi
author_rank: Super Contributor
timestamp: October 12, 2019, 09:37:50 am
content: Did you read the article I have posted, didn't you?

author: 001
author_rank: Super Contributor
timestamp: October 12, 2019, 09:42:34 am
content: Quote from: Yansi on October 12, 2019, 09:37:50 amYours link is about common transistor line regulators, not teterodes, isn`t it?So I read this. But why heathkit use 6l6 as tetrode?Quote

author: Yansi
author_rank: Super Contributor
timestamp: October 12, 2019, 09:59:05 am
content: No, I meant this:QuoteNot sure what the heathkit designers did, but having a constant Vg2 lets you optimize the tube characteristic by a lot. If you vary Vg2 from zero up, you can affect the maximum passable current with zero control grid voltage.Too small Vg2 and you can't pass enough current through the pentode.Too high Vg2 and you need silly voltage swing at the control grid from 0 to -Vcutoff voltage, from fully open to close the tube completely. Silly voltage swing at the control grid means you need even more gain in the control loop of the regulator, resulting also in poor regulation.By changing the Vg2, you can affect the control grid cutoff voltage like 10:1 ratio (from -100V to -10V).With a triode, you can't optimize the characteristic by any other means, than replacing the tube by a different type.A little shame usual datasheet do not provide plots with Ia vs. Vg2 at a constant Vg1 and plate voltage, or the cutoff voltage vs others.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 12, 2019, 10:10:19 am
content: The effect of the screen grid is to flatten the plate curves, which in the case of the cathode follower, gives good PSRR.The screen acts as a virtual plate, so that varying the screen grid voltage varies the plate current in the same way varying the triode plate voltage varies plate current. (It is no accident that "triode mode" behavior results from tying screen and plate together.) Meanwhile, plate current is less dependent on plate voltage, i.e. the plate resistance is very high.Another way to look at it is, the cathode current is the sum contribution from control and screen grids. The screen grid has lower gain, by a factor of mu_g2g1. If you don't have screen grid curves available for a given tube, then you can translate: an increase of mu_g2g1 volts on the screen is equivalent to 1V on the control grid.Plate current is then cathode current minus grid currents, which are generally small, and the screen current is normally the most significant.Screen current however gets extreme at low plate voltages, usually below V_g2 / 2 for beam tetrodes. This is the main downside to a tetrode/pentode pass regulator: besides the required screen supply (which has to be referenced to the output, so is usually a separate winding and rectifier), you need to be careful not to run into saturation (dropout), lest the screens go toaster-grid on you. (Some protection can be provided, like supplying screens from a modest value series resistor, and perhaps adding a diode from screen to plate, so that when plate voltage is low, the screen voltage is pulled down; this gives a triode-like saturation characteristic. Better-than-triode saturation can be had with a more clever protection circuit, but this does require more parts, including preferably a PNP transistor, which kind of begs the question, why the tube in the first place...)Tim

author: 001
author_rank: Super Contributor
timestamp: October 12, 2019, 10:19:51 am
content: Thanx againMy question is about this situation:if 2nd grid voltage is about 250vbut voltage drop at pass tube is 90vis grid current became greater then anode?i.e. is minimal regulator drop is limited by 2nd grid voltage?

author: Yansi
author_rank: Super Contributor
timestamp: October 12, 2019, 10:34:36 am
content: 2nd grid voltage of 250V is the silly business I was talking about. 50 to 100V as Vg2 may be more then enough and prevents becoming a toaster grid.//EDIT: To see on an example: PL500 as a pass tube: Maximum g2 dissipation is 5W. At 60V screen grid voltage, you get about 60mA current there, with zero Vg1 and Va. That is still safe within those 5W (3.6W actually), without considering any g2 protection resistors.Yet the tube still passes about 100mA at 20V plate voltage, under the same conditions. More then good enough. (Ig2 falls to about 13mA at this point).PL500 datasheet so you don't have to search for:https://frank.pocnet.net/sheets/030/p/PL500.pdf//EDIT2: Note I am not saying that no g2 resistor shall be used, just saying, less is sometimes more. Any series resistor with the screen grid may be beneficial, also as a means for preventing unwanted oscillation of the tube.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 12, 2019, 11:22:52 am
content: Good illustration of why 6L6s aren't very good here. You want high perveance types like sweeps and, well, voltage regulators (most pass reg types are triodes though), so screen voltage stays low and the plate saturation is low as well.V_g2 / 2 isn't a hard and fast rule, by the way; 6V6 saturates quite low (~30V) even with a high screen voltage (I forget if that was at 150 or 250V or what). EL34 not so much -- true pentodes seem to perform worse. Sweeps typically saturate in the 50V range, with screen in the 100-200V range.An equally useful definition of "saturation" might be, the point where screen current becomes unacceptably high...Oh, and sweeps have higher screen dissipation too, so you can push further into that region, safely.Tim

author: 001
author_rank: Super Contributor
timestamp: October 13, 2019, 08:39:07 am
content: Great idea!

author: 001
author_rank: Super Contributor
timestamp: October 14, 2019, 10:09:44 am
content: Quote from: Yansi on October 12, 2019, 09:59:05 amCan You describe this effect more? (some practic links will be ok too)My disturb is about 2nd grid destroyIs 2nd grid CURRENT rise then 2nd grid voltage more then drop voltage at pass tube?I.e. low drop = damage with 2nd grid. isnt`it?