Why Should You Use a Thermistor in a Tube Amplifier?

Video telling what a thermistor is and why it is helpful in a tube amplifier.

[@Modwiz125]

Thank you for a thorough explanation. The diagram of where it is inserted into the power circuit was very helpful and pulled it all together.

[@robertjames1306]

Thank you so much , great video . This information will help keep my tube gear going ! Robert

[@GroverCricketDaisy]

Great explanation really appreciated the effort you went to.

[@littlegiant6869]

I love your FULL explanation. I finally feel confident in adding one to my old amp. THANKS!!!

[@ianbutler1983]

Thanks, you have a knack for explaining things in a way that I can understand.

[@Kc6vbv]

Great information. I just replaced a thermistor in a Mac 225 and wanted to know how to size one for future use, so this was very helpful.

[@timka880057]

I've been watching you for a while, I have got back into electronics, had some bad times I won't go into, I was in installation of exteriors and construction not by choice, but because I had to to pay bills. But anyway. Don't apologize for any length of teaching, I learn more everyday, some things I had forgotten about electronics. Electronics is my passion I have a little shop to work on Tube products, it's growing with new gear every other month as I can afford... I love music, music is my passion too.. I play and I have a small studio, mostly I play guitar and keyboard synthesizers and program sequencers and drum machines. Anyway. I love your videos, keep them coming Mark, God bless you man!!

[@Frustratedfool]

When I built my own bench power supply, it had a 240v-240v isolation transformer included in the build, as well as voltage outputs. This popped my circuit breaker randomly and for months I would strip out the variable voltage units and replace them. I eventually gave up. Months later, someone on a forum suggested an NTC... problem was solved. I never considered the inrush current from the transformer. I guess you learn a lot from your mistakes. These components are really useful. I have seen some circuits that ‘time’ the cool down, so it would not let you switch off and on too quickly. Thanks for the video.

[@mfr58]

Thanks. Good warning about how hot they might get, in terms of mounting position.

[@frederickcwinterburn1837]

Thank-you, The thermistor is a great idea. I don't have much experience with tube amps, but I'm currently repairing a 1929 Eveready model 32(this one made in Canada)and was contemplating how to improve the filtering without blowing the rectifier tube on start up or damaging a transformer etc. This old radio has 3 selections for different line voltages up to 125 volts but the PTC will add insurance.

[@morrisonAV]

Thanks Mark. Loved the info and really loved the cricket!

[@nonsuch]

Thanks as always for the info. I have a topic you can enlighten us all about if it's on your radar and that's the use of a RED LED in place of a cathode bias resistor. Thanks man!

[@todddembsky8321]

Great information, as a newb, this is one component that I have not heard of.

[@oldricky]

excellent presentation. Thanks

[@choctawhatcheekid2030]

Thanks for another informative video. I use a 10 watt 500ohm ceramic resistor wired across a switch to act as a soft start on my guitar amp builds. Select value to reduce primary voltage to half in the same place in circuit where you have thermistor. After I flip the ON switch I count 7 to 10 seconds then flip the soft start switch that shorts across the resistor giving full voltage to primary. This reduces current spike on filaments. Mr. Carlson's Lab has a video on inrush current on tube filaments and it shows at startup a single filament in a 12AX7 preamp tube has 1amp spike. That's 2 amps per preamp tube. So thermistors could also be used in filament circuit. Also I would like to see you do a video on a series pass circuit to reduce B+ voltage to power tube plates and screens.

[@100amps]

Heya, good video. Just wanted to mention I have some old gear here from the early 1950s that's rated for 117VAC, so it's not such a big deal for us amp folks as it is for old radio restorers dealing with pre-WW2 units. Not sure when the AC supply went to 117V though. Before my time. Having said that, I see 125VAC fairly commonly, and I don't think that was common in the 60s, at least I don't remember it, so the same problem exists and the added resistance is worthwhile. I've also seen folks add thermistors to the secondary side as a soft-start for the B+. I haven't tried that myself so I'm not sure how that behaves.

[@JesusIsLordofMybeing]

Great explanation. Thank you

[@michaelinglis8516]

I love odd components for pedal builds ever since I built a set of optocouplers for a harmonic tremolo out of light dependent resistors, leds and black heat shrink lol. Today I saw something about thermistors and thought I'd look them up and I found your video after awhile. I completly see the value of slowing that surge of current at start up. My main amp that I use 99% of the time is a Marshall DSL100HR. I'm still under warranty so the closest this to a mod I've been able to do is roll the tubes and replace the HT fuse when it blew following a stock JJ el34 blowing(not a fan of JJ EL34'S at all, in my experience I've always had poor sound quality with them sounding muddy and the longest they've lasted before one if the matched quad died on my was about 6 months. And that's with a reasonable bias that took into account the actual plate voltage etc of the amp while plugged intok the power source it would be using in operation. My power was dropping to as low as 104v in summer so I got a boost/buck transformer that boosts by 10v at 116v and bucks by 10v at 126v. That along with a fresh matched quad of EHX EL34'S and a rebias taking into account the possible source voltage fluctuations and I haven't had a power tube blow in over a year.) Anyways I was wondering if using thermistors was common practice with any big name amp builders? I've been inside my amp and looked over the schematic and I don't remember seeing any thermistors around the switch, fuse or input caps. But at the time I wasn't aware of thermistors so I'll have to go back over the schematic now. Luckily it seems like a thermistor could be added at a point that would be less invasive but if hate to get rejected on a warranty repair cause of such a small modification. I swapped out the footswitches in the channel/fx loop 2 button switch that comes with the amp and changed the fx loop to a green led instead of red so the channel an fx loop can be identified in a darker setting if needed. Plus while I was at it I added some led bezels. I mention those "modifications" to note that even with those small mods I worry I might be rejected for warranty repairs. I think it would be rediculous but you never know how far a company will go to save a buck. Great video btw, as soon as I'm able I'll be implementing thermistors in my amp and also when I get around to building my first tube amp from "scratch" (meaning from parts I source myself and using a schematic with modifications to comolement my playing style and needs).

[@djamettp2011]

Thanks Mark! very clear and useful information. Could you pls consider for a future tutorial explaining with more details why do you use a 473j cap in the on/off switch? bye!

[@richard-patphenix3067]

Thx Mark.Good info.

[@rádiosantigos1958]

Very informative video. Thanks.

[@kardeef33317]

Thx 4 the info, I finally fully understand them! .

[@williamstevens7090]

Thanks, Mark. Best discussion on thermistors out there.

[@stephenneff2926]

Nicely done! Now I can install one into my headphone amp. Thank you!

[@littlejohnny1498]

Thank you, I have been wondering about this for a while and what size to use and also fuses in line with your transformer wires and backup diodes for your 5y3.

[@robnic52]

Brilliant topic, clearly explained. I needed this for a small valve guitar amp so many thanks. I'll design a well ventilated site in the casing for a thermistor. I wondered how reliable these devices are? I f you covered that, my apologies.

[@Kalkaekie]

Good video !

[@jjo5917]

Good tutorial thanks

[@zbaby82]

Excellent.

[@OldIronShops]

Great video I'll have to put this to use.

[@robertanderson8613]

Mark I love the videos thanks for your time my kids house got there food ruined and more in that storm anyway my question is could you talk about multiple output tubes like the Cayin A100T keep up the helpful videos thanks

[@RobertKohut]

Nice!! Added one to my st-35...

[@frankstone919]

Thank you for the video. I just ordered a couple of the CL-90's to play with. Not real cheap though on eBay , $13.42 with tax and shipping. I've been using bucking transformers for many of my old power transformers. I found that adding a 0.1ohm 5watt resistor to 6.3v filament windings get them right from 7.2v down to 6.3v. One on each leg of the winding. Amps that I run el34's in voltage is no big thing but , being able to drop a a couple volts on my smaller amps is a must. I recently discovered the transformer impedance thing for tube rectifiers. "Rt" Example: I just built a amp with a old Voice of Music power transformer. That transformer had a ez81 on it but , with 122-123v wall voltage the B+ voltage would have been up there. The transformer had a pretty low resistance for a ez81. I used a couple 250ohm 5watt resistors on each leg of the HT and it knocks about 18volts off the B+. I should have used maybe 180-200ohm resistors but , 250's were all I had on hand. It's only a 120ma secondary and the little 0.1ohm resistors on the 6.3v windings , probably 3.5amp. That amp will plug into the wall. I made a post on el34world forum about it. "Orange AD15 Style Amp" A good subject for a video , with a bit of math , would be determining power transformer impedance for tube rectifiers.

[@jimcatanzaro7808]

You can’t use resistors on ac period

[@mattgarcia440]

Good video...

[@goodun6081]

Many solid state hifi amps use one or more (paralleled) relatively low-ohms-value but large-wattage resistors in series with the incoming AC line, that get shunted out of circuit after a couple seconds by a large relay wired across the resistors(s). This works quite well as long as the relay contacts are clean, so that the relay handles all of the AC feeding the power transformer and the resistors have no voltage on them because of the "closed" relay shunting the voltage around them. However, once the relay contacts become a little oxidized, then you have some voltage flowing through the resistors (with a corresponding voltage drop) and *also* some voltage drop occurring directly at the relay contacts which themselves have formed a resistor; and so, the resistors may overheat sufficiently to break down their solder connections or even burn the board, and the oxidized relay contacts can also start to arc and burn. (This circuit also requires a basic short-duration timer to delay the relay slightly, for just a second or two at most, so it doesn't turn on instantly when you throw the switch on the amplifier). Still, this type of inrush-limiting circuit works quite well, especially for big power amps with very high inrush current due to big transformers, solid-state rectifiers and massive amounts of filter capacitance, *as long as the relay is heavy duty and the contacts get cleaned every 5 to 10 years or whatever*. By the way, one highly regarded and collectible Hi-Fi power amp I know of actually uses thermal fuses in line with and physically touching each of the four or five resistors so that if they start to overheat significantly (because of the relay having becoming too oxidized to form a good low-resistance shunt) the thermal fuses will blow and disconnect power to the amplifier. A quick voltmeter check will tell you if these resistor Bank slash shunting relay circuits are working correctly, ideally there should be no voltage measurable across either the resistor Bank or the relay, which are of course in parallel with each other anyway, but due to very slight contact resistance at the relay you might have 5 or 10 millivolts of AC across these devices. Anything more than that showing up on your meter is telling you that you need to clean that relay.

[@mosfet500]

Thanks!

[@johnlimbach9267]

Thanks for the informative video. I'm a great believer in inrush protection particularly for expensive transmitting tubes, e.g. 3CX1500 etc. Don't use those much anymore but primarily restoring vintage receivers and tube guitar amps. One problem I have since moving to this location is a very high (126VAC) line voltage. So to get to a nominal 110V input I have to drop about 15V. I've been using 2-5 watt wirewound resistors in the hot lead of the line which seems to work well enough and calculating the value is very simple. What's been your experience with this kind of problem? Is there a better way? Thanks.

[@gbass7328]

Item 3: Stripping may not be happening because there is no current flowing in the tubes until the heaters come up to temp, well after the thermistor warms up. So for that 3-4secs the therm has already heated up. Thermistors: I also don't like the saggy affect it has on the power supply. Does help a solid state rectifier PS sound a bit more tube like lol but for punch, you don't want sag. btw - a lot of classic guitar amps didn't have them at all. Best is a timed resistor/relay circuit or a physical switch for a series resistor ie Inrush switch. 110vac vs 120vac line voltages may be true, so if the rails are too high, drop down with an outboard transformer or variac to achieve the actual expected B+. If you're running old enough amps then expect to have a crazy setup.

[@frankgeeraerts6243]

WRONG

[@chuckhanning1174]

Thanks for your video; very helpful. I have a tube preamp that the b+ starts at 590v and 40 mA and then drops to 400 v at 23 mA once the tubes are warm and draw a total of 23 mA after about 30 sec - 1 min. I have four 1.2k resistors 5w to drop the voltage in the power supply and was thinking of changing one to a 1k ptc thyristor that the resistance would rise during the initial over current. After seeing your video, is that the wrong approach? (The heaters are dc and limited to .3 amp )

[@scottjamable]

I have a question. what would be better in a guitar tube amp, a thermistor or a Varistor? Mesa used a snr 130k20 (varistor) from the fuse shield to the neutral terminal in the power supply on old Mark series amplifiers. I want to add protection to an even older mark series amp that I wired a new 3 prong cord into. I love your videos. I live in Raleigh btw. Thank you

[@filo5813]

I'm new to your channel, that was a great video. I want to learn about tube amps, hopefully this channel will get me there. Any suggestions where to start your videos

[@byensbutler]

If you haven't found it yourself: check out Uncle Doug here on YT

[@arnonschreiber3088]

Could you please compare aluminum versus steel chassis for audio applications? Thank you.

[@pauldow1648]

Thanks. Wondering if the thud I hear when turning on marantz 2230 is related ?

[@moodyga40]

You could use a MOV witch will help with brown outs and over voltage

[@stuboyer1901]

Thanks Mark, I've been thinking about putting one in my Marantz 2325 solid state. It has an 8 amp main fuse, what size thermistor would I use? When I did the restoration I raised the main capacitors from 15K to 18K uf 70 volt and a 150 Ohm 125 Volt snubber cap but it keeps blowing the power switch (it freezes on, won't shut off). Any help would be appreciated.

[@mattharrison4238]

How quick do they heat up when placed after the fuse, as you suggest? Does it add a noticeable amount of time to the filament ignition, or is it negligible? Either way, it’s a great idea that I never would have considered, thank you for sharing all of your knowledge with us every week! Keep up the great work! 👍🏻

[@MrMannakin]

It's usually a few seconds

[@oldscoolamps4243]

Can a thermistor's coefficient reverse when failing? I have a modern amp that will not come back on when hot ;yet it isn't blowing fuses . And comes back on when cool. It has a c60 in series on the neutral leg.

[@scottspencer4018]

Can you run multiples of these NTC CL thermistors in series to increase their resistance and steady state current ratings, in case you wanted to knock more voltage down without going with a bucking transformer?

[@ericnortan9012]

I am rebuilding speaker crossovers on a set of large speakers, 15' woofers so they can take some power. The tweeter and mids are each protected by a PTC thermistor. I understand you want to protect the circuit but is there a better way? Obviously, there is the benefit of it being "self-resetting" and cheap.

[@pastamatt]

Thanks for this! If I have an amp that uses a 5 amp fuse, can I use a a CL-80 (3A) and a CL-90 (2A) in series to handle it for inrush protection and mains voltage reduction? (because it's an early 70s amp and I often get 125VAC at my house.... Do Thermisters work that way? Can they handle the larger current by being in series? And if so, do they need to be right next to each other, or ought they be one on each of the PT primary wires?

[@Willam_J]

A few points to consider, when you’re using (or replacing) thermistors (ICL’s) on circuit boards: 1. Don’t jam it down hard on the circuit board. Keep it about 1/4” (6mm) above the board. This will help prevent burning the board, and will also help dissipate heat from the legs, before it gets to the solder joint underneath. Don’t raise it any higher, though, because you’ll risk creating a fractured solder joint, due to vibration, when the amp is transported or being played. 2. After inserting the thermistor, fold the leads over the solder pad. Again, this will help prevent fractured solder joints due to vibration and heating/cooling cycles. When folding the leads over, be careful to trim away any excess so that you don’t bridge over to another solder pad.

[@goodun6081]

William J., thermistors really should not be placed on circuit boards, not unless they are strapped to a heat sink anyway. They can run hot enough to scorch and burn the board, or at least break down the solder joints, especially if lead- free solder has been used. Anyway, legions of "reissue" guitar amps and "modern" tube-type hifi amps aside, circuit boards don't really belong in tube type amps, especially guitar amps, or at least not in the way that they are typically utilized nowadays. I cringe whenever I see an amplifier that has sockets soldered directly to the board without any flying leads, so that the tube pins are unable to "float" or move around a little. And then, the chassis is often a little too small for the size of the board, with the board shoehorned tightly into place, and often physically secured to the front of the chassis by the volume/tone/effects pots (which you might risk damaging or breaking after you unbolt them and try to angle the PCB out of the chassis); and typically you can see the component side of the board but not the foil side, or vice versa, and therefore you will likely have to remove the board in order to change any components ---- but you might have to undo or unsolder a dozen or more wires to get the PCB out for service! Finally, in my opinion, tube type amplifiers should never use double-sided boards, never ever.

[@Willam_J]

good 'un - I agree with you, completely. I wasn’t referring to home-brew designs. I was referring to repairing modern amps, which already have the thermistor soldered to a board. One brand, which comes to mind, and often does this, is Crate. About two years ago, I worked on a Crate amplifier. (I think it was a ‘Crate Club 30’, but I’m not sure.) The guy who owned it, really abused the amp. It first came to me with a blown ICL. I replaced it with an exact replacement, checked the transformers, tubes and basing. Everything checked out and it left my shop. About every six months, it returned to my shop for another ICL. After the second time, I mounted it on a terminal strip, because I was tired of spending two hours disassembling and reassembling the amp, for a two minute repair. He finally blew the power transformer, which there was no replacement for. Crate, themselves, admitted that their power transformer was underrated, which is why I didn’t want to replace the ICL with a stronger component. I wanted that ICL to be the weak link. There were no aftermarket replacements, either. This customer ‘dimes’ every control and, self-admittedly, “beats the hell” out of his amp. #dontabuseyouramp To help ease his pain, I purchased his amp, to use as a donor unit. The speakers, output trans, tubes, cabinet, knobs, etc. where all in good condition. I did, however, promise him that I would sell the amp back to him, if I ever find a replacement transformer. (I’ve know this customer since he was 14, and played in bands with my nephew.) Unfortunately, this isn’t an isolated case, by any means. A lot of modern manufacturers are placing ICL’s on their circuit boards. You’re very right about them being a pain in the ass to get to. Like a lot of other electronic equipment, the designers didn’t give very much thought to servicing their products. The spirit of my post, was to point out how to minimize future trouble, when faced with replacing PCB-mounted ICL’s. Thank you for your input. I’m glad that I’m not the only person that sees the problem with PCB-mounted ICL’s. (And tube sockets! Aargh!!!) I highly suggest, that when people are replacing these ICL’s, that they move them off-board, if they have the room. A three terminal strip, plus hardware, costs about 35 cents. It will prevent further harm to the PCB and make future replacements much easier, especially when you have a customer who abuses their amp. Thanks and take care! :-)

[@goodun6081]

William J. , that Crate amp Transformer probably has a thermal fuse inside of it. Usually they are a little metal can that looks sort of like a cross between a radial capacitor and a small resistor, tucked down in to the windings at the top edge of the Transformer on the primary input side,. Sometimes if you peel away some of the paper or plastic tape, or cut away the white plastic former at the top of the Transformer with notches where the wires come out, you can actually see one end of the thermal fuse it, and occasionally you can even pull it out of there with pliers, and sort of stuff a replacement thermal fuse in its place, but they do tend to be wedged in tightly and varnished in there to some extent, and perhaps difficult or impossible to remove. Usually, the thermal fuses' connections to the Transformer primary and to the power cord are exposed and visible on the outside top edge of the Transformer, and you can probably suss them out with a meter and jumper around the dead thermal fuse with a piece of wire. I don't know how old that amplifier is, but pretty much all modern Transformers from the past couple of decades more or less have thermal fuses tucked into the windings somewhere (to prevent a fire if it is inadvertently left on at home), . it's worth a look, and be aware that almost never does the schematic show the thermal fuse inside the Transformer. For what it's worth, I recently repaired an older Denon turntable that wouldn't spin at all although the automatic arm functions seemed to work. It turns out that there is a thermal fuse inside the direct drive motor unit, encased in some thick shrink tube style rubber and pressed hard up against the windings. We replaced the thermal fuse, and then the platter would spin, except it was going at least a hundred RPMs, due to a leaky driver transistor;, that's probably why the direct drive motor overheated and blew the thermal fuse. (ps, if you ever have occasion to work on a Denon turntable where the arm doesn't have any automatic functions working, you can probably forget about it, they have cadmium photo cells in them that are no longer available and known to fail.) some Denon turntables also use optoisolator or optocouplers that aren't available either.

[@Willam_J]

good 'un - The thermal fuse was still good. He burnt a winding in the secondary of the transformer. One of the few times that I’ve ever seen a secondary winding let go. I peeled back a few layers and couldn’t find a break. When I spoke with Crate’s techs about obtaining a new transformer, they told me that they had a lot of problems with those specific transformers, which is why there were no replacements left. In fact, they even expressed how surprised they were, that it lasted that long. In a desperate attempt to find a new transformer, I even called every Crate ‘authorized’ repair shop, and they told me the exact same thing. It had really odd output voltages, as well, which is why I couldn’t match it with anything. Honestly, though, it would have been futile to even try, because that amp wasn’t worth installing an aftermarket transformer. I played through it, every time I repaired it, and it really didn’t sound very good. It wasn’t even an expensive amp, to begin with. It was loud, but that was about it. I’ve been an EE for 31 years. If there was a reasonable way to salvage that amp, I would have done it. Sometimes, you just have to cut your losses. :-)

[@bernhardnizynski4403]

Would it be safest to use the CL-90 in a medium to high power amplifier (15 to 50 watts) - a chart would be helpful?

[@brothermine2292]

If the tube amp's power transformer is rated at 120V instead of the older 110V, where would be a good place to insert a thermistor? Perhaps after the Standby switch, if the amp has a Standby switch?

[@dri50]

Quick turn on/off: The thermistor may still be warm and not protect like when it's cold, but your tubes are still very warm too. So I suspect the inrush current will not be as dramatic as when things are stone cold. Thanks for the vid. Don

[@BlankBrain]

And the filter caps shouldn't be fully discharged.

[@rich1051414]

It's not about 'protection', it is about slowing the heating of the filament. Fast filament heating is the cause of catastrophic tube failure. You must heat the tubes slowly, you cannot start with the full heater voltage current right out of the gate or the tube will flash, potentially popping it. Slowly ramping up the current with heat is precisely how tubes should be heated. I highly doubt a thermistor will cool slower than the higher mass tube, in any situation xD

[@valentinla1305]

Transformer will be your worst enemy during a quick turn off/on. If the initial voltage polarity and the residual magnetization of the transformer match - the resulting inrush current will be much larger than during a cold start. With the transformer it's also a bit of lottery it may start gently or vice verse depending on the initial AC voltage phase. However, residual magnetization of the core makes things worse.

[@Maxxzinno]

I have some KCC CL190 25ohm 2.4A thermistors. I want to install in my fisher km-60 tube tuner and Heathkit w4-am tube monoblocks. You think it’s ok ? Thx

[@davidnguyen5028]

Would you recommend adding a thermistor to an ADCOM class A amplifier?

[@kenfoland]

Some valves, mostly European-made, have suffered a phenomenon known as "filament flash". If these valves are repeatedly subjected to instant power-on while cold, the filament lifetime is reduced. I suspect this would be useful in that case as well.

[@frankgeeraerts6243]

Yes

[@God-CDXX]

I like these MF72-120D15 i can use these in amps that draw up to 150 watts no prob

[@larryshaver3568]

I bought a Magnavox 4 tube stereo power amplifier and two resistors going to ground from a 6bq5 tube burned right in half[this happened before i bought it.] can you tell if this can be salvaged?

[@beeluv9072]

3 questions: 1) Is there any value to adding thermistors to old Solid State equipment? 2) Why not insert several thermistors in series to reduce voltage down to say 110v? 3) Should I add a thermistor to to my old Hammond organs?

[@jimcatanzaro7808]

I use a b+ relay heating the tubes and putting power is the best I believe

[@richardhansen469]

For even more protection for my first smoothing capacitor and tubes if using diodes instead of a rectifier tube... Can I add an additional thermistor on the DC side between the diodes and first smoothing cap or input choke? This way I would be reducing inrush on both the primary ac as well as filling up the first smoothing capacitor more slowly. What are the advantages and disadvantages of using both an AC and DC side thermistor in place?

[@richardhansen469]

Also can I use these to slow the heater and B+ voltage rise only. But have the bias come up immediately, so that when the heaters are coming up the bias will already be fully up and present to reduce cathode stripping over time. What is the ideal sequence for applying power to the amp using thermistors for some delay?

[@6AM_YT]

Why not before the switch?

[@tommybewick]

I've seen people put big resisters in front of the transformer on 1930's radios to reduce voltage in theory to lengthen the life of them because they originally ran on lower house voltage in the 110 range, this seems like a better option, would you concur or do you think there is no reason to drop the line voltage for these old radios? Great video, thanks!

[@goodun6081]

K2bew, the real reason for using a series resistor in line with the AC power cord was if the radio was designed for series string tubes *BUT* the filament voltages of all the tubes tied together in the circuit didn't equal 110 volts or thereabouts. a typical so-called All-American 5 radio would have a 50L6 or 50 C5 output tube, a 35Z5 or similar rectifier, and three 12-volt tubes, all of which together added up to about 120 volts. However, some radios used only two, or even just one, 12volt tube, which meant that you needed to add a large resistor to drop 12 or 24 volts off of the incoming AC Supply. some radios used a special AC cord that itself functioned as a resistor, and those cords could get quite warm in operation and often turned dangerously brittle with age. one wonders how many house fires were started by these types of radios!

[@gcaligula6653]

I use a 25 watt 8 ohm resistor in series with the power transformer of my Leslie 122 amp, to drop the line voltage. I've been doing that for years because my 6550s tubes would burn up in a month. Now I don't have that problem.

[@God-CDXX]

on small amps you can use more than 1 in series this tends to extend there delay

[@drtoolm]

I was wondering the same thing. Also helps to drop the 122 V at my place, a little more.

[@God-CDXX]

What i call a small amp is a 6L6G push pool 35 watt amp i used 2 of these (( MF72-120D15 )) in series

[@johnsenchak6384]

i would love more deeper discussions on electronics www.eevblog.com/forum/chat/why-should-you-use-a-thermistor-in-a-tube-amplifier-(temperature-coefficient)/

[@boonedockjourneyman7979]

Genius. How the Hell do I keep my wife awake for this? She needs to understand these ideas. You think tube gear matters? Check out your wife’s kitchen. Please, no intent to offend for people in various relationships. You feel me here. I’m an old man.

[@kenfoland]

First!

[@kenfoland]

Awww... Are we ticked-off because I beat you? Or, is that your trigger word?

[@countryhamop4580]

Sorry, but line voltages haven't been 110 since the 30s, not the 60s.