Tube heaters on PCB 2025 - Page 1
Forum section: electronics / projects

author: AlfredHitchcock1999
author_rank: Newbie
timestamp: May 22, 2025, 03:59:11 pm
content: There's been many suggestions over the years toneverroute the heater voltage on a printed circuit board in tube amplifiers. But now, in modern times when even 4 layer boards are quite affordable, what about to use the inner 2 layersjust and onlyfor the heaters (for example AC 6,3 volts from a transformer)—2 huge copper planes (filling the complete PCB) with just a few holes leading up to feed the tubes heater pins all over the board, why not also with a power amp with tube sockets soldered to the pcb (let's now skip the "quality"/heat factor in this case), with for example 2 or 4 power tubes and this might lead to 2 to 5 ampere heater current in total on this pcb with this high AC current (50 or 60 Hz) floating between PCB layer 2 and 3—what will happen?

author: Terry Bites
author_rank: Super Contributor
timestamp: May 22, 2025, 05:02:48 pm
content: Who are these naysaying infidels? Marshall and Fender have been doing it for years.I'm not convinced there's anything to be gained from using power planes for the heaters.A 5mm trace in 2oz copper will handle 5A over a distance of 250mm without tears.I've seen quite a few pcbs go bad because the valves are cooking the board via the socket pins. Cracked off pads and dry joints a plenty. Brown is not a good look nor is that smell which will follow you to the grave.One sensible fix is to use ordinary wired sockets on standoffs. Ceramic sockets of course. Use small coils of thick Cu wire to thermally isolate the socket pins from the board. Get a bit of breeze up there!Maybe consider splashing out on an Aluminium core pcb too, they're coming down in price.

author: AlfredHitchcock1999
author_rank: Newbie
timestamp: May 22, 2025, 07:31:35 pm
content: Thanks, the idea here was more to try something new and make the routing more simple, why not use 2 big planes of otherwise unused copper, if this doesn't make a total chaos on all other connections on the top and bottom layers, that was actually the question here.Btw, what is "ordinary wired sockets on standoffs." ? Sounds interesting from a guy who knows about burned out tube PCB's and the smell which will follow the responsible down in the ground

author: TimFox
author_rank: Super Contributor
timestamp: May 22, 2025, 07:43:19 pm
content: Traditional tube sockets have a mounting plate with two screw holes surrounding a large insert for the socket pins.One can move the socket away from the PCB using two tall washers or similar stand-offs between the plate and the board surrounding the screws.

author: coppercone2
author_rank: Super Contributor
timestamp: May 22, 2025, 10:48:45 pm
content: I don't see any reason for a large power plane for a heater. What is it doing having inrush?? It theoretically loads things by capacitance too. Its like the least important thing to control. Its unregulated transformer voltage. Make them as big as required for current and nothing more.I guess you can put decoupling capacitors too, if you think something is going through the heater.Putting lots of conductor of electricity near where its not needed is typically the opposite of sensible design practice.And it always make failures worse. The last thing you need is some charred PCB connecting god knows what to god knows what.Its kind of like making your whole house a water bed?Planes always make me think of the reactor in Aliens. One stray bullet and things get ugly very quickly. "lets just put cooling jacket.... everywhere". On mechanical things having cooling jackets is like a nuisance and hazard.I think of electricity on a PCB like wiring in the house. How insane would it be if you had like... wide thin bus bars in the wall of your house behind the sheet rock? Having thicker wires makes sense and the overall detrimental effect is minimal. I guess you need them now, to some extent, but its still like best to use them rationally. not 100 paint over coverage, if you can, IMO. I started to get a feeling there is a bit of paranoid 'rule of thumb' design with undetermined concerns over EMC to ensure there is less revisions ?Even PCB heat sinks have limits of utility. If you make them big enough there is diminishing returns. And these planes are focused for like... high speed digital circuits, with very fast pulses to control transistor gates.

author: TimFox
author_rank: Super Contributor
timestamp: May 22, 2025, 11:11:41 pm
content: "What is it doing having inrush?"Vacuum-tube heaters have substantial inrush current when cold, settling down to the advertised value in roughly 11 seconds.

author: coppercone2
author_rank: Super Contributor
timestamp: May 22, 2025, 11:26:15 pm
content: What is it doing to help a heater coil (light bulb)?And by inrush I mean nano second digital things. Power planes are discussed in the context of high speed digital design, where tiny gates and nano seconds are relevant. It is probably going to be measured in pico farads if you try to do it. on a heater. ..13 seconds is like 10 billion years to the relevant circuits (that is if you ran the tubes on DC at that), with AC its even less relevantIf you wanted to decouple it for some reason, a 10nF capacitor would beat the power plane to death. I don't think there even is a point to think about a power plane helping a heater for any filter reasons

author: TimFox
author_rank: Super Contributor
timestamp: May 22, 2025, 11:40:08 pm
content: In the context of heaters of all kinds, "inrush" refers to the initial high current when the element is cold.The only reason to consider large planar areas for tube heater feeds is reducing the series resistance to avoid undervoltage after the tube reaches operating temperature: better done with AWG18 wire.

author: coppercone2
author_rank: Super Contributor
timestamp: May 22, 2025, 11:41:45 pm
content: I was thinking in the context of power planes and their usual application to high speed digital logic (gate inrush for transistors)

author: AlfredHitchcock1999
author_rank: Newbie
timestamp: May 23, 2025, 05:44:52 am
content: That movie clip visually explain this which seems like a bad idea spreading out 50/60 Hz all over the place, and as you say like making a whole house a water bed

author: coppercone2
author_rank: Super Contributor
timestamp: May 23, 2025, 06:00:57 am
content: I mean its like a minor design detail I am just saying that its best to keep planes... reasonable. Most people are satisfied with "polygons" that is a limited big trace. You can't really go without them with modern digitalAnd I don't think its always better, always keep in mind how cheap people are with electronics. Exploiting the PCB as a heat sink, interconnect, capacitor is all possible, and done, but its IMO not always the right choice.Some examples:1) edge connectors (instead of proper connectors) - the fiberglass on the edge can build up 'fretting' from insertions and become conductive because its a rough unfinished 'sponge'. Then suddenly top and bottom can get connected if you use it alot. This appears to have happened with my DE-5000 LCR meter insert. And its not very durable or fixable compared to a real connector.2) heat sinks - small and cheap to use PCB as heat sink but it has to be done smart, I.e. put a heat sink on the other side for special parts that get hot, and use the right kind of VIA (themrally conductive via). You will find its easy to get into a 'treez' position with this where you run chips hopelessly overheated, and have a generally hot PCB (hot caps, etc) because you refuse to buy heat sinks. If the PCB is kept at elevated temperature, face it, the likelyhood of potential problems is magnified3) capacitors/power plane - complex interaction that might make board revisions difficult because of EMC design changes difficult, and its a short circuit risk when there is damage, marginal boards, etc. However it is probobly essential for ALOT of electronics. Sometimes the data sheet expects it. But keep in mind, the data sheet is advertising and it very often can be out smarted4) filters/inductors - with the right PCB you can make antennas, RF filters, etc for high frequencies. But suddenly you are at the mercy of board quality, you might have uncontrolled behaviors because of aging and drift of dielectrics. ALso a big corrosion risk. I have recently seen QC on a bunch of stuff that had antennas on the edge of the board. When it went into the salt spray machine, many failed at the antenna. I don't know why exactly, but it just seems that it liked to eat the antenna trace.5) PCB stacking - (i.e. like using a PCB as a SMT part) - these like to get cracked solder joints for some reason, even if it looks great. Like you get those boards that you 'perimeter solder' for wifi modules and stuff. Plating/pcb is just weaker then stampings6) right angle PCB connectors (put a PCB in a slot to make a solder on 'daughter card') Again, fragile. Its not a remotely proper solder joint unless you have edge plating (soldering relies on two solderable things being in close proximity, its not supposed to be like a "weld' where you stack the bead, as you do with the right angle 'hack'). And even then its two PCB materials that are soldered together. Get header pins instead.7) TVS. You can get a low quality 'arc discharger' by making PCB with little spikes that act as spark gaps instead of a GDT. But use your head, its not very reliable compared to a ceramic and tungsten specialty inert gas filled discharge tube module (available for $3 from littlefuse etc)8 ) switches - gold plated PCB switch like for ranging. How many problems do people have with these? The gold is never thick enough to actually be a good switch, it contaminates easily, etc. I wonder how many fluke meters were thrown out by now because of problems with this mechanism (which has been being 'perfected' for 50+ years). Study real switches and you will see that its basically low performance as far as switches go, but difficult to do without because of compactness (and mostly cost). Great feature for putting Flukes in the hands of people that can't afford them though. Low current switches do get a little bit 'funny' though9) motors - I don't have any experience with this. But they make motors that are built into the PCB (windings are PCB with traces and vias). I assume the main problem is that you are trusting a ton of electroplated via over a enamel wire (soft very high quality copper, I think the reliability must be low compared to a real motor)10 ) contacts - using the PCB as a part of a pushbutton switch or as a 'target' for a spring. Very susceptible to corrosion, not durable compared to some material like brass or bronze to make a real 'target'./You have to think alot about how to leverage it for good implementation and I think as usual, there is inherent deficiencies in the process (as there is with using regular parts)All these solutions also lead to modifications being difficult, more time consuming, etc. PCB are not the most stable material either (sometimes warps, delaminates, etc).The more mass produced it is, the more you see it because they love those kinds of cost savings, and they have huge volume so they can statistically trouble shoot these things to a degree. CAD software manufacturers love pushing this because it gives you 'value' from the program being able to suddenly 'make parts' on your PCB. Board manufacturers get more volume. So you do see alot of great advertising for these techniquesEssentially the mechanical equivalent is being creative with sheet metal bendings. Do you like metal pins for a hinge, or some kind of fancy bent sheet metal pseudo cylinders, for something like a handle? Or do you want a flexi-plastic hinge? It's all about the $$... Of course there is some awesome light weight aerospace designs and stuff, but its kind of specific to actually want those features, they always have a down side.I am sure there are other things that you can get PCB to do, these are just the ones that come to mind.

author: coppercone2
author_rank: Super Contributor
timestamp: May 23, 2025, 06:42:54 am
content: and boards with large planes can behave differently then boards without them mechanically

author: AlfredHitchcock1999
author_rank: Newbie
timestamp: May 28, 2025, 05:13:29 pm
content: What about (on a 2 layer board) route the 2 pretty wide tracks exactly on top of each other? Any benefits or actually the opposite this way?

author: floobydust
author_rank: Super Contributor
timestamp: May 28, 2025, 06:58:13 pm
content: Go back a hundred years and look at how tube gear was wired - there's a few well known (in the day) practices that are lost knowledge and bungled carrying over to PCB's.AC filament wiring was done using a twisted-pair, as far away from the signal path as possible.Once you understand the benefits of a twisted-pair, then duplicate that on a PCB. But today, AC heaters are usually only for the high current, say output tubes in an amplifier.DC-powered filaments prevail today for pre-amp sections. It's common to bias the filaments up to say 60VDC for lower noise, but this can inject noise if it's not well-decoupled.In general, a PCB ground-pour is a no-no with tube circuits. Because it adds enough parasitic capacitance to load down the high-impedance nodes and cause HF rolloff. Even a few pF can be a big deal with the megaohm impedances.Modern PCB layout relies on the ground pour, but instead you have to think hard and design around using one. Nobody like to think hard nowadays, so much mental effort.Modern VOX amplifiers arcing and carbon tracking because the HV spacings are inadequateIt's at the HV secondary power transformer input to the PCB. OOPS.Single-point (star) grounding, certainly for the high current stuff like output tube cathodes, filter caps etc. is a must. Fender knew even back in the 1950's if you stare at the wiring in old amps, you can see they did not just slap together the point-point wiring, it was well planned by an EE.I do see a lot of chinese copies of for example this KSL-M77 clone but look closely at the PCB, it's done poorly. Way too much ground-pour. Filaments are DC so those traces don't matter much.Don't let the boutique parts fool you, I chuckle at the 2N3055 an old RCA part of course for better sound lol.

author: AlfredHitchcock1999
author_rank: Newbie
timestamp: May 28, 2025, 08:30:31 pm
content: Wow yeah, though I have heard many time point to point constructions sound softer and more "calm" and "balanced" (less high end) – in guitar oriented products compared to PCB made similars (=wished for similars). Why then not add copper ground planes to add what you are warning for in this case? And what about putting 6.3V AC heater PCB routes on top of each other to eliminate eventual hum? Or will it not help, or get even worse?

author: floobydust
author_rank: Super Contributor
timestamp: May 28, 2025, 08:43:19 pm
content: I think running the heater traces as a pair (either adjacent or on top of each other) will keep the radiated B-field low but then the E-field is a problem, so you need shielding (ground pour) over them as well. They would have 3-6VAC on them which is terrible in a high gain, pre-amp kind of thing, causes hum everywhere.

author: TimFox
author_rank: Super Contributor
timestamp: May 28, 2025, 08:45:34 pm
content: Twisted pair of wires has an advantage for both H and E fields.

author: AlfredHitchcock1999
author_rank: Newbie
timestamp: May 28, 2025, 08:56:11 pm
content: Thanks, this is quite difficult, in other words? Almost impossible without twisted damned wires...

author: TimFox
author_rank: Super Contributor
timestamp: May 28, 2025, 09:02:41 pm
content: Twisting wires is not difficult, and the twists/cm are not critical.

author: AlfredHitchcock1999
author_rank: Newbie
timestamp: May 28, 2025, 09:12:08 pm
content: Well twisting, wires in general, seems in this case not needed, must be possible to make it quite easy on a PCB, progress.

author: coppercone2
author_rank: Super Contributor
timestamp: May 28, 2025, 11:21:25 pm
content: you can kind of do it using a bunch of vias it will just be like unreliable crumbly wireyou need to build a wire twisting tool, it started to hurt my hands doing it. Take a PCB, put a bolt to it to grab with a drill, and then solder two alligator clips to it (put them upright in hole) and then you can put wire in the clips and spin it with the drill. works well for fine wire. for thicker wire you might need something different then alligator clips. just need new clips that are good and close rightprogress usually goes where its needed, trying to eliminate twisted pairs (with their awesome performance) is like not easy and there is little reason to do it.PCB's use twin runs at best, trying to twist it means alot of vias and its probobly not going to work out too great, and its not the same as a helix.

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: May 28, 2025, 11:31:36 pm
content: Tektronix ran heaters on PCBs on some letter-series plugins such as the 1S1.

author: floobydust
author_rank: Super Contributor
timestamp: May 29, 2025, 07:33:55 pm
content: It's either off-board twisted pairs, or just go DC:diyAudio - filament wiring tube heaters on pcb's15 page threaddiyAudio Heater Wiring - the Good the Bad and the UglyUsing a bunch of PCB via's like this pic I found, it looks uglyand you have a couple amps through them.Then you have these boutique builds that still get it wrong.Something subtle to note is because the filaments connect to most tubes, any RF will get coupled between them. Then it's capacitively coupled to the cathodes.I've seen it typically as AM radio and power line hash. Unbypassed or (not RF bypassed, just an electrolytic) cathodes get affected.Tried 0.1uF disc caps to GND and it actually made it worse. Ahhh in the days of the ARRL you'd use chokes and feedthrough caps etc.