Designing a PCB for a tube amp - Page 1
Forum section: electronics / beginners

author: dazz
author_rank: Frequent Contributor
timestamp: October 29, 2019, 07:07:20 pm
content: I'm in the process of designing a pcb for a tube amp that has a JCM800 preamp and a single ended EL84 power amp. Apparently this thing has quite a lot of gain and is prone to being noisy.I've been doing some reading on ground planes, ground loops and all that stuff and this is where I'm at (see attached pictures)I tried to keep the power supply away from signals and the input section, and my main concern at this point is how to handle my ground connections, but any suggestion or correction will be very much appreciated. I've only designed a bunch of low signal guitar pedal pcb's, and this is my first one in Kicad, so I don't really know what I'm doing.

author: andy3055
author_rank: Super Contributor
timestamp: October 29, 2019, 09:02:11 pm
content: The filament supply can introduce a lot of hum as they are AC. In the old amps, the wires are twisted so that the AC hum will get cancelled. You could probably run the filament supply on separate wires and twist them instead of putting them on the PCB. Having said that, I have noted that some guitar amps have them all on the PCB (that is the most recent I have seen a tube amp). I guess it is a matter of trace layout/design and how you can minimize the effects of AC. In any case, keep the signal traces as far as possible from the filament supply traces.That rendering and design looks fantastic! What software did you use?

author: dazz
author_rank: Frequent Contributor
timestamp: October 29, 2019, 09:25:06 pm
content: Quote from: andy3055 on October 29, 2019, 09:02:11 pmThanks Andy. That's Kicad's 3D viewer, really cool stuff!I put the filament traces on the bottom layer and most of the signal traces at the top, not sure if that will be enough separation, they cross at a couple of spots. Maybe narrower filament traces would help prevent hum there? They're 1mm wide right now

author: andy3055
author_rank: Super Contributor
timestamp: October 29, 2019, 09:41:42 pm
content: Quote from: dazz on October 29, 2019, 09:25:06 pmThanks for the reply. The filaments take a higher current and so, you have to keep some thickness in those traces if not they might start burning and also damaging the board. Putting them on the bottom of the board will help . Make the board as thick as possible. Since we have no experience in this area, only building it and trying will help to make a determination. Worst case, you can run wires later. Of course, you prefer to have all on the board

author: schmitt trigger
author_rank: Super Contributor
timestamp: October 29, 2019, 10:16:59 pm
content: Some people swear by DC voltage applied to filaments, specifically on high gain stages, to minimize hum.

author: andy3055
author_rank: Super Contributor
timestamp: October 30, 2019, 01:28:14 am
content: Quote from: schmitt trigger on October 29, 2019, 10:16:59 pmOther than the fact that you need some heavy duty diodes/bridge rectifier, I can't see why not. May be in the old days, it was too expensive to do that and they only had selenium rectifiers that were costly.

author: dazz
author_rank: Frequent Contributor
timestamp: October 30, 2019, 02:36:27 am
content: I have the filaments referenced to ground via resistor divider. Other than that, I don't know if there's anything else I can do to keep them from inducing noise. Hopefully I don't need to go DC for the filaments cause I would need an external power supply for that. Thanks guys

author: andy3055
author_rank: Super Contributor
timestamp: October 30, 2019, 03:34:07 am
content: Quote from: dazz on October 30, 2019, 02:36:27 amWhy do you need an external power supply? You already have the filament winding. The two resistor thingy with the ground tap works. Actually, you should get a wire-wound pot so that you can start at the mid point and move it one way or the other to see where you have the least hum.All you need is a bridge that can handle the total current of the filaments.

author: vk6zgo
author_rank: Super Contributor
timestamp: October 30, 2019, 03:47:57 am
content: Quote from: andy3055 on October 30, 2019, 01:28:14 amIn tne old days, you could count on new tubes to not have heater / cathode leakage, so any injection of hum from the heater was negligible.Sadly, these days, that is no longer the case.On the subject of PCBsIn the early days of this technology, manufacturers seized upon it, & produced equipment with tubes on boards.Much to their consternation, (or more correctly that of their customers), the PCBs quickly began to deteriorate beneath the tube sockets, due to heat.In the best case, tracks became hard to solder to, in a slightly worse case, tracks peeled, & in the worst case, delamination & charring of the board occurred, with most boards progressing through the three stages over time.These were Phenolic boards, & FR4 is a lot better, but by no means, immune to this problem.

author: floobydust
author_rank: Super Contributor
timestamp: October 30, 2019, 03:52:42 am
content: That's some nice hard work, it's very compact.With no negative feedback loop (from the loudspeaker back to the pre-amp) this will have much hum, noise and gain, very hard to tame. The original Marshall has push-pull with loop feedback. The EL84 would need some extra circuitry if it drives the output transformer with no NFB. I'd consider adding a radiator wire too if you like a bit of wail, something Mesa does.A ground-pour does not cure all ills, and adds to problems with high voltages and high impedances. You must have proper spacings and better off with no ground fill on some traces/sections. Vox AC15 screwed that up and has arcing and carbon-tracking problems:https://www.eevblog.com/forum/repair/working-on-a-smoked-vox-ac15-guitar-amplifierThe via's coming from your rectifier diodes does not have decent creepage/clearance.Roughly, 400VDC/AC is about 1.5-2mm minimum. This is just for the HV terminal block, rectifiers, B+ feeds, and plate circuit. You can have much less spacing on signals as they are low voltage, but even a few pF of PCB stray capacitance can roll off the highs.Note your filament trace (V1, V2 pin 9) runs real close the input parts (i.e. R2) and overtop sensitive pcb traces (to C2), which is a no-no. You would have a ground guard-band around all sides of the filament trace(s) and run no sensitive signal traces crossing over top. You get capacitive coupling of hum. DC filaments for the pre-amp tubes is the quietest; the tubes with cathode bypass capacitors it doesn't matter to them. Some product designs use flying wire only for the filaments instead of the pc board.I don't see you grouping the high-current grounds together to keep circulating currents out of the pre-amp section. The two rectifier's anodes, C7, R20, C16 should be tied together as a "power ground". The approach is to have two star grounds, one for high current and another (quiet), then tie them together at one point. The JCM-800 used chassis points for this, Fender knew to do this even in the 1950's and point-point wiring.I'm not sure about some part values, R2 is huge and will add noise, C3 is usually many uF. It must be a thin sound.

author: TERRA Operative
author_rank: Super Contributor
timestamp: October 30, 2019, 03:56:48 am
content: I wonder if it is worth putting some holes in tje PCB in a ring around each tube to help airflow?

author: vk6zgo
author_rank: Super Contributor
timestamp: October 30, 2019, 04:01:05 am
content: Quote from: TERRA Operative on October 30, 2019, 03:56:48 amThat was certainly done in the old days, & helped, especially if a fan was mounted under it.The latter wouldn't be too good on soft musical passages, though!

author: floobydust
author_rank: Super Contributor
timestamp: October 30, 2019, 04:14:43 am
content: I tried a ring of 8 of 3/16" holes and found it doesn't work, there is so little convection airflow possible through small holes, and the pcb standoffs can be too low.Pearl tube coolerswork ~50C cooler, would use on the EL84 if there's room.Likely OP might will place this upside down in the cabinet.

author: vk6zgo
author_rank: Super Contributor
timestamp: October 30, 2019, 04:34:02 am
content: A smaller number of larger holes would be better--- that's what they did in the old days.It still wasn't perfect, though.

author: Cubdriver
author_rank: Unknown
timestamp: October 30, 2019, 05:32:13 am
content: I'll second (or third or fourth) the suggestion to get the tube heater supply lines OFF of the PCB, and to run them externally using twisted pair wiring. They draw a relatively heavy current, and are inclined to spread hum around. Twisted pairs are somewhat 'self shielding' in that the twist distributes the radiated fields such that they tend to self-cancel. Either put holes with pads immediately adjacent to the heater pins on the tubes and solder them there, or solder them directly to the socket pins where they come through the board. Get them up and away from the board a bit, too.This technique is shown below in this photo of an ST-70 clone amplifier. Unfortuantely, I didn't take a good picture of the heater wiring specifically, but the brown twisted pair near the large silver coupling caps just to the right of center at the top of the picture shows what I'm referring to (there is also a green pair to the left, feeding the other phase splitter tube):Nice work on the design and render!-Pat

author: dazz
author_rank: Frequent Contributor
timestamp: October 30, 2019, 11:32:23 am
content: Thanks so much for you input, guys.Quote from: vk6zgoWhat a shame.You would think that, as time goes by, the manufacturing process would improve, but we live in the era of planed obsolescence.

author: Gyro
author_rank: Super Contributor
timestamp: October 30, 2019, 11:42:58 am
content: Luckily, at least in audio stuff, the Cathode circuits are usually the lowest impedance part of the design (a lowish value cathode resistor, often bypassed). This helps to minimise the effect of heater-cathode leakage.Biasing the heater supply to be positive relative to the cathode voltages ,say, +20-50V does help though.P.S. You might want to consider including footprints for grid-stopper and cathode stopper series resistors for U2A. Cathode followers have a bit of a 'reputation' for poor sound quality, but this is usually down to parasitic oscillations when driving a capacitive load. You can always fit jumpers if not needed.

author: dazz
author_rank: Frequent Contributor
timestamp: October 30, 2019, 12:31:04 pm
content: Quote from: floobydust*compliments my work... proceeds to demolish it with devastating criticism*Thanks so much! love thatQuote from: floobydustYeah, I came across a message of yours in a similar thread here from 2017 where you mention that wire, but isn't it meant to do the exact opposite? isn't the wire there to push the amp into feedback oscillation instead of taming it? At any rate, it sounds like a great idea to try, and adding negative feedback too. I will google the original schematic to find out which stage does the NF plug to.Quote from: floobydustOh, I thought the issue was just stray capacitance of tracks+gnd plane rolling off high freqs. Charring sounds a lot scarier!Unfortunately that link is broken, but I'll look that upQuote from: floobydustOh, that's something I thought I got tackled. I thought I was good with 1mm spacing for my vias since they will be coated. My PWR netclass has 1mm spacing, but that won't be enough separation between bare pads and tracks or vias, right? bummer. I'll see what I can do about that. I wonder if there's a way to set different spacings for bare pads and coated vias/tracks in KicadQuote from: floobydustI see what you mean. Another great piece of advice, thanks again. The first stage is good, I think, but C2 is at the input of the 2nd stage, which becomes the first in the low channel, so yeah, that's a screw up (I assumed that the signal would be large enough after the 1st stage of amplification -some 30dB- that it wouldn't matter, is that wrong?). I'm not sure there's a lot I can do about it though. No matter how I route this thing, I'm going to have some signal traces on top of the filament's, I reckon. Because it doesn't make a difference if the track that crosses the filaments is the one that goes to the grid, the cathode or the plate, right?It's increasingly looking like I should indeed get those filament traces out of the pcb and hook them up with external wiring.Quote from: floobydustYou don't see me doing that because I had no idea about that.So I guess my grounding scheme is all wrong. I thought I needed to keep the power supply ground plane connected to the rest right at the star point, close to the power supply. Wouldn't it do same thing you suggest if I added another keepout polygon between the output tube and the preamp section?

author: dazz
author_rank: Frequent Contributor
timestamp: October 30, 2019, 12:33:56 pm
content: Regarding heat management and adding holes around the power tube, I read somewhere during my research that ceramic sockets help a lot. Here's hoping that's true, cause I don't have enough room for holes.EDIT:Quote from: floobydust on October 30, 2019, 04:14:43 amOh!, that's pretty neat! I'll see if I can find them here

author: exe
author_rank: Unknown
timestamp: October 30, 2019, 12:37:07 pm
content: I'm not sure the ham is induced by the traces. The frequency is only 50/60Hz, so I'd be surprised that a 20-30cm trace would be an effective antenna. I'd expect ham to come from the environment, or somehow else gets coupled into the circuit, but not directly radiated by traces/wires.

author: dazz
author_rank: Frequent Contributor
timestamp: October 30, 2019, 12:47:09 pm
content: Quote from: Gyro on October 30, 2019, 11:42:58 amThanks Gyro, I'll need to look up "cathode stopper", that's a new one on me

author: Gyro
author_rank: Super Contributor
timestamp: October 30, 2019, 01:02:46 pm
content: You're welcome. Basically it is a low value series resistor that helps isolate the cathode (well the grid-cathode voltage) from the capacitive load. The ECC83 is a fairly high gain tube which is being used as a unity gain buffer in a cathode follower so stability can become an issue. You will often see similar things in transistor emitter followers and opamp outputs where unity gain into a capacitive load can be an issue. go for something like 47 - 100 ohms, that doesn't make any practical difference to the circuit impedances.The golden rule for stopper resistors is to put them as close to the tube as sensibly possible.

author: dazz
author_rank: Frequent Contributor
timestamp: October 30, 2019, 02:21:53 pm
content: Quote from: Gyro on October 30, 2019, 01:02:46 pmAdded the grid stopper, but I still don't understand what a cathode stopper is. I already have a cathode resistor in R17, would it be a resistor between the grid and the cathode?

author: Gyro
author_rank: Super Contributor
timestamp: October 30, 2019, 02:50:25 pm
content: No, just aseriesresistor on the cathode, just like the one on the grid (doesn't matter whether you put it before the junction with R17 or after).P.S. I'm really talking belt and braces precautions here.

author: dazz
author_rank: Frequent Contributor
timestamp: October 30, 2019, 02:56:58 pm
content: Quote from: Gyro on October 30, 2019, 02:50:25 pmAh, I see what you mean now. Sorry, I'm a bit slow sometimesETA: And of course, the capacitive load you were referring to is the tone stack. It's starting to sink! finally!

author: Gyro
author_rank: Super Contributor
timestamp: October 30, 2019, 03:06:44 pm
content: ... and just general PCB parasitic capacitances (hence putting it close to the tube).

author: dazz
author_rank: Frequent Contributor
timestamp: October 30, 2019, 03:36:23 pm
content: Quote from: Gyro on October 30, 2019, 03:06:44 pmThanks again, this is the kind of stuff I'm looking for. The end product, the pcb, the amp are secondary, it's all about learning.Found this article that seems relevant to the issue:https://www.analog.com/en/analog-dialogue/articles/techniques-to-avoid-instability-capacitive-loading.html

author: floobydust
author_rank: Super Contributor
timestamp: October 30, 2019, 07:07:58 pm
content: Analogue electronics has historically always been prototyped or breadboarded, see the master's Jim Williams or Bob Pease's work. That's one advantage of point-point wiring, turret strips, terminal strips etc. is you can move parts and wiring around and experiment, learn, try new things, almost instantly.Nowadays people want to go straight to making a PCB, which is like working with concrete - one it's done hardening, you won't be moving traces around and learning from that. Troubleshooting hum and noise due to a few traces or bad ground strategy is nearly impossible. Your circuit must work beforehand, be tested.In reviewing a PCB layout, I also look to see if the circuit makes sense. If I think a circuit isn't going to work, I hit the brakes on the pcb layout. Design Review meetings take many hours with a roomful of engineers.The architecture is three gain stages with overdrive possible at any one of them, so it's designed for rock and roll, the Marshall sound of my wasted youth. But swapping in a single-ended EL84 section may not be right. Have you built this, tried it out? You want similar gain of the EL84 stage to the push-pull pair and phase-splitter/presence control/NFB of the bigger original.

author: Audioguru again
author_rank: Regular Contributor
timestamp: October 30, 2019, 07:17:56 pm
content: Where will you buy a power transformer, an output transformer and first and many replacement vacuum tubes for this antique amplifier design?Negative feedback reduces the gain, might cause oscillation and reduces distortion that the design had because the distortion was part of its design.

author: dazz
author_rank: Frequent Contributor
timestamp: October 30, 2019, 07:49:32 pm
content: Quote from: floobydust on October 30, 2019, 07:07:58 pmI haven't built it myself, but a friend has built a different (one layer, etched at home) pcb, and he managed to make it work after a lot of fiddling, cutting traces, rewiring grounds, etc. I can't say how closer it is to the original jcm800 tone, but it does sound pretty damn good and marshally when it works. It won't be the same, of course, if only because the power stage is so different, but that doesn't matter to us. At this point it's a bit of a personal challenge for us.BTW, I pulled the schematic and I can't find the negative feedback anywhere:https://www.thetubestore.com/lib/thetubestore/schematics/Marshall/Marshall-JCM800-50W-4010-Schematic.pdfPerhaps it's there in some other jcm800 model, not sure

author: dazz
author_rank: Frequent Contributor
timestamp: October 30, 2019, 07:51:52 pm
content: Quote from: Audioguru again on October 30, 2019, 07:17:56 pm12ax7 and el84's are readily available, and the output transformer won't be the same as the original since de power amp is different too. I don't think that will be an issue, unless I'm missing something here

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 30, 2019, 08:24:23 pm
content: Regarding ground planes: this is actually one of those situations where it's less preferable. Because of the high impedances (1kohm+). At the very least, the clearances must be higher (50 mils?) to deal with the voltages. Trace widths can still be thin (I'd go with 20 mil I think, just so there's some meat in case something gets scratched).That leaves shielding, between input and output, and between stages. And between heater supply, I suppose. It's certainly still valuable to have ground filling these locations. You can still pour ground with these rules set, and then route manually where the clearance prevents the pour from connecting. Set the rule for pours, with a modest exception for traces (maybe 20-40 mils, whatever the maximum required to route to, or between, pins is). A brief stretch of closer proximity won't matter as far as capacitance is concerned, and low voltages (most grids and cathodes) don't need as much insulation clearance.Regarding the layout as shown:Don't do single point or star grounding nonsense, it takes an expert eye to resolve correctly and you're probably setting yourself up for failure otherwise. Just pour everywhere given the above rules, stitch over traces where needed, and trust that ground is filling well enough that any voltage drop will be negligible (and that no RF resonance will reach a high enough impedance for anything to oscillate -- the aforementioned grid stoppers help greatly with this, too).Try to route the heaters separate from the rest of the circuit, and route them close together to keep induced voltage small (if AC is used).Tim

author: dazz
author_rank: Frequent Contributor
timestamp: October 30, 2019, 09:44:54 pm
content: Quote from: T3sl4co1l on October 30, 2019, 08:24:23 pmI have decided to leave the heaters tracks there. Well, sort of. I'll spin another version of the board with no filament traces, and will panelize them so I'll get 5 of each. I might even do a 3rd version and order 5 panels of 3 different boards each. That way I can experiment with a few different things and see how they work.20mil (5mm) is what I have right now for the signal traces, and I'll follow your lead there and increase the ground plane clearance to 50mil.I also moved the filament traces closer together where possible, and managed to avoid crossing signal traces in a couple of spots.Sorry, but I don't know what you mean re: grounding. If I shouldn't use a ground plane, nor star grounding, but I can still do copper pouring...Are my keepouts where longer signal traces run, not enough to keep stray capacitance from effecting the tone of the amp?

author: dazz
author_rank: Frequent Contributor
timestamp: October 30, 2019, 09:54:52 pm
content: Does anyone know how to set the clearance for the board outline independently of the netclasses? I've set my power netclass clearance to 2.5mm to avoid arching but the traces that are close to the board edge trigger DRC errors, which makes no sense to me. I'm I trippin'?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 30, 2019, 09:56:58 pm
content: Not that youshouldn'tuse a plane, just that it's less preferable, which is really to say, that there can be more distance between signals and ground. The openings you've made do help with distance. What you want to avoid is crossing signals over wide gaps or splits -- the slot near the input is a concern, and the slot by the power supply would be a concern if they were signal traces (but they are power, carried on resistors of high value and bypassed at point of use, which is an excellent mitigation).I doubt you'll notice anything wrong, honestly: at these frequencies and impedances, you'd have to have either very strong interference to be perceptible to the circuit, and what little crosstalk or feedback may arise across such a short piece of metal (the path around the ground slot) will be similarly negligible.Tim

author: dazz
author_rank: Frequent Contributor
timestamp: October 30, 2019, 10:20:06 pm
content: Quote from: T3sl4co1l on October 30, 2019, 09:56:58 pmSorry to be such a pest, but can you please elaborate on this? Do you mean I should avoid running long signal traces over gaps in the ground plane? If that's the case, isn't that the whole point of adding gaps in the gnd plane, to avoid adding low pass filters to the circuit? Is the concern at that point the lack of shielding that the gnd plane would provide but doesn't anymore?If the above is right, what I was trying to do is to avoid stray capacitance, sacrificing shielding in traces that will either already have significant gain, or won't be amplified all that much (low channel)Let me know if I got it wrong, pleaseQuote from: T3sl4co1l on October 30, 2019, 09:56:58 pmUnfortunately, if that was the right thing to do, it happened by pure chance, because I don't know what any of that means

author: schmitt trigger
author_rank: Super Contributor
timestamp: October 30, 2019, 11:10:42 pm
content: There is actually a pair of *Canadian* companies that are at the forefront of vintage electronics.Hammond Mfg in Ontario is an excellent source for classic power and audio transformers, and chokes:https://www.hammfg.com/electronics/transformers/classicAnd Sphere Research in British Columbia has tubes and vintage parts galore:https://www.sphere.bc.ca/test/tubes.html

author: floobydust
author_rank: Super Contributor
timestamp: October 30, 2019, 11:49:50 pm
content: EdcorUSA for low cost transformers, and parts (M6 laminations, end bells).Sowter UKis excellent, probably the best selection I've seen but shipping is expensive across the Atlantic.For simplicity and low risk, I'd consider wire for the filament connections or using DC for the pre-amp tubes. Marshall switched over to DC filaments there. Many circuits to peruse at:https://el34world.com/charts/Schematics/files/marshall/Marshall_Schematics.htmI disagree on the big ground pour, the originals going back to the 1950's Fender, Marshall etc. always used two or more star grounds, bare minimum a power ground and signal ground. You don't want ripple current from the main filter cap or output stage getting near the pre-amp. I would think we can do the same or better. It's just keeping three or four nodes close. OP has most of it reasonable.This pic of a Marshall chassis shows the signal ground bus at the front, filament twisted pair at the far back, and chassis ground for higher currents.They could have just grounded stuff anywhere convenient to the chassis, which is much thicker than pcb copper...

author: dazz
author_rank: Frequent Contributor
timestamp: October 31, 2019, 12:18:02 am
content: After following your wise advice, this is what the pcb looks like at the moment. Moved things around a bit to dodge the filament traces where I could. Then increased the clearance to 1.5mm for HV signal traces from the plates and made sure there's at least 2.5mm clearance between all high voltage tracks and pads with everything else. Added some ground pads in different spots just in case I need to bridge the keepout gaps for some reason. Also the cathode follower got it's grid & cathode stoppers. And I'm probably forgetting some other things I changed. Hope at least I didn't make it worse than it was before!I'm considering the two completely separate ground planes for the preamp and the rest of the circuit, then running two ground wires to chassis gnd. I can always add a jumper to connect both planes.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 31, 2019, 02:31:34 am
content: Quote from: dazz on October 30, 2019, 10:20:06 pmRight, you're making a bastard plane at this point. It should still have reasonably good conductivity between all points (and especially along the directions of signals routed over it), but it should be somewhat distant from those signals, which is where the holes come in.Consider if you built the circuit using twisted pairs to carry signals (with ground) between sections (or perhaps twin lead, which at Zo ~ 300 ohms has much lower capacitance than regular ~100 ohm twisted pair). All the grounds tie together at each local ground, and pairs route between localities. (A locale, a subcircuit, would probably be a given amp stage.) Which will naturally lead to some ground loops, but small ones, and controllable by means of collecting together pairs that are running in similar directions. (Or a ladder architecture where power flows right to left, and signal left to right -- as drawn on the schematic -- which is a quite good way to do it. No worry about star grounding, the ladder architecture already guarantees that!)Effectively, you can expand that a bit by using a pour -- it could also be a "mesh" or grid pour at this point -- giving more conductivity than the pair-routed case: less voltage drop for a given current flow, and so less need to take stock of the voltage drops across ground.QuoteYeah, it's not far off, just the cases I mentioned.It's hard to talk through this without a framework; I can approach this from an RF-port perspective, where a signal is a transmission line over ground, and the low-frequency equivalent circuit arises out of the impedances and delays of those transmission lines. This is very general, and hard to express in terms of a textbook "here's a voltage and here's an  ground" circuit. (Partly for precisely that reason, that the textbook circuit assumes uniform ground.)At low frequencies, transmission lines become reactances based on their characteristic lengths and impedances. Which in turn come from the dimensions of the circuit, and the frequencies of interest. At DC, the textbook circuit is indeed the correct case (given you can write down the ideally small, but still nonzero, resistances of ground itself). But that's not a very interesting case. So at AC, you can look at important dimensions, like how close signals are to ground, and their lengths; or the spacing of signals, compared to ground (which doesn't depend on transmission line behavior, very much: avoiding crosstalk or oscillation by sheer distance between signals, or shielding by routing ground inbetween them, works at all frequencies!).The hazard occurs when, at a frequency of interest, a ground slot (say) is long and wide enough to drop significant voltage, and a trace crosses over that gap -- then the voltage drop across that gap is impressed upon the trace, and interference gets in. In works perfectly symmetrically, where a signal crossing a gap carries a current across that gap, exciting it (allowing interference to get out: say, RFI from a digital circuit).Probably, you're nowhere near the frequency * length where this even can be an issue, and so it's merely "best practices" -- not anything pertinent. (I mean, I would like to think that's enough of a reason, but obviously, it doesn't have to be.)Tim

author: dazz
author_rank: Frequent Contributor
timestamp: October 31, 2019, 06:18:40 pm
content: Awesome stuff, thanks T3sl4co1lI think I see what you mean now when you say that the slot at the input is a concern.So the trace that goes to the gain pot and back, ends up like 30mm farther away over the other side of the ground plane. Hopefully the plane is beefy enough to avoid any significant voltage drop between those two spots, I would think it is, we'll see. I have added a bunch of gnd pads in different spots just in case I need to bypass the gnd plane gaps for some reason.OK, I just rewired the power supply cause I obviously had one of the caps in the wrong place, and a couple other minor tweaks. Guess I'm gonna call it good and go order those pcb's.For the umpteenth time, thanks heaps to all who helped me with this thing, you guys rock!

author: dazz
author_rank: Frequent Contributor
timestamp: November 30, 2019, 12:26:22 pm
content: Guys. The PCB was a total success! Thanks everyone.Here's a quick demo by a friend of mine (I can't play anywhere near as good) who just put it together

author: schmitt trigger
author_rank: Super Contributor
timestamp: November 30, 2019, 06:27:06 pm
content: Cogratulations!!That is exactly how a tubed guitar amp should sound like.And your friend was really, really happy:"un trabajo encomiable, encomiable. El tio es un reloj. Al primer tiron y a puro pelo. Estoy anonadado".Love those Castilian phrases.

author: dazz
author_rank: Frequent Contributor
timestamp: November 30, 2019, 06:35:59 pm
content: Quote from: schmitt trigger on November 30, 2019, 06:27:06 pmMuchas gracias!I would have never pulled this off without your help, guys

author: andy3055
author_rank: Super Contributor
timestamp: November 30, 2019, 08:52:25 pm
content: Great job! I like how that PCB turned out for you.

author: dazz
author_rank: Frequent Contributor
timestamp: August 12, 2020, 03:52:42 pm
content: Hey guys & gals, after finishing this pcb, I have since designed a push-pull version of the amp, then modified it to mount the tubes on the top layer and the components on the bottom layer to make it easier to mount on a header chassis. Quick question, please. I want to have the ground plane on the bottom layer so that the signal traces on the top layer are sandwiched between the chassis and the grounded copper pour, for isolation purposes, right?

author: Bud
author_rank: Super Contributor
timestamp: August 12, 2020, 04:05:16 pm
content: These 3D rendering thingies have become pretty good these days

author: BrianHG
author_rank: Super Contributor
timestamp: August 12, 2020, 07:05:46 pm
content: Quote from: dazz on August 12, 2020, 03:52:42 pmSo long as the PCB GND and the chassis GND are shared and bolted together. If you are operating based on a star GND system where the optimum central GND reference is the middle of the PCB, then a bolt at the middle of the PCB, the only one with a GND connection should be tied to your frame.When routing your PCB, remember the 'star' GND rules for at least the power supply and you should be able to cancel out any looping GND interference or injected hum from the PSU DC rectification.IE, your GND fill is your signal GND and the PSU GND & rectification caps should have their own GND path on the other side of the PCB, going to that GND star point before feeding the rest of your signal GND.However, this strategy is taking things to extremes as if the amp was mimicking old fashioned point-point wire design.(Also, if you do not have series resistors on your PSP diodes, add a 10-100nf cap in parallel with each one. This further imunes your from a possible faint buzz injected during the diodes switching which may come depending on the harshness of your AC supply. This was never an issue with tube-rectification, or some really slow old diodes.)

author: dazz
author_rank: Frequent Contributor
timestamp: August 12, 2020, 07:36:48 pm
content: Quote from: BrianHG on August 12, 2020, 07:05:46 pmI don't have a star ground scheme in this one, here I simply poured two separate ground planes, one for the PSU and pentode cathodes, and another one for the preamp. The output GND connectors are marked in blue, those will go to the star ground bolt in the chassis. Or that's the plan anyway.The inputs are at the bottom left, as far away from the power supply as possibleETA: Thanks for the tip about those caps in parallel with the rectifying diodes, I'll see if I can make room for them

author: BrianHG
author_rank: Super Contributor
timestamp: August 13, 2020, 03:40:40 am
content: A trick I use to help me visualize how and where to section my GND is first not to use the flood fill at all. I use thick traces. Many times really thick in places like when I designing an amp. Note that I use the shortest/most direct and thickest traces where the most current flows.Once everything is done, I do select and do GND fill where necessary as the PCB cad software polygon fill automatically go right over my GND tracks anyways. (So long as the option 'pour over same net' is enabled.)This way, I can see is a star GND scheme makes sense and I will also see if my PSP section needs any such fill at all.

author: vk6zgo
author_rank: Super Contributor
timestamp: August 13, 2020, 04:27:09 am
content: Quote from: schmitt trigger on October 29, 2019, 10:16:59 pmIt was never necessary in the old days, but has become a "sacred cow" amongst those who are "semi-educated" in tube technology.Possibly this is because of the poor quality of most tubes available today, some of which are tarted up "pulls" from operating equipment, which may well have been removed from service due to heater-cathode leakage.I've never been a fan of PCBs used with tubes, as the heaters produce quite a lot of heat, sometimes leading to damage to tracks & board delamination.With the old phenolic boards, "sometimes" could be replaced by "inevitably"!Ps: Oops! Didn't realise this was a zombie thread!

author: schmitt trigger
author_rank: Super Contributor
timestamp: August 13, 2020, 02:59:20 pm
content: In the good old days, there was a trim pot inside the amplifier, which was called AC balance or something like that.It was a low valued trimpot, whose cursor was grounded, and the two endpoints were connected to the 6.3 volt heater supply.One would adjust the trimpot for lowest hum. It usually was only applied to the high gain phono preamp stages.I have a H H Scott amplifier with that adjustment.

author: TimFox
author_rank: Super Contributor
timestamp: August 13, 2020, 03:49:46 pm
content: Besides the tube sockets, another potential heat problem with vacuum-tube PCBs are the power resistors, which may run hot. Heathkit used to tell the assembler to cut a small piece from the cardboard box (maybe 5 or 6 mm thick) and use it as a temporary spacer between the power resistor and the board while soldering, and then remove the spacer so that the body of the resistor did not heat the board material directly.

author: dazz
author_rank: Frequent Contributor
timestamp: August 13, 2020, 06:12:30 pm
content: Thanks for the great ideas and suggestions everyone.In this design, I have the filaments referenced to ground on one end of the 12.6VAC, so that it can be rectified to some 16VDC which is what I use for the grid bias. I guess I could have used the plate or screen supply for the fixed bias to have the possibility of adding that trimpot in the filaments supply.We've already built this amp, only that it was the original version with the tubes in the same side as the components, and there was no hum according to my friend, the guy who put it together and tested it.Also, I have the largest power resistor mounted vertically. I think I'll do the same thing for the other power resistors, I mean, why not.

author: dazz
author_rank: Frequent Contributor
timestamp: September 02, 2020, 09:26:43 pm
content: Back again with a question about the power transformer needed for the EL84 push-pull amp.Going by the simulation, I estimated that I needed 220VAC and 200mA in the secondary for some headroom. Does that seem about right? I'm having trouble finding a Hammond tranny with those specs here:https://www.tube-town.net/ttstore/components-10662/transformer/power-transformer/audio/There's thisHammond 369HXwith a 150mA 225V winding. Would that provide a full 300V for my anodes at máximum gain & volume?Thanks

author: AlfredHitchcock1999
author_rank: Newbie
timestamp: May 22, 2025, 03:23:37 pm
content: In modern times, now, when even 4 layer boards are quite affordable, what if simply use the inner 2 layers just and only for the heaters (AC 6,3 volts)—2 huge copper planes with just a few holes leading up to feed the tubes, why not with a power amp included with for example 2 or 4 power tubes, for example approx 2 to 4 ampere heater current in total on this pcb, what will happen?

author: SteveThackery
author_rank: Super Contributor
timestamp: May 22, 2025, 03:32:18 pm
content: I just wanted to say that I have built valve amps with DC to the heaters. Not sure it's worth it, but there were no unwanted side effects.Also, if memory serves, lots of valve amps float the heaters: no connection to ground. I think.

author: AlfredHitchcock1999
author_rank: Newbie
timestamp: May 22, 2025, 03:40:16 pm
content: Thanks but in this case I mean just AC straight from the transformer, the thing so manyexpertssay we absolutely should avoid. I better start a new thread in this topic.

author: eugene
author_rank: Frequent Contributor
timestamp: May 23, 2025, 02:24:54 pm
content: Quote from: AlfredHitchcock1999 on May 22, 2025, 03:23:37 pmOne advantage of using twisted wires is that they can be placed far from the delicate music signal traces. Putting amperes of 50/60 Hz current in a plane between two signal layers seems to me to be going in the wrong direction.I have put traces for heaters of preamp tubes on the PCB, but I put them along one edge of the board, far from everything else, with both occupying essentially the same area on adjacent layers. This seemed to work, but ultimately I decided that the bast way to do this is to:Mount the tube sockets directly to the chassis for heat dissipation and run wires from the sockets to the board. This may be more important with power and rectifier tubes, but you can buy sockets with long PCB mount leads. You mount the sockets to the chassis, then mount the PCB on standoffs over the sockets with the long thu-hole leads going through the PCB.Run the heater wires (twisted) off the board.Run a big fat ground buss (10 AWG solid copper) above the board connecting the separate sections. The ground buss starts at the GND end of the first capacitors in the power supply and ends at the input jack. Each section of the circuit has it's own set of GND traces which includes one end of the cathode bias resistor, the grid reference resistor, and the power supply capacitor which is near the section that it serves. I do not put all of the PS caps in one place like Blackface Fenders.Run the B+ power on a separate wire above the board connecting the separate sections. It starts at the positive end of the first PS caps - the same caps where the GND buss starts - and connects the stages in order, just like the GND bus.I've tried running a wide GND trace along one edge of the board, possibly on both sides, as well as a wide B+ trace along the other edge. That works too, but I've decided that a few flying wires works as least as well, maybe better, and is not too difficult to implement.

author: TimFox
author_rank: Super Contributor
timestamp: May 23, 2025, 02:33:01 pm
content: Quote from: SteveThackery on May 22, 2025, 03:32:18 pmFloating the heaters is not a good idea: tubes have a finite heater-cathode voltage rating.It is common to bias the heater feed to a positive voltage with respect to ground and the cathodes to reduce hum.A simple resistor divider to the transformer secondary center-tap or a resistor artificial center tap suffices.

author: SteveThackery
author_rank: Super Contributor
timestamp: May 23, 2025, 06:23:03 pm
content: Quote from: TimFox on May 23, 2025, 02:33:01 pmAh, yes, that's absolutely right. I remember it wrongly - my apologies. The Quad 22 and the Leak TL12 Plus both connect the heater winding centre tap to ground.

author: vk6zgo
author_rank: Super Contributor
timestamp: May 23, 2025, 10:51:09 pm
content: Quote from: AlfredHitchcock1999 on May 22, 2025, 03:40:16 pmIn the heyday of tube amplifiers, anything but ac to the heaters was almost unknown, except in some esoteric test equipment.In those far off times, anybody could go into an electronics shop & buy tubes with minuscule heater to cathode leakage.Today, any tubes/valves that you may buy are probably "pulls" from working equipment, either so acknowledged, or often "tarted up" & sold as NOS, (they were often "pulled" from equipment because of heater cathode leakage), "seconds" from tube factories that somehow avoided "landfill" back in the day, or crappy ones from the dying days of tube manufacture, when a lot of secondary tube manufacturers of doubtful pedigree had their output released on the general market.In the old days, obtaining well filtered & regulated low voltage DC was very difficult, but now it is relatively easy, so some of those out of spec tubes can be used, & are by the so-called "experts", but it is a "work around", nothing more "high tech" than that.As to using PCBs with tubes---this was common in the early days of such boards, but the constant heat from the heaters wreaked havoc on the old Phenolic material, & can still adversely affect FR4.PS:- Looking back, I see that I have posted a similar comment back in 2020 on this "zombie" thread!