Introduction: Series of Universal PCBs for Tube Amp Build
Tube circuits were a crucial step in the development of the electronics. In most areas they became completely obsolete in comparison to cheaper, smaller and more efficient solid state technologies. With the exception of audio - both reproduction and live. Tube circuits being relatively simple and mostly mechanical work being connected with making a tube amplifier they are ideal for self-building - DIY. They are surely connected with high voltage and so can be dangerous, but if some basic guidelines are followed, most of the danger can be avoided.
First approach to tube circuit building was so called point-to point, where element leads were directly fixed to tube sockets, pots, jacks .. with the help of various terminals. To facilitate mass production then companies started to put the elements on different boards (some approaches still caled point-to point, although not really being such). Nowadays most of the electronics are made as PCBs - printed circuit boards. Even most of the mass produced tube designs are made on PCBs nowadays. But PCBs have certain disadvantages for the tube world:
- tubes produce a lot of heat when on, so even in normal function they are prone to greatly reduce lifespan of the PCB
- mostly tube circuits are so simple and straightforward, and the used (high voltage) elements so big that it doesn't really make sense to produce tube circuits on whole boards - there would be mostly empty space and few traces with some pads - really a waste of FR4 material
- a lot of components of the tube circuit are too heavy or too bulky to be mounted directly to the PCB (transformers, chokes), others are unsuitable for PCB due to mechanical stress (tubes whose sockets are mounted directly to the PCB have to be exchanged with care)
On the other hand it is sometimes hard to solder directly to the amp parts, and some tend to get damaged in the process (I have succeeded in ruining quite a bunch of switches when soldering to them). It is also hard to troubleshoot and service classicaly point-to-point built devices, even more if they are not built with extremely good planning. PCB gives a solid and from-chassis-detachable way of fixing elements.
So the situatution calls for a way of half point to point wiring, similar to what they did it in known guitar amps like Marshall or Fender. A lot of builders still use their approach with great results. But Fender - Marshall approach has some drawbacks:
- they mostly use axial components, which are rare and so less affordable
- most of the circuit elements are paralelled, which causes a waste of space and can lead to noise, oscillations and element coupling
- there are long exposed leads on the boards
- this board is then often mounted in the center of the chassis, pushing all the tube placement out of it, which is again suboptimal
Simple and quite similar design of most hi-fi and guitar circuits enables us to use modular approach in tube amp building, using PCB modules. Studying the schematics helps us to design PCBs, where there is no space wasted with paralelled elements, but follow the rules of trace routing. Double sided design enables us to make modules smaller and use both sides of the board. We can solder connectors to PCBs, which makes it even easier to troubleshoot and service devices.
For a DIYer it is not practical to design a PCB for every project, it would be quite expensive! But simplicity and similarity of commond tube designs enables us that we design PCBs, which are useul for the most of applications.
Here is a "collection" of some PCBs which I designed to facilitate tube amp making.
- double triode poin-to-point PCB
- tone stack PCB
- stompswitch PCB
- two switch PCBs
Step 1: Double Triode / Noval / Preamp PCB
Preamp section is quite similar in most tube applications and usually consists out of series of double triodes in noval packages, often being 12AX7 tubes. Sometimes there is a cathode follower setup, but mostly there are only different combinations of grid stopper +plate resistor+ cathode bypass cap+bias resistor+ coupling cap values. It is so not such a demanding task to design a pcb, which would be quite universal for the preamp part of the amp circuit - or for the noval tube (nets are made in such a way that also most of the noval non-double triode tubes can be used with ease).
PCB was designed to fit a 1U rack enclosure (tube being horizontal)- otherwise it would be beneficial to make it a bit larger.
It is up to the user which elements go to which side of PCB. Silkscreen is here only as a help with orientation.
The pcb is designed to go together with noval Belton socket. It is fixed through the socket (so exchanging the tubes is not a strain for the PCB). It is to be fixed to the sockets with some standoffs in between.
One end of certain element leads is soldered directly to the socket, other(s) are soldered to the PCB. There are few additional pad-trace groups (common name is net) on the board to assist with different setups. To further explain the PCB it is probably best to go through the tube pins.
- on the "south" of the PCB there is a "ground bus" with few traces going to the corresponding places on the PCB
- on the "north" there are two nets provided for B+ - there has to be a jumper (white line) installed to connect them (that detail makes this PCB useful also for non double-triode noval tubes)
1 - plate1 - (white line marked with 1 on the opposite side) - made in a way to have the wire going to the marked net on the pcb, then there is the place for the plate resistor (marked R7) and the stage coupling cap can be soldered in one of the "reserve" nets
2 - is grid1 (white line marked with 2) - coupling cap or grid stopper can be mounted directly to the solder lug of the socket if needed - R1 is drawn to be a grid leak resistor - R1 pad to ground can also be used to connect the screen from the shielded cable
3 - is cathode1 (white line marked with 3) - designed so there is cathode resistor and a bypass cap soldered on the socket lug and in the ground pad directly on the other end
4 and 5 are not marked, 9 is marked but doesn't have a dedicated net - 4,5 and 9 are heater pins - as a firm believer in DC heating, I always connect only 4 and 5 in my double triodes and suplly 12,6V - wires for heater go directly to the socket solder lugs, but pass two big pads as a form of strain relief
6 - is plate2 - same function as 1 - is made to have a wire going to the dedicated net, then there is R9 as a plate resistor and you can use one of the "reserve" nets to fix the stage coupling capacitor
7 - is grid2 - the same function as pin2, but insted there is R8 drawn as a place for grid leak resistor
8 - is cathode2 - the same function as pin3
(9 - is central tap of the heater in double triode setup, in some noval tubes having the other function. Usually I omit this pin or even break off solder lug from the socket)
From the Alembic I have got the habit of adding a power filter capacitor as a part of the circuit, so I have included some big pads connected with both ground and B+ on the eastern edge for this. .
Step 2: Tone Stack PCB
In the schematics of most tube guitar amps you notice "tone stacks" are pretty similar. Depending on the output impedance of the previous stage there are two main designs (with slight variations, known as Fender and Marshall). I combined them both in one PCB. I also wrote most comon values of the used elements in silkscreen table on bottom layer. (
The reason I designed a separate PCB for the tone stack is that all other preamp parts are gathered around the tube, but tone stack is made around the potentiometers. From my experience there is quite a possibility to mix up the wiring in this part of the circuit. Elements used in tube tone stack are high voltage and so tend to be too big to be practically fixed on the pot solder lugs. Also being high voltage I don't feel dafe to leave them dangling against the (conductive) front plate. On the other hand having them together with other preamp elements around the tube brings long lengths of unnecessary wiring.
PCB is made for PCB mount potentiometers - some purists are against that, but this pcb is so small and light that there is no chance turning the pots would srcrew up the connection. For the faint of heart there are three mounting holes provided. The smaller non-plated holes on the pcb are ment to be strain relief for the wires.
R1, C1, C3 and C4, together with the pots VR1-3 are ordinary parts of the circuit, pots arrranged in TMB manner.
There is no volume pot place - I was limited to 10cm width to the board to get it at the sale price ... And volume pot is not always directly after the tone stack - there is J3 to connect it, north the signal, south the ground. C2 is there to bridge C1 with additional capacitance, which makes mids a bit higher - it can be switched on the J2. The big sqare pad in the ground net is there to enable input screen connection
Step 3: Switch Header PCB
I don't believe I have ever fried a single electronic element with solder heat, and everybody warns so much about it. ICs, transistors, diodes and so on can take quite a lot of thermal abuse before quitting on you.
With the exception of switches and potentiometers (plastic Piher ones). The wire doesn't stick well, you put your soldering iron on the lug one more time ... and the lug moves in its place, you have melted soft plastic around it. There is a good chance switch will start sticking and cracking sooner or later. With all the elements, for which it is most practical to have them soldered directly to the switch (remember trying to solder a component in series with the switch) it is much more likely that you will ruin it. Or make a messy nest on its lugs. Next problem is wire strain - you finish your project, put all the wires in nice sharp order and then catch on one of the switch wires by accident and it breaks - adieu efforts of the last hour, you have to screw it out of the front plate (or a pedal) and resolder the wires. Sometimes it is practical to have a chance to use an ordinary connector on a switch, not unsolder it every time it needs to be removed. And if there is excessive force used on the wire, it doesn't break, but the connector lets go - and you just reconnect it.
So instead of a solder lug switch you use a PCB mount one. You can solder all the wires in place and solder also switch pins without the fear you will destroy the switch.
Connection is arranged in the form of well known one row 2.54mm header - you can use it to make internal connections or install a connector. There are four big plated through holes, which can be used as strain relief for the incoming wire or to make additional needed connections.
There are two variants of this PCB, low and high voltage one. HV is not made with the 2.54mm pattern, as this violates needed standardised creepage / insulation distance. I ordered those PCBs to be only scored, not cut, so I can make whole rows or columns effortlessly if the use of more switches is desired.
Made for the (most used) DPDT switch.
Step 4: TB Stompswitch PCB
I know nobody uses stompswitches in tube amp builds, but this PCB was in the same batch - and a part of the same mindset. Let's say an upgrade of the previous DPDT switch banter. It is just my render of the small PCB every pedal kit seller is offering for a nauseating price.
If wiring switches generally can be a nuisance, it is twice the nuisance to nicely wire a 3PDT stompswitch for true bypass. It can take you the same time for soldering the whole pedal circuit as it takes to make the jacks and stompswitch wiring. And it's the same pasta every time, not the nice adventure of making a new circuit.
This PCB features:
- pads for a PCB mount 3PDT stompswitch
- separate in and out jack connection pads with strain relief holes - the jacks will be neatly wired at last and the wire will not break off even after removing the circuit for the 10th time from the enclosure
- 4 wire single line 2.54mm pin header pads. This enables you to put a connector on one or other side of connection with the main effect pcb. The strain relief here is one big rectangle because I like to use ribbon cable for this connection. The pinout (I-gnd-B+-O) suits my standrad pinout when making pedals from scratch.
- provison for LED dropper resistor and LED not to make those connections an unhealty mess hanging in your pedal enclosure
- zero distance to the switch perimeter on the south edge to allow you to mount the switch as close to the enclosure wall as possible - to give place other important segments.
Step 5: I Want to Make Them Too ...
google me for gerbers or PCBs if you need them.
Ones asking for the schematics certainly don't understand the concept of those PCBs. They are made to be universal, multi-appliciable or whatever you name it. You take the schematic you want to use, analyze it and then choose which element goes where in my board to make it optimal. You don't ask where to put you socks when you buy the drawer.
Participated in the
PCB Design Challenge
Question 6 months ago
I would love to find these designs somewhere?
3 years ago on Step 5
Great!! I'm more into the hifi game, do you consider proposing PCB for that kind of use? Like small amp or headphone amp?
Thanks for sharing
Reply 3 years ago
Thank you for the response. This is "universal modular" build, so those PCBs can be easily used also for HiFi.
- there would probably be no stompswitch in your HiFi setup :D
- switch PCBs could be used for different switching means (toggling options as loudness ...), but if you are trying to build a preamp, there is a main switching PCB missing, namely, source rotary switch PCB (probably you would use a 6/2 switch) . As those are usually quite big and there are rarely any elements on them, there is really no need to have a PCB for it.
- tone stack here is really not suitable for HiFi as both the possible designs include a mid-cut, which is natural for the electrical guitar, but a big no-no for HiFi world. "James TS" (more eg: http://arkhamsound.com/james.html) would be much more appropriate, but I don't plan to make a PCB for it soon. But it is not so hard to make on pots with the help of a short terminal strip.)
- "main board" can be used for any HiFi triode setup, you just have to use the schematic to determine, which element goes where. Well, I must check, how is this board suitable for different types of phase spllitters ...