As promised we will go over the power supply and getting our signal to the driver on this budget stand alone spring reverb, but first I have a confession to make. I rarely put effort into a power supply design. On rare occasion I find myself having to hit the books because I decided to go in a strange exotic direction with my amp design, but this link is my go to most of the time. http://PSU Designer II 2.20.0 Build 74 – Download, Review, Screenshots (softpedia.com)
Be Aware that power supplies are dangerous! Never design a power supply without a transformer to isolate you from mains supplied at the wall socket if you value life.
You can use this to design just about any amp with tube or solid state rectification, but first we need a target voltage and current draw. In this case we know the reverb driver designed last week used 160 volts at 8mA. Our preamp will also use 160 volts at 0.3mA and our cathode follower half of the 12AX7 draws about 1mA a 9.3mA grand total. The rectified DC voltage will be higher than the input AC voltage. Our transformer is going to be the $13 120v transformer from mouser: https://www.mouser.com/ProductDetail/Hammond-Manufacturing/187D120?qs=%252BXxaIXUDbq3kPZhR0cJ2rQ%3D%3D discussed in this article here: https://crunchytubeamps.com/diy-all-tube-amp-with-13-power-transformer/.
Highlight the transformer and rectifier area in the upper right and select the change box when it appears. you will then see a ‘select type of source” box like the image above. These are all of the possible rectifier types you can experiment with in this program. For now we’re going to choose Solid State and Bridge.
Highlight the R1 5kΩ load area then select type of load constant current.
Highlight the current tap I1 and change value 1 from 100m to 9.3m based on our circuits current draw. click ok and then click Simulate in the left corner. Click the VC1 box to see our results!
The graph on the right displays your voltage and ripple. V(C1) is displaying an RMS voltage of 165.67 which is perfect for our 160v needs. The original schematic has an extra filter stage with a 10KΩ resistor, but we are not using it. By highlighting a section of RMS voltage on the graph you can zoom in on the ripple voltage. There is less than 0.5v which we use to calculate our percent of ripple (0.5v/165.67rms=0.003×100=.3%). We need to keep the ripple under 5% so we could use a smaller capacitor if wanted, but I will stick with the 220uF/200V. Defiantly play around with the PSU designer and you will quickly realize that this is a handy tool.
This is the approximate load line of our preamp. the operating point tells us the current draw needed for for our power supply calculations. The black load line is our plate voltage 160V divided by the load resistor 220K which you can find on the schematic which is about 7.27mA (I rounded up to 7.5mA). From here we could guestimate approximate current draw to half our load current or about 3.135mA, but here is a more detailed approach. The operating point is halfway between where the load line crosses the 0v grid line and 0mA on the 160v line. We can guestimate about -1.2v at .3mA for the operating point based off the cross-section of the black and purple lines. This preamp will be perfect to deliver the signal to our driver. If you are interested in a deeper understanding of load lines here is the link: https://crunchytubeamps.com/use-the-preamp-tube-of-your-choice/
We still need to account for the second part of our 12AX7 which will be the cathode follower, but I will discuss that in part 3 along with the parts list. For now we will just add 1 mA, but we did a good job of breaking down the circuit up to this point. From series heaters and driving a spring reverb in part 1, to the software used in our power supply and getting the signal to the driver with our preamp, we have covered the basics of this circuit. Please comment with your questions and I will do my best to address them while we dive into this budget stand alone spring reverb:)
