Budget Stand Alone Spring Reverb 3 of 3 (updated 12/8/24)

The final countdown

We’ve covered design of, the power supply, series heater, reverb driver, and the preamp. We can now wrap up this budget stand alone spring reverb with a cathode follow and the parts list for those following along. The schematic below is the most basic design for a stand alone reverb, but will it work? Of course it will! Let’s put it together.

Cathode follower

During the power supply design we decided the cathode follower draws 1mA. Cool except what’s a cathode follower? Looking at our wet and dry circuit you might stop to question how the gain from our circuit will affect our signal going into whatever amp we are using. The effect would overdrive our signal with unwanted gain and noise unless we can reduce it. Enter the cathode follower. We can reverse the effect by turning a tube on it’s head and the output will be close to unity (like the gain never happened). We are just going to look at current draw, but you can deep dive into this subject starting on pg. 242 here: http://tubebooks.org/Books/zimmerman-mason_1959_electronic-circuit-theory.pdf

Current draw

For those not interested in understanding how we figure out current draw of a cathode follower please skip to the parts list at the end. Everyone else look at the picture above. For all intensive purposes this is just another load line. For Rk we are using a 33KΩ load from cathode to ground. The load line is then Ebb/Rk or (160VDC/33KΩ=4.8mA). The 160VDC from our power supply is applied directly to the plate. Below is our 12AX7 Cathode Follower load line, but to find the slope you will need to stack another data graph on top because this graph ends at 4mA. The operating point is half the current of where the load line crosses P on the 0 grid volt line at 2mA, so 1mA current draw at 125V is the operating point Q. That’s it:)

Following up

The load line has a steep slope, but it’s fine because we are not concerned about gain. Most cathode followers use a load of 10KΩ at higher voltages, but this is usually for 12AT7 or 12AU7 as long as the load line doesn’t operate beyond plate power dissipation. I know, it seems too easy to find current draw, but now you can experiment with different load values to AB test for whatever sound suites you best.

The output

A quick word on the budget part of our budget stand alone spring reverb. Most designs would use a mixer stage involving a triode tube, but this circuit utilizes a resistive mixer. The 100KΩ pot combines the wet and dry signals. In the middle position the pot send half the wet signal into the 100KΩ resistor to the output, and half the dry signal to the other 100KΩ. The other halves of the signal are sent thru the center lug to ground. The wet signal can be reduced by adding resistance between the top lug of the pot and the 100K resistor. The dry signal can be reduced by increasing the 1MΩ resistor connected to the bottom lug. This is how you can control unwanted gain into your amp, or stop the wet signal from completely killing your volume.

Component parts

The list below is for the electronics. Additionally you will need hardware, power switch, ac cord, .5A fuse and holder, and a chassis. I had a lot of parts on hand and my budget was less than $80. I would think this entire project can stay under $150 with some skillful shopping.

   18      Transformer  1ea mouser.com part# VPP12-400   https://www.mouser.com/ProductDetail/Triad-Magnetics/VPP12-400?qs=BAwhWtRySQL6%2FFJrOyJ1tw%3D%3D

19      Resistor 1/2W   1K   1ea   R20   

20      Resistor 1/2W   100 2ea   R18,R19

Final touches

There are many different types of cathode follower designs you could experiment with, and designs with tube mixers in place of our resistive mixer. I hope you can use this basic design to branch out into more complex builds to get that sound you’re looking for. Enjoy your budget stand alone spring reverb, and please address questions or concerns to the comment section so we can all learn and grow together.