Every amp is better with a stand alone spring reverb. The ability to add reverb to my 1950 Sherwood Master feels like I just used defibrillator paddles to shock rock n rock back to life! This article will cover the series heater set up, and how to drive the spring reverb unit. Let’s keep the budget under $200 and knock this thing out.
All tube heaters are series 12.6V hook up. The resistor needs to be at least 10 watts, and this is how we determine that. we need 25.2v for both tubes(12.6 x 2 tubes). We can grab 60v from the center tap of our 120v transformer, and use a resistor to drop 34.8v. The entire circuit draws 0.15A so 34.8v/.15A=232 ohms or about 220ohms. 220ohm will drop 33v and dissipate 4.95 watts (220ohm/.15A=33v) and 33v x .15A=4.95 watts. Even a 10 watt resistor will get very hot in this situation. The resistor needs to be between the transformer and tube heaters, and you might even use 2x 110ohm 10 watt resistors instead, but 220 will work fine. Check out “voltage considerations” section of the article linked below for more series heater info:
https://crunchytubeamps.com/diy-all-tube-amp-with-13-power-transformer/
Accutronics makes the very small and cost effective AMC2BF3. It has 2 springs and medium decay. We need 150ohms to drive it, and I plan on using both sections of a 12AU7. The above load line is for one 12AU7 section. Since we’re using both sections in parallel, the load resistance will be cut in half from 23,750 to 11,875 ohms. We can use a transformer to match our 11875 ohm output to our 150 ohm spring reverb. The transformer ratio is found by dividing the squared root of our output by the squared root of our input (√11875/√150), or about 9:1. That’s about the equivalent of 700 ohms of impedance to 8 ohms out transformer. Read the “what about an output transformer” section of the article linked here for closer look at universal transformers: https://crunchytubeamps.com/diy-all-tube-amp-with-13-power-transformer/
Let’s figure out this drivers’ cathode bias resistor value. While the load was cut in half by adding a parallel tube, the operating current has doubled from 4mA to 8mA. Looking at our load line, the operating point is approximately at -6.5v. No problem, we just divide 6.5v by 0.008A to get 812.5 ohms. An 820 ohm resistor will be perfect. I am using a 50uF capacitor to bypass the cathode resistor but you could use 100uF or even 220uF if you choose. Link below to a downloadable spreadsheet calculator to figure bypass capacitor value.
https://www.facebook.com/groups/485856047509004/permalink/486622537432355
We can heat our tubes and drive our reverb, but we still need to power our circuit and feed a signal to our driver. Our next steps will focus mostly on rectification and how we can apply it to any circuit. The final part will go into wet and dry signal and parts list. By the end of this 3 part series you should be able to construct your own stand alone spring reverb using the parts of your own choosing. https://www.facebook.com/CrunchyTubeAmps/videos/2003860206308536
