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Danelectro Spring King Delay Mod

The Final Product

The Danelectro Spring King is a Spring Reverb pedal with an actual springs inside (three to be precise). Along with the reverb unit, it contains a chip to produce Analog(ish) Delay, which is setup to produce a slapback delay. Immediately after listening the slapback, the first thing that came to my mind is whether there is a way to manipulate the delay. After some searching on the internet, I found a post on the Surf Guitar 101 Forum describing how to do it (plus a drip mod which I haven’t tried yet). Searching a little deeper, I found a post on a site called DIY Audio Circuits providing all the information anyone could ever want for the PT2399 Chip that handles the delay part for the pedal. (Both are linked on the end of this post)

Disclaimer

The actions described in this post will void your warranty and can damage your equipment. I take no responsibility for your actions.

The Connection

It turns out the mod fairly easy to do if you have enough soldering experience to be comfortable desoldering SMD resistors. All we need to do is to replace two resistors with pots. The pin 6 of the chip is VCO with a decription of “Frequency Adjustment Pin”. In short this means that the value of the resistor which is connected between it and Ground determines the delay time, with smaller values resulting in shorter delay times and larger values in longer. The resistor we need to replace is labeled 362 (3.6K)

Time Resistor

Time Resistor

For the feedback, we need to replace the resistor labeled 153 (15K) connected to pin 12 of the IC. Here, smaller values produce more feedback, while larger values less.

Feedback Pot

Feedback Pot

After removing the resistors and soldering the wires, we need to to find out what values should the pots have. First problem, the datasheet informs us that on bootup the resistor on pin 6 should be at least 1k and since I wanted to be able to have so short delay that is actually unnoticeable, I added a 1k resistor in series with the pot. Next step, determine the pot value/longest  delay. I just went through my bin to try to determine the longest delay possible with acceptable noise (the longer the delay, the more degraded the repeats are). I really liked the the length that could be achieved with a 100k pot, but the noise level made it unusable so I reverted to a 50k pot (with probably 20k being the wisest choice for most people). Finally, we need to find out the values for the feedback pot. Again, we needed to have a resistor in series with the pot, in order to limit the feedback/oscilation. The safe choice here is to use the same value as the resistor replaced (15k), but I decided to use a little smaller (10k), allowing the pedal to go to self oscillation. The value of the pot could be again 50k, but I didn’t like something in its behaviour, replaced it with 100k, didn’t see much difference, got stuck with the latter.

Testing

Testing Potentiometer and resistor values

Layout

For the final step, we need to figure out a nice layout for the pots, drill the holes, add the knobs, label them and we are done

Layout

Layout

Pots Installed

Pots Installed

The Final Product

The Final Product

Result

What we are left with is a far more versatile pedal than the original (which was almost a one trick pony). We now can get from surf/garage types of sound to some more ambient/experimental (the kick pad lends to this type of music). One final thing to note is the function of the knobs (and why I relabeled them). The volume knob determines how much sound gets into the spring, the tone is the tone of the reverb, the reverb is actually the wet to dry mix (including both the delay and reverb), and time and feedback are self explanatory.

h3>Sources

Surf Guitar 101 Forum Post (Mod Instructions)
DIY Audio Circuits (PT2399 Info)

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