Danelectro Spring King mod

Posted by in equipment, mods & repairs, Studio

The Danelectro Spring King is a small spring reverb, but there’s also a slapback delay in it, using a PT2399S chip. Thanks to some documentation on the internet I could easily mod the reverb to control the delay. The idea is to be able to get only the reverb signal if we want, and in a general way to take advantage of the delay by changing the Time and Feedback as we feel.Control Delay Time:

Between PIN6 (VCO) and Ground, there’s a CMS resistor „362“ (3,6k). Once it’s taken off you can put wires instead to a pot.
»It seems that a PT2399S can freeze if, when powered on, PIN6 is connected to the Ground. So either I will have to put a small resistor in front of the pot, or simply remember not to put the pot to „0“ before powering on. In any case, you just to unplug and replug to get it back.
Control Feedback:
There’s a „153“ (15k) resistor just after PIN12 (OP2-OUT), replacing this one should work fine.

50k pot + 10k resistor in series for the FEEDBACK
50k pot + 3,3k in series for the DELAY TIME

Disclaimer: try at your own risk! This is tricky due to very small components, and you run the risk of ruining your pedal if you’re not careful!

Okay, here are instructions, all in one place:

The Danelectro Spring King is a hybrid delay and spring reverb.
It uses a pretty typical slap delay circuit, with a PT2399S delay
chip and a fixed delay time and feedback setting. It feeds the delay
signal through the springs and then out to a gain recovery stage.
Controls: The Reverb is wet/dry mix for reverb and delay. The
volume controls reverb (the strength of signal going through the
springs), but not delay. If it’s all the way down, you only get the
delay. All the way up and you still get the delay signal, but it’s
totally drowned out by the reverb unless you kick the delay up to
a more obvious setting via the mod.

I have found that a slightly shorter delay time than stock, with
perhaps a slightly lower feedback level sounds just great. It sounds
tighter and more “real.” It’s easy to make the Spring King sound
trashy, but it can also sound surprisingly good. With the feedback
and delay time at the minimum (essentially no delay sound at all), the
springs still sound good and give off that dripping faucet sound.
Feedback controls how many repeats the delay gives you. If you
turn it up it will start to self-oscillate.

The 3.6k resistor (marked “362”) at pin six determines the delay
time. Replacing that resistor with an external potentiometer allow
you to you control delay time. Shorter delay time “tightens up”
the overall reverb sound, while a longer delay time makes it sound
“looser.” I think this is intended to simulate shorter and longer

I use a 10k pot for delay time (which stays within the realm of slap
back at all positions) and a 50k pot and a 15k resistor in series for
feedback so that at max feedback, it’s still getting the original 15k

Note that by “wire to pot,” I mean first remove the resistor, and
then solder two wires onto the two solder pads left in its place,
and run those wires to the potentiometer. It’s a little tricky because
they’re surface mount, but totally doable if you’re careful. I use
double-sided tape to secure the wires to the inside of the enclosure
so they don’t droop down and mute the springs.

The delay resistor

The feedback resistor

Wide view

Unmodified springs & wiring

The Spring King is compatible with any spring reverb tank with
an input impedance of 8 to 10 ohms. You may want to upgrade the
spring tank to the larger unit like the Accutronics 4AB3C1B, the
same type used in the Fender 63 outboard reverb. Compared to the
Spring King’s original springs, the 4AB3C1B has longer springs
and a longer reverb time, and a nicer drip. I think upgrading the
reverb pan is a very worthwhile improvement. Accutronics has
many different units to choose from. Visit their website and learn
how to read the codes on the tanks. The codes tell you how many
springs there are, reverberation time, input and output impedance,

Of course, a larger reverb will require a new housing. You could
houseing it in a bread box (a tool box or something similar might work), or if you have fabrication abilities, build a custom
enclosure for it or rehouse the entire Spring King and new reverb
pan. You’ll want to use some type of shock absorbers at mounting
points to insulate it from external vibrations.

To connect to an external reverb pan, you need to drill a couple of
holes in the case and install two RCA (phono) jacks (available at
Radio Shack.) Locate the two pairs of green and black wires, that
go from the PCB to each end of the springs. Cut those wires, strip
the ends, and solder them to the RCA jacks, going from the PCB
to the jacks. The green end is the signal, the black end is ground.
If you want to still be able to use the internal springs, you can
send those wires to a 4PDT toggle switch (available at Small Bear
electronics.) Again, be sure to prevent the wires from drooping
down over the springs and muting them.

Switch diagram

Once your RCA jacks are installed, you can run RCA cables
from the Spring King to the Accutronics reverb pan (they come
with RCA jacks already in place.) If you plug them in and get
no reverb, try reversing them – if they’re backward you’ll get no

Then plug in and enjoy your new improved Spring King!

Drip mod:

Increasing the low end going to the springs will result in a more prominent drip. A cap and a resistor set the lower end of the filter – larger values = more bass. Be careful, as too much bass may cause your spring to feedback.


The stock is a 51k resistor with an unmarked capacitor. Here are some notes I made of different combinations I tried.

48k + 560pF = moderate drip
48k + 680pF = More drip
47k + 1120pF = Heavier drip
48k + 1220pF = Heaviest drip
48k + 440pF = airy sound, less drip

Some of the heavy drip combos will force you to back off the volume control to avoid feedback. You still can’t get as wet as a tank here (I think due to weaker recovery circuit, not 100% mix option, and not enough treble response in tone control), but you can get quite a respectable drip.