How do those Spring Reverbs work in guitar amps?

Yes, the transducers are like a speaker and mic. The sound travels down the spring taking some time. The signal is picked up at the far end giving a delayed copy. This delayed copy can be run back to the input where it is delayed again. By controlling the gain of this feedback the reverb can be made more or less pronounced. If the loop gain exceeds 1 it will create an ever growing echo that gets out of control like feedback, but not at a high frequency.

--

Rick C

** The long springs transmit torsional waves, twisting back and forward so must be driven the same way. Tiny tube magnets are made to twist by the dri ve coil assembly while at the far end a near identical transducer converts the twisting back to an electrical signal.

The drive assembly is a kind of motor that makes the magnet rotate on its a xis about +/- 30degrees. A torsional wave travels fairly slowly down a spri ng, making for long reverb times.

A major benefit of this method is that external vibrations do not induce to rsional waves in the springs but only lateral ones and so are largely rejec ted by the pick up assembly.

AFAIK the idea was patented by Hammond in the late 50s - then made famous b y Fender in the 60s guitar amps.

BYI I agree, it is not easy to find the above info on the net since the Ac cutronics site disappeared.

.... Phil

Prickman is WRONG again:

** No, there is no loop feedback around a spring a reverb unit.

The torsional waves simply reflect at each and and travel back and forward many times before fading out.

A reverb control only mixes signal from the unit with direct signal to vary the effect.

.... Phil

Your followup just now made me aware of that a company named Accutronics used to exist. There's a couple of spring reverbs in my bone pile. The metal tanks are stamped with "ACCUTRONICS GENEVA, ILL." The tanks originally came from PAIA as part of a kit:

This site seems like a good source of information:

Thank you,

--
Don Kuenz KB7RPU

** Funny how it totally fails to mention the principle of operation.

... Phil

On a couple of guitar amps I had to invert the phase of the speaker(s) beca use of feedback. Does this mean I actually had a bad reverb tank and covere d up the problem doing that ?

I think Peavey was the one that put their tanks in a sort of leather bag at the bottom to isolate it. I had one break loose somehow and the tank was s tuck to the speaker magnet and made one hell of a howl.

I also noticed on a few amps that the level of echo is controlled at the in put side of the tank. Even if the echo is turned all the way down you can s till get feedback or shock induced noise from it. Why they would design it like that don't ask me. I don't remember what brand it was but I am sure of it.

** That never works.

** Fender did that waaaayyy before Peavey copied.

It doesn't help much, a cloth bag is hardly a sound barrier.

BTW

is there no stupid, self promoting, narcissistic bullshit you will not post ??

Just wondering.....

..... Phil

Nope.

** That never works. "

Worked for me a couple of times.

But what are you saying, that the reverb tank was bad and I should have replaced it or what ? All I know is that the feedback stopped. Well maybe not the feedback, but its effects which were quite annoying.

Take a look at the reverb unit schematic for the Roland Jazz Chorus:

The spring is large in this one so the transducer is driven by a small power amplifier - you need to put a decent amount of energy into it to drive the acoustic waves.

More "refined" designs like this one have a current-sense resistor in the transducer ground leg and wrap the drive amplifier feedback loop around it to even out its nonlinear response.

The output signal is picked off by a regular op-amp amplifier in non-inverting configuration.

** The spring unit is made by OC Electronics, has a folded spring but otherwise is very similar in operation. ** Complete bollocks.

** Cut the crap - it's just simple current drive.

The input coil used has a nominal impedance of 8 ohms at 1kHz, much lower at 100Hz and much higher at 10kHz.

Drive power requirement is around 10mW, some as other designs, but the low impedance means it cannot be driven fully by a single, regular op-amp.

The reverb units in Roland amps and tape echos are easily driven into distortion - needing a resistor of about 56 ohms in series to eliminate.

See pic of the inside of one.

.... Phil

That makes sense. How much average voltage would one expect to see between terminals 4 and 5 of the tank input prior to the unit distorting with a guitar signal?

The Bandit in this unit from a Q on sci.electronics.repair seems to be using a single 4558 section to drive the tank. Also with current feedback

The collector of Q3 is connected to ground, while Q4 is going to what I guess is the negative supply. Interesting.

BOY-YOY-YOY-YOY-YOYING...

In the Roland circuit that is

** 500mV rms maybe.

** You posted the wrong schem.

Most PVs have nominal 600ohm reverb tanks that are comfortably drive by half a 4558.

.... Phil

Accutronics is now known as Accu-Bell Sound Inc.

There's a link at the top of the home page "How Reverb Works", which describes the operation of a spring reverb and why it sounds like it does.

I still have a few reverb tanks stored from the days of building my own guitar amps. Fun stuff! It's really spooky how the length and number of springs affect the sound you get from them.

Cheers, Dave M

** Fraid that link is dead.

.... Phil

** Not available to me.

PV forum members only.

There are plenty of PV Bandit schems on Google Images.

.... Phil

.... Phil

I saw one amp, don't remember the brand, but it used a 6V6 to drive the tank. Had its own output transformer to boot.