Wurlitzer 270 // “Passive-Output” Conversion

There’s no doubt, the Wurly 200 series keyboard is a classic R&B piece …

For a while, I used the cheezy sounding on-board speakers on this “Elephant Grand” 270, got quickly tired of those and moved up to a Peavey stereo-Chorus 212 amp I was given // using the headphone output jack on the Wurly at first … that was passable for the most part // and, of course, it sounded mo better through a PA … recently, I been plugging the Wurly into my 360+/361+/1×15 bass rig and finding that to be the best by absence of a horn/tweater … it’s way more natural sounding w/o any harshness in the dynamics

that last setup provided improved fidelity and balanced ergonomics (strong/tight bass, no ice-pick top end, etc …) … still, … I recently did some recordings with the Wurly and upon checking out the playback I noticed the output sounding quite dirty in sections // … especially with certain note combinations, almost sounding like inter-modulation terms, etc … further fidelity improvements were needed

I had studied the Wurlitzer 200 series preamp schematics before, and came to suspect that a much better signal could be obtained by going directly from the “source” of the keyboard’s electro-mechanics // (a popular theme for me; as my Rhodes, Hammond, Baldwin Electropiano, are *all* modded this way in the front-end …)

>> bypassing all ensuing electronic circuitry in the process was probably (AGAIN) the way to go … no hesitation this time (!)

WurltPreamp

all that is needed to pull this off is this :

(i) … access to the bias source (150vdc in the schematic, 138 vdc in my Wurly 270) for polarizing the reeds … and also, in lieu of the existing preamp circuit and controls, we simply need

(ii) … an appropriately valued cap, resistor, and clean wiring job to extract signal while making sure hum and other noises don’t creep back in along the way …

passive_out

luckily, I found an unused vintage NOS Mullard 0.33uF / 160v film capacitor in my stock pile (!) .. a 1 Meg / 1 watt film resistor would do for bias feeding … that’s all, parts wise

WurlyCap

the keyboard now receives its own (direct) bias lines to feed to the reed bar and GND connections from the original PSU section … again, all other electronic circuits are simply disconnected and left unused //

the instrument now becomes a simple mono source device with NO Volume or Tremolo control // all in exchange for a super-pristine signal

WurlyReconnect

bypassing all the original electronics means we can use better/modern circuitry instead … in my case, from the (new) output on my Wurly I go to a 360+ Lite preamp and 361+ power amplifier bass rig // … yielding fantastic sound, if I may say (!!)

and, with absolutely NO issues at all // … well, actually I have to leave the reeds biased all the time or else I get intermittent crackling noises for the first hour after plugging in … after that it’s all good

ie., no AC hum … hardly any hiss (Signal/Noise is much higher than before as a result), and at this stage of the game the Transient Response is supremely accurate … couldn’t be better

A real joy to experience …!

WurlyFront

NOTICE

A few players have chimed in to inquire about modding their Wurly … I will say the following for now

(a) I don’t necessarily advise doing this to an older “tube” type Wurlitzer piano // that is, UNLESS you really want to experiment with sticking FX insert jacks and the amplifier // … if I had a tube model Wurly that’s what IO would probably do, as the built-in Wurly amplifiers are capable of great sound (unlike my possibly sick 270) and a good phase-shifter between the reed and the amplifier would sound great I’m sure … that being said the similar results obtained with mine (ie., from going direct out) could be obtained with a tube unit as well if similar mods were done // so, yes, it is an option but I don’t consider it “de rigeur”…

(ii) for those who have busted 200 series electronics, if the power transformer is still kicking but the circuit board is in bad shape I would strongly recommend exactly duplicating the high-Z filter part of circuit that provides the reed bias voltage … it provides better ripple filtering than the tube circuits did, and it is the best way to guarantee that you’ll have the correct output voltage when using the original power transformer

CUSTOM REED BIAS REPLACEMENT CIRCUIT // ON-BOARD PREAMP BYPASS MOD …

200MOD-SCHEM

wurlyHVfilter

wurlyHVhookup


8 comments on “Wurlitzer 270 // “Passive-Output” Conversion
  1. Matt Normand says:

    Hello, I am interested in updating the filter caps but do not know the equivalent values I should buy. There are two 500 MF @ 50V and one 1000 MF @ 50V. I am familiar with replacing filter caps in guitar amplifiers, but here is where I am stuck with this piano.

    If you can tell me the equivalent values, that would be awesome.

    Thank you!

  2. jcm says:

    HI MATT,

    SORRY, NOT SURE WHICH CAPS YOU MEAN – DOESN’T SOUND LIKE THE ONES IN THE HV FILTER I’M REFERRING TO … IN THE MODEL 200 SCHEMATIC WE SEE THE HV “REED BIAS” CIRCUIT FILTERED BY THREE 0.33UF CAPS GOING TO GND // TO BE SAFE, THESE SHOULD BE RATED TO 400 VOLTS … THIS HV FILTER IS FOUND ON THE MAIN BOARD AND FEEDS ABOUT 150 VOLTS DC (THIS CAN VARY) TO THE REED ASSEMBLY

  3. Gabriel Donohue says:

    Can you show a pic of exactly where the wires get connected.

  4. jcm says:

    HI GABRIEL,

    HAVE A LOOK AT THE SCHEMATIC AND PICTURES I ADDED FOR YOU …

    THE KEY GOAL HERE IS TO ELIMINATE ALL POTENTIAL SOURCES OF HUM … THIS INCLUDES DISABLING THE PSU MODULE TO THE ON-BOARD PREAMP/PWR-AMP, ETC … AND LIFTING THE GROUND TO THE POWER CORD, SO THAT THE ONLY GROUND COMES FROM THE OUT-BOARD PREAMP OR POWER AMP

    THE POWER TRANSFORMER IS NOW ONLY USED TO PRODUCE THE HV VOLTAGE THAT BIASES THE REED ASSEMBLY, WHERE THE BLUE AND RED-YELLOW FEED A NEW HV-RECTIFIER/FILTER BOARD … ALL OTHER WIRES (RED) ON THE POWER TRANSFORMER ARE DISCONNECTED FOR GOOD MEASURE (NO CURRENT FLOW, NO RADIATING FIELDS)

    THE CHASSIS CONNECTION SHOULD BE OBSERVED TO HELP SHIELD THE KEYBOARD ASSEMBLY, IN THE CASE OF THE 270 THERE IS CONDUCTIVE PAINT THROUGHOUT TO HELP SHIELD THE CAVITY, A GROUNDING SCREW IS THEN USED TO TIE THIS PAINT TO THE CIRCUIT GROUND LINE … THIS IS ALL VERY SIMILAR TO A PASSIVE GUITAR OR BASS, EXCEPT IN THIS CASE A DC VOLTAGE IS USED TO BIAS THE REED ASSEMBLY AND A SIGNAL CAP IS USED TO FEED SIGNAL FROM THE REED TO A GROUND REFERENCED LINE, AND THEN TO A LOW-NOISE PREAMP, POWER AMP, ETC …

    (SEE ADDED PICS ABOVE)

    HOPE THIS HELPS

  5. Mantrak says:

    Hi, I would like to eliminate the original amp and replace it with Blackstar ID Core guitar head (stereo + efects + sims). I would transplate it into my 270. I also intend to replace speakers with fresh ones with more range. I want more aggresive tone with more treble. Maybe the best would be to canibalize a Blackstar ID Core 40 combo (20W + 20W). I am a bit puzzled by your mod. Why you need keeping bias voltage? Why Fender Rhodes does not need it? I am not entirely clueless about electronics but I do need this explanation. Huge thanx!

  6. jcm says:

    HI MANTRAK,

    THE WURLY REED SYSTEM OPERATES ON AN ELECTROSTATIC PRINCIPLE, MUCH LIKE A CONDENSER MICROPHONE DOES // AND IN BOTH CASES REQUIRES “DC” VOLTAGE BIAS ON THE ELEMENT TO PRODUCE “AC” SIGNAL … A FENDER RHODES OTOH OPERATES FROM MAGNETIC (PICKUP) COILS WHICH REQUIRE NO “DC” TO DO THE SAME … GOOD LUCK ON YOUR PROJECT !

  7. Josh says:

    Great article! I was wondering if you could speak a little, or maybe make a different post, outlining how you bypassed all the electronics in a baldwin electropiano. The electronics seem to be the weak link for these pianos. Are the pickups passive and its simple, or is it a little more complex? Thank you!

  8. jcm says:

    HI JOSH,

    I’LL TAKE SOME PICTURES NEXT TIME I’M AROUND MY BALDWIN ELECTROPIANO …

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