Wurlitzer 200a preamp clone

[DiggyFresh]

Hi peeps,


i've been working on restoring a Wurlitzer 200a and i thought to clone the Preamp/vibrato/line out section and make a stompboxe
for use with other instruments (guitar, pianet, rhodes).

The circuits are very simple, but i need help figuring out the eq curve of the preamp  (the preamp response is compensated to make the piano even, i think it attenuates the low notes to match the volume of the high notes better. and a little high freq roll off).

here's the preamps schematic:


Can you more experienced guys help me figure out which capacitors do the eq curve?  i would like if possibleto be able to bypass some of them to have flatter response to use with other instruments.


I think c3 and c4 are "Miller feedback capacitors" and do a high frequency roll off.

C1 probably deals with low frequency roll off (coupling).


No idea what C2 is doing.


Thanks alot guys

[Kipper4]

Even blown up lots, its very hard to make out whats in the picture.
Please can you post a bigger one or a link thanks.

[DiggyFresh]

weird, i see it perfect on my computer and phone. Here's a slightly bigger picture:



Thanks

[slacker]

I simulated it and it's pretty much a flat response up to about 15kHz or so then rolls off, the resistance between the points 1 and 2 in squares controls the gain. There's a bit of the circuit that's unclear though so what I've done might be wrong, see the break in the line between the bottom of the 220p cap and the 33k, if you just connect it together the circuit doesn't seem to work, which makes sense because the first transistor has no DC path through it. If you ground that point it seems to work as you'd expect and the DC test voltages match those on the schematic. I looked at a few schematics and they all seem to be copies of the same one, with that missing bit of track.

[DiggyFresh]

Thanks Slacker!

Very interesting! I'm not good enough to do simulations yet so that helps immensely!

Can you tell what the input impedance would be with the simulation? That would be good to know.

I hadn't even noticed tha break in the line, with my limited knowledge i thought it was just a blemish on the print..

Unfortunately, i don't have the piano with me anymore to verify if they are connected togheter or grounded..
Very strange.

I am guessing the diode is there to protect the transistor from over voltage (?) (There is 150v on the other side of c1 to polarise the reed bar pickup).

Should this diode be grounded to protect or connected to the 33k at the emiter of q1?

Also, Slacker, what about the frequency response if you ground both the 33k and c2 (220p) ?? 

[slacker]

Can you tell what the input impedance would be with the simulation? That would be good to know.

Looks like about 400k

I hadn't even noticed tha break in the line, with my limited knowledge i thought it was just a blemish on the print..

I think it is just a blemish but just fixing the break the circuit doesn't seem to work, maybe there is some other error in the schematic or maybe I did something wrong, I'll double check.

I am guessing the diode is there to protect the transistor from over voltage (?) (There is 150v on the other side of c1 to polarise the reed bar pickup).

It looks like it's to clip negative signals to protect the transistor from large negative signals but I'm not 100% on that without knowing what's going on with that break.

Also, Slacker, what about the frequency response if you ground both the 33k and c2 (220p) ??

That's what I did, I grounded the bottom of the diode, the cap and the 33k resistor.

[DiggyFresh]

Ah ok ok. Thanks infinite man! Realy helpfull!


Like you said, connecting to point 1 is the vibrato. Just a variable resistance to ground. (An ldr/led with the led beeing controlled by a simple 2 transistor oscillator).

Did you try simulating a resistor there to see if it changes something?  Something around 25k would be around the middle of the ldr/pot combination.

I'm still confused about c2 tho (220p), what does it do to the eq curve when grounded?  You said response was semi flat to 15k when grounded. I don't understand its purpose.


The help is very appreciated!

[PRR]

There is clearly a missing ground at D1 C2 R5.

Just by experience, I say the 100pFd caps do nothing in the audio range.

Input impedance is near 470K||3,000K or 400K as said. 0.02u in front makes it flat to 20hz.

C2 is probably to kill radio reception when reed-bar is disconnected, or instability in the amp. The effect of 220p here depends on what is to the left of "in". If a perfect voltage source, 22K+220pFd goes out to 30KHz. I suspect the original is a *capacitor pickup* (note 150V bias!!), then 220pFd extra does not change response, only level. If you jack a guitar in here, 220pFd is like another 7 feet of cable; this type input is super common on guitar amps so would be fine, and fine-er for most other sources you mention.

Overall gain depends on R10 56K and the part you cut off. Diddling pin[1] can give midband gain from several hundred to nearly unity.

[DiggyFresh]

Amazing Prr! Thanks!

A lot clearer now, i guess i was confused about the "eq curve" because i kept reading on forums it had one, even manufacturers of "remake" upgraded amp mention it.  I doubt it is in the following amp or the aux out amp.

@Prr. 
I could not tell you if it represents a "perfect voltage source"  but It is indeed a "capacitor pickup" polarized with 150v.


Thanks guys for helping to find the error on the schemo and the analysis.  Can't wait to see how it sounds on other sources!

[GaryM]

Nothing to add except to say that I'm also restoring a Wurlitzer 200A. Did a recap of the electrolytics, increased the output bias to eliminate some crossover notch and replaced the output fuses with resettable fuses. Next, I need to find some chrome legs, a sustain pedal and some original style knobs.

Here she is. 

[DiggyFresh]

Good luck Gary! 

If i can help,   (if you are willing to go full on!)
I was able to DRASTICLY  reduce the noise by also:


- replace transistors in the preamp by low noise (2n5089 or mpsa18)
-  replace signal resistors by metal film in the preamp.

- refresh the resistors and 0.33uf caps in the 150v supply
- twist the ac wires
-redo most of the signal wires away from ac with high quality
Shielded cables
- hunt down the internal ground loops.

I was able to realy lower the hiss and move the hum below that, making it a very low noise piano.  This is not info available on the net.  No need to replace the Whole amp by an aftermarket ones like most people seem to suggest. Reealy worth it.

[GaryM]

Thanks for the suggestions, DiggyFresh. That's a great idea to go with lower noise resistors and caps.

On the twisted power lines, my model comes with those routed inside a metal tube, so they are shielded for EMF.

Only problem I'm experiencing is some low frequency distortion. On lowest notes, the waveform going into the power amp is not what's coming out, per my oscilloscope.