Wurlitzer 200 Amp Schematic/Layout Questions

[Precision87]

I am currently rebuilding the amp of a Wurlitzer model 200 that was plagued with pretty bad hiss. I am in the process of replacing all the resistors and capacitors.
Part of the issues is that somebody had already worked on it years ago (not very well, i might add...). I can tell what has been fussed with, and by using the original schematic and layout, have been able to get almost everything back in order.

First question: What are the values of R75 and R76? those numbers don't seem to coincide with the schematic. Do I have the "correct" schematic?

Second question: Are D2 & D3 drawn in backwards? These don't match the schematic, and It's basic electronic knowledge that the cathodes should be facing the positive side of an electrolytic cap in a standard full-wave rectification application... Just wanted to be sure.

[Alan Lenhoff]

Let me make a couple of points -- and then hope that some of the people who actually make a living repairing these things will chime in with better information.

First, I believe there were at least three versions of the 200 amp.  So, it's possible you're working off an incorrect schematic.  Does yours indicate a serial number range?

Also, these weren't ever great, quiet amps. There were good reasons that Wurlitzer kept re-designing it -- and then moved on to the Model 200A.  (And there are good reasons why companies like RetroLinear manufacture quieter, modern replacement amps for the 200.) My point is that many people find that re-building these amps doesn't satisfy their desire to have a truly quiet amp. You are limited by its original design.

Alan

[cinnanon]

I second that. Go Retro or Vintage. Not original. They're plagued.

[Precision87]

The serial number of my model 200 is: #59245.
I think it's great that there are companies making improved boards for these old EPs.
My plan is save up and go with one of these if the rebuild board is unsatisfactory.
That being said, I seriously doubt that this amp was this noisy when it left the factory all those years ago. I realize it probably won't be as wonderful as some of the redesigned boards, but i'm curious on how much better I can make this one first.   
Obviously there is potential in these old boards, otherwise companies like vintage vibe wouldn't sell "rebuild" or "hiss killer" kits for them.
I have a fair amount of experience in electronics, just not specifically the Wurlitzer 200 circuit (not yet, at least).
I'm not just going to throw up my arms over a couple inexpensive resistors and spend $270-$389 on a redesigned amp.
If anyone is willing to help with my specific questions, I would be greatly appreciative. If you don't know a ton about electronics, even if it is just a photo of your 200 amp (particularity a close up of the components in question) would be a big help.   
Thanks.

[Alan Lenhoff]

>>My plan is save up and go with one of these if the rebuild board is unsatisfactory.>>

Sounds like a good plan.  Only you can judge whether any improvement you make in the amp is good enough to meet your needs/expectations. 

Your piano's serial number indicates that it was built roughly 1/3 of the way through the Model 200 production run.  I don't know when the presumably improved amps were released, but it's quite possible you have one of the earlier amp versions.

Alan

[Tim W]

Hello,

I feel like I should answer here as I designed the RetroLinear/Warneck Research EP200 amp.

First to answer the questions:  Yes, diodes D2 and D3  are reversed on the layout drawing.  Good catch.  D4 and D5 are also reversed in that drawing.  Somehow D1 made it though correctly.  Second, the two resistors in question were changed during production.  The final and correct values to use are 6.8k for R75 and 2.2k for R76.  If there is a 5uF cap in series with the wiper of the volume pot and the terminal post where R75 is soldered, replace it with a jumper as shown in the layout drawing provided above.

There is unfortunately very limited potential in your original amp. 

When I got my first 200 when in college, I bought all new premium quality parts to rebuild the amplifier.  I rebuilt it properly and carefully, doing everything right along the way.  When I flipped the switch, I still had a hissy hum bucket.  The vibrato would huff and puff, and the amp still didn't have considerable output power.  As I continued to study analog circuit design as I went onto graduate school in EE, I went back and looked at the 200 schematic to figure out why it was so bad.  It was then I realized the flaw- there was no way, based on the architecture of the circuit design that the amp could ever be low noise- no matter how many caps or premium transistors you put in there.   And the amp was designed to rely on characteristics of transistors that drift over time and temperature!  So that means that you could not expect consistent performance over the life of the amplifier.  Not a sound design, but the best they were doing at Wurlitzer in the early 70s.

From there, I did an analysis of the pickup structure in a Wurlitzer, and then designed and built a low noise amplifier that matched well with the pickups and had the necessary characteristics for excellent performance.  The vibrato feature was then added after I had a good preamp, and finally a redesigned power amp was put on the backend to drive the speakers.  Our EP200 amp was born.  Adding the comb pickup shield plates to the EP200 amp kit knocked the last bit of hum out of the piano.  Silence, unless you play a key.

You may want to consider that some companies sell parts and kits simply to make money from as many people as possible- and they do not have to provide technical support for the electronic parts they sell if you do the work yourself, since there is no guarantee that the parts get handled, soldered, or installed correctly.  Parts are sold at a considerable markup so that they can still make money from people who are not willing to commit to buying a new amp.  Everything is completely on you, and the improvements from many of these kits are usually marginal, insignificant, or non-existent. 

A careful look at the other amps available on the market will reveal that they are basically duplicates of the original Wurlitzer design, with fuses and higher power resistors added.  No change to the original failed circuit architecture!  One may also note that the other boards out there are starting to look more and more like our design (with solderless terminal blocks, etc.) but still with no change to the circuitry.

The original 200A had a much better preamp, but the power section still depended on transistor characteristics that drift.  No good, so we adapted our 200 design to work in 200As.  Our 200A amp provides significant improvement in the power section and still has an improved noise floor over the original 200A.

So, for $389 you get everything you need to make your Wurlitzer sound as good as it possibly can as far as the electronics go, pickup shield plates (for the EP200), no time invested in working with electronics except for the install, support from us if needed for the installation, a full 2 year warranty, and peace of mind for the next few decades.  Our amps are assembled and fully tested here at RetroLinear before they go out the door.  Many, many Wurlitzers you see on stage and in studios now have our boards in them- all working quietly, reliably, and bringing out the best each piano has to offer.

I think this is a pretty good deal for an instrument generally selling for $1000+ on the 'used as-is' market and only heading up from there. 

Here you can find several notable users of our products:

http://3knobwurly.com/artists.html

[pianotuner steveo]

Can you still see the color bars on your resistors or are they burnt? If you can read them, then you can find the same value replacements. As for the diodes, I would say go with your gut. I only had 5 years experience as a bench tech ( back in the '90's) but I seem to remember diodes either burning up or the amps just plain not functioning if installed incorrectly.

[Precision87]

@Tim W,
   Thanks for the input. I didn't know that other replacement amps are pretty much the same circuit. That's good to know. I also didn't realize that your price included the reed bar shields. That definitely makes it an even better sell too.
I plan on following through with this rebuild first (due to being half way already). That way, if i plan on going with a Warneck Research, I can at least sell an operational old one on ebay.

[cinnanon]

I'll make the prediction that you'll be buying one soon.

[Precision87]

@pianotuner steveo,
              On my board, those resistors showed obvious signs of poorly being replaced (several resistors stack to create various values). They did not look burned. The equivalent resistance was way off compared to what Tim W listed them as. 

[Ben Bove]

I can't reiterate Tim's statement enough.  I feel that the original amplifiers in Wurlitzers were much like the Rhodes action - an ever-changing model because they were trying to improve a naturally flawed design.  I can't even keep track of how many iterations of 200 and 200A amps there were ... it was the early days of solid state, and they got them out the door.  The keyboard collecting community is much different than guitars, so keeping an original amp or anything that might "affect value" doesn't even matter, it usually in fact increases value to restore or improve the keyboard.

I put one of his boards in my 200, and I haven't been back since.  It is the most requested Wurly rental.

I get wanting to recover some money on a working amp on ebay... Maybe a bit over $100?

[Precision87]

Quick for Tim W. (or anybody else that can answer this:
    What is the vibrato rate frequency of the Warneck aftermarket wurlitzer 200 boards?

[Tim W]

Designed for 5.5Hz, but may vary up to +/- 0.5Hz due to component tolerances.  This is in line with what the original amps produce.

If you want a custom fixed frequency, we can easily change it to whatever you want.

We designed the VariVib add on so that you can get a range of less than 1Hz to 15Hz while maintaining the identical amplitude modulation envelope on the audio across the entire frequency range. It sounds authentic (like Wurly vibrato should) other than the speed variation now available.

Best,
Tim

[cinnanon]

while maintaining the identical amplitude modulation envelope on the audio across the entire frequency range.

I noticed this with your amps, and like it.  Your volume remains the same, whereas in the original 200A amps the vibrato knob magically makes the volume noticeably louder as you turn the vibrato up.