Help identifying/testing my preamp

Started by Brewski, June 23, 2022, 12:32:22 PM

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Brewski

Hello all. I just bought a 1978 Rhodes 73, which I believe to be a Satellite model. It has legs like a stage and a tremolo preamp like a suitcase. I don't have the power supply or cable required to use the preamp, so I don't even know if it works. I can use the 1/4 plug directly from the pickups and bypass the preamp, but what fun is that? I want to use the preamp and tremolo with my guitar and/or keyboard amps.

1) How can I identify which preamp this is?
2) Is there a way to test the preamp before spending money on an aftermarket power supply?
3) what are people's favorite aftermarket power supply/interface?

Thanks so much!

Brewski

Vintage Vibe tells me this is the Peterson preamp. They also said Rhodes was no longer using this preamp in 78 so the rail is likely not original.

I still wonder if there's a way to test before buying the power supply. Maybe I have a friend with a suitcase speaker.

spave

Quote from: Brewski on June 23, 2022, 12:32:22 PM1) How can I identify which preamp this is?
2) Is there a way to test the preamp before spending money on an aftermarket power supply?
3) what are people's favorite aftermarket power supply/interface?

Hi Brewski,

1. VV is correct on the Peterson not being original to a 1978. That being said, most people prefer the Peterson so it's not really a bad thing.

2. The best way to test the Peterson without a power supply is to take it to a local tech or send it to Avion Studios to have it refurbished. Even if the preamp is currently working, it still has 40+ year old parts and will sound a lot better after being gone through. I sent one of mine to Avion and am very happy with the results.
https://www.avionstudios.com/rhodes/main-in-service-janus-preamp-recapped-rechipped

3. If you ended up sending your preamp to Avion then you could also buy their power supply and save a bit on shipping. https://www.avionstudios.com/rhodes/peterson-preamp-power-supply-output-box
I haven't used it personally but I've heard good things about it.

Alternatively, VV also makes a power supply that usually gets good reviews. https://www.vintagevibe.com/products/4-pin-rhodes-external-power-supply

Or you could get VV's check block power supply if you don't like the look of a bulky power supply box: https://www.vintagevibe.com/products/fender-rhodes-4-pin-cheek-block-power-supply



Also, two other things:

1. You can plug the preamp back in then just put the 1/4 into accessory 1 which goes straight from the pickups. Looks a bit cleaner than running the 1/4 past the lid.

2. Satellite speakers do show up for sale occasionally so if you have the space, a set could be a nice alternative to the power supply box.

Hope this helps  :)
1969 KMC Home Rhodes Prototype
1971 Suitcase Fender Rhodes
1977 Wurlitzer 270

sean

#3
Brewski,

If I had this preamp in my house, I would test it with battery power.  It is a lot easier than any other way.

The hardest part is slapping together a wiring harness that plugs in to the preamp.  Since I don't have any four-pin plugs on hand, I would solder little test leads to the back side of the connector, or I would try to cut down some spade lugs to shove into each terminal of the connector.  You can also use alligator clips, but I would recommend the little J-hook style test leads. 

Be careful if you connect to the back of the 4-pin connector - it is very easy for the little clips to slip off and bang into another pin.  It is very easy to accidentally dump the full DC power into pin2 or pin3 and damage your test power amp.  For that reason alone, I use a cheap 9V-battery-powered toy amp as my testing amp.  Also, connect your clip to the preamp first, then connect the other end to the battery.

I like to use the least-fiddly setup as possible.  Get a box of little connectors from Amazon: WAGO 221 LEVER-NUTS.  Scroll down until you see the pack of 25 for thirteen bucks.  You can plug various size wires into these connectors, lift the lever to change connections, and tape the connector to your bench to keep things from moving around.  For benchtop testing, these connectors are re-useable.  (Mark them as used, so you never install a used connector in an AC box in your house.)


You just need to get the positive terminal of your battery to preamp pin 1, and the negative terminal of your battery to pin 4 (which is also the audio ground for the two channels).  Connect a 1/4" jack between pin 4 ground and pin 2 audio out, and another jack between pin 4 ground and pin 3 audio out.

You can use three nine volt batteries in series to get a little more than 25V, or you might have a power tool that uses a 24Volt battery.  If you are using three nine volt batteries in series, don't expect them to last for long lighting up those tremolo bulbs.  If the bulbs don't get to full brightness, you can put another group-of-three batteries in parallel with the first group for more current at the same source voltage.  Put a switch or quick-disconnect in the circuit to cut the power easily.

Also, if the audio sounds tinny (no low end), add a "bypass" capacitor across the power supply (battery) terminals.  Honestly, you don't need a huge electrolytic cap, something smaller than 50uF will probably be fine for testing with 9V batteries.  You should be able to notice the improved bass response by using a little 10uF MLCC ceramic cap for bypass (if you have a handful of them, put them in parallel).  The bypass capacitors are easier to manage on a little breadboard, but it isn't too hard to manage if you use the 5-hole Wago connectors for the +24V and Ground bus connections.

A few years ago, I did some calculations to predict that the Peterson preamp will probably draw less than 300mA:
https://ep-forum.com/smf/index.php?topic=9995.msg55633#msg55633

Study this schematic:
https://www.fenderrhodes.com/org/ch11/fig11-8.jpg

BTW, I have lots of experience with Rhodes power supplies for the Janus preamps.  See https://ep-forum.com/smf/index.php?topic=9657.0


Oh, I guess I didn't mention this:  take the preamp out of the piano before testing.  Remove the whole name rail, and remove the knobs, and unscrew the preamp from the rail.  If you need to run a long cable from the harp connector to the test rig, you can plug the long cable from the harp into the Accessory 2 (lower) jack of the preamp (and leave the existing RCA cable that hangs out the back of the preamp sitting there idle).

Wait - one more thing: this testing rig will be a little bit noisy, because nothing is shielded nor truly grounded.  I find that tolerating the noise is easier than trying to eliminate it with aluminum foil and ground wires.

Whatever you do, take photos and post them!

Sean

pianotuner steveo

Is the DIN Jack above the word input there? In the photo it looks like it is missing.
1960 Wurlitzer model 700 EP
1968 Gibson G101 Combo organ
1975 Rhodes Piano Bass
1979 Wurlitzer 206A EP
1980 Wurlitzer 270 Butterfly Grand
2009 73A Rhodes Mark 7
2009 Korg SV-1 73
2017 Yamaha P255
2020 Kawai CA99
....and a few guitars...