Author Topic: Help selecting tone pot capacitor for Rhodes Piano Bass  (Read 817 times)

Offline Brewski

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Help selecting tone pot capacitor for Rhodes Piano Bass
« on: September 17, 2018, 11:09:16 AM »
Hey folks,

I'm trying to get a '76 Piano Bass back into serviceable condition. The tone knob does nothing, so I was planning to change the capacitor. Any thoughts on what type of capacitor I should use? A schematic I found online calls for 2X100nf in parallel. I was going to look for a 200nf poly film cap, perhaps this one:

https://www.mouser.com/ProductDetail/Panasonic/ECW-FD2J564JQ?qs=sGAEpiMZZMv1cc3ydrPrF95ZAHiPXMKgXoGu%2frWisocxoFAzmhM6rQ%3d%3d

Any advice appreciated.


Offline Brewski

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Re: Help selecting tone pot capacitor for Rhodes Piano Bass
« Reply #1 on: September 17, 2018, 11:22:28 AM »

Offline sean

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Re: Help selecting tone pot capacitor for Rhodes Piano Bass
« Reply #2 on: September 17, 2018, 05:09:05 PM »

Try again.  That one is not stocked.


You should have no trouble finding a film capacitor.  You will pay more for shipping than the cap.  Arrow is desperate enough to give free shipping, I think it requires a $20 minimum, but maybe not.  Arrow's website sucks, but you can eventually find what you need there.  0.2uF is not as common today as 0.22uF, so Arrow has few to choose from, but this would work:
https://www.arrow.com/en/products/ecw-f4204hl/panasonic 
 
Mouser and DigiKey have lots to choose from, including:
https://www.mouser.com/ProductDetail/Panasonic/ECW-F4204JL?qs=sGAEpiMZZMv1cc3ydrPrF0%252bjlB8SXIRuBCtI0kuTyXA%3d
https://www.digikey.com/product-detail/en/panasonic-electronic-components/ECW-F4204JL/P12106-ND/585173

Those are all similar PolyPropylene caps.  If you want a ceramic cap, then the choice of two .1uF caps becomes more obvious - 0.2uF is not a common value in ceramic capacitors.  Values of 0.1uF or 0.22uF are much more common. 
________________________


BUT WAIT!!!!    What makes you think that the capacitors in your Piano Bass have failed?  These caps should be pretty reliable in this situation.

Have you checked the pot with a meter to verify that it is functioning correctly?  Have you sketched out the circuit to make sure that it makes sense and is all still connected right?

Anybody got a schematic of what the Piano Bass name rail volume and tone controls should be?

Sean 


Offline Peacefrog35

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Re: Help selecting tone pot capacitor for Rhodes Piano Bass
« Reply #3 on: September 18, 2018, 08:54:10 PM »
I have a schematic somewhere. Give me a day to locate it.
current gear:
1965 Fender Rhodes Gold Sparkle Piano Bass
1965 Fender Rhodes Gold Sparkle Piano Bass
1962 Fender Rhodes white top piano bass
1966 Fender Rhodessilver sparkle piano bass
1968 Gibson G101 organ
1966 Vox 301H wood key conitnental
1968 Vox 301E Continental
1967 RMI 300A Electra-piano and Harpsichord

Offline Alan Lenhoff

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Re: Help selecting tone pot capacitor for Rhodes Piano Bass
« Reply #4 on: September 18, 2018, 09:52:26 PM »
Here's a schematic someone posted on Fenderrhodes.com.  I can't vouch for the accuracy of it.

http://www.fenderrhodes.com/img/service/manual/piabassscm.gif

Alan
Co-author, "Classic Keys: Keyboard Sounds That Launched Rock Music" (Coming November, 2019)

Learn about the book: http://www.classickeysbook.com/
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1965 UK Vox Continental;1967 Gibson G101 organ; 1954 Hammond B2; Leslie 21H; Leslie 31H; 1974 Rhodes Mark I Stage 73; 1972 Rhodes Sparkletop Piano Bass; 1978 Hohner Clavinet D6; 1968 Hohner Pianet N II; 1966 Wurlitzer 140B; 1980 Moog Minimoog Model D; 1977 Fender Twin Reverb; Vox AC30CC2X amp.
(See the collection: https://vintagerockkeyboards.com/ )

Offline sean

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Re: Help selecting tone pot capacitor for Rhodes Piano Bass
« Reply #5 on: September 19, 2018, 07:28:18 PM »

Wow.  If that really is the tone control circuit in the Piano Bass, I bet the tone control is unimpressive.

Circuit simulation leads me to believe that the circuit rolls off everything above 2700Hz at roughly 6dB per octave, and the tone control knob just brings that rolloff down toward 880Hz.  So it removes high frequencies that are not really dominant in the source signal.  It wouldn't really affect the ballsy low bass tones of the long tines in the Piano Bass.  It might knock out some pingy overtones maybe?  Doesn't sound very useful to me. 

Was this circuit really used in all years of the Piano Bass?

Sean

Offline Alan Lenhoff

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Re: Help selecting tone pot capacitor for Rhodes Piano Bass
« Reply #6 on: September 19, 2018, 10:35:48 PM »
>>Doesn't sound very useful to me. >>

Sean, what you've described is exactly what they were trying to achieve -- filtering out the upper harmonics to leave a more pure fundamental bass tone.  Without that, the PB would sound like the lowest 32 notes of a Rhodes 73, rather than like a bass guitar.

Alan
Co-author, "Classic Keys: Keyboard Sounds That Launched Rock Music" (Coming November, 2019)

Learn about the book: http://www.classickeysbook.com/
Follow us on Facebook: www.facebook.com/ClassicKeysBook/

1965 UK Vox Continental;1967 Gibson G101 organ; 1954 Hammond B2; Leslie 21H; Leslie 31H; 1974 Rhodes Mark I Stage 73; 1972 Rhodes Sparkletop Piano Bass; 1978 Hohner Clavinet D6; 1968 Hohner Pianet N II; 1966 Wurlitzer 140B; 1980 Moog Minimoog Model D; 1977 Fender Twin Reverb; Vox AC30CC2X amp.
(See the collection: https://vintagerockkeyboards.com/ )

Offline sean

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Re: Help selecting tone pot capacitor for Rhodes Piano Bass
« Reply #7 on: September 20, 2018, 04:47:26 PM »

AHA!

Now I get it.

Sean

Offline Jenzz

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Re: Help selecting tone pot capacitor for Rhodes Piano Bass
« Reply #8 on: September 21, 2018, 11:07:21 AM »
Hi .-)

The above mentioned schematic was done by me many years ago. It was taken from what i found in an original PianoBass.

Keep in mind, that in addition to the high frequency roll-off, the caps form a resonant circuit together with the pickups inductance, because if you dial the tone pot, the caps get more or less parallel to the pickups.

The fix 100nF cap together with the pickups (2,24H) forms a somewhat flat curved resonant circuit at roundabout 330Hz,.

Jenzz
« Last Edit: September 21, 2018, 11:18:28 AM by Jenzz »

Offline cinnanon

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Re: Help selecting tone pot capacitor for Rhodes Piano Bass
« Reply #9 on: August 17, 2019, 02:16:48 PM »
Hi all, I have a December 1969 Piano bass (sparkle top) that i'm working on. My circuit differs from that found above. Here is mine vs. the one above. I don't know what the difference in sound would be, or which is superior. Mine looks original too. Looks like the tone pot is wired the same just reversed, but the volume pot is slightly different (tapers are different too).

« Last Edit: August 17, 2019, 02:30:26 PM by cinnanon »