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Old 4th July 2014, 12:07 PM   #1
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Default Farfisa Combo - Reverb Replacement

Hi - first time posting on these forums but I've been lurking for a while and have already learnt a lot from the knowledgeable people on here.

On to the actual subject of this thread: I recently came into the possession of a Farfisa Combo Compact organ that baring a few minor issues was in relatively good condition. Unfortunately, the reverb unit was dead. It was an old patent-eschewing piezo design and the elements themselves had failed, so no hope there.

Of course, these days it's no great loss but it would still be nice to have an on-board spring reverb. I wondering whether it could be replaced with a standard tank of the Fender type. Being a piezo design the associated circuit utilises high impedance input/outputs:

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I was thinking of perhaps installing a transformer on the driver and replacing the 120KΩ resistor on the output with a lower value one, but my experience with tubes is limited and there are no doubt nuances I'm missing. I know that the ECC83's limited output power may be an issue.

Anyway, any help/thoughts are greatly appreciated. As I say, if it's a completely lost cause it's of no great consequence.
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Old 4th July 2014, 03:29 PM   #2
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I think accoutronics are still on the market. You could look a reverb spring with high impedance coils - but maybe this is not worth the money.
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Old 5th July 2014, 01:39 PM   #3
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Hi, yeah surprisingly there's company near me that sell some standard Accutronics tanks and they're not too expensive (I can always find another use for it if this doesn't work). Not the highest input impedence but hopefully the transformer I've requisitioned up should be able to supply enough current.

I'm guessing the input could do with a bit of high pass filtering too?
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Old 6th July 2014, 12:12 AM   #4
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Traynor and Ampeg to name a few used capacitor coupling on reverb tanks.

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Old 6th July 2014, 07:46 PM   #5
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Quote:
Originally Posted by Baby Squid View Post
I'm guessing the input could do with a bit of high pass filtering too?
Yes, a roll-off between 300~500Hz should be fine!
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Old 18th July 2014, 08:11 PM   #6
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I've been fiddling around a bit and it just isn't working; I just don't think the tubes are up to the task. I was thinking of implementing a solid state driver circuit with discrete transistors but I'm in two minds about it.
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Old 18th July 2014, 10:31 PM   #7
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What does fiddling around mean?

What type reverb pan are you using? For a high impedance pan, one like the 4FB2C1B would be the choice. The "Fender type" pan has the lowest impedance driver coil and would never work here. And transformer???

Here is an Ampeg model that uses the cap off a plate drive that your amp already has:
http://bmamps.com/Schematics/ampeg/Ampeg_v4preamp.pdf

The "delay line" is just a spring reverb pan, and the ground side of the input and output jacks is not drawn.


What is wrong with it now? Reverb has two parts, drive and recovery. Which part is not working for you? or are both failed? Is a strong signal reaching the drive end? If you touch the wire to the recovery stage, does it make hum?
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