Farfisa Compact Replacement Spring Reverb Driver

[tardishead]

Having restored a few Compact organs one thing that very rarely survives is the original reverb tank
It was made with piezo disks not magnet/coils.
Both sides of a 12ax7 tube were used to get a big voltage swing needed for these piezo tanks.
I was wondering about different options to power a replacement Accutronics style spring reverb tank.
There is a 8vdc supply available which supplies about 110ma to all the circuitry of the organ
And then there is a 210VAC winding off the transformer which supplies 270VDC for the two 12ax7 tubes with an idle current of 1.5ma or so. One 12ax7 is used for reverb driver one for output preamp and reverb recovery amp.

So one option would be to power a LM386 reverb driver from the 8vdc.
But I was looking at this circuit from Valve Wizard. Seems like a waste to not use the unemployed tube socket and would be great to replace the reverb driver with a tube powered one.

Idle current for the 2 halves of the 12au7 is stated as 6ma with peak I of 12ma
Filament current for 12au7 is the same at 300ma.
Would this be a risky modification in terms of available power from the HT winding? Which was only meant to power a couple of ma from 12ax7 tubes? But its only 2-3 more VA demanded off the transformer core which Im sure must have been built with a fair bit of headroom. Then theres the HT winding wire diameter - that will limit the available current obviously! What was general winding practise? Is this risky?

 

[joaquins]

  You are taking the old driver out so you sure have some power to play with. You already have a driver for the piezo one, tweaking the parameters of the same driver to get lower impedance is an option, I don't know the original circuit. A transformer is an option, a high input impedance tank another, so you don't need lower than the tube can handle. The IC option should be fine as well if you don't mind having some Si inside a Farfisa...

JS

 

[PRR]

A LM386 or a tube with a reverb transformer will want a 8 Ohm tank. Opamp on >20V or tube without a transformer will want the highest tank impedance.

Any of these can work. I "like" the idea of tube drive and recovery but I am not sure anybody hears it (except the builder). The wound-tank will need more total gain than the piezo-tank. Be prepared to experiment.

 

[tardishead]

Read up about the "1 tube reverb" idea that is going round guitar amp websites where people are using one half of a 12ax7 to drive the standard Fender reverb transformer (25k:8) and using a high impedance tank 4fb.... instead of a low one
I thought I could use the available 12ax7 socket. Wire the 2 triodes in parallel (to decrease the output impedance and increase the power) and then the reverb transformer and high impedance tank. What do you think?
The bonus of this is that I won't be using any more current off the HT transformer than what was there originally.

 

[bibi]

I've done this before and ended up using a constant current driver similar to what you posted.  I used a transistor LND150 as the recovery amplifier. 
I think the circuit you are referencing with the 12AX7 uses a transformer.  There may not be enough space to pack that in the organ. 
Actually a pentode/triode tube works great as then the pentode can be the current driver and the triode works as the recovery.  There are loads of cheap p/t tubes out there that would work. 

 

[tardishead]

I've done this before and ended up using a constant current driver similar to what you posted.

Ah ok! With a Farfisa Compact? So constant current driver does not fry the HT winding ?

Actually - there are 2 versions of the Compact PSU/Reverb?preamp schematic. One uses 2 triodes of 12ax7 for reverb drive, one triode for recovery and one  for preamp. The other uses one triode for reverb drive , one triode for recovery and 2 triodes for preamp. I had better check what one I have at the moment.

 

[bibi]

The one I did was also a Farfisa Combo Compact and I didn't have any frying. I doubt it would be a problem even with a lower impedance tank, but if you use the F version you just really need a lot of voltage swing and very little current. 

Now that you mention the two schematics, I looked again at my notes.  I had to use a transistor to get the signal voltage up as the original tank has a very high impedance input and output.

Good luck and let me know if there is any other info you need. 

 

[tardishead]

Checking the Compact I am working on it is actually the one using 2 triodes as the preamp and the other 2 12ax7 triodes as reverb driver and recovery
This limits my options if I want to mod the reverb circuit only which I do.
This leaves one b9 socket for drive and recovery
PT tube? 6bx8 or similar. Big increase in current used I think a bit too risky!
12au7 constant current driver without transformer and high impedance tank and SS recovery amp - possible
But I think I am more inclined to use lm386 driver and 12ax7 recovery with a low z tank. I might parallel 2 triodes for what it's worth