Organ Amp - Tone Control and Guitar Input add

[kagliostro]

A guy at DIYItalia has some Farfisa Organ Amp parts, he asked how to use the Power Section of a Farfisa Foyer with the Keyboard of a Farfisa Compact Deluxe having on the Power Section also a Tone Control (for the Organ, despite the fact the Keyboard has a Tone Control by itself) and a Guitar Input

Farfisa Foyer - Power Section


Farfisa Compact Deluxe - Keyboard Output


About the Tone Control initially I was thinking to a Voigt, then I remembered that in my archive I've a Davoli Krundaal Organ Bass schematic from wich borrow the Tone Control
(note that the Presence Control was labeled that way in the Davoli amp, a bit odd to me as label)

Davoli Organ Bass


To drive the Tone Control with the less signal lost I've think to use a CF, I find the version you can see on my draw on Merlin's Book, it is claimed also to have a very high input impedance

As preamp, at the moment, my choice was to use the first triode of Merlin's Medium Gain Preamp


I mixed this things and this is the result
(NOTE there wasn't a coupling cap between CF and Tone Control, FIXED, see schematic on following post)

(remember, the principal use of this thing will be Organ Keyboard at the input)

There is a thing I don't like very much on the Tone Control, the use of a 2.5M pot, I've seen that a Lesa 2.5M pot can be find, but it is old component and I would like to know if it will possible to mod the capacitor/capacitors value/values to obtain the same result with easier to be find pot, I know that something can be done but it is out of my Know How

Obviously I need to hear your opinions, councils, consideration etc. etc. about the whole thing

Thanks

Franco

In the attached pdf the whole documentation about Farfisa Foyer and Farfisa Compact Deluxe

[PRR]

You want a blocking capacitor after the cathode follower or DC will leak through the 470k resistors to the power amp.

That is quite a complicated tone control. If you really can not get 2.5Meg pots, try 1Meg, and be ready to try changing the 470Ks to 330k, maybe increase some capacitors 30%-50%.

[kagliostro]

Many Thanks PRR

I completely forgot to put a coupling cap, thanks for the advice

Will a 0.1 cap be adequate in that position ?

About the Tone Control, yes, I admit, is a complicated one, I tried to find something usable on the organ amps schematics I can find around but it was a no useful.search

May be I can swap it for a Voigt .... there are other Tone Controls that may be an alternative ?

Franco

[sluckey]

0.1µF will be fine. I would use an Alpha 2M potentiometer. Should be able to find them over there. The pot is just being used as a variable resistor so it is not critical. I would prefer a 3M potentiometer but the only ones I saw had a reverse audio taper.

[kagliostro]

Many Thanks Steve

2M I'll suggest to the guy to try with it Thanks

Here a revised version with the 0.1 coupling cap added AND corrected connection of the Organ Keyboard Female Input



Franco

[kagliostro]

I forgot to add a coupling cap between the CF and the Tone control and this kind of Tone Control require a copling cap as PRR pointed to me

Then I added a 0.1 coupling cap between the CF and the Tone Control but ... but looking to the schematic a question come to me

which will be the better way to add the coupling cap ?

here you can find two solution, one I've previously posted and one is new, in the new version the 0.1 coupling cap is connected directly before the 470K resistor and, in my mind, it didn't affect the value of the 0.001 cap that is already present in the Tone Control



Am I thinking wrong or overthinking ?


BTW


I tried to examine how acts this Tone Control but wasn't able to find any more than how acts the High Control

Thanks

Franco

[PRR]

I think you are over-thinking.

> how acts the High Control

Does it act? It looks like it would load-down the highs from the driver. But that has been changed to a cathode follower, very low impedance, hard to load-down. Maybe something was lost from the original plate-loaded version?

[kagliostro]

Ciao PRR

THANKS

I was thinking the effect is to shut to ground the high frequencies acting independently from the induced load to the CF

There are examples of TS following a CF in guitar amps but may be the components values aren't the same of the same TS placed after a Plate Follower

About the position of the 0.1 coupling cap I may be overthinking but about the Tone Control I must investigate more

Franco

[kagliostro]

The search for the 2.5M pot was hard, I didn't find it (initially I finded an old Lesa long shaft pot but then no more) I was searcing without result for a 5M dual ganged pot (to put in parallel) but also this was without result, the only I've find are dual 2.2M long shaft pots ....

PRR pointed me also that the Davoli Organ Bass Tone control was preceded by an Anode Follower, not a CF so I've think to swap for one other design, the Passive Baxandall that I've seen also connected at the output of Cathode Followers

so, here is the new proposal



Suggestions ?

Thanks

Franco

[kagliostro]

On the Foyer Reverb part of the circuit, there is an arrow labeled "From ON/OFF Switch Reverb)



Looking to the whole schematic I identified a part that seems to correspond, composed by something that may be a Switch (draw in a way I don't understand), a cap and a pot, seems that the reverb circuit is connected to ground via the cap and the wiper of the pot or shorting the Switch






Have I argued correctly ?

Franco

p.s.: Attached the whole Farfisa Foyer Schematic in pdf

[sluckey]

Switch first. The black dot is the movable switch contact. Push the switch lever one way and the black dot connects to the two upper switch contacts (which are connected to nothing). Push the lever the other way and the black dot connects to the two lower switch contacts (which are connected to ground).

The cap and pot is a tone network (probably preset at the factory?). This tone network is connected to the black dot which is always connected to the output of the reverb tank. So, the switch simply grounds the output of the tank when set to reverb off thus killing the reverb. Switch the reverb on to remove the ground and allow glorious reverb sound.

The whole operation could be accomplished with a simple SPST on/off switch, but the reverb switch is part of a multi function switch assembly and it's just easier to make all the switch sections identical.

[PRR]

Yeah, that's a funny switch symbol probably to support features used by other organ parts (vibrato, pipe rank, chording, string filter...). As said, they just buy (draw) one basic switch for all functions.

Has to be a tone (treble) control (cap and pot across tank output), and a mute switch (short tank output to ground). Like this:

[PRR]

56k tank drive resistor does not sound like the classic Hammond wound-coil tanks. Some other amps used piezo crystal drivers.

If you have the Farfisa tank, this is just a reminder to take good care of it. If you are restoring a sick reverb, it may be tricky.

[kagliostro]

Many Thanks Steve and PRR

Now I can confirm what I was thinking

Franco

[kagliostro]

One other question

The guy who is adding the Preamp I've draw to the Farfisa Foyer Organ is asking to me with which B+ he will feed the added preamp







My intention was to simply use the 200v B+ that is present on the Organ but ... in the optic to feed the new triodes near as planned by his author (Mr. Merlin) and avoiding to add a new PT I had this idea

In the Organ the PT is a 190 - 0 - 190 rectified using an EZ81 and B+ resulting in 200v

The triode at the input of Merlin's preamp (I borrowed) in the schematic is feed with around 260v B+

So why not double the rectifier maintaining the EZ81 tube ad obtaining the ~260v of B+ using a pair of Solid State Diodes , like here



I think it is feasible but not sure about

May be it really a solution ?

Thanks

Franco