attenuators

[Paul Joppa]

Why does the wire from the selector switch enter the "Out" of the attenuator?  Wouldn't the wiper be the normal "out"?

The Foreplay III is not wired as a normal attenuator. It's a shunt-more attenuator, and it's not a normal one of those either. This particular wiring sets it up so that a momentary failure of the switch contact removes the source; in a normal shunt mode connection such a failure acts as a momentary full-volume setting.

Actually, this is a variant of a classic "ladder" attenuator, which is (I'm sorry, it's not my fault!) NOT what is called a ladder attenuator today. You have to go back to the fifties to fine the classic ladder, which is a series of pi attenuators. It used to be the standard of quiet attenuators, used especially in live broadcast situations.

As you can see, there really are many attenuator wirings that are not "normal" but are still used in many situations.

 

[intojazz]

Hello Paul,
Indeed the pre was working correctly prior to these changes. Prior to the GP install I modified the pre for use with 6SN7 output tubes. Paul J. posted:
"All the pins are different between the tubes. You have to wire the triodes as in the circuit diagram, which means you have to choose which triode is the voltage amplifier and which is the cathode follower. Once you have done that, you will know which grid takes its input from the level control." How do I determine that?
John

 

[Grainger49]

Right, the FP III is the different kind of volume control.  The usual output is the input, the usual input is the output and ground is still ground.

I just refreshed my memory by looking at the schematic.

 

[Yoder]

Right, the FP III is the different kind of volume control.  The usual output is the input, the usual input is the output and ground is still ground.

I just refreshed my memory by looking at the schematic.

Hmmmm.... I built the following diagram with some help:

It also seems to follow DebK's images of her GP mods. But when I sat down and traced where the wires went, the attenuators connect to the "A" and "B" tubes which feed the RCA output jacks. So, in support of what Grainger says, the correct diagram should be (please correct me if I am wrong):

 

[Paul Birkeland]

The top diagram is consistent with the schematic, but in actuality both diagrams will function in the preamp, with the bottom diagram providing constant input impedance, with the top schematic having better range (IMO).

The OP should check to be sure these controls have a variable resistance between GND and Out and Out and In through the rotation of the control.

 

[intojazz]

Thanks Yoder,
The new diagram was helpful.
John

 

[Yoder]

John: does it work now as it should? I have not tested mine yet. I am finishing up the Eros and then will start all of the testing of the Para's, FP III, and Eros...then the truth shall be revealed!

 

[intojazz]

Hello Yoder,
Just got the pre working this morning. I had it wired using the upper diagram and had no volume control. I re-wired it using the lower diagram and volume control is normal for both channels. It seems fine now except I've picked some emi. I think the unshielded  wiring between the selector and attenuators is a possible cause? It was shielded in stock form so.....  any ideas for just shielding it?  piece of foil with a drain wire wrapped around it?
Thanks for the help,
John