Concertina Circuit Question

[alexage]

Attaached is a schematic I made of the driver and concertina splitter from an old Lectrolab amp that was given to me after the last tech gave up. There is no available schematic of the amp, model S 950.

The 47K resistor at the bottom was unconnected at one end, as shown. Can any one tell me:

- What funtion this resistor serves? The circuit would look familiar to me without it.

- Why the driver cathode is connected to the cathode end of the concertina?

Thank You![/img]

[Phil_S]

I don't know much...it looks weird enough to think it's a mistake. We know what the typical concertina looks like and this isn't it. Remove that wire and the 47K. Ground the 100K. See if the thing works.
Phil

[dartanion]

Could have been some sort of presence control, however you normally see NFB injected into the driver and not the inverter.

[tictac]

The schematic is drawn incorrectly is what's causing confusion.

The 100K on the cathode should be directly to ground and not connected the the cathode of the first triode.

The 47k is setting the negative feedback level and should be connected one end to the secondary of the OT the other end to the cathode of the first triode at the ungrounded junction of the 680 ohm resistor.

TT

[Phil_S]

Drawn incorrectly...that certainly seems reasonable. So does NFB.