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!

[teemuk]

The 47K resistor looks like negative feedback (which someone has diconnected). It should connect to speaker output (I'm not sure which impedance tap though).

Also, to make the circuit sensible, the 100K at the cathode circuit of the phase inverter triode should most likely connect to ground (not to the cathode / feedback of the 1st triode as shown). This would be a very basic scheme one could expect a simple guitar amp has. Recheck your sketch. I'm quite sure you made an error while drawing it.

[alexage]

The 47K resistor looks like negative feedback (which someone has diconnected). It should connect to speaker output (I'm not sure which impedance tap though).


Thank you. I have seen feedback applied to the driver tube cathode, but not the concertina itself. Where would that connection be made?

Also, to make the circuit sensible, the 100K at the cathode circuit of the phase inverter triode should most likely connect to ground (not to the cathode / feedback of the 1st triode as shown). This would be a very basic scheme one could expect a simple guitar amp has. Recheck your sketch. I'm quite sure you made an error while drawing it.

This makes complete sense. However, I have triple-checked those connections! The connection between the 100K and the 47K at the bottom of the concertina is a long wire from the tube to the power/output area of the amp, which is where the 47K is left hanging. The wire is almost certainly part of the original build because of the type of wire, and how it is bundled. All the connections in this circuit appear to original, possible exception being the 47K. I thought of three reasons to run that wire from the preamp section to the power area:

1. to connect the splitter to ground at speaker ground. Makes sense, especially if NFB is involved, but why have the 47K involved, and where is the NFB?
2. To connect the NFB (but then I'm left with no ground for the concertina)
3. to pick up a V- to set up the bottom of the concertina for greater swing through it, but this seems a bit extravagant for a guitar amp.

So I'm still baffled.

And, independent of all the above, why connect the cathode of the driver into the cathode-side of the concertina?

Thank you again!

[Chuck H]

This is the best I can figure. Remember, this amp has already been worked on...