preamp question

[overdrajv]

Hello!

Im going to build something similar to this amp:

http://www.ax84.com/static/sel/AX84_SEL_080729.pdf

and I dont understand why would someone amplify a signal, and
then halve it with two 220K voltage divider.

Any ideas?

Merry Xmas,

Miha

[Frank Berry]

It's not a voltage divider ... It's a high frequency boost circuit.
That's the circuit after the first volume control. The second pair of resisters is a voltage divider. Only the designer knows why it's there.

[flg]

There is also a cap there Mids and Highs are not attenuatted but the bottom end is cut 6db.

[lineup]

This is not unusual for Tube amplifiers.

When using 1 tube and signle end configuration. Same for amplifiers using one single JFET transistor.
In these cases there are reasons to try to find the optimal working condition for the single transistor/tube.

With biploar transistors, it is very common to use emitter resistors to reduce gain.


In the shematic is used no less than 3 tubes gain stages!
No wonder there is too much gain.

You are basically right.
As any voltage amplification is bound to degrade signal
we should not amplify more than we need + a bit extra margin.
Because more amplification will only add more distortion than what we want

[overdrajv]

Wouldnt it be more logical to reduce the gain of the stage by lowering the plate resistance? Maybe there would be less distortion operating the tube with less plate swing, thus greater linearity, than first generating too much signal and then atteunating it..

Anyway this is a guitar amp, so more or less distortion is what one needs.

What about the loop around the cathode follower with the diode?
Whats that for?

Thanks!

Miha

[Miles Prower]

Originally posted by overdrajv
What about the loop around the cathode follower with the diode?
Whats that for?

Since this has a SS power supply, the positive rail will come up very fast, much faster than the heaters can warm up. They included that to prevent overvolting the grid of the second 12AX7, which can't stand positive grid voltages and grid current. Once the heaters are hot, the diode is reverse biased and drops out of the circuit (except for its junction capacitance, of course, but who cares? It's a gee-tah amp).

I took care of the problem of SS power supplies and DC coupled stages by making separate heater and HV supplies. Power up the heaters first, then turn on the HV, and you don't have to worry about it.

[overdrajv]

But this amp does include separate switches for heaters and B+ ..

[Merlinb]

Originally posted by overdrajv
Hello!

Im going to build something similar to this amp:

http://www.ax84.com/static/sel/AX84_SEL_080729.pdf

and I dont understand why would someone amplify a signal, and
then halve it with two 220K voltage divider.

Any ideas?

Merry Xmas,

Miha

The quality of overdrive generated in a valve is partly related the anode voltage. For a 'lead sound' it is desirable to use a large anode resistor, so the anode voltage swings low, which helps generate a 'hard' sound.
However, the output signal is too large to deliver directly to the next stage (particularly at low frequencies), so it's usual to include some sort of attenuation, which can also include treble boosting too.

The diode/resistor protects the cathode follower from arcing, which might occur if the standby switch is not used.