Pre-Amp Build

[paul_b]

Cathode voltage on the choked tube?

 

[Wntrmute2]

6.6V on the choked side. 3.9V on the resistor side.

 

[Deke609]

Cold the low voltage gain be a consequence of the -ve feedback? I seem to recall an early schematic that had a switch for adding/removing the feedback loop.

cheers, Derek

 

[paul_b]

Now try replacing the cathode resistor on the choked stage with a 1.8K part. This will bring the bias voltage up at the expense of drawing a little less current in the output stage.

If you remove the feedback network, the step-up from the first stage will be visible.

 

[Wntrmute2]

I am thinking I don't really need more gain but will try the above suggestions. I am enjoying the learning here!

 

[Wntrmute2]

I did not have a 1.8K resistor but I put in a 2.0K. That made thins worse - now my voltage at the grid of the second stage is 4.4V and at the output at the second stage is 3.0V.
Phone screen broken but hopefully will post pics in a bit.

 

[Deke609]

Leaving aside all of the scope traces, how does it sound? Did you notice a difference between resistive-load and choke-load?

Tangent question: when you are comparing 1st stage and 2nd stage waveforms, are you aligning both resulting waveforms at their zero-crossing point, or at the bottom of their negative peaks (valleys)? If the latter, I think this might exaggerate the degree by which the top of the 2nd stage waveform is smaller than the first -- i.e., I wonder whether the lower voltage output of the 2nd stage is more symmetrical.

cheers, Derek

 

[Wntrmute2]

I have not put it back into the system. I am aligning the waveforms at the bottom for ease. The weird thing, too me, is that the voltage coming out of the second stage is the same or lower than coming out of the first. I know the feedback would cause some of this phenomenon.

 

[Wntrmute2]

I just measured the cathode voltage with the 2K resistor instead of the 1.2K one that was in there before.
Voltage was 6.6V (choke side) now 7.6V (choke side).

 

[Wntrmute2]

Playing around, I put the two resistors in parallel and got pretty nice traces with a slight step-up in PP voltage. However my cathode voltage has dropped to 435mV.
Bellow are the traces at close to full power.

 

[Deke609]

So you put a 2K in parallel with a 1.2K as a combined cathode resistor, and got more output voltage swing?

cheers, Derek

 

[Wntrmute2]

Yes but I wonder if the cathode voltage of 435mV is an issue?

 

[paul_b]

435mV DC? At idle? You had 7.6V DC before? Always put units on numbers and add specifics.

 

[Wntrmute2]

I had 7.6 VDC with the 2.0K cathode resistor. 6.6 VDC with the 1.2K resistor. 435mV with the two in parallel (750 ohms). This is at close to maximum output. I'll recheck those measurements but I'm pretty sure of my measurements. The curves LOOK the best with the 750 ohm cathode resistance and there is a 0.5 volt of step-up.

 

[Wntrmute2]

Remeasured with the choke and 750 ohm cathode resistor (1.2K + 2.K in parallel) and got a more realistic 5.8VDC at the cathode. I don't know why I got such a low number before.
Does this seem appropriate?
How do I calculate plate dissipation again?

 

[paul_b]

You don't want a 750 ohm cathode resistor. You want 1.8K for the output section with the choke load.

 

[Wntrmute2]

When you say output, you mean the cathode resistor correct?

 

[paul_b]

Yes.

 

[Wntrmute2]

Thanks Paul. I'll get some ordered.

 

[Wntrmute2]

Not to argue but when I calculate the mA across the 2C22 with the 750 ohm cathode resistor I get 8mA of plate dissipation which seems comfortably within the 11mA maximum rating. When i do the same with the 1.8K cathode resistor, I get around 3.6mA. Seems very low. What am I missing? I want to understand what is going on here.