Simple CF with 5687/6900/E182CC

[ervinl]

Hi,

I have successfully build AF 5687, and would like to improve sonic by adding a simple CF with similar tube (5687/E182CC/6900).

                                       --- B+
                                      A
In -------- R stopper ------G
           \                          C 
           R grid                    |--|C|-- Out
           /                         Rk
           |                          | 
           ------------------------- 0 / GND

B+ = 250 volt
R grid = 470k

Now I would like to know what range of Rk will suit to 5687/E182CC without overloading the tube due to absence of R plate in CF topology.

I would not like to add some R below Rk to - (minus) supply, as I would like to keep the design simple (not to add another PSU).

I also do not want to add another Cin for input. Is it possible?

Thanks,

Ervin L

 

[ervinl]

Hi,

Just try to mod AF to CF, using the same circuit (R anoda 10k, Rk 470 ohm), 5687 tube 240 B+, originally out from C coupling to anoda, try to switch to CF by connecting C to cathode.

Why do I hear weak sound? Gain is lower, it is OK (understand, unity gain), but the sound (to me) is weaker/less power than direct/without CF.

Is it because I still use R anoda of 10k? I am still afraid if I bypass this 10k, the tube will be overloaded in CF configuration.

Thx,

Ervin

 

[Arnulf]

When using identical load values you cannot theoretically get more output power from cathode follower - gain is below 1 or at best very close to 1 and current is the same, the product of the two can only be the same or lower.

What are you trying to do (what the heck is this "improve sonic" anyway - quantify it so we can offer suggestions as to how to achieve your goal) ?

 

[SemperFi]

If I understand you correctly, you want a CF after the common cathode gain stage you now have.
In the 'drawing' in the first post, the grid is connected to gnd. That leaves very little voltage at the cathode. No wonder it sounds bad! You need to lift the grid to a higher voltage. Easiest way to do so is to direct couple it to the anode of the gain tube. This anode should sit at a voltage a little higher than 1/2B+, (actually around 2/3 B+ for max swing).
Lets say you have 130volts at the anode of first stage, the CF will have some 135volts at it's cathode. For a 5687 I'd rush 10mA thru it, as a starting point. So the cathode resistor should be about 13kohms in this example. Now you'll have a cathode follower that can do some work.

That said, a 5687 makes great gainstages without the need for a CF, since it gives pretty low Zout. Unless the load is extremely low for some reason. Never add a stage if not needed, it just makes things complicated and can probably make the total worse than if left simple.

 

[ervinl]

If I understand you correctly, you want a CF after the common cathode gain stage you now have.

That's right.

In the 'drawing' in the first post, the grid is connected to gnd. That leaves very little voltage at the cathode. No wonder it sounds bad! You need to lift the grid to a higher voltage. Easiest way to do so is to direct couple it to the anode of the gain tube. This anode should sit at a voltage a little higher than 1/2B+, (actually around 2/3 B+ for max swing).
Lets say you have 130volts at the anode of first stage, the CF will have some 135volts at it's cathode. For a 5687 I'd rush 10mA thru it, as a starting point. So the cathode resistor should be about 13kohms in this example. Now you'll have a cathode follower that can do some work.

Currently, the AF section, I use around 8-9k resistor (AB resistor, parallel of 16k 2 watt, measured total around 9k due to AB's character tend to stretch).

If I use 13k in the CF, does it mean we have higher impendance than AF output side (9k vs 13k)? CMIIW.

That said, a 5687 makes great gainstages without the need for a CF, since it gives pretty low Zout. Unless the load is extremely low for some reason. Never add a stage if not needed, it just makes things complicated and can probably make the total worse than if left simple.

If I already have 5687 (later try to upgrade with 7119 or E182CC) AF pre amp, then what may improve the sonic and more capable to handle various output impendance (various Power Amp)? May be I could add some module to drive small headphone, or another power tube amp like EL34 directly.

If I add SRPP module with 5687, should it in front of AF, or after AF module? Or just go with all AF, but different kind of tube (with more transconductanse/lower Zout)?