White Cathode Follower sound

Help Support GroupDIY:

abbey road d enfer

Well-known member
Moderator
Joined
Jan 22, 2008
Messages
12,976
Location
Marcelland
Don't forget that "R=1/gm" is only an approximation that assumes zero load resistance. For any other load the optimum will be larger. Also the true optimum resistance is usually slightly larger than small-signal theory predicts anyway, so 1/gm is always an underestimate.
And gm is an uncertain and varying value.
 

merlin

Well-known member
Joined
Aug 17, 2011
Messages
174
The reference I was actually looking for is another EMI invention (they must all have been smart fellas at EMI) and is
"Cathode Follower Circuit Using Sceen Grid Valves" published in 1947 so, if anyone has it?
You mean this?
 

Attachments

  • 1947_Cathode_Follower_Screen_Grid_Valves.pdf
    692.4 KB · Views: 48

abbey road d enfer

Well-known member
Moderator
Joined
Jan 22, 2008
Messages
12,976
Location
Marcelland
You mean this?
I understand that, regarding input impedance, it acts in a similar way to an active shield, but I have a few questions.
To me, it seems the second tube has its screen-grid and plate connected, making it a triode, which is in contradiction with the title and ptemises, or the two screen-grids are connected together, but not to the plate, and in that case I wonder how it can predictably work.
 

merlin

Well-known member
Joined
Aug 17, 2011
Messages
174
To me, it seems the second tube has its screen-grid and plate connected, making it a triode, which is in contradiction with the title and premises,
The first valve is the cathode follower using a screen-grid valve. The second tube is just a 'helper' that needs to provide some linear gain (maybe 20 to 30 dB), so there's no need to run it in pentode mode which would just add complication.
 

abbey road d enfer

Well-known member
Moderator
Joined
Jan 22, 2008
Messages
12,976
Location
Marcelland
The first valve is the cathode follower using a screen-grid valve. The second tube is just a 'helper' that needs to provide some linear gain (maybe 20 to 30 dB), so there's no need to run it in pentode mode which would just add complication.
So the output is taken from the 1st tube's cath...
So weird.
 

MisterCMRR

Well-known member
Joined
Jul 1, 2019
Messages
119
Location
Ventura, CA
That suggests using tubes with Gm about 1.2 mho (typical of a weak 12AX7), which somehow defeats the purpose of a WCF.
Some mfgrs would use 12AX7 everywhere, in order to simplify inventory/purchasing, but this is not in accordance with the image Mastering Lab tended to project. And anyway the output tube is 2 sections of E188CC in parallels, for a Gm of 25 mho, which suggests the use of a 40 ohm upper anode resistor.
There seems to be a big incoherence here.
On re-reading this thread, I think the "big incoherence here" is that you've misrepresented the transconductance of these tubes. For example, the 1.2 mho for a 12AX7 (implying 1.2 A of plate current per V at the grid!) should be 1.2 mmho (or 1 mA per V). Correcting those figures will, of course, make the resistor values fall into the same "ball park" as those seen in real applications.
 

abbey road d enfer

Well-known member
Moderator
Joined
Jan 22, 2008
Messages
12,976
Location
Marcelland
Of course I meant mmho (actually mS). My comment is nevertheless valid, as the values indicated are about a factor 10 out.
Using 1.2mho or 1.2S would lead to very low values, about 0.8 ohm. I believe this point has been covered already.
 
Last edited:

Winston OBoogie

Well-known member
Joined
Jun 3, 2004
Messages
2,639
Location
UK.
Don't forget that "R=1/gm" is only an approximation that assumes zero load resistance. For any other load the optimum will be larger. Also the true optimum resistance is usually slightly larger than small-signal theory predicts anyway, so 1/gm is always an underestimate.

Yep.
With a significantly high load then Ra = (rp + 2RL)/mu
 

Winston OBoogie

Well-known member
Joined
Jun 3, 2004
Messages
2,639
Location
UK.
Looking at the schematic that My3gger posted in reply no. 99 of this thread, it certainly looks to me like the Ra of 2K could be a factor of 10 off optimum.

I don't know the operating point of the 6414 as used here so just picked a nominal and conservative point using a 250 - 300V supply and a -2V grid bias, which showed approx. a little under 6mA/V for Gm on the Raytheon data sheet. I rounded this down to 5mA/V to allow for underestimating of the theory and I get 200R for Ra


Edit: attached 6414 data sheet and corrected schematic reference

 

Attachments

  • 6414.pdf
    952.7 KB · Views: 18
Last edited:

MisterCMRR

Well-known member
Joined
Jul 1, 2019
Messages
119
Location
Ventura, CA
Of course I meant mmho (actually mS). My comment is nevertheless valid, as the values indicated are about a factor 10 out.
Using 1.2mho or 1.2S would lead to very low values, about 0.8 ohm. I believe this point has been covered already.
1.2 mS would make the resistor R = 1/Gm = 1/1.2E-3 = 0.833E3 = 833 ohms). That doesn't seem far astray from the values in real circuits being discussed here.
 

MisterCMRR

Well-known member
Joined
Jul 1, 2019
Messages
119
Location
Ventura, CA
The EMI precision compound cathode follower (what I'd call it) is very clever. Using the gain of the second stage, it drives the screen of the first stage (follower) to subtract the effect of a constant screen current in the first stage. This constant current (as explained in the text) effectively reduces the followers voltage gain below the desired 1.00. But, as the author points out, too much gain in the second stage can drive the circuit into oscillation as the small-signal output impedance at the cathode becomes negative. This circuit would have to be carefully designed to keep it from oscillating when a tube is replaced, for example ... it may be why the circuit is rare in production equipment.
 

Winston OBoogie

Well-known member
Joined
Jun 3, 2004
Messages
2,639
Location
UK.
The EMI precision compound cathode follower (what I'd call it) is very clever.

I think so too :)
I'd seen the reference to it in RDH4 while looking for info re. pentode common anode circuits, but never the actual write up.
Thanks again to Merlin for digging it up.
 

Latest posts

Top