Designing a Two-stage Tube Circuit With Compressive Cathode Follower.Jeremy “Tubegeek” Epstein March 2014
This design process is directly inspired by Merlin Blencowe’s analysis of the Fender Bassman-style two-stage combination in his terrific book “Designing Tube Preamps for Guitar and Bass” 2 ed. I am indebted to Merlin for his clear and thorough explanation of the design process as well as his insightful demonstration of the distortion which results from the typical Bassman two-stage circuit. All mistakes are mine; Merlin has cheerfully agreed to permit me to refer to his explanation and method. Thanks Merlin!
Some design goals and parameters:A) To design a two-stage tube circuit including a gain stage, direct coupled into a very compressive, soft clipping cathode follower, as described in Merlin’s book. Essentially to replicate the Bassman-style circuit's tonal qualities, using different tubes suitable for a very small build.
B) To use subminiature dual triodes I have on hand: 6N16b.
C) To use a small 12V:120V power transformer on hand, fed by a 120V:12V wall wart. In other words, a simple linear power supply design. The 12V will feed the heaters and then the stepped-up 120V will be rectified, for a B+ voltage of approximately 160V DC after filtering.
Note that this transformer could also be configured for 240VAC output, I expect to use the lower B+ though.
Also note that the final unit will utilize four triode sections: I have 6V-heater tubes on hand and a 12V heater supply. My expectation is that I will use one two-stage circuit to drive a tone stack, and a second two-stage circuit to make up gain and drive the output. This is still TBD.
Design steps:1. On the standard anode curves for the 6N16b, plot grid-anode curves (shown in red, these curves are nearly vertical for a cathode follower.)
2. Choose a B+ value, and add a reversed scale for the cathode voltage (red scale.) B+ chosen to be 160V (rectified 120V minus a few volts for filtering.)
Iteration I.3. Plot a load line in the normal fashion (blue line, Rk = 10K.)
4. Mark 0 V end of load line on both voltage scales (left and right green crosses.)
5. Locate approximate center bias point on load line (center green cross at approximately Vgk = -2.5V, Vak = 109V, Vk= 51V.)
6. Point A: Vk = 95. Point C: Vk=0. Output V = 95 V pk-pk.
7. Point A: Vga = -75. Point C: Vga = -160. Input V = 85 V pk-pk.
8. Gain = 85/95 = .895
9. Operating point at B: Vgk = -2.5, Vk = 51. Vg = 48.5 Ia = 5 mA
Direct Coupling to previous stage:
Va must be Vg for the cathode follower = 48.5 V
10. Using Ra = 10K and B+ = 160V, plot load line.
11. 2.5 / 5 ma = 500 ohms Rk, 5 mA * 10K = 50V. (Not shown, no good.)
Evaluation:One design goal is to try the grid-current mode for the cathode follower. With 2.5 VBias, we are far away from the grid current region.
Iteration II.Using a different bias point, at B”’ we will have about .8V Vbias which should get us some grid conduction and the following other results from steps 3 and 9-11. (We will skip steps 4-8 from here on out because we aren’t really interested in linearity here and because a huge output swing also isn’t needed.)
Vga = 78V
Vgk = -.8V
Vk = 79V
For the gain stage, using Va = 160-79 = 81V and Ra = 17K
1.5V/4.7 mA = 320 ohm Rk
Iteration III.Iterating again with small Vbias (hotter bias on CF.)
20K load line (blue line)
A’ at 0Vgk. B’ at 1Vgk, Vga about -73. Vga = 92. Vgk = -1. Vg = 91.
Direct Coupling to previous stage:
Va must be Vg for the cathode follower = 91.
Using Ra = 20K and B+ = 160V, plot load line (same blue line.)
(At E) 2.2 / 3.4 ma = 650 ohms Rk, 3.4 mA * 20K = 68V.
This looks like one good candidate.

We will iterate again, using Merlin’s suggestion of a lower value for Rk on the CF. We’ll try a load line of 8K this time.
Iteration IV.8K load line for CF
7 mA Vgk -2 (at B”)
Vga 104
Vg 56
Vk 58
Va for gain stage = 56
load line = 160 / 2mA = 80K
VBias for Va = 56V = -1.5V
1.5/1.3 = 1.15K Rk
Iteration V.8K load line for CF
(At B*)
Vgk for CF = -1
Vga = -90
Vg = 70
Vk = 71
For gain stage, Va = 70
80K load line
Ra = 80K
VBias = -2 V
-2V/1.1mA = 1.8K = Rk
Iteration VI.The Bassman cathode follower has equal Rk and Ra on the two stages. It looks like a load line of somewhere around 15K might get that done - let’s see how that works out. 16K makes the line easy to draw...
Rk = 16K
For Vgk of -.6V at X, Vga = -65, Vg = 95
For the gain stage:
Va = 95 VBias = 2.2V Ia = 4 mA, Rk = 550 ohms.

Using these values, here is most of the circuit. Merlin’s suggested grid stoppers and build out resistors are shown here. Tone stack TBD.


Next step is to breadboard the circuit above. The 550 ohm Rk's will need to be tweaked to dial in the bias most likely, however, it's been my experience that direct-coupled circuits are nowhere near as tricky to set up as you'd think. They'll always settle in
somewhere!Comments welcome. Thanks again to Merlin Blencowe = merlinb !