Now the amplifier circuit. Here is the schematic:
Potentiometer:
This will be a 25K Goldpoint stepped attenuator. The volume pot acts as a voltage divider between the driver tube grid and ground. That means turning up the volume pot will increase the resistance between the input signal and ground, so more signal is sent to the driver tube grid, resulting in increased signal and volume. Turning down the volume pot decreases the resistance between the input signal and ground, so more of your input signal goes to ground and your volume decreases. With the volume set to 0, all of the signal is lost to ground.
Input stage:
It will feature my new favorite tube, the MH4 and its cousins. It will be cathode biased, also known as auto-biased. Without getting into the nitty gritty details, this will alow the tube to find its own bias point based on the value of the cathode bias resistor and allows the tube to maintain that bias point as it ages. This biasing scheme requires a cathode resistor bypass capacitor, which acts as a high-pass filter, among other things. The value is appropriately high to allow all audible low frequencies to pass. This will be an Audio Note Kaisei electrolytic. The 56kohm resistor on the plate is known as, you guessed it, the plate resistor
this acts as the load for the tube and is usually 2-3 times the internal resistance of the tube. From experimenting with GOTL modifications, I have found that changing this resistor has a very audible effect on the sound. I will be using discontinued Japanese Riken carbon composition resistors on the MH4 plates, considered by many to be the best sounding resistors made.
I am going to briefly explain how the bias point is chosen for the input tube using "load lines". Below is the current vs. voltage operating characteristics of the tube with the load line drawn in red. The far right point of the line represents the maximum voltage across the tube, equal to the high voltage B++. The far left line point represents the current flowing across the 56kohm plate resistor with zero voltage across the tube. This load line tells us the plate current for any given voltage on the tube and our bias point will be on this line. The curved lines on the graph represent the grid voltage, and the grid voltage at our desired bias point will determine the value of our cathode bias resistor.
For the input tube, we want to choose a
linear point on the load line, meaning we want to choose a point on the load line where the distance to the next nearest grid voltage line on both sides is about equal. The tube will swing along the line right to left with the positive and negative AC input signal.
I chose to bias the MH4 with -3V on the grid, which corresponds with 145V on the plate, a plate current of about 1.75mA, and a cathode bias resistor of 2kohm. This is the green dot on the load line. I may adjust this in the final build to a grid voltage of -2.5 or -2 depending on the distortion measurements.
The input stage is coupled to the output stage via a Jupiter copper foil 0.47uF 630VC capacitor. This allows only the AC output signal to pass and blocks the 145V DC from the MH4 plate. If that DC current reached the grid of the next tube, it would "red plate" and burn up!!!
Output stage:
The output stage of this amp will feature various pentode and beam tetrode power tubes as strapped triodes. This includes EL34, KT66, 6L6G, KT77, and the possibility of others. A Goldpoint selector switch will be used to change the cathode bias resistor for the optimal bias point for each tube. The switch has six possible positions, so I can add resistors for two more tubes later if I please
in reality, these tubes could probably all be used with the bias optimized for one tube and no switch. Rarely are headphones going to require enough power to push the tubes to significant distortion at an unoptimal bias point, this is much more important for higher power uses like speakers.
But screw that!!! I will optimize the bias of each tube and choose the appropriate resistor. Audio Note Kaisei will be used again for the bypass capacitor. For strapped triode mode, the screen grid is connected to the plate via a 100ohm resistor and the suppressor grid is connected directly to the cathode (in beam tetrodes, this is an internal connection as there is no true suppressor grid, the suppressor grid is actually generated by the beam of electrons flowing through the tube!!! Very cool, look it up).
Using our load lines again, we can choose the bias point for the power tubes. However, it is a bit of a different process since we do not have a plate resistor acting as the load for the tube, the AC load is the impedance of the primary winding of the output transformer, 4.6kohm. Also, the primary winding has a very low DC resistance, so the plate of the power tube essentially sees the entire 250VDC B+.
Like the input tube, we can draw our load line with the full B+ voltage on the right and the maximum plate current on the left, the blue line on the graph. But our true operating point is not on this line! We cannot choose the plate voltage since there is no plate resistor, the plate voltage is FIXED as the B+ of 250V. If we operated the tube here, the bias point would be the far right of the load line, there would be no place for half of our input signal to go and it would be lost.
So, we have to draw additional load lines at the same slope as our initial line, up to but not exceeding the large curve at the top of the graph. This is the maximum plate dissipation of the tube. If we exceed this line with our operating point on the load line, the tube will melt. We want to choose a load line that is below the maximum plate dissipation and allows for maximal left and right excursion along the load line. The upper limit on the right side of the line this time around is where the grid curves start to get
squished, which will cause significant 2nd harmonic distortion. The leftmost limit is the grid voltage where the grid will begin to draw current, which is BAD. By maximizing the equal left to right swing across the load line, which are getting the most peak-to-peak voltage out of the power stage, which means more POWER!
I will have to redo my bias points since I made changes to the power supply, but for the sake of example, here is where I might choose the bias point of the EL34, green dot on the red load line. This point represents 250V on the plate, 85mA plate current, a grid voltage of about -14V, and a cathode bias resistor value of ~165ohm. This will be done for each power tube to find the correct cathode bias resistor value for the switch.
Doing some quick math, at this bias point, this gives us roughly a peak-to-peak voltage of 220V, which is about 77VRMS, which means about 1.3W of undistorted output power. If I were to push the output tubes closer to the max plate dissipation, could get something like 2.5W out of them with this output transformer. Either way, it is way overkill. The tubes will live a longer life at this bias point and a power output of 1.3W.
Okay, wrapping this up, the output transformer will be a Lundahl LL2765 with a 4.6kohm primary and a 32ohm secondary winding, which will be the output impedance of the amp.
Well there you have it, the amplifier circuit. I know that is technically dense, but hopefully somewhat interesting.
Next step will be building the parts list and prototyping this bad boy!!!
BTW, this is still new to me, so please if Glenn or anyone else spots an error feel free to let me know
The higher distortion at higher voltage operating points can be compensated to some degree by using output transformers with higher primary impedance (however, higher primary impedance means lower max output power, so always tradeoffs).