307A Push-Pull Amp

Description: A single-ended tube headphone amplifier utilizing the TTVJ Millett 307A. Various single-ended and push-pull power amplifier circuits have been breadboarded with the 307A over the past year. The 307A was primarily used in a WWII US Army field radio, with limited datasheets available, mainly from Western Electric. The author has not encountered a WE-branded 307A, despite having a collection that includes Ken-Rad, Sylvania, Raytheon (also known as RK-75), and National Union brands. The WE tube appears similar to the Sylvania tube, suggesting a possible rebranding. The 307A can be challenging to find, with stock sourced from England, as most spares ended up in Europe during the war. The amplifier has been tested in pentode, ultralinear, and triode modes, with triode operation yielding the most favorable results, producing characteristics akin to a 2A3 or 300B tube. The plate dissipation rating is 10 watts, which seems conservative, as the tubes have been operated at or slightly above this rating without issues. The design features a conventional input buffer driving a phase splitter transformer (Sescom MI-19), followed by a differential driver stage using 7W7 loktal pentodes that couple to the 307A grids via capacitors. Notably, the B+ is fed through a choke, with a capacitor connected to the filament end of the cathode bias resistor, allowing the driver stages to be fed without decoupling. This configuration has been found to lower distortion significantly. The driver stage utilizes a differential pair of 7W7 pentodes sharing a common screen grid dropping resistor and a small bypass capacitor, which has proven more effective than traditional methods. The input stage employs a triode-connected 7W7 in a near-unity-gain plate follower configuration to mitigate driving source impedance effects, ensuring high impedance (100k) and resistive input for optimal performance.

The single-ended tube headphone amplifier circuit based on the TTVJ Millett 307A is designed to deliver high-quality audio reproduction with a focus on low distortion and stable performance across various operating conditions. The use of the 307A tube, with its historical significance and unique characteristics, provides an interesting foundation for the amplifier design. The triode mode of operation, achieved by connecting the G2 and G3 to the plate, results in sound characteristics that resemble those of well-regarded tubes like the 2A3 and 300B. This choice of configuration maximizes the linearity and musicality of the output, making it suitable for high-fidelity headphone listening.

The input buffer stage is critical, as it isolates the amplifier from the source impedance, ensuring that the frequency response and total harmonic distortion (THD) remain consistent regardless of the source characteristics. The selected phase splitter transformer, the Sescom MI-19, is noted for its performance, contributing to the overall fidelity of the circuit.

The differential driver stage, utilizing the 7W7 loktal pentodes, is designed to provide robust drive capabilities to the 307A grids. The shared screen grid dropping resistor and bypass capacitor configuration is a unique approach that has been empirically validated to reduce distortion more effectively than conventional methods. This design choice reflects a deep understanding of tube amplifier behavior, particularly regarding the interaction between the screen grid and the output stage.

The implementation of a choke in the B+ supply line, along with the strategic placement of capacitors, plays a significant role in maintaining a stable operating point while minimizing noise and distortion. This approach allows for the modulation of the B+ supply with common-mode signals, which can help maintain balance throughout the amplifier circuit.

Overall, this single-ended tube headphone amplifier design showcases innovative techniques and thoughtful engineering, resulting in a high-performance audio device that leverages the unique properties of the 307A tube while addressing common challenges faced in tube amplifier design.A single-ended tube headphone amp using the TTVJ Millett 307A. I`ve been breadboarding various SE and PP power amp circuits with the 307A for a good year or more, but just haven`t had the time until now to come up with anything I like. As far as I can tell, the 307A was only used in a WWII US army field radio. Even though the only datasheet I can find is from Western Electric, I`ve never actually seen a WE branded 307A (and I have a pretty good collection of them). I have 307A`s from Ken-Rad (the most common), and Sylvania, Raytheon (AKA RK-75), and National Union. At least in the pictures the WE tube looks exactly like the Sylvania tube, so I`m guessing that WE may have just re-branded Sylvania tubes (or vice-versa).

Finding 307A`s can be a bit of a challenge. My stock came from England. I believe that most of the spares wound up in Europe since this is where the radio was deployed in the war. There are small quantities available at US tube dealers, and I know of at least one guy in Europe who seems to have a big stash.

Anyway, back to amplifiers. I have run the 307A in pentode, ultralinear, and triode mode. I have not been impressed with pentode or UL operation - it works fine but nothing to write home about. But triode connected - in this case by connection G2 AND G3 to the plate - you get a very nice triode, somewhere between a 2A3 and a 300B in characteristics.

I found some triode curves somewhere on the net. I`d like to credit whoever made these but I cannot figure out where they came from - if it was you, please let me know so I can ask permission (and forgiveness) to post it! The plate dissipation rating is 10 watts, but that seems to be a very conservative rating. I`ve been running them right at or slightly above that point for some time with no problems. This design is pretty conventional. An input buffer drives a phase splitter transformer (I`m using a Sescom MI-19, which performs very well).

A differential driver stage using 7W7 loktal pentodes drives the 307A grids via coupling capacitors. The 7W7 is a relatively unknown (until now ) jewel. I got a couple dozen for $1. 00 each at RES ! A couple of oddities you might notice: The B+ is fed through a choke, and a capacitor (call it ultrapath or whatever you`d like) connected to the filament end of the cathode bias resistor. This B+ is used to feed the driver stages with no decoupling. So basically from an AC standpoint everything is tied to the 307A filaments. I found experimentally that this scheme resulted in significantly lower distortion than other connections.

I believe that what`s happening is that the B+ is being modulated with common-mode signal which tends to force balance in the entire circuit. Another oddity is that the driver stage - a diff pair of 7W7 pentodes - share a common screen grid dropping resistor, and a very small (0.

001uF) bypass capacitor. Again, experimentally, this worked considerably better than the usual choices, including a VR tube to regulate the screen supply, and separate screen dropping resistors, and using large bypass capacitor(s). Interestingly, using a big bypass cap increased THD significantly across the board. Adding a small cap lowered THD at high frequencies (above ~10kHz) without increasing distortion at low frequency.

The input stage uses a triode-connected 7W7 in a near-unity-gain plate follower configuration. I did this to remove any dependencies on the driving source impedance - the frequency response and THD at high frequency is very dependent on how you drive the phase splitter transformer. With the buffer, the input is high impedance (100k) and resistive, so the source impedance doesn`t affect the amp performance.

Almost any triode will work - a 6SN7 / 6J5 / 7N7 / 7A4 works fine, but the higher Gm of the 7W7