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Following is a list of links and articles relating to Self-Split Push-pull valve amplifier output stage configurations. A key advantage of the self-split technique is to eliminate the phase-inverter tube(s) for a push-pull output stage. Typically, the driven push-pull tube operates only in Class A, and the other push-pull tube has a cloned signal applied to it - so max output power level is lower than for Class AB. Furthermore, cloned signal linearity can suffer as grid-conduction and cutoff regions of operation of the driven tube are approached, so acceptable output power can be even lower.

The screen driven self-split technique was patented from 1936 by E.L. Clark from Philco in US2109021, with an improvement patented from 1941 by W.E. Gilbert in US2361282. Philco used it in some of their radios from 38-3 (circa 1938). Magnatone used it in their Melodier 109 (late 1950's) and Davis 106 (circa ?).

'Improvements in or relating to Electric Amplifiers' British Patent 492,407 by Albert Jesse Biggs of General Electric Company, 1938. - description and design details for common cathode push-pull configurations including grounded grid split and anode signal driven split.

'Improvements in and relating to Thermionic Valve Circuits of the Push-Pull Type' British Patent 508697 by Horace Dudley McDonogh Ellis of BBC, 1939. - description for common cathode push-pull configuration with grounded grid split and biased grid split.

"Novel 10 watt amplifier for P.A." by W.N.Williams, Radio and Hobbies, April 1942. - Early practical amplifier using screen driven split 6V6G PP.

Common cathode self-split design in Radio-Craft, 1947.

Anode take-off self-split design in Radio-Craft, Feb 1948.

"Phase Inversion Circuits" by John W. Straede in Radio-Craft, 1948. - describes some self-spilt techniques.

The Parry Cathamplifier used a cathode coupling transformer to generate a grid drive signal.

Radiotron Designer's Handbook 4th Ed (1953), pages 585-586 - description of self-split configurations.

Philips Radioplayer 176 circuit with self-split technique, circa 1955. Philips Radioplayer models 174, 175, 176, and 185 use a self-split 6M5 PP stage with fixed back bias, where the cathodes are connected to the speaker output winding to generate the clone signal. The 15k PP OT in the class A 6M5 design presents very close to textbook 7k loadline. With idle plate voltage of 260V, screen at 205V, and idle bias of 6.4V, the cathode currents are 32+32mA for 7-8W anode dissipation. Each speaker is a Rola 8H 3.5ohm at 6Wpk spec, so a 4Vpk swing on the speakers would give 4.5W output and remain out of grid-conduction and cutoff regions for the driven tube. The 64mA cathode idle current would not be an issue in the OT secondary and the speaker voice coils. Bob Weaver designed an intriguing self-split single tube amplifier in 2010 using a beam-deflection pentode along with a SE operated driver transformer and a PP operated output transformer.

Link to www.tubecad.com - a description of three self-split configurations: screen driven, ultra-linear driven, and split input driven.

Link to Merlin's website - Merlin's Links and Downloads page has two articles relating to self-split PP stage design.

Link to Stephen Keller's website - Stephen has a section on self-split circuits, including Magnatone's 109 amplifier schematic with a screen-driven split, and also the Davis 106 schematic (http://home.comcast.net/~stphkeri/davis106.gif). And the 1946 Bustard paper is a good summary of techniques.

Link to article on Triode Electronics website - Compact Hi-Fi Power Amplifier, by Melvin Liebowitz - a practical article on 6W self-split with grounded grid.

Link to DIYParadise website - The Projects list has Simple EL84 Integrated/poweramp project for a self-split with grounded grid.

Link to Guitar Amp Builders Forum (originally AX84) website - the Firefly project uses self-split 12AU7 PP output stage with grounded grid split. There is also a High Octane version.


Following is a list of articles relating to Valve Reliability.

"The life testing of small thermionic valves" by Thompson, Dudderidge & Sims, 1926.

"A New Secondary Cathode" by Bull & Atherton, 1950.

"Radio Valve Life Testing" by Brewer, 1951.

"The life of oxide cathodes in modern receiving valves" by Metson et al, 1951.

"A Study of some Properties of Materials affecting valve reliability" by O'Donnell Roberts, 1954.

"Some experiments on the breakdown of heater-cathode insulation in oxide-cathode receiving valves" by Metson, Rickard & Hewlett, 1955.

"Valve Life" by Metson, 1955.

"The relationship between cathode emission, cathode resistance and mutual conductance in receiving valves" by Holmes, 1956.

"The Life and Reliability of Valves" by Rodenhuis, Santing & Tol, Philips Technical Review, Dec 1956.

"Cathode/Heater-insulation failure in oxide-cathode valves" by Genty, Newson & Oldfield, 1965.

"Thermistors for the gradual application of heater voltage to thermionic tubes" by Gano & Sandy, 1958. How MIT tried to minimise valve stress and failures in early valve computers such as the 'Whirlwind 1'. The only known reference to valve heater filament failure statistics related to turn-on stress. Gano did his Master's working on Project Whirlwind 1, and then continued to work on power coordination with Lincoln Labs. Thermistors for filament cycling was a project by Sandy starting in 1955 (9 and 23 Sept and 4 Nov 1955 biweekly reports).

"Some factors affecting transmitting valve life" by Basnett, 1955. - Summary discussion of failure mechanisms in transmitting valves.

"High-Power Transmitting Valves With Thoriated Filaments" by Walker et al, 1960. - Detailed review of thoriated tungsten filament in transmitting valves, incuding assessment of BBC reliability data.

Extract from "Thermal Analysis & Control of Electronic Equipmnt" by Kraus & Bar-Cohen, 1983. Discussion on tube cooling and the influence of tube shields.

"A comparison of the noise and voltage coefficients of precision metal film and carbon film resistors" by T.R.Williams, 1959. Measured noise results for 20k and 50k 1W resistors.

"Measurement of battery noise and resistor-current noise at subaudio frequencies" by K.F.Knott, 1965. Measured noise results for 2k carbon 1/4W resistors.


Following is a list of interesting valve related articles.

"A 3000 Watt Audio Power Amplifier" by Bereskin, 1956.
With screen dissipation hitting 150W and plate loss reaching 2kW, this will keep your house warm!

"An Experimental 9000 Watt Airborne Sound System" by Martin et al, 1956.
The 304TH tubes were supposedly used in stage amps for Pink Floyd - now I know where they got the idea from!

"High Power Audio Transformers" by J.F. Peters, 1936.
If you can make an amp powerful enough, then this might help in making audio transformers up to 180kW in power rating!

"Audio Amplifiers" by Langford-Smith, 1956. Classic.

"A note on noise in audio amplifiers" by Woll & Putzwrath, 1954.
A good summary.

"A note on induced grid noise and noise factor" by Harris, 1950.
Discussion on grid noise theory versus experimental.

"The nature of the uncorrelated component of induced grid noise" by Talpey & Macnee, 1955. Discussion on induced grid and shot noise.

"A study of noise in vacuum tubes and attached circuits" by Llewwellyn, 1930. Discussion of shot and thermal noise.

"Optimum air gap for various magnetic materials in cores of coils subject to superposed direct current" by Legg, 1945.

"The incremental magnetic properties of silicon-iron alloys" by Carter & Richards, 1949.

"The measurement of choke coil inductance" by Wright & Bowditch, 1927.

"The practical design of iron-cored transformers and chokes carrying d.c." by Palmer, 1946.

"Characteristics and applications of selenium-rectifier cells" by Harty, 1943.

"Analysis and reduction of output disturbances resulting from the alternating-current operation of the heaters of indirectly heated cathode triodes" by McNally, 1932.
Analysis and measurements relating to reducing mains hum in audio equipment.

"Heater cathode insulation performance" by Klemperer, 1936.
Discussion and measurements of the heater insulator interaction with the cathode, including oscilloscope VI curves showing resistance change characteristic around zero volt differential and asymmetric characteristic.

"Examination of conditions which give rise to hum", Radiotronics No.119, May 1946.
Analysis of heater voltage to grid via capacitive coupling and humdinger.

"Magnetic shielding of transformers at audio frequencies" by Gustafson, July 1938.
Discussion and measurements of the effectiveness of magnetic shielding around signal transformers.

Simulated schematic of stray magnetic field from transformer from M. Breschi and A. Cristofolini.

Frequency modulation of resistance-capacitance oscillators by Maurice Artzt, 1944. The start of faxing.

Some articles I've put together:

An article on Output Transformer Protection. Rev: 16 June 2023

An article on Choke measurement. Rev: 4 Jan 2024 - with Details of a Choke measurement jig - Rev: 31 Dec 2023.

An article on renovating PA amps. Rev: 6 Oct 2023

An article on a shunt load for choke-input filter applications. Rev: 5 Jan 2024

An article on microphonics in valves. Rev: 12 Jan 2023

An article on Magnatone vibrato design and varistors. Rev: 2 Apr 2023

An article on Hum in Valve Amplifiers. Rev: 18 Oct 2023

An article on Fusing Valve Amplifiers. Rev: 23 Feb 2023

An article on valve amplifier power supply issues. Rev: 29 Sept 2023

Williamson amplifier design info Rev: 14 Nov 2023 - see Williamson Amplifer page.

An article on output transformer measurements for the Williamson amp. Rev: 12 Oct 2023

Update: 5 Jan 2024
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