The transformerless Philips balance amplifier was first described by Arnold Peterson and Donald B. Sinclair in the Proceedings I.R.E. January 1952 (page 43). After that, a special tube was developed in the Netherlands, the EL86, with which the load impedance of the circuit dropped to 800 ohms.

Philips introduced the EL86 tubes in 1956 at a Holland exhibit. (later UL84, PL84 is matching with EL86, except filament heating , statement: “Radio und Fernsehen” 5/57.)

The pentode tube is similar to the EL84, but is designed for higher currents, allowing transformerless designs in the single ended push pull. Schematic can be understood from here. Single Ended Push Pull has nothing to do with Push Pull or SE schematic. Is a quasi complementary amplifier circuit, which requires an inverter and a capacitor in the output as DC bias with tubes to that easy. el86 datasheet – single ended push pull circuit suggestionDownload

Later EL86 datasheetDownload

The products at this time were HF306 (preamp) and two mono blocks of HF303 OTL (EL86) or the HF304 (EL84 with output tranfromator). Philips also offered construction sets like: HF10 (EL84), HF302(EL86, mono 10W), HF309 (EL84, with output tranfromator)

AG9014 was manufactured in 1958-1959, which was a fully integrated amplifier. First with AD5038, later AD5046 800Ohm loudspeaker. On the front panel there are 4 rotary controls for Treble, Bass, Volume and Balance. Furthermore, an On / Off slide switch, push buttons for Tuner, PU and Tape selection, Stereo / Mono keys, a yellow Distortion light and a green Mains light. The RIAA input had at the back selection for crystal or MM type of cartridge (EF86, active riaa). The Tuner input works with a low pass filter at ~12kHz + with an additional serial resistor to lower sensitivity. This amp is well explained in the Rodenhuis book, where he is fingerpointing how hard was it to measure the distortion and how suitable is the circuit for HI-FI. The pre-pre amp is done with a double triode ECC83, with a local negative feedback right before the tone control. The final stages are equipped with an ECC83 double triode. The first triode acts as a pre-amplifier, the second triode as a phase reversal tube. The latter drives the “upper” EL86. The power tubes operate in class A setting, and each receives an anode voltage of 165 Volts at a current of 75 mA.

The screen grids of the power tubes are each “fed” via a choke coil, so the efficiency of the amplifier is higher than a circuit with screen grid resistors. The return is close to the theoretically feasible (50%). The two chokes are combined on one core, so no pre-magnetization occurs. The inductance of the coils is so high (60 Henri) that the effect of this is only visible below 20 Hz.
The auxiliary winding (S5a) on this choke coil / transformer is connected to the output of the amplifier. The winding ratio of the transformer is 1: 1: 1. Due to this construction, DC voltage is never present at the output.

The feedback circuit of the power amplifier consists of a combination of positive and negative feedback. The positive feedback arises through the common cathode resistance of the triodes. The negative feedback occurs by returning the output signal via a resistor to the cathode of the first triode. This type of circuit is in principle very sensitive to phase shifting, but because an output transformer is missing here, this is not so bad in practice.

More details can be found in the schamtics section. (specs, Rodenhuis book, ect…)

AG9015 came later, 1959-1965. Very similar construction, but GZ34 replaced the 2 mono rectifier tube. The tuner low pass/sensitivity lowering was built in into the negative feedback of the pre-pre amplifier and it was made more symetrical compared to the predecessor. The yellow distortion lamp was dropped. The most important that it had now an output transformer for the 8-16Ohm operation (while keeping the 800Ohm output), which gave space the double choke, supplying screen grid for the pentode. So this bootstrap solution ensures perfect sound quality also in the 8-16Ohm operation.

There was mainly 3 hardware variant, you can distinguish the first type based on the 8uF caps , where length matches (same voltage rating) and has lower voltage at preamp because of R36 (220K!). If one 8uF is differing in size in the pair (from the 2 top laying caps, the not blue-insulated type is longer), than it is from the newer builds (from 1962). Later versions had the RIAA sensitivity switch components also moved from back to the EF86 input section (R43, R45) leaving the teminals at the input unused. These have teh connector wired with an shielded cable…

The earliest versions have through hole balance potentiometer (see schematic pdf doc for details). Laters were replaced by a non through hole, or repaired based on factory recomendation (they could break, however my works still fine.)

The power tubes operate in class A setting, and each receives an anode voltage of 210 Volts at a current of 55 mA.

AG9018 came in ~1963-1965 with an teak wood box, else exactly the same as the AG9015.

Factory recommended loudspeaker: for very short time AD5041 (weak on power AD3700BM in 2 way with Ad4200A) AD5046, later AD5050 (These are AG9710AM 800ohm types) from 1964 AD5055 (2way with AD5201, 2x AE37011). These are 800Ohm speakers, benefiting from the transformerless operation. AD5046 is interesting sound on jazz, but the continuous lack of bass is too much penalty. I think at 8 Ohm, the Tannoy HPD315 (example my Chattsworth 12″) was the best selection (or it’s 15″ big brother) at that time.

Factory recommended turnatble: Initially the AG2209 (with AG3401 pick-up) , but the 22GA230 (AG2230) from later was a much better selection. (AG2230W without anti skating). Nowadays I would recommend Lenco L75, PTP5, Garrard 401, Micro seiki, and and and. ect…

Factory recommended tuner: A5X83A from 1958 (mono) or A5X93A since 1963 (stereo)