February | 2016 | simplepleasuretubeamps

“Shiny Eyes” Parallel Push-Pull EL34B is my first medium power push-pull monoblock . It was sponsored by my audiophile friend Rocky.

First, I want to thank Kevin Carter from K&K Audio for his technical support and his awesome sounding push-pull driver, an all balanced driver using the Russian pentode 6J49P in triode mode and very good Lundahl transformers.

It uses six EL34B per channel delivering a maximum of 50Wrms into 4 or 8 ohms. The output audio transformer is a 120W Hammond 1650TA. The driver stage is full balanced push-pull input stage kit from K&K Audio using all Lundahl transformers LL1544A for input and LL1692A for output. Driver tubes are 6J49P carefully selected by Kevin Carter.

The power supply consists of a 375VA potted Plitron toroidal transfomer with secondaries at 340V/0.8A and 6.3V/ 12.8A. HT is SiC diodes rectifed and CLCLC filtered, using Hammond 159V chokes and MKP 50uF 900V capacitors. The driver has its own shunt regulator. The high voltage is delivered with a 30 seconds time delay based on IC555.

The enclosure that holds together the mono-blocks was delivered by Modushop in Italy, model Pesante 5U 400. On the back panel one can observe the IEC main power jack and the fuse holder on the left, the RCA input jack in the center, and on the right the Cardas speaker binding posts and two switches, one for 4 and 8 ohms and one for speaker on/mute. On the front panel there are two meters: the center one is a VU meter, and the right side mA meter shows the total anode current of the six EL34Bs. This mA meter should show not more than 420mA ( typically between 350 and 400mA). On the right of the front panel one can notice the main power switch and light indicator.

The case of the mono-block amplifier is running hot in few spots that are close to the group of the six EL34Bs power vacuum tubes. It is not recommended to touch the top of the case above the power tubes where the temperature can reach values that are not safe for touching. For this reason this amp should not be left unsupervised. Each monoblock weights about 60-70 lbs .

This a medium power amplifier ( 2x50W max), providing a transparent, detailed, good dynamics sound. The EL34Bs are running self-biased, class A triode mode.

Here are some measurements:

Input Voltage (Vrms)	Output Voltage (Vrms) on 8.3 ohms	Power	THD (%)
0.5	6.23	4.67	0.3
0.73	8.8	9.3	0.53
1	12.5	18.8	0.95
1.45	18	39	1.87
1.55	19.2	44	1.76
1.66	20.5	50	2.29