Mar 23, 2016

EL84 Push Pull amplifier, the little giant

The EL84 is NOT a DHT, but still a fascinating tube for many triodes lovers (I belong to this group) who recognize its sonic qualities.
Great manufacturers like Dynaco, Scott, Leak, Loyez just to name a few, sold thousands of amplifiers using this cute little pentode and for many serious listeners of that time these amps were by far superior for music reproduction to their more powerful concurrents using EL34, 6550 or KT88.
Yes this tube is very involving, it as the speed ( that usually lacks to the bigger tetrodes and pentodes) the naturalness and ease so many people do like in the very best triodes. It reminds me the 10, the E130 or the RS242, it makes music living like and it's power limitation is it's only drawback. Remember that in the 60's there were not so many speakers with 97/99dB sensitivity to perfectly match a 10 watt amplifier.

In the past 20 years I made half a dozen amplifiers using this tube, always with great pleasure, and that's why I decided to make a post about it.

Also because I always keep an EL84 amp to drive my beloved Wharfedale Super 12 RS/DD. Magnificent full range speakers especially on open baffle.

The amp I will talk about is not a design of my own, it belongs to Harman Kardon. I just made some modifications for safer use and better sound reproduction (I built one exactly like the original for comparison).
Why the HK amplifier ? I found during all these passed years making electronic stuff that simplicity is usually the best way to fidelity and the HK20 design is really simple and very smart.

It operates with a single 6CG7 as driver and phase splitter, two EL84 and a rectifier.
You can't do more simple and when you add the qualities of the 6SN7 family tubes you are right on the way for a good sounding amp.

HK 20 original schematic

As you can see, very simple with some tricks to prevent the use of a decoupling cap in the first stage. V1a bias is taken through R10 in the cathode of the phase inverter V1b. The current of the two sections flows in this resistor, the result is a smaller value that it would have been if directly grounded the usual way. This helps to lower the effect of degenerative feedback thus giving a higher gain to the first stage. Elegant, efficient and easy to calculate if you want to change the tube (I tried the E80CC instead but preferred the 6CG7 after a long listening comparison). For a perfect cathodyne balance a single R11 resistor in parallel with R8_R9_R10 will give the exact value of R7.

To calculate R11 with (R8 + R9 + R10) = Req   use formula   R11 = Rreq X R7/Req - R7

My own HK20

I was not comfortable with EL84 operating voltages.
370V on anode and 350V on screen make the tube work very hot and very hard. I am not sure the usual EL84 will last very long under such a fire.
Even the professional and super strong Mazda 7320 reach their limits, furthermore when bread boarding an amp for tests I had some hum that I could not cancel. So I decided to modify the whole amplifier supply and to pay great attention to time constants, I also preferred the EL84 UL to the pentode mode, I reduced NFB to get decent THD (1,5% @ 10W, mainly third) and a more mellow sound. I suppressed R1 potentiometer (always a source of trouble) and replaced it by a voltage divider to match the output of my line preamp.
Now with such a supply and mods the amp is dead silent, punchy, open with a wide sound stage.
Very detailed heights, lush mid, rock solid bass make this amp an all purpose companion.

Power supply revisited.

Inside this little giant, AB resistors and Aerovox V161 caps give a vintage look without compromising sound qualities.
Reliable Siemens capacitors to smooth out the ripple, way better than the usual electrolytics (MKV and MP J/S are my preferred) and Hashimoto OPT.

Philips Holland killer tubes, Mazda GZ32 and shielded 6CG7

The extra socket was for a buffer stage intended to be used with a 600 ohm symmetrical input transformer. I finally did not populate.

Personal EL84 review

I am lucky enough to have different brands in stock to make a comparison.
I wont write a lot just give my impressions, for an extensive test read  Vacuum Tube Valley issue 8

Mullard soft and round sound, not my taste.
Philips RTC the very best with this amp, very well balanced.
Philips ( Amperex ) Holland top of the line with Philips RTC.
Mazda 7320 strong and powerful sound, can be tiring.

TFK the worst nothing else to say.
Tungsram excellent performer, lacks a bit of punch in low end, the best on voices.
Russian Reflector 6P14P/K (and only this suffix, means vibrations proof. NOT EB or ER) a real surprise, very good sounding, on par with Philips RTC.

Some great CD's to listen with


  1. Bonjour Fred,
    très bel ampli, comme d'habitude. Je découvre aussi les vertus des EL 84, il y a des arguments même en face d'un 300B SE...

  2. Hi Fred. Nice blog. I would like know if it is possible downsize this el84 pp design. I have enough with 4 or 5 watts, and extend a little tube life too (I suppose). With down a little the PS will be enough?. Work points change, I suppose, so, what are the best place to change in schematic?
    Like simplicity, as you. And like 6CG7 too. This schematic is what I searched.
    Thnak you
    Juan Antonio

    1. Hi Juan
      think the best would be triode mode.
      Connect a 100 ohm resistor between anode and g2, connect k and g3. Do not lower DC to stay in the linear region of characteristics. You will probably have to change biasing to draw about 20/22 mA per tube.
      Make a try and let me know.