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A push-pull triode EL84 schematic by Thomas Martens

Posted by LowRedMoon (A) on May 24, 2009 at 10:16:49

It's been while since I completed this build and I think that a post about it is well past due.

Three winters ago, while I was on an extended work assignment away from home and holed up in a Boston hotel with ample time on my hands, I grew antsy to begin a complete tube amplifier build. Somewhere along the line, I began dialogue with Thomas Martens about a PP triode connected EL84 amplifier design that he either had on hand or in his mind - I don't remember exactly what the case was at the time. After a brief period, he actually snail-mailed me photocopies of hand-drawn schematics and I quickly started ordering parts.

This was my first complete amplifier build. Because I'm reasonably young and was between residences at the time, I had to complete the task with a meager set of tools, which included no more than a borrowed drill press, a hack saw, a few common hand tools and a soldering iron. Needless to say, the enclosure ended up being a mild hack-job of an aluminum plate attached to the top of a hard wod frame, as was dictated by my limited tool resources and eagerness to complete the project.

I followed the design exactly. I used K40s as coupling capacitors. I used a ground bus method that made sense to me at the time, which was a 10ga. solid copper wire shared by both channels, earthed to chassis at the first filter cap ground and run up to the signal input ground. All other ground connections were made with the highest current connections closest to the filter cap ground and lower current connections closer to the sinal input ground. Now, whether this is necessarily the most effective or optimal grounding method is unknown to me - I know that there are others - it is just what I used at the time.

I was nervous powering it up for the first time (imagine a guy wearing safety glasses, standing as far as possible from an amplifier with his arm out-stretched toward the amp's power switch, ready to dive for the power cord in response to a fireball of an exploding amplifier), but the amp powered up without incident. It was DEAD QUIET in every way. With the volume fully maxed, there was a barely audible hiss at the speaker - one that you had to really focus on to hear. The speakers were only 91dB/watt rated (not optimal for the amp, but what I had at the time), but this quietness was also evident when I eventually connected them to a pair of Heresys a year later.

The next point deserves better emphasis than I am abe to convey with words.
This amp, fed by a humble Onkyo 320DX CD player, presented the sweetest sound I had ever heard coming from a stereo. The most immediately noticed quality was tonal smoooothness, which goes a long way with me. I'm not big on audiophile jargon, but sometimes you can't help but spill it out. This little amp really presents a clean, focused, smooth and snappy picture with all the immediacy and floating cymbal sounds that we love. Its bass production was not loud and in-your-face, of course, but was tonally clean, which I think is more important.

Now, given that it is rated for about 4.5 watts, the amp runs out of steam quickly if you have insensitive speakers, as I did at the time. It did a good job of filling a 12x16 room with glowing sound through 91dB/watt@1M speakers, but rocking volumes were not permitted without distortion. More sensitive speakers are required for that.

At one point, I connected the EL84's as pentodes, which did beef up the power a bit and sounded very good, but I'm presuming that it is the lack of feedback that allowed more distortion to be present. This amp, as it is written, should be kept as a triode amplifier.

Of note:
The original schematic called for a Hammond 370JX power transformer that is rated 300-0-300 @ 250mA, with a 200-500 ohm 25W resistor between the rectifier and first filter cap. to adjust B+. I eventually replaced it with a 370EX (275-0-275 @ 125mA) in order to eliminate the resistor. After using a variety of power transformers, I fully endoese the hammond 300 series. In my experience, hey run cool and quiet at rated voltage and current.

I have also used a 5U4GB rectifier in this amp, which does mate very well with it. It causes a more romantic and slightly more soft and colorful, yet still clean and fully enjoyable musical presentation.

Enjoy your Memorial Day weekend everyone, and be well.

Adam

I'm not an audiophile. I'm a sound junkie, a music lover and a tinkerer - but definitely not an audiophile.

Edits: 05/24/09 05/24/09 05/24/09 05/24/09 05/24/09

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