Steve's Valve Amplifier

[SteveTheShadow]

OK
Perhaps I should have posted here in the first place.

So...here are the schematics for my balanced, push-pull EL34 amplifier.

The voltages it contains are LETHAL so this is for information only and I'm not advocating that anyone builds it.
Besides, being fully balanced (to eliminate the phase splitter and make things simple in the extreme) it is of fook-all use to
anyone in the domestic audio scene.
It was built to serve my own purposes, i.e. work with a digital source feeding a balanced output DAC.

Audio Circuit:


Power Supply:

[jammy395]

It looks Awfully Complicated Steve..

But then im only a mere Sparky - I can take a supply to anything...!!!

[SteveTheShadow]

Well the audio cct is straightforward.
The signal enters at the left hand side, via the pos and neg terminals. The pos and neg signals are in opposing phase with each other. The two signals enter the grids of the pair of 12SJ7s where the gain of the valves causes the small signal at the grids to modulate the high voltage 370V DC at the anodes, producing a magnified replica of the input signals. The amplified signals are then passed to the EL34 grids via th 100nF coupling capacitors. The signals again modulate the high voltage 502V DC at the anodes, of the EL34s causing a greatly magnified replica of the input signal to appear. This time the two signals flow into the output transformer. The High Voltage DC to the EL34s is fed in at the centre of the primary winding and one half is wound opposite to the other, which means that one of the two out-of-phase signals gets reversed, which puts the whole lot in phase so the signals add together, and voila we have a large, high voltage signal ready to be stepped down to drive the speakers, via the 8ohm speaker winding.
The 470K resistor between the anode of the EL34 and the 12SJ7 side of the coupling cap is calculated to feed a portion of the output signal back round to the EL34 grid, via the said coupling capacitor, and is necessary with pentodes, to reduce output impedance and distortion, otherwise we get a rising response trend, which leads to overly bright sound. The high output impedance without feedback also produces underdamped low frequencies, leading to ill controlled, boomy bass.

As with transistors, some form of feedback (not necessarily the way I have done it) is also necessary with pentodes, which causes the single ended triode mafia to dismiss them and look down on them as somehow less worthy.

[jammy395]

Nice steve - i take it the amp works rather well with your Metronome speakers....

[SteveTheShadow]

Yes it works great with the Metronomes.
There is room for improvement in the regulation of the bias supply, which is the next item on the (very short) list of things I need to sort. Only needs a suitable resistor/zener shunt reg.

[Andrew]

There's something really beautiful about valve amps. Seeing that wonderful glow when the music is playing always moves me. Have you had any/many bought valve amps, Steve? Just wondering how they compare to the stuff you can build?

[SteveTheShadow]

Hi Andrew,
I started with an Icon Audio Stereo 40 in 2004, which was joined by a World Audio Design Phono 2 valve phono stage built from a kit. I have been hooked on the sound of valves since then.
I have built around twenty different projects, all designed by my own hand, as I felt that it was the best way to learn, rather than copying someone else's circuits. So in effect I taught myself, valve electronics, with the help of the good folks on the Audio-Talk forum, and a goodly dose of old books and Morgan Jones "Valve Amplifiers"
The first fifteen amps I built could be bettered by equivalent commercial units. It is only with the last five builds that I consider my own efforts to be able to surpass the equivalent commercial valve amps. It is with these builds that I finally cottoned on to the vital importance of the power supply as the number-one contributor to the final sound of a project.

As the Doc has said, you can't just bodge something together and hope. Yes it will work, but if you want the thing to sound half way decent, you have to actually apply yourself and learn to know what you are doing and why you are doing it. If you are not prepared to put in the sweat and the reading, and buy some test gear, then you have very little chance of making something that could be called hi-fi.
Building your own gear is a great hobby, but like most things that are worthwhile, you only get out what you are prepared to put in. Put in the effort and the rewards are fantastic.

I have discovered that to get great sound requires good engineering. Throwing a set of flavour of the month NOS expensive valves and a few exotic resistors and caps into a circuit copied off the internet is a sure fire way to waste money and get crap sound.
Just a bit of personal philosophy thrown in there

[Andrew]

Hi Steve. Great to hear that you have bettered commercial designs through your concerted efforts. You often hear of people throwing a few bits together and thinking they know all there is to know about valve amps. I'm really glad it takes a bit more than that to excel.

Other than a "Music Angel" amp I took in to help a mate, I've really liked all the valve amps I've owned. Some have been amongst the best amps I've owned: The Audion Sterling was the best of the bunch to me at least. There's nothing like sitting listening in the dark with the valves glowing beside you. All the better I'm sure, if you have managed to make them glow and sing all by yourself

[SteveTheShadow]

Recently I've been doing a lot of work on my push-pull valve amp, making the whole thing a bit more sophisticated. The front end has been changed for a set of cute early 1960s Brimar small-signal pentodes with top cap control grid connections.
The input stage screen grids have acquired a nice solid-state voltage regulator fed by by a mosfet, cascode, constant current source, feeding a zener string shunt arrangement. The regulator was originally designed by Morgan Jones and was implemented as a small credit card sized PCB, by Nick De Smith "Jack" on the Audio-Talk forum.
Driver circuitry has been substantially upgraded, courtesy of a pair of dual triode cathode follower valves.

A useful upgrade to the sound quality has been obtained.


Power Supply


Audio Circuit


Get yer soldering irons out chaps.

[SteveTheShadow]

The top cap grid valves I am using on the input stage, have an interesting history.
The National Valve Museum has some info here: http://www.r-type.org/exhib/aab0164.htm

They were designed apparently for incompetent customers
Not a good move by Brimar, as being seen to have used them in your hi-fi amplifiers, would have labelled you as a know nothing idiot in the eyes of the cognoscenti.
Consequently there are still plenty of them about at cheap prices.

Eventually the instrumentation industry found a use for them in electrometer applications as they were not quite as concerned with image as the hi-fi industry. Shows how little has changed really.