"NO! I am not Bob Ross's Brother, and I do not paint... But if I did, it would be in one color only."
Here is a circuit that I have been wanting to do for some time - An EL84 driving another EL84 single ended. Sakuma San is the real innovator here - his theory being that each family of valves sounds different. Say you are listening to an amp that has a 12AX7 driving an EL84 - what you hear isn't either valve by itself, but what each contributes to the circuit as a whole. If you want to isolate the sound of an individual valve, you have to either drive it by itself, or with another identical valve. Sakuma's most famous circuit is probably his 300B drive 300B circuit. Google 'Sakuma and 300B' and you can find all the details for yourself.
I'm not looking for anything quite so exotic - I'd be happy with a clean 3-5 watts per channel from a fairly straightforward and simple circuit that utilizes commonly available and widely manufactured valves. I have lots of 6BQ5/EL84s around for tinkering, as well as plenty of ~5k single-ended transformers, so that is what is going in.
The initial iron layout is shown below - vintage VM power transformer, TV swinging choke, and some nice beefy air-gapped VM output transformers.
The power transformer has 5V filament taps, so why not go with a valve rectifier as well?
Here is the Power circuit configured on the Duncan Amps Power Supply Simulator (PSUDII):
You can see that I had to model the multiple taps on the swinging choke as two separate entities. Normally you would want the higher inductance value upstream, but in this case the DC resistance was simply too high to overcome with the 100mA load from a pair of EL84s. Tweaking the capacitor value directly downstream of the rectifier easily lets one set the operating voltage for everything downstream of the choke - another reason to love choke loaded power supplies! With a 3.3uF capacitor, you get a B+ of just under 320 volts for the output tubes (red line) , and around 275 volts for the preamp section (green line). Just right for what I want. Notice in the graph below that the current loads are modeled as stepped loads that come online at 10 seconds time, and that the the B+ floats up to well over 550 volts under a no load condition. HV caps are a definite must...
Below you can see one channel of the circuit fleshed out.
The front end is triode strapped, and at the given operating voltage, yields about 20 dB of gain – plenty for just about any quasi-line level input used. A 100k log potentiometer acts as the volume control. If you have a variable gain source, you can omit the pot and replace it with a 100k resistor.
The 5k grid stopper can probably be omitted, but contrary to old wive’s tales it does not reduce gain or output response. It does however, effectively block RF and HF oscillations from getting into the signal path. A great primer on grid stoppers in general can be found here: http://www.aikenamps.com/InputRes.htm
The output section is pretty much cookbook from the RCA manuals – standard pentode setup with an idle current around 40 mA per tube, with plenty of cathode bypass capacitance.
Since I blocked local feedback at both stages of amplification, I did add the provision for global NFB feeding into the cathode of the preamp stage. Just remember to remove the cathode capacitor on the preamp if you decide to use it. The trimmer pot will allow the end user to adjust the amount of feedback to suit their personal listening preferences. Looks good on paper, time to start building…
IF NFB IS USED.
Below are some pics of the completed layout. It finished up quite nicely. I shot the chassis in a nice RAF blue that complements the 'Au Naturale' output transformers and choke. After some listening with the original circuit, I did alter the power supply circuit a little to increase operating voltages just a little more - there is now a 22uF cap right off the rectifier in place of the 3.3uF that was originally modeled. B+ is now closer to 350 volts, and the sound is definitely tighter. I did a lot of listening with differing amounts of negative feedback, and in the end eliminated it entirely. The amp is now running completely open loop, with no local or global feedback of any sort. What you hear is nothing but the pure sound of EL84s with no tinkering or processing - the signal goes in, gets amplified, and goes out. Sakuma would like this amp. I like this amp. You will like this amp.
At the rated output of 3 watts, Total Harmonic Distortion is just under 1%. Output power at saturation is just over 5 watts with 4% THD. If you look at the distortion chart, you will see that it is almost entirely second order distortion. No odd order grating sounds at all. With feedback, frequency response across the audio spectrum (20 Hz - 20 KHz) is within 2 dB across the board - a tribute to the vintage Hammond output iron. Open loop, the bottom -3 dB point climbs to around 60 Hz, but still doesn't sound flabby or lossy. HF response is essentially the same, with or without FB. If you build one incorporating feedback, I recommend that you are very careful adjusting the trimpot - at low resistance settings, you can short the cathode across the output transformer windings if you go too far in adjustment. Best practice here is to use the trimpot to find the best sound, and then replace it with a fixed feedback resistor once you find a value you like sonically. Don't forget to disable the bypass capacitor on the cathode as well.
I like the sound best with a pair of VM branded Sylvanias in the front, driving a pair of Telefunken output tubes. The sound is unlike anything else I've built or heard before, making it hard to describe. The even order harmonics are certainly noticeable, but not at all detracting from the music. Soundstage is bigger than life, with smaller nuances to the music being much more noticeable and present. There is no hum or other naughty sounds at all - the choke loaded power supply and careful wire routing really paid off.
If you are looking for purity in the single ended circuit - this is it.
I have completed the 40 hour burn-in, and the sound is for the most part unchanged. Maybe a little crisper on the high-end. Highly addictive - I've been burning through my entire music library, and female vocals and early stereo recordings are definitely my favorite on this amp. The Zombies, The Gap Band, Ofra Haza, and even Israel Kamakawiwo'ole will all blow your mind. Old favorites sound new again. Definitely one of my favorite works. Drop me a line if you have or are interested in building your own - I'd love to get your take on the sound.