Piccolo tube amp
Parallel Single ended EL84
Introduction
Dear Music lover,
I hope the story of this fantastic tube amplifier will convince you to finally build
one yourself. Maybe you were already planning this but the high cost of such a project
(> 5000 euro / dollar) for a build amplifier stopped you from going any further.
I was also afraid for this.
Then after a long search ,I bought a second hand amplifier with the famous 300B
tubes, which was only a few weeks old.
The great sound of these tube amplifiers are in my opinion much closer to reality,
the 'soul' of the music is much 'stronger' than a regular amplifier.
The circuit of the this amplifier was a parallel single ended type.
As I'm a DIY freak I always wanted to build a tube amplifier myself.
When I was searching for some components I came by a company called
Aquablue. They gave me lots of hints, tips and I got to listen to some tube
amplifiers which sounded fantastic. This company proofs that you can
have a superb tube amplifier without the need to win the lotto.
They offer this wonderfull amplifier build or as a kit, and the fun of building your own is just fantastic so I bought the kit and within 2 evenings work I had my amp up and running.
In the next chapters I'll explain you my experiences with this amplifier,
which is build around two EL84 penthodes in pure class A, single ended.
The power output is about 8 watt, more than enough when you have
speakers with a sensitivity of over 89dB.
By working orderly and by choosing the right components, you'll have a
wonderfull amplifier; which exceeds many commercial tube amplifiers !
Have fun !!!
A word about the schematics
First Stage V1 2C51/396A
At the input we find R1 270 ohm the grid stopper resistor as this tube has a high transconductance
The negative grid tension is obtained by R2 ( 270 ohm)
The working point lies at 7.5mA / 2.1Volt
With R4= 7.5K/ 1 watts the power supply voltage is adjusted at +/- 240 Volt
With R3=15K/ 1 watts the anode voltage is fixed 130V
This stage has a no feedback and enough gain to drive the output tubes
We use a very high quality Silver foil Mundorf 0.1µF capacitor to bring the audio signal to the outputstage.
The output load is obtained by the load charging the output transformer , a high quality Lundahl double C core LL1664 gapped for 80mA
The powerstage with the 2 EL84 parallel connected tubes:
Here we find a Class A bias with 35mA current for each Penthode / -11 Volt
Capacitor C5 220µF/25v Elna is paralleled
over the cathode resistor. There is an MKP capacitor added in parallel as it is very good to use smaller value to improve for the sound quality.
The input impedance of the output transformer is 3Kohm, which means
for each penthode 6Kohm. (a little lower than the optimal load, which
reduces a lot the 3th harmonic.)
Since the load impedance of a 8ohm loudspeaker is more about 6ohm,
it's possible to use a 3Kohm primary impedance for the output transformer
without any problem.
Some experimentations with the amplifer:
the importance of the power supply:
A lower first capacitor reduces current spikes and improves greatly the sound, just as if using a tube rectifier
The 20 H/80mA coil has a huge influence on the sound quality.
The use of diodes for the filament AC/DC conversion does not seem
to change the sound quality either.
The use of a diode power supply:
Use big capacitors (Elko's) behind the coil gives you more spare
current => more punch and stability.
(even with one EL84 you can get a lot of power).
The influence of the coupling capacitors:
The capacitors in the signal route need to be of good quality.
ex: MKP. the differences are very clear and can change totally
the sound. (good or bad).
The influence of the capacitor paralleled with the cathode resistor of
the EL84:
When you use only one EL84 one can hear it well
When only using an ELKO one can hear some instability at low
frequencies. (the ELKO starts to resonate)
When using 2 penthodes in parallel this phenomenon is partially absorbed
by the differences in the tubes. But also here it's better to put more
MKP's parallel with the ELKO, the sound get faster and more stable !
Daniel Desanois