DIY All DC coupled EL84 SE Amplifier
The two stages of the Aikido driver are DC coupled and I thought it would be nice to have the
final stage DC coupled as well. With fixed grid bias on the EL84 also the bypass capacitor
required in case of cathode bias can be ommited. This way the amplifier is almost free of
capacitors in the signal path. The only remaining one would be the power supply filter cap.
The idea came to me, to use the Aikido circuit to drive the screen grid of the EL84 instead
of the control grid.
Screen drive has been used before, not only for
HF Amplitude Modulation
but also for audio amplification, see related links below.
I tried it out and after a couple of iterations I ended up with the folowing circuit.
It works very well, DC coupled through all stages and without feedback. Input tube is 6N2P
followed by 6N1P.
To build this one I re-used the experimental setup of the
Aikido driven EL84 SE Amplifier
. The power supply is also the same.
Since my approach to this circuit was purely experimental, having an independent variable +B supply
for the driver stage would greatly simplify things. This way it is possible to modify the idle
voltage at the screen grid of the EL84 and also attain a larger voltage swing at the Aikido's output
-if need be- without having to change a lot of resistors. To account for these, an extra high voltage
adjustable power supply was built that also provides 6V for the grid bias. Note that a change of
the idle voltage at the EL84 screen grid will in turn affect the idle current through the EL84.
In the end the +B used for the driver was 300V. It may well be that a somewhat higher voltage may
increase output power a little.
A further aspect is the screen grid current draw. With the present circuit the screen grid current
at 1KHz full drive is about 5 mA peak-peak. Distortion becomes audible when the cathode resistors of
the cathode follower are made too small (about 100 Ohms). A value above 220 Ohms was found to be acceptable.
Luckily for me, tubes are very indulgent, otherwise such an experimental approach would not be possible
and with my lack of knowlege and time this hobby would remain far less accessible to me.
Schematic of the variable high voltage power supply
It is based on Michael Maida's design, see:
National Semiconductor. Linear Brief 47 (pdf)
All R6 resistors are 2W types (what I had at hand). The chain helps distribute power among them. In normal
operation they stay cool. With this circuit the output voltage can be set between around 210 and 310 volts.
Drawing about 10mA from this circuit all components stay cool, still I mounted the darlington onto a heat sink.
This power supply is so handy that I will build one for the shack. The multiturn pot allows very accurate voltage
setting. A handy thing to have.
This is a flee power amplifier, it does not even reach the 3W I hoped for. Yet it achives a good sound level
with 91 dB efficient speakers in a mono setup. And I dare say it will reach comfortable listening levels
for most music types as a stereo version.
The amplifier seems particularly picky concerning the OPT. The OPT choice I had for testing was limited to
a Hammond 125ESE and a TSE-4 from Raphael Audio (german company supplying tube amp kits). The 125ESE
is oversized for this application and introduced a kind of distorsion that tired my ears quickly.
With the TSE-4 the sound is beautiful, smooth and fast. The mids and highs are a dream, delicate, fluid, clear
with plenty of detail and in spite of playing mono, I perceived a sense of depth or room information I was not
used to before. The downside of it is the bass. With the chosen windings on the TSE-4 (ratio 4K:15) the bass
is deep and strong but tiring muddy.
With a 5K2:5 ratio the bass is a lot better. It appears much tighter yet the mids become a bit too present
for my liking.
Assuming a nominal speaker impedance of 8 Ohms the 4K:15 ratio results in a 2K plate load while with
5K2:5 it is 6K4. I guess the sweet point lies somewhere between 3K and 4K. I wish I had more OPTs
at hand to try this out.
Update Nov. 13th 2010
Eventually I acquired an output transformer that I thought would meet the 'sweet point' that
would make the desired 'all dc coupled Aikido driven SE amplifier' come true. I picked the '53.60-4'
out of the range offered by
. It has an impedance ratio of 2.4K/4. Loaded with
8 ohms speakers a nominal 4.8K would load the plate of a 6P7S running at 60mA. I hoped also for
a bit more output power this way...
Unfortunately this one is not going to be a happy ending story. Although the final result delivered
more power that its EL84 cousin and the high and mid frequency range stood up to the expectation,
the bass stays the weak point. The output impedance is still much too high and makes the bass sound
In the picture: At the left hand side is the stabilised variable power supply
'a la Maida' used for the Aikido driver. In the middle the new Reinhoefer 53.60 OPT. To the right
the power amplifier with the 6P7s tube and the Aikido using 6N2P and 6N6P. In the front the
power supply for the 6P7S output stage using the EZ81 rectifier tube.