Seductor Single Ended EL84 Amplifier Kit Specs
What is SEDCTR?
SEDCTR stands for Single Ended Direct Coupled TransResistance. It is a 2-stage tube amplifier technology that uses pentodes or beam tetrodes.
Local feedback is used to make the output tube act as a transresistance device, with current input and voltage output. The driver stage is a transconductance device (a pentode) with voltage input and current output. The stages are direct coupled and each stage has its own local feedback. There is no global feedback.
How do SET amplifiers differ?
In an SET both driver and output tubes are voltage in, voltage out devices. The output tube has a lower internal resistance than the load it drives, and the best triodes are highly linear, so they can be used without feedback. SET amps also have very graceful overload characteristics, which is not possible when an amp has a large amount of feedback. We believe SETs to sound the best, but they are costly and (due to a modest damping factor) match well with relatively few speakers.
How does SEDCTR differ from typical pentode/beam tetrode amplifiers?
Feedback must be used in any pentode/beam tetrode amplifier to obtain a damping factor suitable for driving loudspeakers, and to achieve an acceptably low distortion.
Typical pentode/beam tetrode power amplifiers use global negative feedback from the output transformer secondary to the input stage to produces low output impedance and low total distortion. However, too much of the distortion is in unpleasant higher-order products, and the wide variation in phase response of the complete amplifier makes instabilities of various sorts common.
In SEDCTR amplifiers, almost all the feedback is around just the output tube. There are no coupling caps in the loop, and only one bypass cap. This very simple phase response produces a stable circuit in spite of the large amount of feedback. The compact loop also reduces high-order distortion products.
Because of the linearity of the output stage produced by this local feedback, the driver dominates the distortion character. The driver stage is operated into a low load impedance, which causes it to produce predominantly second-harmonic distortion, similar to that of triodes. There is a small amount of current feedback local to the driver stage to reduce that distortion. The output transformer is outside the loop entirely.
Why is Bottlehead using SEDCTR?
There are few highly linear power triodes, and they are expensive. They are made in small numbers, to high mechanical precision. Power pentodes and beam tetrodes are made in much larger numbers since most tube amps and guitar amps use them. They are not very linear but are widely available and much less expensive than triodes. By using these tubes, we can make amplifiers that are significantly less expensive for a given power output. And with SEDCTR technology, we can make them sound a lot like SET amps. We can also obtain a lower output impedance and higher damping factor than is possible with no-feedback SET designs. This means the amps can successfully drive a wider variety of speakers, since almost all speakers are designed for high-damping amplifiers.
And so you see that what we have in mind is a very versatile, bang for the buck tube amp kit that still retains single ended goodness. You could buy two, strap them each into an 8 watt monoblock and add a Smash preamp for less than the price of a pair of our premium 8 watt per channel Paramount 300B amps.
The chassis arrangement is similar to our other kits. We provide the usual high quality level of American made transformers, an American manufactured 10” x 10” chassis panel with our cool new random brushed artisanal finish and lovely Pacific Northwest milled alder wood base that glues together easily and looks great with a wide range of builder-applied finishes. And we use our usual high quality resistors, capacitors and tube sockets.
Specifications
- Tube complement is 2xEL84 and 2X6AU6
- Power output 4 Watts per channel at 10% THD
- Gain 15.5dB
- Sensitivity 0.9V for 3.5W output (5%THD)
- S/N -94dB
- Input impedance 249K ohms
- Speaker output taps 2,4,8 and 16 ohms
So how does it sound? Just as its price indicates the sonic character of the Seductor falls between the Quicksand and the Paramount in terms of resolution. It has the Bottlehead house sound –neutral yet musical, not flabby or bright , resolving but not edgy. It will make a great match with the Smash preamp, and will also work nicely with Quickie or BeePre if you have the urge.
Here’s what one builder has to say –
“Wow, this thing has some power! Coming from a hopped up Sex amp with an Enhanced Foreplay 2 through some tweaked Hammer Dynamic Super 12’s I had to back off the volume quite a bit. Lots of nice detail and heft to the lows and just a little bright up top, but I just started the thing up an hour ago! It should smooth out nicely with some burn in. I guess I would say this amp has gravitas!
Best build yet for a bottlehead kit. Everything checked out with the meter and it fired up right out of the gate, no issues. Very quiet too! Now to build that Smash Pre kit and see if that matches up better than the old Foreplay.
Thanks Doc and Eileen and crew!”
A bit later he writes –
“As with most gear, things have broken in nicely over the past few months. I now have the amp paired with a wonderful Smash pre-amp. This combo is more resolving than the previous combo of Sex and Foreplay.
It’s amazing to me that just when you are satisfied with your system and think things can’t get much better, you can still hear major improvement when you replace some key components. I have better imaging and can hear more subtle details in the music. I have just done the Smashup upgrade and as Doc mentioned this was also a major improvement. More power and detail yet again!
For now I’m very satisfied with this combo and feel I’m set for many years of exploring my music collection all over again.”