Those of you who have been following the Smash preamp thread may have seen discussion of an upcoming amp designed to mate with Smash. Well, we got the prototype finished and we're ready to begin production.
Seductor is a stereo single ended EL84 amp. Tube complement is 2xEL84 and 2X6AU6. Power output is 4 Watts per channel at 10% THD. Gain is 15.5dB. Sensitivity is 0.9V for 3.5W output (5%THD). S/N is -94dB. Input impedance 249K ohms. Speaker output taps are 2,4,8 and 16 ohms. The photo below is of our functional prototype. The production model will have very slightly different layout and venting.
Why Seductor? I am going to let PJ explain that -
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 cheaper that 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 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 retail price of Seductor is $569, but we have decided to offer special pricing on the first ten orders, of $499. We are working on getting the web page up for this kit, so in the meantime if you would like to be sure to get one of these specially priced first kits you can call us at 206-451-4275 or contact Eileen at queen at bottlehead dot com. The guys will be working on the assembly manual in the next week or two, and delivery of the first kits is estimated to begin in mid to late October.
« Last Edit: September 22, 2014, 11:20:29 AM by fullheadofnothing »
Dan "Doc B." Schmalle
President For Life
Bottlehead Corp.
