6AS7 SET amplifier

This very special, one of a kind, SET is based on the 6AS7 twin power triode. A two stage amplifier with a 12AX7 driver. Only two valves in the audio circuit and valve rectification. The rectifier valve is a rare JAN CBXL-5R4GY (brown base). The power valve is an English United Electric and driver valve, 12AX7 Australian made AWA.

Valve rectification and a reactor filtered power supply ensure true musicality. Combine this with an internal power filer, vibration isolated power tranni and OPTs. This makes a very quiet and dynamic amplifier.

There is no input capacitor. The critical inter-stage capacitor, joining the driver stage and power stage, is a large and expensive Jensen, pure copper, paper and oil type. Silver wire is use to bring the audio to the driver stage and component-2-component wiring is used throughout. No PCBs exist in this amp!

After much listening I am convinced this is the best amp I have ever built. Not only based on its detailed mid-range and extended highs but its deep and weighty bass. The volume of bass I was not expecting. The sound stage is deep and wide both behind and in front of the speakers. Sibilance is natural and sustain and reverb. extremely well rendered.

A lovely Mirbeu timber base (custom built especially for this amplifier) holds something very different. A blue top plate carries exquisite but bulky componentry. Though only a few watts, with the right speakers this simple amp will delight and surprise you. With a frequency range from 12hZ to 50,000hZ it is easy to understand why this amp sounds so good. And why the bass feels extended and full. I true audiophile SET amp.

6SN7 remote driver stage for the Be Bamp

The 6SN7 driver stage has enriched the musical performance of the Be Bamp. The bass weight is excellent, mid-range texture is extended and sound stage depth dimensional . Treble is controlled, detailed and mellow. Cymbals are naturally rendered and sibilance well controlled.

This remote driver stage is an improvement over the 6N1p stage (see other blog entries). Some may prefer the original driver stage to this with its brighter top end and crisp mid-range. The natural musical qualities of SE UL tubes amps is evident with the combination of this 6SN7 driver stage and the KT120 based Be Bamp.

For more technical information about the 6SN7 remote driver stage and Be Bamp see entries below.

6SN7 remote driver stage for the Be Bamp

Having the flexibility to change driver stages allows many different types of valves to use with the Be Bamp KT120 power section. The original 6N1P drivers stage can easily be unplugged and other suitable drivers stages plugged in. The power amp does not have to be turned off but the amp is placed in stand-by.

This stage is based on the 6SN7 twin triode valve. It is an extremely popular valve used by audio craftsman. The valve sits atop and a small metal enclosure. The base is sprayed with an epoxy enamel paint and the finish is hammered copper. Once again Russian military paper, foil and oil inter-stage and low ESR capacitors are used.

There is no hook-up wire or PCBs. used and once more component-2-component wiring is employed. The driver stage is compact and attractive and brings a whole new sound scape to the Be Bamp.

Be Bamp with KT120 Tung-Sol tubes

The KT88 Be Bamp is an excellent sounding amp and if taken no further would satisfy most audiophiles. But the Be Bamp with KT120 valves, replacing the KT88s, just takes excellence to a new level.

In previous iterations of the Be Bamp improvements were made to bass response and over all musical weight. Now with the replacement of the KT88s with the over-size KT120s so much more of the music is presented to the listener. Mid range and bass have lifted and the treble has received extra air.

Now bass is extended and controlled, mids are crisper, brighter and better defined and treble just a hint airier. Power has increased in the amp and though efficient speakers and still required to obtain voluminous listening levels, greater musical weight is heard right across the audio spectrum.

Classical, Opera, Jazz and other genera are brought into the listening area with greater presence than previously felt. Emotion from each track, weather CD of vinyl, is embraced with greater immediacy. Detail is better etched into the sound stage and the sound stage deeper than before. Simply a better audio amplifier from retro-thermionic.

Remote driver stage for Bread Board amplifier

To increase the volume level of the Bread Board amp (Be Bamp), else where on this bLog, a driver stage was built. This is the original cct. used in the Merlot amps with some components upgraded. The is no volume control or input cap and pure silver wire used in the signal path.

The Be Bamp can deliver full volume to the point of clipping. Well short of clipping a high volume level can be obtained with large 91db sensitive 3-way tower speakers. The over-all sound of the system, which also combines a two stage valve preamp, is extremely musical. The sound is very transparent with good bass extension and weight, broad well imaged sound stage and natural tones of voice and instruments.

With the driver stage being remote other driver stages can be matched to the power stage and share the power supply. This provides another level of experimentation and flexibility with the over-all system.

Remote driver stage for Bread Board amplifier

Because the Bread Board amplifier (Be Bamp) performed so well, though at low volumes, a driver stage was built for it. This stage is remote from the rest of the amplifier and is joined to the amp by interconnects. A multi-pin, high grade, plug and socket connect the 400V HT+ and the 6.3V filament voltage to the driver stage. This single stage front-end has been constructed in a Twinings Tea Box to match the two stage valve preamp else where on this bLog. Having the driver stage remote allows the trial of different driver stages while sharing the power stage and the and a common power supply.

The same drive stage as in the Merlot amp was used. This employs a 6N1P Russian tube. Parts of the original schematic were upgraded and Russian military paper, foil and oil caps were used for inter-stage connection. There is no volume pot or input capacitor. Low ESR grade caps were used where possible and pure silver wire used in the signal path. Construction is all component-2-component with no PCB or any other cct. brd. The timber Tea box is bitumanised Al foil lined.

KT88 Bread Board amp with no driver stage.

This is the first bread board amp (said Be Bamp) under the retro-thermionic brand so it had to be a different to other BB amps and stand out. Also it was designed to be used with a high gain preamp because, in this model, there is no driver stage. It has been tested with the retro-thermionic Tea Box two stage valve preamplifier. Also all wiring is hidden under the board to give a clean neat look. To further enhance the look the two open OPTs are housed in a metal enclosure. The enclosure doubles as a mounting point for speaker and input terminals, all gold of course.

The driver stage has been removed to reduce the number of stages the music has to travel though before appearing at the speakers. This has resulted in an increase in transparency and detail. In future releases of the Be Bamp a version with the driver stage will be produced.

Though the wiring is hidden there is plenty of components above board. Also the high voltage (400V) rectifier is exposed so electrocution is possible both here and from other components. For a twist the HT standby switch, which must not by operated under one minute of warm-up time, is covered with a red rocket launcher cover. The board is Australian Pine and has been treated with Australian beeswax.

Main components are Hammond (over specified) power transformer and 10H Hammond, high current, choke. Edcor open OPTs are used. The Be Bamp will work with a number of different valves; 6V6GT, and EL34s being just two. A matched pair of JJ Blue Glass KT88s are shown in the image. The amp will produce about 5W if driven fully.

The retro-thermionic Fostex Back Loaded horns were used to evaluate the Be Bamp. Voice and all acoustic instruments display a very dimensional sound with voice sounding crisp and earthy with very controlled sibilance. The amp is absolutely hum, buzz and hiss free and is the quietest amp I have ever built. With a result of the deepest black back ground well know tracks sound new again and display extended fine grain detail.

On request retro-thermionic will provide this amp as a kit with rectifier module and power valve module assembled and tested. Ever part will be provided for the DIYer to assemble.

KT88 Bread Board amp with no driver stage.

The bread board amp (Be BAmp) is just as attractive from the rear as from the front. The enclosure which houses the OPTs is also used to mount the speaker binding posts and input RCAs which are all gold plated and the RCAs are insulated. The fuse and mains switch are also visible from the rear. Only the power transformer is connected to the mains earth.