807 parallel SE amplifier

In the summer of 2002 I decided to build a pair of mono block parallel single ended DIY tube amps, using the 807 tube. Actually, that was my third DIY tube amplifier project. It all started with my decision to use the less known tube 807 , a tetrode whose features and sounds I was well aware of, from a specific DIY project (with one tube per channel wired in triode) made by Christos Spathopoulos.

Christos’ tube 807 amp played very smoothly in the whole spectrum (perhaps the best sound I had heard until then ) but, unfortunately, it lacked power.
We discussed my idea with Giannis and Christos Spathopoulos so we focused to build a tube amp with two parallel 807's per channel in a cathode follower connection, wired in tetrode - class A, using negative voltage in screen, based to the characteristics according to RCA and SCT...

My Parallel single ended 807 vacuum tube amplifiers at a glance :

• two 807 tubes / channel (parallel single ended)
• two independent power supply stages per Channel
• full tube rectifiers PSU
• pure class A
• No feedback
• two independent lines for tube 807 biasing
• CNC chassis design
• hand made output transformers P&K audio

807 Tube amplifier building

For nearly two months Christos and I worked designing the amp -- having in mind to solve two major problems. First, it was my idea to build the power supply using tubes as rectifiers, and the second concerned the bias circuit for the final tubes.

Finally, we decided to use two independent lines for powering the system (using GZ34 for the drivers and 5U4G for the power stage). We also built the Bias circuit, based on two SPECTROL 22K micro potentiometers for adjusting the bias (one for each tube ). Each tube 807 is biased at -15.5 volt (grid). Plate voltage is about 505 volts, and screen voltage is 228 Volts. Next step was to construct the chassis. I designed the chassis in auto cad and my friend Gregory (who has a CNC) did the whole work. The whole construction is hard wired with one exception of a small board for the power supply capacitors. All of the most critical path of amplifier have been wired using silver-Teflon cables (especially the high voltage paths).

Find the 807 tube amp schematic in amp schematics pages and the 807 tube data in tube data pages.And the Power Supply Unit schematic.

Driving - pre amp

The use of a NOS pair of RCA 807s tubes improved the clearness a lot instead of the westinghouse 807s I used before. The driving section uses the 12ΑΤ7 (general electric) (one per channel) and the pre-amp is the well known PV10A Conrad Johnson.

I have to mentioned that Christos Spathopoulos triode SE807 amp was a derivative work based on RH807 by Alex Kitic

costas sarris

The tube power supply project

This project was an attempt to build a tube amplifier power supply for my 6550 push pull dual mono amps. I already had a really ugly tube power supply in my rack and in early 2000 I decided to design and build from the scratch a new tube PSU.

The importance of using tube rectifiers for the power supply

As we know solid state diode rectifier circuits, are not very effective for tube amps, because they generate fast impulsion that pass through the circuit. Using a full wave tube rectifier circuit for producing direct voltage, we can prevent hum pulsations, and we can achieve more accurate filtering.

Voltage regulation and Filtering

The output voltage of a tube power supply always decreases as more current is drawn. This happens not only because of increased voltage drops in the transformer and filter chokes, but also because the output voltage tends to soar the peak value of the transformer voltage as a result of first capacitor charging. Proper filter design is the key for eliminate the soaring effect.
Voltage regulation is expressed as a percentage between the change in output voltage with load.
Percent regulation = 100*(E1-E2)/E2
where E1= no load voltage value and E2= full load voltage value.

Power supply Filtering

There are two main power supply filter classifications : Choke input filters and capacitor input filters.
Capacitor input filters are useful with solid state diode rectifiers.
Choke input filters permits better utilization of tube rectifiers, because higher load current can be drawn without exceeding the peak current rating so we can achieve better voltage regulation results.

Choke inductance

A choke input filter usually act as a capacitor filter unless the input choke has at least a certain minimum value of inductance called critical value. For full wave 60Hz tube supplies this value is given by
L(Henrys) = E (volts) / I (mA)

E=supply output voltage
I=the current drawn through the filter

this is the tube power supply schematic:

costas

Push Pull 6550 DIY audio tube amps

The idea of building a DIY audio vacuum tube amplifier based on high-quality materials begins in the early 90's. Initially had to answer the question "Classic Push Pull or a Single ended amp ?" So starting the design somewhere in 1994 I decided to design οn a blank piece of paper, a pair of monoblocks push pull tube amps with a power supply in a separate chassis which would not use solid materials but rectifier tubes.

In the first photo appears the two channels of my tube amps DIY design which consists of 2 mono blocks with a tube power supply in a separate chassis.

The construction began in 1995 and completed in 1997.It's based on the draft circuit layout dynaco st70 but with several variations.These concern both the use of materials and in topology, because from the start I decided that the construction be implemented in three different units, two for the power amplifier , and one for the tube Power supply unit.
The whole project is housed in three P-shaped chassis which have been coated by 10mm thick rose wood attached to an internal "nerve".
The chassis is aluminum, thickness of 2mm and screwed on the side walls of the wooden coating with stainless screws.

In Power supply unit, because of the transformer weigh used to transverse aluminum are veins to avoid possible damage of the chassis.

The design is simple (Push-Pull) uses the 6550 power output tube, while the phase splitter and the driver stage uses an EF86, and an ECC83 per channel respectively.
You can find the 6550 Svetlana tube data in tube data pages.

Tube amp schematic
This is the final schematic diagram

and the PSU

costas sarris

How I tweaked my full range Back Loaded horn speakers

Almost 4 years after the construction of my Back Loaded Horn speakers, I decided tweaked them adding tractrix horns using compression drivers for the high frequencies.
Searching the web and asking friends who had already experimented with such drivers, I decided to implement a structure based on a tractrix horn using an 1" (inch) High Frequency compression driver.
For this purpose I chose a cheap but very good European driver the BMS 4524. Another reason why I chose the particular driver was the low cost which is around 80 euro / pair , so I can comfortably experiment without wasting a lot of money...

According to the manufacturer (BMS) the 4524 driver achieves a linear frequency response up to 20 kHz, featuring 113 dB sensitivity 1 W / 1 m.
The specific drivers were placed in a pair of horns ( tractrix ).
I got the tractrix horns from Rainer Flock, model T 1040. The horns are made from solid maple. The diameter of the neck is 1 inch while the diameter of the mouth is 10.4 cm.

More info about the tractrix could be found at Rainer's web site.

Next step was the construction of the crossover. The crossover point is at 2600 Hz (1st order butterworth design). A 1st order crossover filters 6 db / octave and has good behaviour in transients. ( c1=7.5 uF L1=0.5 mH )
While not expecting the BMS driver is a diamond. The sound in my living room has changed completely. High frequencies became more clear, sweetest and most relief, while mid frequencies sound more plausible.
It is amazing that a budget less than 180 € You can upgrade a pair of BLH and listen even more quality sound.

costas sarris

acknowledgements :

• Rainer Flo for his effort

The 6L6 simple SE

Even though 6L6 tube is used primary in guitar vacuum tube amplifiers, many manufacturers use this tube in audiophile designs. I decided to build this small single ended amplifier based on 6L6 vacuum tube using a classic three stage single ended design.
The plate voltage is about 320 Volts and the output stage is a classic single-ended cathode follower. Output transformer is 4500 ohms and is set for 8 ohm speakers. We chose this design for one reason : Wiring this tube as tetrode we achieve the greater possible amplification as a result of higher grid transconductance.
Click to the picture below to see more photos and the Schematic.

• one 6L6 tube / channel
• simple construction
• excellent damping factor (Z)output
• pure class A
• 320 Volt plate voltage

This project is a DIY single ended amplifier designed by Christos Spathopoulos

Data :
L3(Mains Transformer in this project) : 150-0-150V / 170mA
Choke : 7H/150mA
Output Transformer 4500ohms primary

This is the final modified schematic (one channel)

Fostex FE206 DIY horn loudspeakers

In February 2006 after I had completed the construction of my dual mono PSE 807, I decided to build a pair of horn speakers using a full range driver in a back loaded horn design.

These speakers could easily be driven by the 807 single ended & 811A tube amps, I had designed.

I choose a design by FOSTEX with the full range unit driver FE206E which has been designed (according to the manufacturer) for use in back loaded horn type enclosures.

Although bass reflex projects have been presented by some manufacturers this driver is generally unsuitable for bass reflex enclosures because of its over dumping characteristics.

Building up the Horn speakers
In this project I build a ‘back loaded horn’ type enclosure following the exact designing instructions from FOSTEX, and I used the FE206E unit.
In this specific DIY horn speakers project I chose for building up the speaker cabinets a type of Finnish birch plywood 21mm thick (15 plies of hardwood veneered plywood layers) for the main section and side panels, which has a sound reduction index more than 23.8 dB according to the manufacturer.
Also I used Dynamat for extra sound absorbency; we coated the place behind each driver unit. The total weight of each speaker is more than 55 kilograms!
The result, however, is a well damped and non resonating enclosure.

Plywood is made by slicing wood into thin veneers and gluing the veneers together to form a sheet.
Its high strength, stiffness and surface dimensional stability make it ideal for making speaker cabinets.
Its cross-laminated construction also, ensures that plywood sheet sizes remain relatively stable under changes of temperature and moisture, making it particularly suited to form work applications like loudspeakers cabinets.
Generally though its construction is difficult and requires very good work (woodworking) the result will compensate you!
Read a loudspeaker review, and watch a video of my DIY loudspeakers here

costas sarris

These are some detailed photos from my DIY speakers :

A Decware EL84 Zen Amp clone

Making a low power single ended amplifier has always been in my designing plans. Obviously such a construction requires the appropriate speakers, particularly horn type designs, so that the output of the amp can reach an efficiency satisfying level.
Once at a certain point, I came across the designs of the Decware SE EL84 Zen amp, which had quite good reviews among the tube amps DIY and Single Ended tube amp builders.

This simple and clever design was exactly what i had in mind. The most convenient way to develop my idea was to use an already existing design.

Tube amp diy

Chassis are made of wood, and i build the whole construction in an aluminum plate (3mm thick). All parts are mounted on the aluminum plate. Finally i coated the aluminum plate with plexiglass (acrylic sheet colored dark gold)
I ordered the output transformers in a local workshop, (Greece / Thessaloniki). All tube sockets are porcelain, and all the critical paths wired using silver-teflon cables. I also used metal film resistors, and WIMA capacitors.

The whole circuit is very simple. It is a common cathode design with the 6922 tube driving the power stage with el84. I changed only the two cathode resistors (150Ω /8 watt) with heavy duty type for heating resistance. In the Power supply I used the 5U tube for rectifier according to Decware manual.
You can find EL84 and svetlana SV84 tube data in tube data pages.

How it sounds this tube amp ?

(back in 2002...)
This simple EL84 tube amp meet his first "crash test" when we put it in the same rack with the GRAAF GM20. We connected it with George's Valmas Coherence II Jeff Rowland pre amp, and we used George's Amati Homage loudspeakers.
Clear voices incredible solo's unbelievable mid and high - frequencies ! absolute separation and purity not expected! But .... we needed POWER !

If you have horn loudspeakers in your living room then its time to build this EL84 tube amp... "monster" because ... sound does matters !

costas sarris

acknowledgements :

• Decware Zen EL84 Amp
• George Valmas for his effort

Giannis Spathopoulos made this one-off treasure audio power amp. This prototype model featuring four JJ EL34 power output pentodes per channel in an ultra linear push pull configuration. With an ECC83 as a pre stage, an ECC85 as a phase splitter, and a 6FQ7 as a buffer, this extremely dynamic and sweet sounding design is rated at 30 Watt in a class A configuration, and 70W in a class AB1 configuration.

The power amp video :

Beautiful construction underneath the new EL34 audio power amp. All hand crafted, uses point to point wiring . OPTs are hand made special ordered for the specific design.

For more info visit Giannis Spathopoulos web page

SET direct heated triode tube power amp

Direct heated triode power output tubes are really interesting. Since September 2008 I've been working in a new project regarding a single ended triode tube power amp using the 811A vacuum tube. Actually it is a project with a long process of experimentation.

The 811A vacuum tube it is a power triode capable to output 10 - 15 Watts in a single ended class A2 configuration, in a three stages circuit using interstage transformer in the driving stage.

It's all about a double project. At first I built a prototype amp using the SV811-3 tube. Afterwards I built another one using the 811A tube.

watch the video : Testing the prototype audio amp SV811-3

Nobu Shishido was one of the first engineers who introduced a design like this. His designs are showcased by the well known WAVAC Audio Labs.My project is based on the Shishido's 811A design as well, but with several changes in his circuit.

Input stage and driver stage

In Shishido's design the input stage consists of an ECC82/12AU7 dual triode connected as a series cascode mu-follower circuit . The signal from the mu-follower stage is directly coupled to the grid of a 6F6 power tube connected in triode , with no use of a coupling capacitor in the signal path.

Instead of using a cascode circuit, I decided to use the EF86 tube ( triode strapped ) for the input stage of the amplifier to provide high-gain voltage amplification. The credit for this choice ( EF86 ) goes to Mr Giannis Spathopoulos. The input stage is coupled directly to the grid of a 6L6GC tube with no coupling capacitor in the signal path. The purpose of this is to minimize low-frequency phase shift in the amplifier and to improve the low-frequency stability when negative feedback is applied.

For the SV811-3 typical operation ( for plate Voltage = 450 to 500 Volts) is A2. Class A2 means that the grid is driven more positive than the cathode for a part or all of the waveform. For the first 5 Watts, operation is A1 and from 5 watts to the max power A2. Grid voltage is about 40 Volts at max power, and this is because grid will draw current from the cathode and heat up. Class A2 also requires a beam power tube in the driver stage , that can supply enough power to the grid.

Driver stage consists of a 6L6GC tube connected in triode running at 260 Volts. The interstage transformer is a C core type transformer, 1:1 5K primary impedance ( equivalent to TANGO NC-20F ). The output stage ( 811A tube ) operates in class A2. Plate voltage is 450 Volts and plate current 75mA when no signal applied.

The new 811A DHT single ended amp :

The SV811-3 prototype :

The Interstage transformer coupling method :

There are three common coupling methods :
a. Capacitor coupling
b. Direct coupling and
c. Interstage transformer coupling.

For the first option, coupling capacitor works allowing only AC signal (audio frequency ) to pass. The major problem is the quality of the coupling capacitor and its characteristic contributing to the audio signal (different sonic and tonal characteristics of the capacitor - even thought using the same brand - of capacitors ).

On the other hand the direct coupling designs allow both DC and AC signals pass to the grid of next gain stage. This seems to be good but the subsonic oscillations (hum and RF) sometimes pass through the final gain stage producing noise and a kind of blear and harsh sound.Another problem is that you have to work with exactly the same parameters in both channels ( matched pairs of driving and output tubes , accurate and exactly the same bias adjustment etc)

In my opinion the use of an interstage transformer is the best coupling method.
This is because the primary and secondary winding of the IT working almost the same. This leads to no loss of the magnetic energy converted into audio signal. Even thought it is the most expensive coupling method the use of an IT is the best coupling mechanism for tube audio amps.
The use of a quality IT is the secret key for high quality sound in Direct Heated Triode designs. ( e.g. WAVAC )

Underneath the prototype amp :

Underneath the new 811A amp :

Output and IT transformers

Except the power transformer , the IT and the OPT (output transformers) ( in all my projects ) are hand made designs made in a laboratory in Beograd (Serbia). Both OPT and IT are special orders according my needs and follow exactly the tube characteristics for each design. They are more expensive than ready made transformers but I know the quality of materials used is excellent.

Negative feedback

Currently No feedback is applied in both projects.

Negative feedback is taken from the secondary winding of the output transformer to the cathode circuit of the input stage (EF86). At the moment there is no feedback applied in the prototype amp. I think the amount of the feedback, could be 5-7dB in order to increase the damping factor, but it seems it is not necessary to apply feedback since we use sensitive speakers .

Power supply

The prototype amp uses an E-shape power transformer covered by the black box at the back. The metal cylinders with bolts are the capacitors ( 2 X 22000uF/16V /channel ) providing filtering for the filament DC circuit. Note that 811A cathode and filaments are together so we need a very careful design to avoiding hum .
In the new amp I m using a toroidal power transformer ( better quality than the E-shape transformer ) and bigger capacitors ( 2 X 33000uF/16V / channel ) for the filaments

The chassis for the 811A valve amplifier

New chassis are constructed from stainless steel 1.5mm thick. The cutting process was done by laser cutting machine. The chassis designed on the computer, and a fellow who has a laser cutting machine did the cutting and bending work.
Laser cutting is ideal because the results are so precise - edges are burr free with minimal gaps and also the rigidity of the base material (stainless steel sheet ) has no impact of the cutting process.
Both sides (left and right) coated by 10mm thick black acrylic.

The new chassis ( for the 811A ) :

The 811A project is already finished. I sent the 811A monoblocks to my friend Scott in Canada.

Here are some photos taken while assembling the 811A amp:

costas sarris

The draft schematic of my 811A single ended tube power amp ( positive grid bias ) class A2:

The PSU

The modified schematic using the Svetlana SV811-3 (negative grid bias ) class A2

The hand made interstage transformers

The prototype amp ( with the SV811-3 ):

2A3 Tube amp: "a tribute to simplicity"

I do not know why but I prefer simple, low power triode designs. May be it's the warm sound produced by these tubes or because I like horn speakers and I know that this relation is erotic. It's a kind of magic, a sweet sound triggers up your feelings, that can reproduced only from a combination of a DHT tube amp and a pair of horn speakers.
So last year I decided to construct a low power SET based on 2A3 tube. I had no previous experience on this specific tube, and this is something that excites me.
While I was looking for the simplest SET design of a top notch Valve amplifier that is based on a 2A3 DHT (Direct Heated Triode ) vacuum power tube, I came upon the "Loftin-White" circuit.
The circuit
This circuit has its roots back on 1929, when published for the first time in the American magazine Radio News, by E. H. Loftin and S. Y. White.
The main characteristic of this amplifier, is the use of a direct coupled circuit.
That means there is no capacitor or coupling transformer between driver and power stages.
For the history the prototype "Loftin-White" amplifier used the "224" tetrode in the driver stage and the "250" triode in the power (output) stage.
The direct coupling circuit It seems easy to implement but It is not. These circuits allows DC and AC signals passing to the grid of the power gain stage, because there is no coupling capacitor. That's the good point affects to sound quality. From the other hand, sometimes the existence of some oscillations (hum and RF) that pass to the final gain stage, affects the sound of the amp, and produces noise or a kind of blear and harsh sound.
To eliminate these problems I had to work with exactly the same parameters in both channels ( matched pairs of driving and output tubes , accurate and exactly the same adjustments, short cabling, etc) and top quality materials only.
Applying this method the final result is amazing. The amplifier reveals an amazing sound stage with warm sound and enjoyable harmonics.
In this specific design the 2A3 power output tube, running at 60mA plate current. Plate voltage is 408V. The driver stage uses the high gain 12AX7/ECC83 dual triode. Input impedance is 100K ohms.
I chose a matched pair of Sovteks 2A3 power tubes and a matched pair of Electro-Harmonix 12AX7.
If you try to build this amp, It is also recommended the "Emission Labs" EML 2A3 (matched pair). All tube sockets are porcelain high quality.
The rectifier bridge circuit in the power supply section uses the GZ34 / 5AR4 full wave rectifier.
The rectifier circuit uses a single section "pi" filter. Combined with JJ or Mundorf capacitors obtained excellent listening results.
The OPT’s
The most critical component in the sound path is the output transformer. Therefore there is no compromise in the quality of the output transformers used in all my amps. For this amplifier I chose, the “ETUDE 1” series, a high quality single ended output transformer, by Monolith Magnetics.
The heart of any Monolith transformer is a high grade grain oriented SiFe dual C-core. The resonance free frequency range is typically better than 7Hz - 75 kHz (-3 dB).
Because of their excellent quality, the result is fast transients, live voices, and exceptional low, mid and high frequencies.
Finishing & Construction
The wood casing (could be oak or cherry) is individually crafted by hand.
Underneath the CNC machined chassis all weldings are made one by one by hand. (point to point wiring)

For inquiries and more Information please contact me via the contact page HERE
Features

An interesting video about the 2A3 tube amp :

A vacuum tube guitar amp it is a "must have amp" for every guitarist. The circuit in this amp is a combination based on the Fender "champ amp" output stage and the ax84 Hi-octane project. I chose the 6L6 beam power tetrode tube to implement the power output circuit.
In this amp there are two preamp stages precede the power amp stage. Both preamp circuit sections uses the high gain 12AX7/ECC83 dual triode.

The amp is under construction so Stay tuned. Until the next part enjoy some music !

The tube power supply project

This project was an attempt to build a tube amplifier power supply for my 6550 push pull dual mono amps. I already had a really ugly tube power supply in my rack and in early 2000 I decided to design and build from the scratch a new tube PSU.

The importance of using tube rectifiers for the power supply

As we know solid state diode rectifier circuits, are not very effective for tube amps, because they generate fast impulsion that pass through the circuit. Using a full wave tube rectifier circuit for producing direct voltage, we can prevent hum pulsations, and we can achieve more accurate filtering.

Voltage regulation and Filtering

The output voltage of a tube power supply always decreases as more current is drawn. This happens not only because of increased voltage drops in the transformer and filter chokes, but also because the output voltage tends to soar the peak value of the transformer voltage as a result of first capacitor charging. Proper filter design is the key for eliminate the soaring effect.
Voltage regulation is expressed as a percentage between the change in output voltage with load.
Percent regulation = 100*(E1-E2)/E2
where E1= no load voltage value and E2= full load voltage value.

Power supply Filtering

There are two main power supply filter classifications : Choke input filters and capacitor input filters.
Capacitor input filters are useful with solid state diode rectifiers.
Choke input filters permits better utilization of tube rectifiers, because higher load current can be drawn without exceeding the peak current rating so we can achieve better voltage regulation results.

Choke inductance

A choke input filter usually act as a capacitor filter unless the input choke has at least a certain minimum value of inductance called critical value. For full wave 60Hz tube supplies this value is given by
L(Henrys) = E (volts) / I (mA)

E=supply output voltage
I=the current drawn through the filter

this is the tube power supply schematic:

costas

Push Pull 6550 DIY audio tube amps

The idea of building a DIY audio vacuum tube amplifier based on high-quality materials begins in the early 90's. Initially had to answer the question "Classic Push Pull or a Single ended amp ?" So starting the design somewhere in 1994 I decided to design οn a blank piece of paper, a pair of monoblocks push pull tube amps with a power supply in a separate chassis which would not use solid materials but rectifier tubes.

In the first photo appears the two channels of my tube amps DIY design which consists of 2 mono blocks with a tube power supply in a separate chassis.

The construction began in 1995 and completed in 1997.It's based on the draft circuit layout dynaco st70 but with several variations.These concern both the use of materials and in topology, because from the start I decided that the construction be implemented in three different units, two for the power amplifier , and one for the tube Power supply unit.
The whole project is housed in three P-shaped chassis which have been coated by 10mm thick rose wood attached to an internal "nerve".
The chassis is aluminum, thickness of 2mm and screwed on the side walls of the wooden coating with stainless screws.

In Power supply unit, because of the transformer weigh used to transverse aluminum are veins to avoid possible damage of the chassis.

The design is simple (Push-Pull) uses the 6550 power output tube, while the phase splitter and the driver stage uses an EF86, and an ECC83 per channel respectively.
You can find the 6550 Svetlana tube data in tube data pages.

Tube amp schematic
This is the final schematic diagram

and the PSU

costas sarris

How I tweaked my full range Back Loaded horn speakers

Almost 4 years after the construction of my Back Loaded Horn speakers, I decided tweaked them adding tractrix horns using compression drivers for the high frequencies.
Searching the web and asking friends who had already experimented with such drivers, I decided to implement a structure based on a tractrix horn using an 1" (inch) High Frequency compression driver.
For this purpose I chose a cheap but very good European driver the BMS 4524. Another reason why I chose the particular driver was the low cost which is around 80 euro / pair , so I can comfortably experiment without wasting a lot of money...

According to the manufacturer (BMS) the 4524 driver achieves a linear frequency response up to 20 kHz, featuring 113 dB sensitivity 1 W / 1 m.
The specific drivers were placed in a pair of horns ( tractrix ).
I got the tractrix horns from Rainer Flock, model T 1040. The horns are made from solid maple. The diameter of the neck is 1 inch while the diameter of the mouth is 10.4 cm.

More info about the tractrix could be found at Rainer's web site.

Next step was the construction of the crossover. The crossover point is at 2600 Hz (1st order butterworth design). A 1st order crossover filters 6 db / octave and has good behaviour in transients. ( c1=7.5 uF L1=0.5 mH )
While not expecting the BMS driver is a diamond. The sound in my living room has changed completely. High frequencies became more clear, sweetest and most relief, while mid frequencies sound more plausible.
It is amazing that a budget less than 180 € You can upgrade a pair of BLH and listen even more quality sound.

costas sarris

acknowledgements :

• Rainer Flo for his effort

The 6L6 simple SE

Even though 6L6 tube is used primary in guitar vacuum tube amplifiers, many manufacturers use this tube in audiophile designs. I decided to build this small single ended amplifier based on 6L6 vacuum tube using a classic three stage single ended design.
The plate voltage is about 320 Volts and the output stage is a classic single-ended cathode follower. Output transformer is 4500 ohms and is set for 8 ohm speakers. We chose this design for one reason : Wiring this tube as tetrode we achieve the greater possible amplification as a result of higher grid transconductance.
Click to the picture below to see more photos and the Schematic.

• one 6L6 tube / channel
• simple construction
• excellent damping factor (Z)output
• pure class A
• 320 Volt plate voltage

This project is a DIY single ended amplifier designed by Christos Spathopoulos

Data :
L3(Mains Transformer in this project) : 150-0-150V / 170mA
Choke : 7H/150mA
Output Transformer 4500ohms primary

This is the final modified schematic (one channel)

Fostex FE206 DIY horn loudspeakers

In February 2006 after I had completed the construction of my dual mono PSE 807, I decided to build a pair of horn speakers using a full range driver in a back loaded horn design.

These speakers could easily be driven by the 807 single ended & 811A tube amps, I had designed.

I choose a design by FOSTEX with the full range unit driver FE206E which has been designed (according to the manufacturer) for use in back loaded horn type enclosures.

Although bass reflex projects have been presented by some manufacturers this driver is generally unsuitable for bass reflex enclosures because of its over dumping characteristics.

Building up the Horn speakers
In this project I build a ‘back loaded horn’ type enclosure following the exact designing instructions from FOSTEX, and I used the FE206E unit.
In this specific DIY horn speakers project I chose for building up the speaker cabinets a type of Finnish birch plywood 21mm thick (15 plies of hardwood veneered plywood layers) for the main section and side panels, which has a sound reduction index more than 23.8 dB according to the manufacturer.
Also I used Dynamat for extra sound absorbency; we coated the place behind each driver unit. The total weight of each speaker is more than 55 kilograms!
The result, however, is a well damped and non resonating enclosure.

Plywood is made by slicing wood into thin veneers and gluing the veneers together to form a sheet.
Its high strength, stiffness and surface dimensional stability make it ideal for making speaker cabinets.
Its cross-laminated construction also, ensures that plywood sheet sizes remain relatively stable under changes of temperature and moisture, making it particularly suited to form work applications like loudspeakers cabinets.
Generally though its construction is difficult and requires very good work (woodworking) the result will compensate you!
Read a loudspeaker review, and watch a video of my DIY loudspeakers here

costas sarris

These are some detailed photos from my DIY speakers :

A Decware EL84 Zen Amp clone

Making a low power single ended amplifier has always been in my designing plans. Obviously such a construction requires the appropriate speakers, particularly horn type designs, so that the output of the amp can reach an efficiency satisfying level.
Once at a certain point, I came across the designs of the Decware SE EL84 Zen amp, which had quite good reviews among the tube amps DIY and Single Ended tube amp builders.

This simple and clever design was exactly what i had in mind. The most convenient way to develop my idea was to use an already existing design.

Tube amp diy

Chassis are made of wood, and i build the whole construction in an aluminum plate (3mm thick). All parts are mounted on the aluminum plate. Finally i coated the aluminum plate with plexiglass (acrylic sheet colored dark gold)
I ordered the output transformers in a local workshop, (Greece / Thessaloniki). All tube sockets are porcelain, and all the critical paths wired using silver-teflon cables. I also used metal film resistors, and WIMA capacitors.

The whole circuit is very simple. It is a common cathode design with the 6922 tube driving the power stage with el84. I changed only the two cathode resistors (150Ω /8 watt) with heavy duty type for heating resistance. In the Power supply I used the 5U tube for rectifier according to Decware manual.
You can find EL84 and svetlana SV84 tube data in tube data pages.

How it sounds this tube amp ?

(back in 2002...)
This simple EL84 tube amp meet his first "crash test" when we put it in the same rack with the GRAAF GM20. We connected it with George's Valmas Coherence II Jeff Rowland pre amp, and we used George's Amati Homage loudspeakers.
Clear voices incredible solo's unbelievable mid and high - frequencies ! absolute separation and purity not expected! But .... we needed POWER !

If you have horn loudspeakers in your living room then its time to build this EL84 tube amp... "monster" because ... sound does matters !

costas sarris

acknowledgements :

• Decware Zen EL84 Amp
• George Valmas for his effort

Giannis Spathopoulos made this one-off treasure audio power amp. This prototype model featuring four JJ EL34 power output pentodes per channel in an ultra linear push pull configuration. With an ECC83 as a pre stage, an ECC85 as a phase splitter, and a 6FQ7 as a buffer, this extremely dynamic and sweet sounding design is rated at 30 Watt in a class A configuration, and 70W in a class AB1 configuration.

The power amp video :

Beautiful construction underneath the new EL34 audio power amp. All hand crafted, uses point to point wiring . OPTs are hand made special ordered for the specific design.

For more info visit Giannis Spathopoulos web page

SET direct heated triode tube power amp

Direct heated triode power output tubes are really interesting. Since September 2008 I've been working in a new project regarding a single ended triode tube power amp using the 811A vacuum tube. Actually it is a project with a long process of experimentation.

The 811A vacuum tube it is a power triode capable to output 10 - 15 Watts in a single ended class A2 configuration, in a three stages circuit using interstage transformer in the driving stage.

It's all about a double project. At first I built a prototype amp using the SV811-3 tube. Afterwards I built another one using the 811A tube.

watch the video : Testing the prototype audio amp SV811-3

Nobu Shishido was one of the first engineers who introduced a design like this. His designs are showcased by the well known WAVAC Audio Labs.My project is based on the Shishido's 811A design as well, but with several changes in his circuit.

Input stage and driver stage

In Shishido's design the input stage consists of an ECC82/12AU7 dual triode connected as a series cascode mu-follower circuit . The signal from the mu-follower stage is directly coupled to the grid of a 6F6 power tube connected in triode , with no use of a coupling capacitor in the signal path.

Instead of using a cascode circuit, I decided to use the EF86 tube ( triode strapped ) for the input stage of the amplifier to provide high-gain voltage amplification. The credit for this choice ( EF86 ) goes to Mr Giannis Spathopoulos. The input stage is coupled directly to the grid of a 6L6GC tube with no coupling capacitor in the signal path. The purpose of this is to minimize low-frequency phase shift in the amplifier and to improve the low-frequency stability when negative feedback is applied.

For the SV811-3 typical operation ( for plate Voltage = 450 to 500 Volts) is A2. Class A2 means that the grid is driven more positive than the cathode for a part or all of the waveform. For the first 5 Watts, operation is A1 and from 5 watts to the max power A2. Grid voltage is about 40 Volts at max power, and this is because grid will draw current from the cathode and heat up. Class A2 also requires a beam power tube in the driver stage , that can supply enough power to the grid.

Driver stage consists of a 6L6GC tube connected in triode running at 260 Volts. The interstage transformer is a C core type transformer, 1:1 5K primary impedance ( equivalent to TANGO NC-20F ). The output stage ( 811A tube ) operates in class A2. Plate voltage is 450 Volts and plate current 75mA when no signal applied.

The new 811A DHT single ended amp :

The SV811-3 prototype :

The Interstage transformer coupling method :

There are three common coupling methods :
a. Capacitor coupling
b. Direct coupling and
c. Interstage transformer coupling.

For the first option, coupling capacitor works allowing only AC signal (audio frequency ) to pass. The major problem is the quality of the coupling capacitor and its characteristic contributing to the audio signal (different sonic and tonal characteristics of the capacitor - even thought using the same brand - of capacitors ).

On the other hand the direct coupling designs allow both DC and AC signals pass to the grid of next gain stage. This seems to be good but the subsonic oscillations (hum and RF) sometimes pass through the final gain stage producing noise and a kind of blear and harsh sound.Another problem is that you have to work with exactly the same parameters in both channels ( matched pairs of driving and output tubes , accurate and exactly the same bias adjustment etc)

In my opinion the use of an interstage transformer is the best coupling method.
This is because the primary and secondary winding of the IT working almost the same. This leads to no loss of the magnetic energy converted into audio signal. Even thought it is the most expensive coupling method the use of an IT is the best coupling mechanism for tube audio amps.
The use of a quality IT is the secret key for high quality sound in Direct Heated Triode designs. ( e.g. WAVAC )

Underneath the prototype amp :

Underneath the new 811A amp :

Output and IT transformers

Except the power transformer , the IT and the OPT (output transformers) ( in all my projects ) are hand made designs made in a laboratory in Beograd (Serbia). Both OPT and IT are special orders according my needs and follow exactly the tube characteristics for each design. They are more expensive than ready made transformers but I know the quality of materials used is excellent.

Negative feedback

Currently No feedback is applied in both projects.

Negative feedback is taken from the secondary winding of the output transformer to the cathode circuit of the input stage (EF86). At the moment there is no feedback applied in the prototype amp. I think the amount of the feedback, could be 5-7dB in order to increase the damping factor, but it seems it is not necessary to apply feedback since we use sensitive speakers .

Power supply

The prototype amp uses an E-shape power transformer covered by the black box at the back. The metal cylinders with bolts are the capacitors ( 2 X 22000uF/16V /channel ) providing filtering for the filament DC circuit. Note that 811A cathode and filaments are together so we need a very careful design to avoiding hum .
In the new amp I m using a toroidal power transformer ( better quality than the E-shape transformer ) and bigger capacitors ( 2 X 33000uF/16V / channel ) for the filaments

The chassis for the 811A valve amplifier

New chassis are constructed from stainless steel 1.5mm thick. The cutting process was done by laser cutting machine. The chassis designed on the computer, and a fellow who has a laser cutting machine did the cutting and bending work.
Laser cutting is ideal because the results are so precise - edges are burr free with minimal gaps and also the rigidity of the base material (stainless steel sheet ) has no impact of the cutting process.
Both sides (left and right) coated by 10mm thick black acrylic.

The new chassis ( for the 811A ) :

The 811A project is already finished. I sent the 811A monoblocks to my friend Scott in Canada.

Here are some photos taken while assembling the 811A amp:

costas sarris

The draft schematic of my 811A single ended tube power amp ( positive grid bias ) class A2:

The PSU

The modified schematic using the Svetlana SV811-3 (negative grid bias ) class A2

The hand made interstage transformers

The prototype amp ( with the SV811-3 ):

2A3 Tube amp: "a tribute to simplicity"

I do not know why but I prefer simple, low power triode designs. May be it's the warm sound produced by these tubes or because I like horn speakers and I know that this relation is erotic. It's a kind of magic, a sweet sound triggers up your feelings, that can reproduced only from a combination of a DHT tube amp and a pair of horn speakers.
So last year I decided to construct a low power SET based on 2A3 tube. I had no previous experience on this specific tube, and this is something that excites me.
While I was looking for the simplest SET design of a top notch Valve amplifier that is based on a 2A3 DHT (Direct Heated Triode ) vacuum power tube, I came upon the "Loftin-White" circuit.
The circuit
This circuit has its roots back on 1929, when published for the first time in the American magazine Radio News, by E. H. Loftin and S. Y. White.
The main characteristic of this amplifier, is the use of a direct coupled circuit.
That means there is no capacitor or coupling transformer between driver and power stages.
For the history the prototype "Loftin-White" amplifier used the "224" tetrode in the driver stage and the "250" triode in the power (output) stage.
The direct coupling circuit It seems easy to implement but It is not. These circuits allows DC and AC signals passing to the grid of the power gain stage, because there is no coupling capacitor. That's the good point affects to sound quality. From the other hand, sometimes the existence of some oscillations (hum and RF) that pass to the final gain stage, affects the sound of the amp, and produces noise or a kind of blear and harsh sound.
To eliminate these problems I had to work with exactly the same parameters in both channels ( matched pairs of driving and output tubes , accurate and exactly the same adjustments, short cabling, etc) and top quality materials only.
Applying this method the final result is amazing. The amplifier reveals an amazing sound stage with warm sound and enjoyable harmonics.
In this specific design the 2A3 power output tube, running at 60mA plate current. Plate voltage is 408V. The driver stage uses the high gain 12AX7/ECC83 dual triode. Input impedance is 100K ohms.
I chose a matched pair of Sovteks 2A3 power tubes and a matched pair of Electro-Harmonix 12AX7.
If you try to build this amp, It is also recommended the "Emission Labs" EML 2A3 (matched pair). All tube sockets are porcelain high quality.
The rectifier bridge circuit in the power supply section uses the GZ34 / 5AR4 full wave rectifier.
The rectifier circuit uses a single section "pi" filter. Combined with JJ or Mundorf capacitors obtained excellent listening results.
The OPT’s
The most critical component in the sound path is the output transformer. Therefore there is no compromise in the quality of the output transformers used in all my amps. For this amplifier I chose, the “ETUDE 1” series, a high quality single ended output transformer, by Monolith Magnetics.
The heart of any Monolith transformer is a high grade grain oriented SiFe dual C-core. The resonance free frequency range is typically better than 7Hz - 75 kHz (-3 dB).
Because of their excellent quality, the result is fast transients, live voices, and exceptional low, mid and high frequencies.
Finishing & Construction
The wood casing (could be oak or cherry) is individually crafted by hand.
Underneath the CNC machined chassis all weldings are made one by one by hand. (point to point wiring)

Start your build with just a € 500 deposit using the PayPal
For inquiries and more Information please contact me via the contact page HERE
Technical data

Features

An interesting video about the 2A3 tube amp :