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[added Mai, 2005]
- Idea & Design by Wim de Haan and Kees Heuvelman.
Notice. This amplifier design is been published in the so-called Elektuur Audio Special 2006 issue. You might like to get hold of this issue, this to check out the final article for more detailed and refined information on this amplifier design. This magazine is written in Dutch only.
This amplifier design was originally published in AudioXpress USA (Mai 2005). The link to the full article is listed down here.
Original AudioXpress 6336A push-pull tube amp as PDF file [7MB]
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Amplifier Design Goals
- All Tube Design
- All Triode Output Stage
- Full Class A Operation
- No Overall Feedback
- Push-Pull Operation
- High Performance Specs / Measurements
- High Performance Sound
- Approximate 20 Watts into 8 ohm
- Good damping
- Exclusively Design
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Input stage
One of my favourite small signal tubes is the 5687. I have used this tube in several designs with great success. It might be a little dark and less detailed as other tubes, but it is really musical and specs are outstanding. Result is a very dynamic and evolving / bold sound.
In general I use the 5687 made by General Electric (GE), but I used also the Philips/ECG with success. In my applications before I did use a standard Grounded Cathode circuit with standard values (Ra=15K and Rk=680E). Best results I had with the two systems in parallel.
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Though in this new design I wanted to use something different. Some long time ago I did order the Tube Cad Circuit simulation program and did play with it on some occasions and did found a really interesting circuit and many interesting details.
The circuit I am talking about is the so-called Current Sourced Grounded Cathode Amplifier. In this amplifier the plate resistor is replaced with a triode based current source, which results in more gain and better PSRR.
According to Tube Cad exhibits each triode a certain amount of transconductance, which is the degree to which the flow of current through the device changes as a response to a change in grid voltage relative to the cathode. By dividing the idle current by the transconductance, an input overload voltage results (Vth). If this Vth voltage is exceeded in a negatively charged way relative to the cathode voltage, the tube stops conducting any current. This voltage serves as a rough figure of merit for a tube circuit, as the higher the value, usually, the better the sound. Because this Vth is much higher than in my standard Grounded Cathode circuit, I choose for this circuit.
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Phase Splitter
A Long Tail circuit is chosen as phase splitter / driver stage. This very common circuit yields fair gain and fair balance. In this application I choose the 6SN7 in a configuration with a current source. The current source is build around a small transistor with LEDs as voltage reference. A potentiometer is added to adjust the circuit for minimal distortion. An additional potentiometer is used to adjust the symmetry / balance of the Long Tail.
The 5687 input circuit and this 6SN7 based phase splitter yields enough gain to make to overall circuit work without overall feedback. Overall gain / sensitivity is just about 20 dB, which is a good figure.
Ouput Stage
In the search for the perfect output tube the choice was initially more difficult than I thought. Because I wanted to add some personal touch and exclusivity to the design I could not use the EL84 / EL34 / KT88 kind of tubes, also because these are not triodes.
However after listening to a 6336-based DIY amplifier, the choice was made just like that. The 6336A tube is intended for power supply use and contains two separate triode systems. Mu is low as 2 and power consumption is as high as 5 Ampere at 6.3 Volts, plate dissipation is 30 Watts. Plate Resistance is very, very low and for this reason the 6336 tube is used in OTL circuits. Best choice would be the 6528, very similar to the 6336, but Mu is much higher, which would make it much easier to drive.
The 6336A is used in full class A using cathode bias. The 6336A would work perfectly with low primary impedance this to yield more output power, however chosen is for a 8K load for low distortion figures with less output power. Output power is found less important. Because the toriodal output transformers are very sensitive to DC unbalance. Meter M1 and a variable resistor is added to the cathode circuit to solve this problem. Selecting the 6336 for balance would be impossible to do so.
Trying several 6336A tubes I noticed that only the RCA yields low distortion and high power output. Cetron and Raytheon measure worse. So the RCA is the one to choose for.
Optional Driver Stage
The 6336A tubes are a rather difficult load for the Long Tail. Measuring the prototype I encountered a high roll of at 70 kHz. The output transformers specs shows us something else, so the circuit limited the high 130 kHz response of the transformer. Adding the cathode follower circuit measured up to 130 kHz at -3dB, the limitation of the transformer. You could use either a 6SN7 or an ECC82 in this circuit. The final amplifier is build without use of this circuit.
Output Transformer
In some former projects I did use transformers of Hammond and Audio Note, both with good results. Though one of the design goals was High Performance specs, so I did choose for the toriodal output transformer made by Plitron.
I did use the original 'Dutch' version made by Amplimo, the VDV 8020, which is intended for EL84 Push-Pull use. Specs are outstanding, frequency response yields up to 130 kHz.
Component Choice
5687 Tube - Philips / ECG
6SN7 Tube - Rogers
6336A Tube - RCA
Coupling Caps - WIMA FKP
Resistors - Philips Metal Film 1% - 0.6 W
Power resistors - Vishay Hi-Power 25 Watt
Toriodal Output transformer - Amplimo Transformers / Holland
High Voltage Capacitors - ELNA
Specifications
All measurements with 6336A output circuit set at +450V DC and using the 6SN7 cathode driver circuit.
17 Watt into 5 ohm at 1 %
F -3dB = 130 kHz
To be added more soon.
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