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Thursday, May 19, 2011

Single Ended Amplifier Concept, Part 7


This article will show a variation of the transformer coupled single ended amplifier concept. It is based on the 46, which was presented as tube of the month in the previous post.

The amp was built mostly with parts which I had available. The parts cost is moderate, even using a heavy duty power transformer, ASC oil caps and all Lundahl signal transformers and chokes, the parts add up to about 1100-1200 Euros, excluding chassis. It uses all the features of this amplifier concept which contribute to the overall sound quality: Multiple choke filtered power supply, high quality caps, interstage transformer coupling, ultrapath caps in each stage.

I still have some power transformers in stock with center tapped secondary and 3 independent 6.3V windings. To keep cost down, I used this one. Only this single power transformer is needed for B+ heater and filament supplies. The 2.5V AC voltage is obtained through dropping resistors from two of the 6.3V windings. The third 6.3V winding is used to supply the heaters of the rectifiers and the 6N7 driver tubes. This requires the heater winding to be referenced to ground. Two 6AX4s are used in a full wave configuration. Since it can withstand a large voltage between heater and cathode, the heater winding can be shared with the driver tubes. The PSU is common for both channels, only the filament windings of the output tubes are separate:

The B+ supply is the usual choke input PSU using two LC sections for smoothing. Heaters and filaments are fed with AC. Since the heater windings have no center taps, a hum buck potentiometer is wired across the filaments of each of the 46. Two resistors are wired from the wiper to the filament terminals to reduce the resistance of the 100 Ohm Pot which I had available.

The rest of the amplifier circuit is similar as presented in previous parts of this series of posts. my favorite indirectly heated driver for small output tubes, the 6N7 is used again, both halves wired in parallel. The output tube is driven by a Lundahl LL1660/10mA, wired almost 1:1 (actually it is 1.125:1). I had a pair of Lundahl LL1682/50mA in stock. These fit quite well to the 46. With the primary impedance of 5.5kOhm and a secondary of 5 Ohm they will provide a good damping factor when used with a 46 on 8 Ohm speakers. The 46 is wired for Class A use, with the second grid tied to the plate at the socket. See the 46 datasheet for details of the tube.

The B+ is around 300V. The 46 is biased at around 30V. With voltage drops across chokes and output transformer primaries this results in just under 250V across the 46.

Below, some photos of the construction of the amplifier. The first picture shows all capacitors, sockets, connectors, switches and resistors mounted and some initial wiring. The power transformer is placed on the top side of the plate and will be hidden under a cover.

The next photo shows the inside with all chokes, interstage and output transformers mounted and completely wired:

The top view of the completed amplifier:

The amp sounds very smooth and delicate. It shares a lot of qualities with the best 45 amps at a lower cost and size. The same circuit can be used with minor adaptions for 45 or 2A3 output tubes.

Best regards



  1. Beautiful and elegantly simple. I would like to clone this amp. The 60Hy choke is a plate choke I believe. Is this critical or just what you had?

  2. Hi!
    You can use other chokes as well. I like high impedance chokes for good decoupling and there the Lundahl plate chokes are just right.

    Best regards ... Thomas

  3. Gorgeous and simple design! Will a 2V 200ohms source be sufficient to drive this amplifier? I would like to operate without a pre.
    Also what is your opion on passive input transformer based pre-amps like the Electraprint PVA?
    Would a simple stage like this work to increase the source voltage and impedence or is it a no no to add another winding in the signal path of a transformer coupled design?
    Thank You!

  4. Hi!

    This amp requires about 1V at the input for full power out. So you will have some gain reserve with a 2V source. That can be done.

    I have no problem with another transformer at the input but in this case it is not needed

    Best regards


  5. Hello Thomas, I have 45's and I cannot seam to find reasonably priced 46's. Any interest in optimizing your circuit for the 45?
    Connect the opt to pate only? Most parts will be purchased new so are there any values you would recommend tweaking? Are all caps acs oil type? Would you see any advantage in using the Clarity Cap TC series?
    I have some nice 600 vct PT's. Would the voltage be too low for the 45 or can the PS be tweaked?
    What are your Resistor recommendations?
    Sorry for the basic questions and thank you! Really lovely design.

  6. Hi!

    I can offer most of the parts required for this amp. Drop me an email and let me know what you need: thomas -at- vinylsavor -dot- de
    I will then also send a schematic for the 45 tube

    Best regards


  7. Hello, Thomas!
    I have been following your work since Sound Practices days.
    I have built on a breadboard your 6N7/46 amp. It does sound great. I used available Hammond 126C interstage iron, and did a capless parafeed ouput, using a bi-filar Hammond 126B interstage to drive the Electra-Print 10K OPT. At the input I used a LL1530 at 1:3.5 step-up, since I'm using a Slagle AVC. The power supply is choke-input, and uses 394A Thyratron tubes, and ASC oil caps. Audyn Q4 pp caps were used for bypass duty on the signal board. I also found 6Y7G, 6Z7G, 1635 plug in for the 6N7 without troubles. The first 2 have higher gain than the 6N7! Thanks for a great design and inspiration for my best amp so far.

  8. oops, forgot to add that the 47 drops in for the 46 and perhaps sounds even better! I also like the globe tubes, except in a 45, where the ST shape sounds better.