A low cost 600 watt ultra-linear amplifier

A low cost 600 watt ultra-linear amplifier for 40, 20, 17, 15, 12, and 10m

This compact little amplifier is the brain-child of Pat Murdoch, ZL1AXB, in Auckland, New Zealand. It is only 11″ wide, 4″ high, and 9″ deep. In most respects it is a typical grounded grid linear amplifier, with the exception of a rather unique tuned cathode input circuit that was developed by Pat over 10 years ago.

To best describe this unique circuit; the 813’s directly heated cathode (filament) becomes an integral part of the resonant cathode tuning circuit by the use of a coaxial coil (L2). The coil is made of 3/16″ copper tubing which is connected directly to one of the tube’s filament terminals, and an insulated #12 wire passing through the interior of the coil’s tubing is connected to the other. The filament voltage is then applied to the “cold” end of the coil, as well as its interior wire, as shown in the schematic drawing below.

The coaxial input coil is resonated in parallel by a 1000 pf variable capacitor (C3), which is mounted above ground and tuned with an insulated shaft. The LC circuit is shunted by a 2.5K ohm resistor to reduce its Q, and tunes from 7 mHz to 30 mHz, thus eliminating the need for an input band-switch. For this reason it is necessary that C3 have off-center rotor plates for ease of tuning the higher frequencies. With the input tap position on L2 set properly, a near-perfect 50 ohm input impedance will be realized on all bands.

Aside from its broad-band input tuning advantage, Pat’s unique input circuit stands apart in two other significant ways: it eliminates virtually all input mismatch distortion, and it increases the efficiency and stability of the 813 dramatically.

The maximum output power of a single 813 would normally be around 300w PEP below 21 mHz. With approximately 50w input, this amplifier produces 600w PEP output nominally, and around 500w PEP output at 29 mHz. Its intermodulation and harmonic distortion products are well below 40 dB down from its maximum power output, and its tendency toward parasitic oscillations is so low that a parasitic plate choke is unnecessary.

To operate the 813 at these output power levels, a sufficient volume of air moving over the tube’s glass envelope is required to dissipate the increased heat. In this case, the tube is mounted horizontally, about ½” above the case bottom and about 1″ from the rear of the case, directly in front of a 3″ exhaust fan that is mounted on the rear panel which draws cooling air around the envelope, from the air holes in the top and bottom of the case (see photo of case below). The air holes in the bottom of the case are located below the rear of the plate coil (L1). A low value 10w resistor can be placed in series with the Fan’s AC supply to slightly reduce the fan speed, thus reduce noise.

A 3kv @ 350ma continuous duty rated power supply is required for the 813 to achieve its maximum output power. A schematic for a suitable HV power supply is shown at the bottom of this page.

L1 consists of two 2″ ID coils in series that are: 8 turns of 3/16″ copper tubing with the windings spaced about 3/32″, followed by 8 turns of #8 copper wire with the windings spaced similarly. Counting from the C1 end, the following taps are placed at: 2 turn for 10m, 2½ turns for 12m, 4 turns for 15m, 5 turns for 17m, 7½ turns for 20m, and the full coil for 40m. If one wants 30m, the tap is at 11 turns.

Since Pat winds his own transformers, the 12v relay winding is integrated in T1 thus a similar transformer may be unavailable. A small 12v transformer suitable for supplying the relay voltage (T2) may be placed below C3’s tuning shaft extension, near the front panel.

The following schematics should provide enough details to construct a similar amplifier. 813 tubes can be found via the internet from around $5.00 used, to up to $30.00 new. 813’s that have gone soft can usually be rejuvenated by applying 10½ volts to the filaments for an hour or two.

It is important to note, this supply is not adequate for the 1kw version of the amplifier. For this, the current rating of T-3 must be increased to 1.5 amps.

Article originally available at http://www.raibeam.co m/ zl1axb / 813_amp.html

22 thoughts on “A low cost 600 watt ultra-linear amplifier”

  1. The web link for this article does not now work,
    but the design certainly does.

  4. Would it be possible to email plans to me as the enlarged images are not clear enough to read.
    Thank you.
    Jim K9JWP

  5. I have the same question as Jim, K9JWP, Do you have any plans available via e-mail? I also am having trouble getting a clear enough image to get the values of the components and designators.

    Thanks

    Milton KE5XA

  6. Hi , Is It Possible To Email Me The Complete Parts List Please , Thanks 73 , George , ” Z S 1 K G B ” Cape Town , South Africa

  9. Hello,
    I’m new to the home brew but I think that this design is one I could do. Would you send me the plans and a parts list along with any recommendations you might have. That’s for sharing your expertise with. The ham community. I look forward to hearing from you!

    “73’s”
    Billy Young
    Kd5csd

  11. Hi, can I use that to amplify all ultrasound wave ( square-sine-triangle etc ..) generated from a function generator and use that for different uses and different frequencies?

    For example also an ultrasonic cleaning machine with 10-25 Ohm piezo tranducer ?

    Pier Paolo

  12. happie419@gmail.com Jim Hansen 4412 Beck Dr. Maumee, Ohio 43537 USA

    Can this be modified for 75 Phone band? I’m new to building amps. This one looks like a good one to start with. I would also like a complete list of parts if that is possible.

  13. All who have problems reading the schematics; Set this website at maximum size and check the schemes again. With Edge I found that they are both legible. With this start a snippet with a free app called Snipping Tool will give you still extra if this by itself is not enough. I wear glasses and use a 3x magnifier.
    You might also have a magnifier app on you browser.

  14. Here is another link to a site using an 813 Tube with a lot of [pictures and the guy even builds his own variable capacitors etc. I’m not sure but it may be the same thing; it couldn’t hurt to take a look. http://www.zl1bjq.co.nz/813saga.html
    If you need anything else I can take a look, retired and nothing to do so to speak. Callsign KN6AOZ (USA)

  16. Simone,
    Thank you for keeping this project accessible on the web. It’s a good tutorial on basic amplifiers using a popular vacuum tube. Best regards, Bill, WS4F

  17. is there pcb board for sale like to make it as i am looking for portable for my yaesu FT817ND

  18. Great idea and concept.
    I would also like to get a detailed readable plan and parts list as ZS1KGB requested.

    Thank you for sharing it with other amateurs.

  19. Would you be able to build a hf 3oow linear amplifier to use on my yaesy ft-891 and like to know how much money it would cost me in Cnadian funds

  21. Hi Simone – I am Interested in constructing the 600watt amplifier by ZL1AXB but I cant see what the mains voltage is, 120vac or 240vac ???? Would you have this information ???? cheers Rob VK4ALL.

  22. In my opinion, the 813 is the best tube ever made, & every circuit it is used in excels.
    I built a 2 tube 813 amp 80 ~ 15 meters, that runs on a 2400 volt supply, puts out 1k to 1.2k pep on high neighborhood power voltage. These tubes run cool, no need for a big hurricane fan, & their made to never wear out, burn out maybe, or broken, otherwise it will keep on like the eveready bunny. Nice work on the construction of that amp. 73’s ars ac3ib

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