Eimac® 3-500Z Tube Linear Amplifier


General Specs:
Class: AB2
Plate voltage: 3.2-4kV
Max Power: 1900W PEP
Bands: 160, 80, 40, 20, 15, 10m
Input-tuned network

The homebrew linear is finished and is in testing.

Enlarged photo

In test

Tank in test

New shunt circuit for Grid Current I kept losing the shunt circuit for grid current, and the diodes were failing. I made a board with four 1N5408 in series opposed in parallel to the pair of 51 Ohm resistors and 0.01uF cap. This seems to do better.

New Bottles

The new 3-500ZG's arrived from RF Parts today Aug-11-2009. They are a matched pair.

New Bottles with original Tank

Also ordered, and installed is a new Load capacitor. The new Load cap is 0-1400pF, and a bit larger width-wise so I had to rearrange parts of the tank to accomodate (new bracket and new holes through the front panel).

(the Load cap is the big variable capacitor on the bottom. The variable capacitor on the top is the Tune cap and is in two sections. The tail (shorter) section is brought in on 80/160m bands only.)

New Load cap fitted in during the Tank re-work
New Tank with new Load cap, and new parasitic chokes
New Tank with new Load cap, and new parasitic chokes (2nd view)

The insides of the amp pizza-box

Construction Notes for W7BRS Linear Amp

Everything is hand made except where obvious. The input tuning board provides the match on the input side (to pin 1 of the tubes). The input tuning is a pi-network for each band. The slave rotary switch on the bandswitch selects the relay on the input tuning board to bring that band into the circuit. I can bypass the input tuning board easily because I used bulkhead BNC connectors through the chassis next to the board. The RF sampling meter circuit is also fed through a similar jack for keeping that signal clear and isolated as much as possible.

My bias is a 50W 6.8V Zener on a 50W heatsink. I have an option later to add another switch on the front panel to switch the bias for a CW or SSB operation. I am also looking at some other schematics/texts on perhaps making the bias even more 'selectable', but not at the moment.

The main coil is 3" diameter. There's a 1.5" diameter coil on the plate side that is silver plated ribbon. The coil is extended to work 160m.

The Tune cap is augmented for 80 and 160m with additional variable capacitance.

My HV outboard power supply is able to provide either 3.2k or 4kV. It's wired now for 3.2kV.

Access to control area.

The HV wires are coarse stranded wire in PVC jacket. OD is 0.140" and can handle an 20kV.

Control area

Some views of the new new panels:

Meter wiring done:
Control bay, More control bay

The two red lines from the center towards the front panel are HV B-. The bigger black line from center towards left to board attached to side panel is RF output for the RF Voltage meter circuit (where the ceramic disc cap is).

The red+white lines to the board attached to the panel on the left are the HV B+ circuit (4 1Meg Ohm in series to the meter with a 1k ohm to ground).

The rotator switch near the bottom left is the multi-function switch for choosing to view RF VOlts or B+ HV on the "round meter" (see last pic).

The input tuning board is seen from the top and above it is the blower.

Each meter is shunted with a resistor (except RF), two diodes, and a 1kV 0.01uF cap.

The thin black lines along the left are the ALC controls, and the small coiled up black line is a 12Vdc spare line for future lamps/controls.

The white/red wire is HV wire (15kV). The big black wire is RG-8X to carry the RF out for sampling. Connections through the chassis wall for HV, and RF are via BNC bulkhead connectors to make it easier to disconnect for service and keep airtightness.


Front panel with ALC, RF Sense, multi-switch (view HV+ or RF Volts), meters for grid & plate current. Front panel


A friend of mine, Jim K7NCG, learned that I wanted to build a linear amp. He came through in fine business by letting go of his project at a modest price. I plan on picking up where he left off and build this linear concordant with the form and function as it was intended.

Originally designed for 10m thru 160m, it represents a lot of work on the part of Jim to collect the parts, design the layout and prepare the power supply.


This amplifier project is a homebrew linear amplifier that is based on a pair of Eimac® 3-500Z tubes.

What I have here is the makings of a really good linear amp. The parts include, most of the RF tank components, the tubes, chimneys, blower, input tuning board, an outboard (and on dolly wheels!) 4kVAC power supply, and bags and bags of smaller parts to make up the control panel, band switches, loading capacitors, and more.

I took pictures of the major parts the day after receiving them and wanted to put them here to document the process for rebuilding the amplifier.

Jim gave me the manila folder full of designs, schematics and information on linear amplifiers and I will absorb as much as I can from it.

On to the pictures.

Pictures of the 3-500Z Homebrew Linear Amp

Eimac® 3-500Z tubes and chimneys

Air blower (grey manifold behind tuning board) forces air under the chassis through a formed cavity to make the air come up around the base of the 3-500Z and through the chimneys.

The input tuning board.
Input Tuning Schematic

The input tuning board.

RF Tank Parts


RF Tank Parts

RF Tank Parts

RF Tank Parts

HV Transformer

Schematic for Power Supply

Name that Connector!

Credits: Original Schematics and Amplifier design: K7NCG

Sources and Advice

There's nothing that wasn't easy to do on this project, but what made a difference was getting the right parts and tools for specific elements.

By far the best source of parts is RF Parts. Their service and expertise is second to none for the amatuer radio homebrewer. If you're looking for parts for your linear amplifier project, get their catalog. You won't be dissapointed.

For general parts and components, I use Mouser because their website has a better search tool than Digikey. Mouser's ordering system is very smooth and delivery options are excellent.

Locally, in the Puget Sound area, there are a few shops of varying degree of utility:

Flea Markets and Building

Don't underestimate the flea-market. I purchased many tools and parts for this amp pennies on the dollar. Meters for $2, wire, tools, ceramic rotator switch hardware, ferrite rods, transformers, power-supplies, coax, ceramic standoffs, and jumper-wire-up boards, copper-clad PCB, etc... Keep a list of what you'll need so the next fleamarket you visit you'll be prepared.

Things to bring to the fleamarket: A shopping bag, a Digital meter, screwdriver, pliers, and a 6" scale, and a scratch pad for taking notes.