4X811HB

The HB stands for Home Brew.

newampon

In its current configuration this amp is equipped with four 811As and puts out 550 watts on 75 meters.

The total investment was about $250 and four weeks.

Purchased parts include:
Plate tuning cap
Antenna load cap
Misc relays
Two meters
High voltage diodes
HV filter capacitors
HV filter equalizing resistors
Parasitic suppressor kit
Vernier drives
Band switch.
Tube sockets
Bypass caps.

All other parts were scrounged or donated. If all parts were purchased at current (2013) rates cost would exceed $700 and the project would not be worth doing.

It is built into a much modified tower computer case. An old computer power supply serves to exhaust air form the case and act as a source of 12 vdc at 15 amps and 5 vdc at 20 amps. The 12 vdc is used for powering relays but it can also be accessed off the rear panel to power other equipment. Like a transceiver to drive this amp.

A second computer power supply case has been gutted of all parts except the fan. It is used to house the filter capacitors and rectifiers for the Plate supply. The plate supply uses the plate transformer out of a Heathkit SB-220 and can be configured as a doubler or a bridge. For the four 811As we only need 1500 volts so the configuration is as a bridge.

The tubes used in this amp are mounted to a removable plate. Additional plates are available for other tubes as they become available. This design can use 811As, 572s, one or two 3-500s, 4-125s and even 4-400s. The basic circuit was taken from: How To Build The DX-811A All-Band Linear Amplifier. CQ Magazine September 1982, BY F.T. MARCELLINO, W3BYM.

A very simple soft start circuit using 120 vac relays is employed. One for the filament supply and one for the plate supply.

Controls include Plate tuning, Antenna loading, Band switch, ans five toggle switches with functions as follows:

Right to left

1. SSB/CW engages primary tap on plate supply transformer. Transformer secondary delivers 1000 vac in SSB and 1200 vac in CW.

2. Turns on ATX PC supply. Provides 12 vdc at 15 anps and 5 vdc at 20 amps. Also powers primary supply relays and three cooling fans. Nothing works until this power supply is turned on/ You can tell iy is on if you hear fan noise.

3. Filament supply ON/OFF

4. Plate supply ON/OFF. Plate supply will not come on unless filament supply has been turned on.

5. Transmit/Standby Interrupts amp keying line to transceiver. Keys at 12 vdc.

Transmit ready light comes on when all conditions for transmit are met.

Tuning controls are equipped with 3:1 velvet vernier drives.

Band switch has 6 positions. Correlation to frequency through tuning chart. Covers 3.7 mhz to 30.5 mhz. Frequency coverage below 4.2 mhz requires addition of a fixed 120pf capacitance to plate tuning capacitor.

Meter on the left is for grid current, measures 0 to 240 ma. Drive should be adjusted to 120 ma which correlates to .4 on the meter scale.

Meter on the right is for plate current. Reads directly from 0 to 1 amp. Typically runs at 550 ma when amp is running properly.