VU2ATN  - Linear Amplifier Using 807 Valves - QRO



 807 Valve QRO Schematic
Power Supply Schematic
  When Dr. Ashutosh, VU21F suggested publication of information on my vintage valve type amplifier for NR60, I was hesitant. I
guess lot many hams, now-a-days, are not at all aware ofsomething called vacuum tubes &hence there would be no takers
for such a recipe. However my choice of such an amplifier aheadof NR60 was based on the following considerations:

1. Cost factor
2. Ruggedness-cannot be immediately damaged because ofmismatch
3. Good possibility of taming for multi-band operation without elaborate &dedicated filters for different bands
4. Low TVI
5. Simplicity of construction, switching etc.

If you are convinced that those are good enough reasons for a little tactical move backward, may perhaps switch on the soldering iron.
However, before you start please make certain that the basic NR60 board is working absolutely OK without any problem. It
must be tested on-the-air with local hams for quality & acceptability. Both the NR60 board & VFO must be shielded. If you use any
premixing technique, the associated circuitry also should be adequately shielded. The power supply should be made in two
separate well-shielded cabinets- one for the low voltage (12V& 25 Vdc &the other for the HV supply for the valves. Last but not
the least, the valves &associated circuitry must also be shielded appropriately with no possible interaction between the grid &
output circuits.

Prospective constructors should refer to Radio Handbooks & other standard literature for more information & confidence. The circuit is straight forward &generated from available literature. Here is the circuit diagram with values marked: The transistorized linear driver is expected to swing up to 60 to 70 mA with signal (AF) input. The transistor has to be biased at 20 mA &must be heat-sinked properly. The 25V collector supply need not be switched while going to AM mode. However the 12 V supply must be switched. Similarly for the valve stage, only screen supply shall be disconnected during receive mode. It is needless to say that antenna must be switched between Tx & Rx as shown in the circuit diagram.

While dealing with vacuum tubes, please remember all safety aspects &go through very carefully the standard literature. The Power Supply: The power supply for the PA stage is straight forward &as shown in the diagram. The power supply uses a single transformer for I supplying the filament supply, plate &screen voltages &also the I bias supply. The transformer shall not be available off the shelf and it has be to ordered with somebody. However, arrangements can be made for supplying the requisite voltages & current from an alternative source. My set-up is based on a circuit for such a
transformer, made available to me by my good old friend Debu, VUPB and the transformer was designed &constructed by grand OM of Amateur Radio Ganeshda, VU2LL(now silent key). Most probably, such a power supply is used in HW 101, manufactured by Heathkit.

The components are mounted on an aluminum chassis over tag strips &no PCBs have been used. All the switches are mounted on front panel including the bias-control & neon indicators. The transformer has been mounted on the top surface of the chassis &all other components &wiring on the bottom side. Slightly oversize components have been used for reliability. For safety reasons, the chassis must be earthed properly &any prospective constructor is requested to please go through the chapter on Power Supplies in any standard Radio Handbook &other literature.

The above configuration (the AF amplifier and the power supply) Has been working satisfactorily for more than 3 years.

I hope there will be some takers, HI. Happy construction!.
Atanu Das Gupta