Apr 172013

After serious thought on the layout of the components on this Linear Amp especially the placement of the filament transformer and the position of the RCA 813 valves for cooling, I decided that the best thing to do was to remove everything and reposition it better within the chassis. Initially my thoughts were to sort the amp out and get it working keeping the same layout, but as I progressed I was not happy with the layout, so this has now actually turned into building a 1kw Linear Amp from some recycled parts as I am now modifying a lot of the old amp.

linear amp chassisThere is going to be major changes to the top of the chassis as the aluminum screen separating the HT transformer from the tank circuit is going to be moved down slightly and the meters in that part are being taken out and repositioned in a suitable place. The HT transformer is going to be moved forward to the front of the linear to make room behind it for the filament transformer, as I didn’t like it being near to the PA tank circuit.

The plate choke is being replaced with a better one that I have made which has considerable more inductance and a very low self resonance frequency achieved with the insertion of a ferrite rod in the middle of the choke.

The valves are going to be moved to where the filament transformer is presently located and a fan fitted into the back of the chassis behind them sorting out my concerns about proper ventilation and while doing this, the vent in the front is going to be closed off and more ventilation put on the top of the amp. (You don’t want warm air blowing in your face when you are operating.)

The PA tank coil is being replaced as I have found a better design that enhances the 10M band which is the Achilles heel of the RCA 813 Valves due to the high inter capacitance of the electrodes in the valve. Using the new coil design should make it possible to get better power on the 10M band.

The last change to the amp is to replace the ceramic band switch in the PA as it is HUGE and takes a lot of room up, this will be replaced with a suitable smaller ceramic switch which I already have.

Although this may seem a lot of work, in the long run it will make the amp better and easier to work on and I only wish I had started the project with this concept rather than follow the original build. I shall put it down to experience and now go forward with the project in a positive manner.

For anyone thinking about building a linear amplifier a good starting point is the Matt KK5DR Website.

Apr 092013

813 Filament CircuitRefurbishing the Linear Amp was still in progress, and because I was changing the old filament transformer for a proper center tapped transformer suitable for the amplifier, I had to change the circuit to take this into account.

Another modification to the amplifier was the cathode bias circuit as the old circuit did not have one. R5 is a 50K 10 Watt resistor that biases the valve at cut of when not transmitting keeping the valves cooler and D5 is a 5 volt 10 Watt Zener Diode which biases the valve for transmission.

For those not familiar with the grounded grid configuration D5 and R5 raise the voltage of the cathode above earth making the grids negative in respect to the cathode. on large transmitting valves, the heater filaments and the cathode are connected so share the same circuits.

D6 – D8 1N5400 3A  glitch protection diodes for the meters in case of a negative going High voltage spike. F4 a high voltage 1A fast blow fuse provides further protection in the event of a catastrophic failure. All capacitors are .01uF @ 1000 volts and M1 = 1 amp meter and M2 = 50mA meter

It was time to check the capacitor bank and I removed it from the Linear Amp and checked each capacitor individually for shorting / OC, I found two resistors were open circuit and there were no protection diodes across the individual capacitors so I decided to strip the bank down and replace all the resistors and add protection diodes. At the same time I hot glued the capacitors to the original perspex in way that the air flow could go round them everything was completed on the capacitor bank apart from the bleed resistors which had to be re-calculated.capacitor bank

This is where the health warning needs to be placed. Warning: Extreme Caution, Lethal Voltages, High Voltage WILL KILL  I checked the HT transformer and found this to be OK with an output of 1.2KV which will give about 3KV with no load on the voltage doubler. Good wire wound resistors are hard to find so I recalculated the the bleed resistors for 2 watts and needed 562.5k resistance so I ordered some 680k 2watt metal film resistors as they would do the trick though the discharge time of the capacitors would be a lot longer.

Diode ChainI clipped the diodes from the diode chain and replaced them with 3A 1000piv diodes, but before this I checked the balance resistors which proved withing tolerance so this was another job done and ready to fit.

While I waited for the parts I had ordered, It was time to refit the valve bases so that more air could pass through and fit the filament transformer and I now also had to make a suitable voltmeter for the 3kv voltage, things were beginning to take shape now on the refurbishment / rebuild.

<<< Part One >>> To be Continued

Mar 282013

Valve Linear Amplifiers WILL  KILL!!!  If you make a mistake there is no SECOND CHANCE, so you have to be VERY VERY careful before you start working inside them.

I am writing this because of some stories I have heard that frighten me, as we are talking about HIGH VOLTAGE in excess of 2.5 THOUSAND VOLTS with high current which is a lethal combination. and I hope that I have managed to scare you into taking notice of the safety measures which should be adhered to. I have worked with mains voltages and above since the age of 16 and have a healthy regard of electricity and know that it can bite so always take precautions when working with it. With the high voltages found in linear amplifiers you have to be extra careful as it does not bite it KILLS.

Before working on a linear amp, if the high voltage circuit has been activated then you have to leave it for at least half an hour for the power supply to discharge before touching anything. I built a 2.5KV high voltage meter into the linear amplifier that I am building so that I would know when it was safe to put my fingers inside it. DO NOT short out the EHT as you can damage the linear doing this. You LIVE LONGER with a little bit of patience.

Recently I heard a story of someone who wore Marigold Gloves as a safety measure when working on a live linear amp, I personally think it safer to work with bare hands with the power supply fully discharged. This same person upgraded his power supply from 800 volts to 3 thousand volts to get more power, his valves actually exploded. If something goes wrong in a linear amp, because of the power there is usually a BIG BANG.

The motto of this story is to Listen to people who know what they are talking about, research your amp so you know more about it and don’t work on a live linear, stay safe to be able to use the linear and make sure it is not sold in a Silent Key sale. (I asked Jonathon G6JUT if he would like to place an advert for his business on this post but he declined as he thought it would be in poor taste.)