Jun 112017

Yes, it was a wet start to our day and certainly very muddy under foot but that didn’t dampen our enthusiasm for putting on the Special Event Station at Fir Park Free Show near Market Rasen.  Members of the club putting on the Special Event Station were – Andy M0KED, Pam G4STO, Les G1LQB, Peter G1FLL, Stephen M6TSJ, Jason G7KPM, Tom G4OSB and Ian G4XFC.  Our Regional Manager Jim G0EJQ dropped by and had several contacts on VHF; we also had help from Andy M0OOO from the RAF Waddington Club. Terry G7JFI and Fiona called in and manned the station later in the day.





There were fundamental shortages in our Special Event Station equipment; G1LQB provided a gas stove and kettle for tea and coffee but we omitted to take a frying pan, bacon and rolls for bacon butties.

We didn’t make loads of contacts but an enjoyable day was had; we had time to have a look round other stands and the vintage vehicles and equipment at the show.  We are looking forward to putting on another Special Event Station at Boultham Park in July; we have also been asked to put on another SES at Fir Park Wings and Wheels in August.

 Posted by at 9:02 pm
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 122013

Because of finding dangerous faults during this project, this Linear Amp was not a refurbishment but a complete rebuild. I would advise others not to do this work unless they are experienced in what they are doing, as inexperience could result in DEATH as LETHAL VOLTAGES are involved. For reference, this linear is not pretty and is being brought back to life as a working project and will only be used safely in my shack, it will not be entered into any beauty contest.

Old Valve Base FixingValve Bases RewiredI changed the way the valve bases had been fitted to provided better ventilation by removing the plate that the bases were attached to and using 4mm 2 inch screws refitted them below the chassis using the screws as spacers (it is recommended that you fit the base of the 813 is below the level of the chassis with just the glass envelope above it).Valve Bases From Side I had to rewire the bases as the grids had been directly wired to earth instead of via a .01uF 1000v capacitor and a 4.7R 1 watt fuse resistor (still to arrive and be fitted) which is a safety precaution as you can’t be too safe with linear amplifiers. Right of the valve bases is a new homebrew filament choke.

Things are plodding along nicely as the bleeder resistors for the capacitor bank arrived and were fitted and a mock up of the power supply using another HT transformer I have was assembled and fired up via the Variac transformer so I can start to re-form the capacitors.

Forming the Capacitor BankThere was no big bang and I am now slowly increasing the voltage across the capacitors till I eventually reach the top voltage with this HT transformer which is 2.5KV and it should then be OK for the HT transformer on the linear amp where the voltage across the capacitor bank will be 3KV. (I just hope there is no big bang or the cat gets electrocuted.)

Homebrew Filament TransformerThe fitting of the hombrew filament transformer shown on the left with the RCA 813 Valve is being delayed as I rewire the linear, as it is heavy enough turning it over with just the HT transformer fitted, I have replaced the filament choke and decoupling capacitors and wired it up and all it needs is wired to the transformer when it is fitted. The principle jobs at the minute are rewiring the mains input circuit and fitting a soft start to the HT transformer, removing unwanted components and making a hole in the front panel for the HT voltage meter so things are just plodding along.

<<< Part One <<< Part Two >>> To Be Continued

 Posted by at 11:43 pm
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

Apr 042013

G2FHM HF Linear AmpThis is a series of articles doing a step by step refurbishment on an old homebrew HF linear amplifier. It has to be taken into consideration that when the Linear Amp was built everything at that time would be valves which were more forgiving unlike the transistors of today. The amp was 2 X RCA813 valves wired in grounded grid configuration and the input was fed directly to the cathodes of the valves with no matching network. (this was common in the old valve days)

Blocking Capacitor Inside blocking capacitor The first step was a visual examination of the linear and I was suspicious about the wax on the chassis below the plate blocking capacitor and the coating of the other capacitor wired in parallel  on top of it had also melted. I removed the blocking capacitor and found that it had completely burned out, I would have to replace this with a suitable door knob capacitor.

Linear Inside topThe large 10v filament transformer shown in the picture above is not suitable for the linear as it is not center tapped and will cause a 50hz modulation to the output, so this must be replaced with a proper 10v center tapped filament transformer. The filament transformers are hard to find, but I have a suitable transformer that I have rewound so the original one will be replaced by my homebrew one.

The 15 watt bulb on the left of the picture above was used as a soft start for the power supply and was in circuit when the linear was initially switched on (the 813 valves did not need the time for heating up as other valves do before HT was applied) when the HT switch was turned on it shorted out the bulb for full power to the HT transformer. This was going to be replaced by a proper soft start relay so that the HT switch would be completely separate from the main switch.Diode Bridge

A 200 ohm potentiometer at the back of the linear was completely burned out and with some unconnected components and wires and two diodes on the rectifier board were shorted out with a piece of wire.I never gave this a second thought, the whole thing was going to be replaced with new components.

Because I was changing the filament transformer this meant that I had to also change the metering circuits, I decided that rather than a refurbishment, for safety this was going to be a major rebuild. I was also not happy with the cooling system on the Linear so I was going to change the layout of the fan’s for better ventilation.

100_0486Turning my attention to the electrolytic capacitors on the power supply, visually they seemed OK and the bleeder resistors across the capacitors were all wire-wound and looked OK. The next stage was to make a new diode chain as I did not like the old one and try to re-form the capacitors and check them to see if they were all alright, I would use a variac transformer so I could slowly increase the voltage across them.

813 valve basesI did not like the wiring on the valve bases so decided to cut all this out and rewire the valve bases and make sure that they had a good earth as I was not happy with this part of the build. I removed all those components and the valve bases and while everything was disconnected it would be a good time to alter the chassis to improve the air flow round the valves.

My final check before starting anything was to check the aerial switchover relay and this proved to have been arcing so a new relay would have to be fitted.

I had set my stall out on what needed doing, all I had to do now was find the time to do it. Time to buy the XYL some flowers and a big box of chocolates so I could work on rebuilding the Linear Amp.

>>>Part Two

Mar 302013

Electrolytic’ Capacitors may smooth out our ripples but they do need some TLC or they will blow your equipments main fuse. They do benefit from a regular top up of electricity but some are neglected and need a little bit of extra TLC to bring them back to life.

electrolytic capacitorThis short article is covering the poor neglected electrolytic capacitors that have not had an electron pass through them for a very long time and is to give you advice on giving them that extra TLC. More information on electrolytic capacitors can be found on the Wikipedia. While this topic is on unused equipment it is also relevant to New Old Stock capacitors recently bought.

Important!! There is controversy about using the methods in this article  on switch mode power supplies. If your equipment has a  switch mode power supply, then use at your own risk!

If a piece of equipment has not been used for some time that has a linear power supply, the electrolytic capacitors electrolyte deteriorates and can in fact blow the main fuse of the equipment straight away. Caution is required before switching on the equipment and a reduced voltage on power up usually helps.

A technique called re-forming is used to bring the capacitors back from the grave and all that is involved, is to apply a small voltage to start with and gradually over time increase the voltage till you reach the maximum voltage. On equipment this can be achieved by powering it up with a Variac transformer and slowly increasing the voltage over time. A quick method that can be used is to put the equipment in series with a 40 Watt light bulb and the same effect can be achieved though not as good as using the Variac Transformer. Basically what you are doing is using electricity to re-form the electrolyte dielectric see Wikipedia.

Warning! It’ is OK to start with reduced input but does not always guarantee ‘no bang or smoke’ !!

Mar 302013

Having recently started being interested in HF Linear Amplifiers I needed a voltmeter to measure the High voltage on the anodes of the valves, I have a Bradley meter but it will only go up to 1.2 KV and I needed a 2.5 KV voltmeter to monitor the voltage on the Linear Amp that I was building.

100uamp MeterI had a 100uA meter in my junk box with a scale of 0 to 25 so I though that I would use that with a series resistor, (forget what it says on the scale as this can be easily changed) so the next thing to do was the calculation for the voltmeter. To find out the value of the series resistor using the formula V/I = 2500/.0001 = 25 Meg-ohms minus the internal resistance of the meter which was 540 ohms so the series resistor would need to be 24,999,460 ohms for FSD and to find the power to be dissipated in the resistors using the formula W=V X I = 25000X.0001=.25W. (I would recommend a 100uA meter because of the power rating of the dropper resistor, a 1mA meter would need a power dissipation of 2.5Watts)

For the dropper resistor I decided on a chain of .5 watt metal film resistors, these have a voltage rating of 500volts so you would need a least 6 resistors in series to be on the safe side. I decided to be extra safe and have a chain of 19 X 1.5 meg-ohm resistors ending with a variable resistor for calibrating the meter. Although not necessary it is best to put the variable resistor and the meter on the earth side of the circuit where the voltage in the circuit is low.Dropper Resistor Chain

To calibrate the meter, I used a Variac transformer to set the voltage on the power supply to 1KV with the Bradley meter then using the variable resistor set the voltage on the 100uA meter to 1kv and the job was done and the meter was ready for wiring into the linear amp.

Although this meter has been designed for 2.5 KV using the formulas above you can measure any voltage. This article comes with a health warning HIGH VOLTAGE KILLS, make sure your dropper resistor chain is properly insulated as high voltage can arc across an air gap. If you want to build a stand alone voltmeter for high voltages make sure you use a good insulated box to put the meter in and hard wire the high voltage leads into the insulated box, High Voltage Probes are essential.

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.)