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High Power Amplifier WS19
A long time ago I bought from a private advertisement a High Power Amp No2 for the Wireless set 19
A written description and no photos I remember it cost a full 2 weeks pension at the time It duly arrived by post I unpacked it to see a hole cut in the front plate ‘for a meter’? mmm? Next surprise, it had been fitted with a power transformer, 2 rectifier valves, choke and capacitors. The job was exceptionally neatly done. Suits me, I want to operate it off the power anyway. It was the 4 x 807 valves type, ‘Nice’ Maybe I can crank this up to transmit a few watts The unit was lugged to a few displays, but not working. I kept an eye out for a suitable square meter for the hole Eventually, I “clicked” on a box assortment of meters as I judged a square meter in the lot appeared as if it might fit It did, but (there is always a ‘but’) the dial was for 8 amps And that I find a strange marking in its self (I digress) Time passes, and I was given a carton of Holden car parts, which got shoved into a corner Again, eventually, I was ratting through the carton (I must have been bored) and low and behold a small round, wait for it - - - The correct dual marked dial scale, you little beauty! We have passed the bush fires and now the floods set in, This is Australia, it is so big, and you can drive all day and be in the same state. Bush fires at one end, floods at the other and drought still holds on in some places, while the population clings to a narrow section around the coast line. A few weeks ago I could have played marbles any where here, and now I can’t find the way to the shed because of the long wet grass! For the want of something to do, I benched the HP amp rfamp-orig.jpg With a little fiddling I got the meter movement and dial swapped over in the housings. So far so good or how lucky they are the same internal diameter Now to replace all the capacitors, (to be on the safe side) before powering the unit on Then I notice that all 3 trim controls are disconnected, another mmm?? I traced and drew the circuit First the input drive (meter trans No4) sensor is gone - replaced with the internal fitted meter? MY Circuit rfamp-rewired.jpg Now I struggle with all things radio, (again the buts) but that can’t be right? I checked and checked, even to replacing some of the all same red wires with different colors so I could follow them better Look closely at the meter wiring (some circuitry omitted for clarity) as was.jpg Have you sussed it???? ONE - The meter is wired directly from bias negative (approx 30 volts) to 12 volt positive supply AND if you haven’t noticed - - - REVERSE polarity !!!!!! Surely the modifier had used the Amp? Another mmm, the meter was missing, right, burnt out maybe? TWO – The meter is across the RF choke to the network onto the cathode? And other circuit peculiarities, Moving on, my meter requires half a volt for full scale deflection and was fairly close to the correct reading on the low voltage when I fitted it So, begs the question, Why have 42 volts backwards across it? OK next There is a bias voltage control, so OK Another ‘but’ the “drive” from the ws19 control is in the grid Here is the BUT, but it is wired full resistance from the input and the wiper varies the bias. I don’t understand at all, How can that vary the drive? Well it would, BUT also vary the bias ???? A nightmare to tune???? I searched the WWW for days about transmitters and linear amplifiers, There is nothing like the wiring WS HP amp, I was learning fast though Well anyway, first rewire the meter, at least with the correct polarity and Why connect it to 12 volts? I did not, I connected it to earth Finding a suitable voltage dropping resistor took some time swapping values Settled on a string to make 8.88k ohms The face is marked for 600 and 15 volts This resistor combination allows full scale at 30 volts Nice - 600 less a 0 and divided by 2 Next the cathode/plate current I rewired the meter from AFTER the RF choke to the network across the load resistor The selector switch and meter fully rewired is now.jpg Now that drive control still puzzles me I rewired it Full resistance from bias voltage to the grid Once bias voltage adjusted that will remain static The wiper to the input from the WS 19 can be adjusted independent of the bias is now better yet.jpg Let’s turn this beastie - - - ON Connect 12 volts DC Valves all aglow Main switch - ON Rectifier valves all aglow Press to transmit Voltage rising slowly HISS HISS ?black snake? Quick, switch off, I smell the smoke now Absolutely no visible signs of anything toasted Sneak it back on, no HT at all Unsolder wires and check sections for the deadly short Long time later, C5a on top of the chassis, behind the coil, yep, I missed that one .1uf 3kv across full HT after the choke Perfect appearance and perfect short Sneaked a new cap in behind the disconnected original and re-solder all the wires All voltages steady, I changed some meter resistors to obtain a more correct reading That leads to - never believe the color code on the resistor One was marked 3.9k but (I like buts) measured 5.8k, I therefore went to 4.7 k and whacked the meter full over before measuring and fitting a string to make 6.2K Also Resistor R9a – 12 ohm 4 watt (the load for the WS19) has been toasty at some time I replace it with 2 of 25 ohm 5 watt carbon types in parallel There is an additional relay and the original relays have been rewired, how-ever they all work so I left them alone I’ll need to do a transmitter section on a WS19 to really power it up Listen for me on 3.5 something Or maybe, I’ve got it all wrong Cheers rod Last edited by Rod Salter; 12-02-20 at 05:28. |
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