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HF homebrew tube amp photo build

Discussion in 'Home Brew' started by Radio Tech, Sep 30, 2009.

  1. ElectronTubesRule

    ElectronTubesRule Active Member

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    I have not done anything like this since I was 14 in Boy Scouts but I am going to ask anyways. Have you considered taking the windings apart just to do a center tap? This would also give you a chance to dry the paper out in an oven at 180°F and you could saturate the paper with resin or polyurethane etc..... if it is not still in good shape. Just an idea though. Like I sad I have not done that since Boy Scouts we where working on the Amateur Badge and we did a lot of things under direct adult supervision. We ended up totally rewinding the transformer but in your case you just want a center tap to make the rectifier design easier.


     

  2. ElectronTubesRule

    ElectronTubesRule Active Member

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    If you cut the secondary winding off with a chisel on the MOT the shunts fall right out after you clear away the copper. if you use insulated normal wire you get a rap to volt ratio on those MOT's secondary of about 1 volt per wrap. So if you take your normal high voltage hook up wire and make 6.5 wraps you end up right around 6.3 it might be a tad under. So you do not need to use enameled winding wire. You are already aware of the size to current ratio I am sure of the conductor you use. Since most tubes can have ac or dc on the filaments you do not need to rectify it at all so it does not matter if you use one or two of them to get the voltage you need. I have been playing around with MOT's for a while and a nice stiff high voltage resistor and plenty of capacitance will clean it up nicely. Use a wire conductor size that gives you close to peak current at full saturation and then you do not have to worry about current limiting either.

    I proposed using MOT's for all of my non-HV power needs due to ease of modifying them and their insanely low cost as in "free".


    I want to actually build a kick but power supply with them at some point just to show what can be done on the cheap. That is down the road though and I will need to find someone with the test equipment to put it through it paces and and do trace plots to show what they can do nicely, what they can be made to do and where they fail totally. Idea's are grand but data is King and it would be nice to have some nice data to look at on these. I am convinced that using identical transformers in a combo of series and parallel is key as well as making sure you limit how much current can be draw at any one time. Keeping them cool is also something to be considered in HV application.

    In low voltage filament use they are almost idiotically simple to make fit most low voltage needs even high current needs! If I was needing a high current low voltage set up for something like the Russian Gs-35B I would use a MOT in a second if I had one sitting around for free over purchasing something specifically for it!


    I have to agree with you CaptinKilowatt the glass envelope tubes are intrinsically sexy and sleek compared to any and all of the ceramic to metal tubes. The glow is almost as romantic as a fire in a fire place. The ONLY and I do mean ONLY reason I am not using them is bang for the buck! The 811,572,813,833 Russian tube counter parts are a lot more expensive when looking at dollar per watt hour then the Military Surplus stuff!I would love to at some point build a bifilur tuned input single 813 broad band table top amp, I would love to do a two 833 1KW no fan or at the most a fan that could be turned off and I think a 4x813 would rock as well! But since I am a single Dad raising 3 boys and money is tight I am doing everything from a bang for the buck view point. Even my component selection is based on unit cost per item......So I order some stuff from Russia, some from Taiwan, Some locally sourced and so on.....I always include shipping cost in the unit price. I often have to wait 2-8 weeks at times for my parts to arrive but I am able to keep cost down by shopping this way. I might only need a 3amp 1000V diode but if I have a glut of 10amp 1000V diodes and using a bigger one is not going to hurt anything I use what I have.

    You will likely be forced into component selection because of your desire to keep it period correct in appearance. Like for instance computer grade snap in caps or bolt in are out of the question. So you will have to find, ship and test maybe reform old oil filled power storage caps and prob. put some modern caps inside the old metal twist in caps as in empty out the old dead twist-in caps install modern ones in the case and then reassemble. So while I know I will like the look and feel of your amp cosmetics will more then likely force your hand a bit in component selection depending on how true to vintage look and topology you decide you need.

    Have you considered using 1/2 or full wave tube rectifiers?
     
  3. ElectronTubesRule

    ElectronTubesRule Active Member

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    Update on MOT's looking for a cheap supply of these in like new or new I have found Ebay not a bad place to start. I found one guy that sells new MOT's that are taken from units damaged in shipping. So for instance he had 4 identical MOT's @ $11 each but $17 to ship each one. Now those are probably for 950Watt units but because they are identical you can link them in series to use full-wave rectifier circuit. You could take 4 of them and configure them in groups of 2 in series then run each series in parallel. This would mean no isolation transformers or other nonesense to make sure the load is shared evenly.

    Just an idea mind you but if you used HV resistors to limit current flow on the supply side filter through some AC rated caps then run into full wave rectifier and some more filter caps and typical load/bleeder resistors it should be stable and clean. Standard soft start board would be a given I think given the not so robust design of MOT's.

    If you wanted to just run them parallel I do not think that one would be looking at any additional expense in the rectifier design and layout or parts used. In the end where you put the diodes and caps is what makes the difference not like the individual parts are any more expensive doing any of the 1/2 or full rectifier designs. I think the money you save on the transformer alone easily more then offset's the need for more filtering stages to clean up the ripples from 1/2 wave rectifier.

    A Lasagna Dinner from Sams club or Walmart takes 45 minutes on high in the microwave. I knew girls in COllege that only cooked with a Microwave from Turkey Dinners to Mac and Chese it was Microwave action 100% of the time. I have yet to see a home burn down because a Microwave transformer went up because it could not handle the output into it's 1200Watt Magnatron! I could be wrong but I think MOT's do not have current limiting other then their center tap to chassis ground. So at full power with the shunts and all in them they have to be at 100% Saturation yet they last for years. I make at least 5 cups of tea a day in my Microwave at 2 minutes a per cup of tea " Huge Cup". My entire family warms up leftovers all the time they see a lot more use then any Amateur radio operators amp does in terms of total hours of use per year.

    My point is that I think the naw sayer's all have valid points they bring up about not useing MOT's but these same guys where the types that poo-poo'ed HEath kits use of a rather light weight transformer and voltage doubler board and conventional fan like the evap fan from Refrigerators. TO this day those same "Junky Light Weight Heathkit Amps" sell like hot cakes on Ebay and most OEM's like Ameritron have done the same thing. So their will always be people that fear change, the unknown or convention. You are not building an HF amp for a Nuclear Submarine you just want to talk about coffee to guys 1000+ miles away once in a while!
     
  4. liquidh8

    liquidh8 Active Member

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    Nice thread, I couldn't see the pictures from my work computer though. I love homebrew builds. keeps us posted.:laugh:
     

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