madamp g3 project issue

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    • madamp g3 project issue

      I have an issue with my project probably due to my fault but I would like your help to continue working on it.
      This is briefly the sad but true story.....
      1. I assembled all the parts and turned on the amp. The fuse was blown.
      2. I resoldered 1 suspicious joint but again fuse was blown
      3. Then trying to find the reason I thought that I had connected the Power Transformer wrong and reversed the red (208v) with the brown wires (6.5v) and got it work with a new fuse (2A).... I know two horrible mistakes.... The result was to burn the resistors 26, 27. In this case, fortunately, I had removed the valves....
      4. I understood the bad connection of the P.T. as it also got very hot....
      5. I got red and brown wires in right position and replaced resistors 26 and 27 with 100Ohm but less watt..... I also found a short circuit between legs 3 and 9 of V3.... Maybe this is the reason of initial fuse blowings... Can it be???
      6. I got the amp to work.... with valves. Valves did work. Started to measure voltage and I had 220V(not 208V)in the rectifier and 7.2V between lugs (not 6.5) within 10%tolerance with wall plug at 226V.
      7. I stoped measuring as the transformer got hot again... I dont know if it was due to the previous mistake or not.
      The P.T. was short circuited (burnt...) Closed the Amp and then found the fuse (2A) blown.....
      Please give me the appropriate advice to go on with my project!! I would appreciate any help. Tell me if the P.T. is due to the wrong connection of the red and brown cables and if the fuse blowing can be a result of a short circuit between lugs 3,9 of V3..... Tell me also, which components must I replace.. and how to get them... Thank you in advance,
    • Hi and welcome to the forum!

      re #5: Yes, connecting the cathodes of V3 can result in high plate current that burns the fuse. But in this case also V3 should "illustrate" this by glowing red inside (not only the heater but also the plates). And you should replace the 100 Ohm resistors with correct power specs otherwise they night also get hot and might blow up

      re #6: What is your voltage at the wall outlet, seems to be <230V AC? If this is the case and the voltage at the rectifier input is 220V instead of 208V, all other voltages are also higher, so this is OK

      re #7: When saying "the transformer got hot again" what about the voltages at points A/B/C and heater voltage, were they OK? And what do mean with "Closed the Amp and then found the fuse (2A) blown"?

      Before being able to determine the parts you need to do some more analysis ;)
      ========== MAD = Martin Adams / Digital ist nicht alles / ==========
      ========= Meine Alben: ========
    • Mr Martin... Thank you for your response!
      I am going to replace the 100 ohm resistors with proper values. How many watts are they rated???. After visual inspection there are not signs of failure in the other components... but may it be better to also replace diodes 1N4007 or they probably have not been damaged??
      re #5 The fuse is burned imediately, and there is no power supply in the amp... so how can V3 illustrate this glowing red??
      re #6 My voltage is 226V and the voltage at the rect. input was 220v when reconnected the power transformer the right way.
      re #7 When reconnected the P.T. the right way.. (..with 2A fuse....) .. it continued to overheat with high rate... the small valves were surely glowing and I am not sure for the large one which may glew a litlle later but as the temperature of p.t. was too high I didn't manage to make any measurement at A/B/C points and turned off the amp. Ther was also a suspicion of smoke in the P.T.
      What is the cause of the P.T. overheat???? May the previously fault connection caused an internal failure or something else that I have not found yet???
      I removed the P.T. and between the red wires I read zero Ohms... so I probably need a new transformer to go on .... Is it so? But, in this case, I am not sure if there is another reason that makes it overheat and I am afraid of a new failure...
      As for the mistake of placing a 2A fuse instead os 250mA.... which components may have been influenced??? Visually all seem ok...
      Closed the Amp.... = Turned off the amp and burnt fuse... (sorry for my English.. wrong phrase)
      I really appreciate your help ... If needed I can attach any photo.... I'd be glad if you could guide me to solve the problem. If any measurements (probably ohm resistance...) between several points can be done to confirm the conformity to the schematic and the good condition of the components ( without power in the circuit and with the P.T. removed)..I can do it....
      Thanks again.....
    • First thing you should do is to remove the power cable from the wall outlet, remove the 200V (secondary) connection to PT, remove the tubes and measure the resistance of each winding (1x primary, 2x secondary) of the PT, also the resistance between the 3 windings (should be indefinite). Next you should connect and power on the amp and measure the free running AC voltages at primary and secondary windings. Then turn off and disconnect the amp from the wall outlet, reconnect the secondary 200V winding to the power supply, remove connections A, B and C, connect and power on the amp, and measure free running DC voltages at A, B, C.
      Please post these values and we will see what might be the next step...
      ========== MAD = Martin Adams / Digital ist nicht alles / ==========
      ========= Meine Alben: ========
    • Mr Martin .... after replacing the transformer, two resistors and the diodes of valve 3 I managed to make measurements but not sound yet... :(. Valves are open... measurements are not so good though. I attach you the measurements and I hope you can help me... probably there is still a mistake or a damaged part due to previous mistakes....
      I also send you photos of the valves.... As for the sound... only a buzz when playing guitar and all pots are turned on (not only gain and volume)
      I' d be glad if you could help me.
      Regards, Ilias
      • madamp g3 measurements.PNG

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    • Ilias,
      looking at your photos it's obvious that something is wrong with your power tube (V3). That tube has two systems (the two grey vertical ones made of sheet metal). The heater wire is only inside the system, so when the whole system is glowing red it means that way to much current is flowing through that system (Plate to Cathode). When seeing that you should immediately turn off the amp, otherwise the tube is very likely to get destroyed in the next 1-2 minutes. Fortunately tubes are much more robust than e.g. transistors so there is a good chance that your V3 tube is still "alive". But you definitely need to checkt the output transformer (measure resistance of primary windings and voltages at both plates of V3 when the tube is removed). Next you should check and measure the resisors and capacitor at the cathode of V3. I assume the problem is there. It seems that you have very little experience with tubes, so I strongly recommend to organize some help of somebody more knowledgeable.
      ========== MAD = Martin Adams / Digital ist nicht alles / ==========
      ========= Meine Alben: ========
    • Hi Martin, thank you for your advice.
      I know my lack of basic knowledge in tube amplifiers. This is also one of the reasons that I decided to start with this kit... as there is a forum for guidance and specific measurements for troubleshooting.
      I think that this kit is for hobbists like me and many people that buy this want to try for first time.
      As for my issue I knew from measurements that something was wrong and also I saw that something was wrong with V3 valve... and this is the reason I sent you the images. To be honest I expected more immediate and substantial help.... as this forum is made for this reason... and buyers of this amp rely on this... "If you have any question to this amp or technical problems, just ask Martin directly in the forum: Madamp support Forum." I think that you should be the "somebody more knowledgeable.." that you suggest me to find.
      I finally managed to find the problem and make my amp work. There was a wrong connection of R21 resistor which I placed in lug 15 instead of lug 16.
      Thanks again. Have a nice day.
    • Hi Illias,
      to manage expectations of anyone reading this thread - due to time restrictions I can not provide a course on electronics in general and valves in particular. There is huge information on these topics on the web and available in books.

      So while I'm providing IMHO a detailed enough documentation incl. layout plan to ease assembly (in German we say it's like "paint by numbers"), you MUST have general knowledge of electronics and valves, otherwise you might not only fail when something goes wrong during assembly but it also migh harm you (it's pretty high voltage inside a tube amp).

      Please keep this in mind when thinking about buying one of my kits.

      @Illias: Regarding "more immediate and substantial help" - in the end YOU have to determine a wrongly connected resistor R21, please accept that I have not and will not check photos of circuits if wired correctly.

      And last but not least, it's good to hear that it now works as it should - have fun with the amp!
      ========== MAD = Martin Adams / Digital ist nicht alles / ==========
      ========= Meine Alben: ========