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    Hi all. Brand new to the forum, forums in general + electronics. I am a guitar builder who wants to know a lot more about the entire instrument from the guitar through the amp. This will be my third amp build – the first two were warm-ups – an “Altoids” amp and a Noisy Cricket – both fun as I get my feet wet. I’ve learned a lot but still have a long way to go.

    OK – on to the questions.

    My PT shows differently than the diagram – it has different colors of wires and 9 instead of 10. Pair of yellow (ok), pair of red (ok), pair of green (ok), pair of black (see below) and one red/yellow (?).

    I checked with Andy (very helpful – thanks!) if I had the correct component and I do. I still need to correlate the PT wiring to the diagram and I am posting a picture of what I have + a marked up diagram.

    Here are the questions:
    1) is the mark up correct?
    2) does it matter which black wire I use for connection to the switch or fuse?
    3) since I don’t have a green/yellow this does not get connected to the ground terminal – right? No substitute or other – right?
    4) connect the red/yellow as shown in the diagram – right?

    Thanks to Andy and Robin and the whole community – looking forward to this build!


    Transformer manufactures sometimes change color coding, so it’s always best to confirm the lead colors with the information you receive with the transformer.

    That said, the general color codes for the secondary taps are: Red = B+ (high voltage), The red/yellow tap is the B+ center tap and goes to ground.
    Green = 6.3v tap going to the heater circuit, Yellow – 5v tap, goes to the rectifier heater. Black = primary winding, connects to A/C source (120v in the USA). If there is no center tap provided for the 6.3v coil (yellow/green on the layout), then an artificial center tap is required to suppress noise is the heater circuit. That’s the two 100 ohm resistors you have added. If a PT comes with additional taps (and many do), do not assume that the multicolored leads are all center taps and wire them to ground. Unused taps should be cut off, cut shout or covered to ensure they do not contact anything.



    Thanks so much. Still learning to read the schematic but it helps to have your words . If I understand correctly, the green/yellow, if supplied, is the center-tap and would not require the pair of 100ohm resistors. This was the piece of info that I was missing in my understanding.


    Now all I have to do is add the two 1M resistors from S1 and S2 to ground.

    I think I’ve got it now!


    The most important part of my previous post is to check and confirm the leads on your transformer by using the supplied info from the transformer supplier.
    The color codes really do change. You could also confirm the taps by checking the turns ratios with a multimeter, there are tutorials on the web that explain the process.

    Regarding balancing the heater circuit, you can use the supplied 5.3v circuit center tap, make an artificial center tap by adding 100ohm resistors to ground to the heater circuit OR add a “humdinger” pot (500ohm) between the heater leads to fine tune the 6.3 v circuit balance.
    Merlin Blencowe’s comments on heater circuits: http://www.valvewizard.co.uk/heater.html

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