Building An Amp How to Rout Signal Wires

Discussion in 'Building the Classics' started by Rotorcraft230, Dec 24, 2019.

  1. StingRay85

    StingRay85 Active Member

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    I just made some plate voltage readings, wow it is a lot.

    EL34: 450V (that's as expected)
    V3: plate 1(pin 6) 263V; plate 2 (pin 1) 246V
    V2: P1 327V; P2 183V
    V1: P1 230V; P2 280V

    I have a feeling this is way too high, but does it make sense when looking at the 2204 pre schematic? As you know I'm a newbie with this stuff
     
    Last edited: Mar 21, 2020
  2. neikeel

    neikeel Well-Known Member

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    You will probably need to add another 1 or 2w dropping resistor after the screens/choke connection. I don't know hat you have now but try a 10k or 15k in series with the existing one, that will lower all the preamp plate voltages a little.
     
  3. StingRay85

    StingRay85 Active Member

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    Yeah that's what I got suggested already, try 25k. But do all the other values make sense?
     
  4. MickeyJ

    MickeyJ Well-Known Member

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    it makes sense if you like it.

    now may be fine tuning time.

    remember, those 2204 plans take into account all the things you've changed ie: everything but the preamp itself.
    the values mean very little any more. just vary the plate voltages on the preamp and P/I to taste, maybe vary the cathode voltages to taste too, you can get cathode voltages wrong and cook the tubes, make sure your cathode voltages are in range..pins 3 and 8 on preamp tubes.
    time for the hard work.

    use your ears, and some resistors. how low is too low? 100v?

    Again, you cannot, on this amp, cheat what the power supply defines the amp as.( change o/t and this changes big time)

    you can fine tune though..there is very little wiggle room though.

    to a very large extent..it is what it is. ( change the o/t , and you change what it is to a large degree)
     
  5. StingRay85

    StingRay85 Active Member

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    I will probably leave the cathodes alone. Just the first drop resistor needs to be increased to a point where I'm at least below 200V on all plates. Trial and error. Drop in 25k, measure. Still to high, try 30k or 35k etc...
     
  6. StingRay85

    StingRay85 Active Member

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    The RFT powertubes were sitting at around 39-40 mA, or 70% which is hot bias but acceptable for the tubes. Brought it down to around 30mA, somewhere between normal and cold.

    A bit crazy that I didnt even bother to check bias on the powertubes up to now. They were an awesome matched sextet at 70%
     
  7. StingRay85

    StingRay85 Active Member

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    I measured my '79 2104 chassis.

    390V EL34.

    V1 P1 203 P2 246
    V2 P1 286 P2 155
    V3 P1 203 P2 210

    Now I know what to aim for
     
  8. StingRay85

    StingRay85 Active Member

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    In the end I settled for 30k. It's about 10V lower what I measured on the 2104.

    Also, I have the voltage selector set at 245, while our net voltage is 230. So if I want to increase voltage on all plates, I can switch it to 225V.

    I also wired in the low input jack. More work than I expected, but I'm glad I did it. Its noise free. I think if you want to use the amp as a pedal platform for clean headroom, this is probably the input to select.
     
  9. StingRay85

    StingRay85 Active Member

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    IMG-20200322-WA0001.jpeg

    All done. Low input channel sounds awesome with bass
     
    neikeel likes this.

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