A modified Magnavox 8802 in the Gillespie style

Discussion in 'Tube Audio' started by thorpej, Aug 22, 2016.

  1. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

    Messages:
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    Dennis -- If you are planning on implementing the EFB circuit with 6V6s, you will want to ensure adequate heatsinking of the 337 regulator. With an estimated 100 mA flowing through it, and the higher bias requirements of the 6V6 family, it will be dissipating 3 watts of power -- which is fine as long as the heat sinking is adequate. As you know, I simply used the chassis, which should be fine, but not knowing your build, it's a point to be aware of.

    Also, you can use the same exact NFB arrangement, but you will want to increase the value of the 100Ω resistor running between the two cathodes of the driver tube to 180Ω to make up for the loss of output stage gain. Input sensitivity will require just over 1 volt to develop full power output.

    Good luck with it!

    Dave
     
  2. gadget73

    gadget73 AK Subscriber Subscriber

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    Location:
    Southern NJ
    Should turn out nice. I hear really good things about the A500. They didn't shirk on the gravity of those things, the complete amp is very heavy and the transformers are pretty stout. I have one amp running 7355's, but its not an H-K.
     
  3. dallen9494

    dallen9494 New Member

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    Location:
    Auburn WA
    Thanks Dave! Do you think I should put a heat sink on the 337? I think there will be enough space, or is it sufficient to mount it to the chassis as long as it has good contact?

    Thanks again for your help,
    Dennis
     
  4. dallen9494

    dallen9494 New Member

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    Location:
    Auburn WA
    Thanks Gadget!
     
  5. dallen9494

    dallen9494 New Member

    Messages:
    17
    Location:
    Auburn WA
    Dave, I just want to make sure I understand, do you mean to replace the 100 ohm resistors on the screen grids to 180 ohm? It appears that the resistor between the cathodes is a 10 ohm on the schematic, I just want to make sure. Also, just to clarify, should I copy the NFB from the 9300 schematic, or the 8802? Sorry for the confusion, I'm easily confused! I just want to make sure since I'm going to use the 9300 OPT's.
     
  6. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

    Messages:
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    Location:
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    No. The 100Ω resistor I'm referring to is a single resistor in each channel running between the cathodes of the DRIVER tube. That resistor determines the amount of NFB applied around the amplifier circuits proper, and needs to be adjusted to account for the Gm of the 6V6s. Use the NFB design of the modified 9300 schematic that retains the original OPTs.

    Dave
     
  7. dallen9494

    dallen9494 New Member

    Messages:
    17
    Location:
    Auburn WA
    Dave,

    I noticed you said driver tubes right after I posted, thanks for the clarification.

    Dennis
     

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