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:
    8,773
    Location:
    Ball Ground, GA
    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 junk junkie Subscriber

    Messages:
    32,431
    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

    Messages:
    29
    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

    Messages:
    29
    Location:
    Auburn WA
    Thanks Gadget!
     
  5. dallen9494

    dallen9494 New Member

    Messages:
    29
    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:
    8,773
    Location:
    Ball Ground, GA
    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:
    29
    Location:
    Auburn WA
    Dave,

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

    Dennis
     
  8. thorpej

    thorpej AK Subscriber Subscriber

    Messages:
    1,357
    Location:
    San Francisco, CA
    OK! In recent weeks, I've referred a few people to this thread, and realized I never followed up with the final schematic. No drastic changes, just a couple of corrections for clarity. I also made a note of which Edcor power transformer would work well if someone were to scratch-build this circuit without any original Magnavox parts.

    [Edit -- updated with a couple of additional notes as requested below.]

    Enjoy!
     

    Attached Files:

    Last edited: Jul 16, 2017
  9. ObinRobinson

    ObinRobinson New Member

    Messages:
    6
    Location:
    Jacksonville, FL
    Thank you for the update! For ~extra~ clarification would you be able to specify which pins are being connected to the tubes in the diagrams? I have only built solid state circuits before and I don't want to risk wiring these tubes backwards and blowing something up! Also how do I know I the tubes are biased properly?

    Obin :)
     
  10. thorpej

    thorpej AK Subscriber Subscriber

    Messages:
    1,357
    Location:
    San Francisco, CA
    That's what I get for using the word "final" :) I just uploaded an edit on the above post with the requested clarifications, and also made a note that you only need ONE of the EFB(tm) cathode regulators (upshot -- you need to use a matched-quad of tubes because that's the adjustment granularity). As for wiring up the tubes wrong -- I always find it handy to have a copy of the tube's data sheet handy when building. Note in my schematic update, I left off the pins used for the tube heaters (because I never draw the heaters in my schematics unless some exotic configuration is necessary).

    As for bias, as noted on the schematic, you want to measure the voltage drop across the 10 ohm 2W 1% resistor that sits between the 6V6 cathodes and the LM337 output terminal. The bias will be adjusted properly when you have a 1.2V drop across that resistor, which represents 120mA of current (30mA per 6V6 of combined plate and screen current). This should be at around +22V on the 6V6 cathodes (which is a bias of -22V, because the grid is pulled down to ground).
     
    ObinRobinson likes this.
  11. ObinRobinson

    ObinRobinson New Member

    Messages:
    6
    Location:
    Jacksonville, FL
    Thank you for that update! One last question: will this circuit work with the 6P6S Russian tubes just as well as the 6V6? The 6P6S tubes are easier for me to find.

    Thanks again!

    Obin
     
  12. thorpej

    thorpej AK Subscriber Subscriber

    Messages:
    1,357
    Location:
    San Francisco, CA
    6P6S are 100% equivalent to 6V6. That is, in fact, what I ended up installing in my son's amp (the original tubes ended up just being too tired out).
     
    ObinRobinson likes this.
  13. ObinRobinson

    ObinRobinson New Member

    Messages:
    6
    Location:
    Jacksonville, FL
    Excellent! I am going to start shopping for all the components. Thank you again for the help!

    Obin :)
     
    thorpej likes this.
  14. gadget73

    gadget73 junk junkie Subscriber

    Messages:
    32,431
    Location:
    Southern NJ
    If you have an option to get matched tubes, go for it. I don't know the 6P6S, but the EL84 equivalents are pretty varied. The ones I have are different enough that they won't even play nice in the amplifier together.
     
    thorpej likes this.
  15. thorpej

    thorpej AK Subscriber Subscriber

    Messages:
    1,357
    Location:
    San Francisco, CA
    Yes that's a good point. In my case, I had a huge batch of 6P6Ss that I could select 4 closely-matched tubes from. If you're just going to buy some random 4 tubes, you might be better off ordering a matched quad of JJ 6V6s from e.g. Eurotubes (my go-to for JJs).
     
    ObinRobinson likes this.
  16. dallen9494

    dallen9494 New Member

    Messages:
    29
    Location:
    Auburn WA
    I finally found some time to finish the amp I built for my son for Christmas. Thanks thorpej and dgillespie for all of your help! I just finished listening to it for an hour or two, I might have to give him some socks instead!
     

    Attached Files:

  17. nj pheonix

    nj pheonix AK Subscriber Subscriber

    Messages:
    7,847
    Location:
    New Jersey
    Nice build! Very clean.
    All kinds of space under the hood, like a mid 60s pick up truck:thumbsup:
     
  18. Lavane

    Lavane AK Subscriber Subscriber

    Messages:
    5,516
    Location:
    northern cal.
    Nice! I used the same hammered silver on a 9303.
     
  19. thorpej

    thorpej AK Subscriber Subscriber

    Messages:
    1,357
    Location:
    San Francisco, CA
    Hey, that looks GREAT! Very nice work! I'm sure you son is going to really enjoy that!
     
  20. dallen9494

    dallen9494 New Member

    Messages:
    29
    Location:
    Auburn WA
    Thanks! I've got a few months to enjoy it before giving it up! I've got a couple of questions though. My voltages are a little low. I basically copied thorpej's P/S except I used a 1.5H 85 Ohm choke in the PS, and did the bucking mod and I've got:
    line voltage 121v
    308v before choke (C1)
    298v B+ (C2)
    288v (C3)
    246v (C4) drivers
    225v (C5) buffer (added from the 9300 dg schematic with help from Dave)
    Heaters 6.3v

    I'm guessing my voltages are low from the bucking mod. The rest of the build copies the final 9300 front end except changing the 100 Ohm resistor on the cathodes of the driver tube to 160 Ohm (thanks Dave!), and adding a 100k attenuator. The EFB follows thorpej's. My questions are these voltages OK? Do I still set the bias to 1.2v?

    I can follow a schematic, but I am still wrapping my head around all this!

    Regards,
    Dennis
     

Share This Page