Totally beginner magnavox 8608

Discussion in 'Tube Audio' started by Jacques_579, Jun 27, 2018.

  1. Jacques_579

    Jacques_579 Member

    Messages:
    64
    FAF12EAF-5A2D-49F5-A588-8CB0DFEEA1DB.jpeg Hi guys,
    I’ve recently acquired a magnavox 8608 from eBay. The amp was coming with a 50/50 uf can cap and 2 Russian PIO uninstal ( which I install when I received the amp ). After installing them the bass was kinda off and was told to change the bias. Now, I’m a total newb with tube amp ( I’ve read ,well tryed too, the Dave Gillespie thread on flea power) and I’m lost. I would like to modified it to the simplest as he did with his first shematic. I’m French and would pretty much like some help but in laymens term ( you guys can get pretty technical ). So please be gentle with me as I’m trying to understand something that seems pretty hard from my point of you lol.
    Thanks
    Fred
     

     

    Please register to disable this ad.

  2. s-petersen

    s-petersen Scott Subscriber

    Messages:
    4,480
    Location:
    Middletown,New York
    Does your amp have the original transformers?
     
  3. Jacques_579

    Jacques_579 Member

    Messages:
    64
    Everything except PIO and the speaker terminal are original. I do want to change them but just waiting a bit before ordering them if I need more thing
     
  4. Brice

    Brice AK Subscriber Subscriber

    Messages:
    1,464
    Location:
    Hoboken NJ
    Jacques, where are you located?
     
  5. s-petersen

    s-petersen Scott Subscriber

    Messages:
    4,480
    Location:
    Middletown,New York
    Are your speakers 4 ohm?
     
  6. Jacques_579

    Jacques_579 Member

    Messages:
    64
    Montreal, canada
     

     

    Please register to disable this ad.

  7. Jacques_579

    Jacques_579 Member

    Messages:
    64
    No, 8ohm
     
  8. frankiebull

    frankiebull Active Member

    Messages:
    211
    Location:
    Oklahoma City, Oklahoma
    Those POI's appear to be in the wrong location. The need to REPLACE the 2 small brown round ceramic disc capacitors. One of which goes from pin 6 of 6EU7 tube to pin 2 of 6BQ5 tube. The other goes from pin 7 of 6EU7 to pin 2 of other 6BQ5. I re did mine per Dave's thread including the new output transformers and choke which are reasonably priced from AES. I just took my time and it was not as intimidating as I thought when I first got started. If you follow his schematic you will note that a lot of that wiring goes away.
     

    Attached Files:

  9. frankiebull

    frankiebull Active Member

    Messages:
    211
    Location:
    Oklahoma City, Oklahoma
    The 2 big white capacitors you replaced with the POI's are part of the wiring that goes away.
     
  10. Jacques_579

    Jacques_579 Member

    Messages:
    64
    I think mine is a bit different then his as I did had two sprague .047 exactly at the same place as the new PIO are now. I’m wondering if I can follow is set up even tho there’s slight differences? And I ain’t that good reading a schematic but trying.
    Thanks
     
  11. FlaCharlie

    FlaCharlie Well-Known Member

    Messages:
    570
    Location:
    Gainesville, FL
    If you read Dave's mod thread you'll notice that the amp performs very poorly if you use 8 ohm speakers because the output transformers have a secondary that's rated at 3-4 ohms. So that's one issue. It's easily solved, however, if you have two sets of 8 ohm speakers. Just connect them in parallel and the result is a 4 ohm load. Put the second pair on top of the first pair and run a short wire from positive to positive and negative to negative so you have two speakers connected to each channel.

    Also, you say you replaced the original can cap with one rated 50/50 uf. The original can has three sections not two. They are 40uf - 40uf - 30uf. The 8300 model, which is very similar, uses a 20uf for the last section. Did you add a third section using a separate cap? Notice there is a dropping resistor between each cap section.

    You will sometimes find differences in various versions of these amps but the basic circuit is the same. Which schematic are you using?

    Reading a schematic can be a bit confusing at first. Think of it like a road map. The lines are the roads (they could be wires or the leads of a resistor or cap) and the various parts (resistors, capacitors, transformers, pots, etc) are like towns along the road. If you follow the road you will come to the town shown on the schematic. Unlike a road map, however, the distance between the towns may be quite different than the schematic might indicate.

    As for the coupling caps, take a look at the schematic. The 6EU7 is a dual triode, two separate triode sections within one tube. The coupling caps go between the plate of the 6EU7 (pin 6 and pin 7 are the two plates) and the grid of the 6BQ5 (that's pin 1 or 2, either pin will work since they are connected internally). So one coupling cap from 6EU7 pin 6 to the grid of one 6BQ5 and the other coupling cap from 6EU7 pin 7 to the grid of the other 6BQ5.

    I haven't built Dave's famous mod but I may at some point. I recently rebuild an 8300 version which I modded to use 12BH7s as output tubes. This works well with the stock transformers and 8 ohm speakers but it's lower power than Dave's mod.

    The 8300 is essentially the same amp but it used a smaller power transformer because it had no AM-FM tuner, only a turntable. Here's a link to my thread on that mod: http://www.audiokarma.org/forums/in...0-or-8600-conversion-to-12bh7-outputs.824135/
     
    Last edited: Jun 28, 2018

     

    Please register to disable this ad.

  12. Jacques_579

    Jacques_579 Member

    Messages:
    64
    I’m waiting for the two output 8ohms and no I haven’t changed the can cap but the 50/50 did come with it. My can is 40/40/20/30 tho
     
  13. Jacques_579

    Jacques_579 Member

    Messages:
    64
    Also thanks for the tip about the schematic and a road map it worked for me. I did made a list of every cap and resistor from the schematic and waiting for them to start it.
     
  14. Jacques_579

    Jacques_579 Member

    Messages:
    64
    Hi guys,
    I finally received all the parts I was missing and assemble the amp the best I could ( being my first attempt at reading a schematic). Now I had a lil trouble figuring out the speakers parts as Dave is using the original terminal and I new ones. I did what I think was the right things, but now when I hook the amp what I hear is loud buzz/interference and nothing else.
    I’ll add some pics so you one of you might help me
    Thanks
    Fred
     

    Attached Files:

  15. century tek

    century tek Super Member

    Messages:
    3,272
    Location:
    The Evergreen State
    Did you replace the big can capacitor? It doesn't look like it to me. If not, this could be why it is humming. If your feedback circuit is messed up, this will also cause squeal, whistle, and other weird things to be heard.

    EDIT: It looks like your speaker binding posts are not connected right. The green wires that come out of both speaker transformers is the positive and should be connected to the red posts, and the black wires are negative and should be connected to the black posts which are both negative and chassis ground. Your feedback resistors need to be on the positive (red) side.
     
    Last edited: Jul 12, 2018
  16. Jacques_579

    Jacques_579 Member

    Messages:
    64
    I didn’t because it was working pretty good before I started the project. Could it have gone bad while working on it?
     

     

    Please register to disable this ad.

  17. nj pheonix

    nj pheonix AK Subscriber Subscriber

    Messages:
    9,743
    Location:
    New Jersey
    You should reach out to member Brice.
    He's trilingual. He speaks English, French, and tubes.
    He should be able to clarify things for you.
     
  18. century tek

    century tek Super Member

    Messages:
    3,272
    Location:
    The Evergreen State
    Ok, so if you didn't touch it before and after, just make sure that it is still connected and properly grounded.

    Short the left & right inputs. If it quiets down, then its a ground loop problem.
     
  19. Jacques_579

    Jacques_579 Member

    Messages:
    64
    It looks ok from what I can see. What do you mean by shorting the inputs?
     
  20. Jacques_579

    Jacques_579 Member

    Messages:
    64
    What I’m realizing now is that I hear music from left channel but loud buzz from right and no music at all
     

Share This Page