Scott 299 schematic error?

Discussion in 'Tube Audio' started by thornev, Feb 3, 2019.

  1. larryderouin

    larryderouin Turn it UP, POP? PLLUUEEEZZZZZEE Subscriber

    Messages:
    22,218
    Location:
    Glen Burnie Md.
    If it's a "SIEMENS", there is a 99.9999999&4/10%certainty it's selenium. replace it with a 400v 4A silicon bridge. You'll need to add a dropping resistor afterward to compensate for voltage drop of the selenium.
     
    thornev likes this.

     

    Please register to disable this ad.

  2. thornev

    thornev Well-Known Member

    Messages:
    765
    Location:
    Mid Hudson Valley, NY
    Larry is everywhere! Thank you, Sir. I ordered an NTE 5310 600V 4A Bridge Rectifier from Parts Express. I read that it fits in a 299A.
     
  3. kvflyer

    kvflyer AK Subscriber Subscriber

    Messages:
    3,247
    Location:
    Fernandina Beach, FL, USA
    Here is the original setup on a first generation 299:

    Original-1.jpg

    And after replacement:

    replaced.jpg

    And the new resistor in the bias/filament circuit:

    Bias resistor.jpg
     
  4. kvflyer

    kvflyer AK Subscriber Subscriber

    Messages:
    3,247
    Location:
    Fernandina Beach, FL, USA
    Above is an early 299 first edition. The only "unprofessional" part is that the bias leads from the power transformer are rather short and you have to pull them tightly to attach to the silicon bridge. You can use an original hole to mount the bridge.
     
  5. thornev

    thornev Well-Known Member

    Messages:
    765
    Location:
    Mid Hudson Valley, NY
    Thanks, kv. So I do have a Selenium rectifier on my 299 version 2 as mine looks like yours. What value dropping resistor R216 did you end up using? Looks like 5W but what ohms? Original is 10 ohm 2W.
     
  6. kvflyer

    kvflyer AK Subscriber Subscriber

    Messages:
    3,247
    Location:
    Fernandina Beach, FL, USA
    I used 33 but had 22and 47 available. I just inserted the correct value to bring the voltage into the correct value.

    BTW, my name is Don Cavey ergo the "KV" which is what my Dad used on his tools so if someone "borrowed" a tool, it was obvious who owned it! The "flyer" thing comes from being a private pilot no secrets, really. When I joined AudioKarma, I didn't know anything about the site and used that "handle". I have been farting about with electronics since the mid 50s but really it's a hobby.
     

     

    Please register to disable this ad.

  7. thornev

    thornev Well-Known Member

    Messages:
    765
    Location:
    Mid Hudson Valley, NY
    Glad to meet you, Mr. Don Cavey. I'm Thorne Ventura. I've always wanted to get into electronics and when I was a teenager (in the 60's) I did things like took two 6" speakers out of my stereo, tied them together with wire and put them on my head...voila...headphones ! Now I've been opening vintage tube gear for just over 1 year and I'm enjoying it immensely, learning as I go.

    So, the dropdown resistor... Where did you put the meter probes to measure the (desired) voltage when you were trying different resistors?
     
  8. kvflyer

    kvflyer AK Subscriber Subscriber

    Messages:
    3,247
    Location:
    Fernandina Beach, FL, USA
    This is a crappy picture but my first edition from below. You can believe that changing the capacitors on the selector switch and the tone controls was a trip! But I took my time and did not break any terminals. Truth be known, there is very little voltage on the tone controls and leakage would not be as big an issue as would a coupling capacitor.

    Chassis_Below.jpg
     

    Attached Files:

  9. kvflyer

    kvflyer AK Subscriber Subscriber

    Messages:
    3,247
    Location:
    Fernandina Beach, FL, USA
    I measured the voltage of the bias circuit and the voltage of the filaments of the preamplifier tubes. For instance, the voltage to the tubes is shown as -44 volts DC (a little low since the four filaments add up to 48 volts. But that is not unusual. Off of the bridge rectifier shows -53 volts. Selenium rectifiers are not as efficient as silicon so there is an increase of voltage. We had an electronics store up until a few years in Baltimore so I stocked up on 5-watt resistors. That way, I could change what I needed to get the correct voltage. Mind you, I used a variac and set the AC voltage at 117 VAC which is what they used as a standard.
     
  10. thornev

    thornev Well-Known Member

    Messages:
    765
    Location:
    Mid Hudson Valley, NY
    I saw this table in another thread and thought it was pretty cool so I made my own. I'm sharing if anyone wants it. Thorne
     

    Attached Files:

  11. kvflyer

    kvflyer AK Subscriber Subscriber

    Messages:
    3,247
    Location:
    Fernandina Beach, FL, USA
    I see that you have similar items as me. I have a Fisher 800B, totally original (in queue), a Fisher KX-200 and matching tuner and I a pair of AR-2Ax speakers as well.
     

     

    Please register to disable this ad.

  12. thornev

    thornev Well-Known Member

    Messages:
    765
    Location:
    Mid Hudson Valley, NY
    Great minds think alike mom always told me ! :thumbsup: I lived in Boca Raton (FL) for a year and a half.
     
  13. thornev

    thornev Well-Known Member

    Messages:
    765
    Location:
    Mid Hudson Valley, NY
    Don - I don't see where the preamp voltage inputs are displayed on the Scott schematic (not the SAMS which is 299 v1). On the 299 v2 they're V2 and V102, right?

    upload_2019-2-11_14-43-31.png

    PS...Oh, -41.5V next to R212 ?

    PSS - PSYCHED !!! I just got all my tubes for the 299. Just need the variac and I can power up to see where I stand. I have the Hayseed can caps so I can do those too.
     
    Last edited: Feb 11, 2019
  14. thornev

    thornev Well-Known Member

    Messages:
    765
    Location:
    Mid Hudson Valley, NY
    I don't think anyone answered this question... Where does the thermistor go in this 299 version 2? Does it go after or before the bridge rectifier? In the 299 there is source electric, then the fuse and accessory electric jack, then some large mystery disc capacitor, then the power transformer, then the bridge rectifier and then the on/off switch.

    I replaced 2 of the 5 can capacitors today with Hayseed, the 2 behind the PT. Forgot to take a picture of the one with 11 leads, but had a picture from earlier today which helped me remember which leads go where.
     
    Last edited: Feb 11, 2019
  15. Tim D

    Tim D AK Subscriber Subscriber

    Messages:
    1,523
    Location:
    Melbourne, FL
    Thorne,

    You want the thermistor to be on the input side of the power transformer on one of the legs of the AC line cord - preferably the fuse side (but doesn't make too much difference). It shouldn't be on the output side where the rectifier sits.
     
    thornev likes this.
  16. thornev

    thornev Well-Known Member

    Messages:
    765
    Location:
    Mid Hudson Valley, NY
    I've decided the big orange thing is a thermistor by MuRata Electronics. I can't find the thermistor in the picture, but the "m" inside the "C" seems to be a MuRata symbol. Unfortunately I cannot determine from the markings how much thermal protection there is.
     

    Attached Files:

     

    Please register to disable this ad.

  17. thornev

    thornev Well-Known Member

    Messages:
    765
    Location:
    Mid Hudson Valley, NY
    Has anyone tried replacing a tube rectifier with a solid state rectifier? I know it means a "slower downer" circuit is required so the tubes don't get blasted with a rush of current, and I'll probably have to re-bias the amp. I'll have to monitor the B+ too as it will probably be higher. I'm interested in how the sound would be affected in a stereo amp, not a guitar amp, and especially the affect on bass frequencies.
     
    Last edited: Feb 12, 2019
  18. kvflyer

    kvflyer AK Subscriber Subscriber

    Messages:
    3,247
    Location:
    Fernandina Beach, FL, USA
    I think that is just a spike supressor, a very high voltage disc capacitor. (This was supposed to have been posted hours ago. Unfortunately, it stayed as a draft, sorry.)

    As for the rectifier, remember, the 340B didn't have a GZ34. It had silicon rectifiers. So, there was no slow start there. Also, the 99 series had a 5U4 rectifier. No cathode there.

    Fisher also used silicon rectifiers in the 800B receiver. Is the GZ34 missing from your amplifier?
     
  19. thornev

    thornev Well-Known Member

    Messages:
    765
    Location:
    Mid Hudson Valley, NY
    Don - Not sure what your point is about the silicon rectifiers and "no cathode."

    My 299 version 2 has a 5AR4, not GZ34 which I believe is what Fisher put in the 299 version 1. I bought a tube set for my 299 but discovered a charred area and melted wire insulation around one of the power resistors so want to replace them before powering up. I should be getting everything this week, weather permitting. I'm really itching to power up that 299 ! I'm thinking I'll be somewhat disappointed after becoming acclimated to that great Fisher sound, but who knows. :) Some people say Scott made the best stereo amps in the 60's.
     
  20. Tim D

    Tim D AK Subscriber Subscriber

    Messages:
    1,523
    Location:
    Melbourne, FL
    Hi Thorne,

    You said, "Fisher put in the 299 version 1". I think you meant "Scott" and not "Fisher".

    The "slow downer" can be provided by the thermistor, but they are only good when cold. If you turn the equipment off and back on quickly the thermistor will already by hot and passing current at its max without the slow ramp up.
     

Share This Page