Scott 299C, no bias pots

Discussion in 'Tube Audio' started by kind, Jun 13, 2018.

  1. kind

    kind New Member

    Messages:
    7
    Hi!
    Few days ago i purchased 1962 Scott 299C amp. I like it a lot, it realy shines after all modern amps I have had.
    I wasn't very lucky: my amp nees some restoration.

    First,
    There is a 50 Hz hum coming out of right speaker. Sometimes it disappears, but it comes back after a while.
    I swapped tubes L <-> R. Hum is there..
    I don't know what to do with that, so I'm going to do full recap and try another 12ax7s.

    Second,
    I measured voltage on pin 6 of 9175. It was -16 V (schematic says it should be -18.5 V).
    Also there is only 300 V on pins 3 and 275 V on pins 8 (should be 420 and 410 V).
    I found that power transformer doesn't supply proper voltage to rectifier tube: 600 V before rectifier, and 300 V after.
    I use 220V-110V transformer to power my amp for my local voltage is 220 V. It/s 2 kWt
    Could the voltage SAG be the reason of low voltages in the amp?

    Third.
    Pity but my amp has no bias pots. It has fixed bias. That's what Scott's site says:
    "Only a handful of vintage H.H. Scott amplifiers employ Fixed Bias that is truly fixed. By that we mean there's not a Bias Control potentiometer to be found! Such models include: Types 208, 222-C, 299-C, 296, and 340 receiver. Assuming everything else is healthy, you're good to go."
    What if I change tubes? Shouldn't I do bias adjustment after that?

    Thanks for any help. I am new in this.
     

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  2. century tek

    century tek Super Member

    Messages:
    3,229
    Location:
    The Evergreen State
    (1)
    Right speaker hum sounds like a bad solder or ground connection somewhere. If it were a power supply problem, you would get hum in both speakers equally.

    (2)
    Low voltages could be a bad power filter capacitor and/or a bad 5AR4 rectifier tube pulling down the B+.
    Pull the 5AR4 tube and see what the AC voltages are coming out of the transformer. It should be pretty much spot on. Your meter should be set on AC volts not DC volts scale for this.

    (3)
    If it has fixed bias, there would be no need to worry about biasing new tubes.

    (4)
    Don't do shot gun (guessing) repairs. It always makes things worse.
     
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  3. primosounds

    primosounds SE KT120 w/ 6J5G drivers. LCLC ps + 2 x 5ar4 Subscriber

    Messages:
    2,484
    Location:
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    An old tube amp is like an old car, it may need to be restored, but unlike new electronics, you can do it yourself because all the parts are there for you to change. From your picture, the amp looks to have original parts. Parts like the carbon composition resistors will usually develop more resistance as they age. This added resistance can significantly affect the voltages in the amp. The fixed bias circuit is also need of rejuvenation. The stock selenium rectifier can also develop excessive resistance which lowers the negative voltage output which according to your measurement is -16v. This selenium rectifier should be changed to a modern silicon diode or even schottky diode. All the carbon comp resistors in the bias circuit should be changed which should make the bias voltage close to the original.
    The coupling capacitors can also affect voltage readings and more importantly sonic performance. IF the caps are leaking some DCV the amp can have hum from that. Anyway, you get the picture. Rebuild the amp starting with the power supplies then working from the power tubes to the preamp tubes. Do one channel at a time and after rebuilding the one channel or tube socket , test your work by playing the amp. In this way you will catch a problem before moving on. The other side of the amp can also serve as a reference to your new work.
     
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  4. kind

    kind New Member

    Messages:
    7
    Thank you for your answers.

    century tek
    The amp has weak power from the begining. The points I measured are:

    upload_2018-6-14_9-14-8.png

    I think it's a step-down transformer can't hold a load. But this is just my guess.

    What I don't uderstand is whether fixed bias good thing or bad thing :)
    What if bias goes beyond recommended parameters aftter tube change?


    primosounds
    Lot of good sense in your words.
    Step by step is a good advice to follow.
     
  5. kvflyer

    kvflyer AK Subscriber Subscriber

    Messages:
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    Location:
    Fernandina Beach, FL, USA
    Failing (leaky, shorted) filter capacitors are likely pulling down the B+ voltage. You might want to attend to them first before anything else. Notice also C207-08-09 and C210. That is the bias supply for the output tubes. It creates a negative voltage for the grids of the 7591 tubes as well as a DC voltage for the 12AX7 preamp tubes. It too may need attention. After that, you might want to concentrate on the coupling capacitors, specifically the .1uFD capacitors which go to the grids of the output tubes. Any failures (leakage) there will cause the output tubes to conduct harder and also drag down the B+.
     
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  6. Dandy

    Dandy Super Member

    Messages:
    4,447
    Location:
    North West, UK
    Welcome kind.

    The above advice is all good. Do the three steps that kvflyer recommends in that order: filter caps, bias supply caps and bridge rectifier in place of the selenium one and coupling caps. There are lots of good rebuild threads here.

    One more thing, this amp expects to see 117VAC, not 110. So your step down transformer might not be right for this purpose. Where in the world are you? I ran my Scott 222C on 110VAC for a while. It worked but not very well.

    Check out the rebuilds by Dave451 here, like this one on a similar Scott to yours. Lots of detail on how to do each step. http://audiokarma.org/forums/index.php?threads/scott-lk-72-rehab.784404/
     
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  7. kind

    kind New Member

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    kvflyer
    Dandy

    Many thanks. Surely I'll do everything you adviced.
    My first step is reading :) and collecting more info in order to understand the very basics. Thanks for pointing to Dave45 thread.

    I don't think my Scott is 100% original. Somebody has already been there :)
    upload_2018-6-14_20-53-43.png

    I haven't seen that green guy in the pictures of other Scotts.
    And C207, 208, 209, 210 are replaced.
    I wish it was original...
    Nevertheless, I think I'll savvy ))

    Actually I have two transformers to feed my Scott. I mentioned the powerful one.
    The another one is about 250 Wt. It shows 118 V on secondary winding without load. When amp is on it goes down to 105 V because of SAG. I think this is not good.
    Apparently I have to buy the third one )))

    I'm in Moscow, Russia.
     
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  8. primosounds

    primosounds SE KT120 w/ 6J5G drivers. LCLC ps + 2 x 5ar4 Subscriber

    Messages:
    2,484
    Location:
    Terra, 21st century CE
    Yeah your second transformer is probably working too hard and will fail sooner or later. You should probably get that sorted first as the AC supply is the most critical issue.
     
  9. kind

    kind New Member

    Messages:
    7
    Yes, I think so.

    Today I've got NOS Mullard GZ34. My previous was USA Tung-Sol.
    I would say Scott sounds darker with Mullard and lacks some dynamics. Waiting for the "rock'n'roll" to come after burn-in time.
    At the moment I would prefer Tung-Sol.
     
  10. Dandy

    Dandy Super Member

    Messages:
    4,447
    Location:
    North West, UK
    As long as you have 105VAC going in, you are within the range from Scott.

    If your capacitors 207 - 210 have been replaced, that is good. as long as the green resistor is the right resistance, don't worry. Maybe the guy before you was a good tech! Did he replace your power supply capacitors too (c205 - 201)?

    Great to have members here from Russia. I am in the UK.
     
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  11. triode17

    triode17 Super Member

    Messages:
    1,049
    As far as the hum goes, try changing the 6GH8s. With my LK-72 (equiv. to 299C), I had to try quite a few 6GH8s until I got a pair that I liked. Ended up using Sylvanias'. If that solves the symptom, you need to solve the cause. First off, check the plate voltage of V3-A. It should be 65v , +/- about 5%. If not, you can adjust the value of R38, screen grid resistance to set the 65v. This biases the phase inverter properly. If the V3-B cathode is at 75v, you really should float the heater supply on about 50Vdc. (The max. H-K voltage is 100v) A two resistor voltage divider from a convenient internal supply can be used to provide the 50v, and add a 0,1uF across it for an AC reference. Now, the H-K voltage is only 25v. This has worked well for me.
     
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  12. kvflyer

    kvflyer AK Subscriber Subscriber

    Messages:
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    Location:
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    A quick word about 6GH8 tubes. RCA used a ton of them in their early color receivers. Service calls were a goldmine for the repairmen. RCA figured out that their 6GH8 tubes were not as robust as those manufactured by Sylvania. So, guess what? RCA bought Sylvania 6GH8 tubes, had Sylvania label them as RCA and installed them in their new color sets. (Not uncommon for tube manufacturers to buy from each other).

    So what does that tell you??
     
  13. Dandy

    Dandy Super Member

    Messages:
    4,447
    Location:
    North West, UK
    I really would make sure the filter caps are good first.
     
  14. triode17

    triode17 Super Member

    Messages:
    1,049
    Yes do that first ( I may have jumped ahead to fast).
     
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  15. kind

    kind New Member

    Messages:
    7
    Dandy, greeting.:beerchug:
    What about Russia vs Enland in the FIFA 2018 final? :)
    We have a rush in here.

    He didn't, and I think this is good news. I prefer to do it myself ))
    Changing caps is in do-it-first list.


    triode17
    Thank you for guideline. I'm going to buy pair of Telefunken at reasonable price. About 10 USD per piece.
    After that I'll measure all voltages.


    It was the beginning of the end of the world (as we knew it). :)
    Same thing with Mullard tubes...
    It means one can get get good tube at half price.
     
  16. Dandy

    Dandy Super Member

    Messages:
    4,447
    Location:
    North West, UK
    I don't follow football, but would watch that for sure!

    I'm glad you are making progress with your amp.
     

     

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  17. primosounds

    primosounds SE KT120 w/ 6J5G drivers. LCLC ps + 2 x 5ar4 Subscriber

    Messages:
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    Location:
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    Say, if you can get 6bl8 Teles at 10 bucks a pop can you get some for me too? cheers.
     
  18. kind

    kind New Member

    Messages:
    7
    primosounds
    ok )) But it's more than 100 USD here. So...
    BTW I don't like how Telefunken 6U8s sound. May be they need time to burn in?
    At the moment they sound dull and saggy.

    Today I replaced old bridge with new one. Plus changed resistor (18 -> 30 Om). Now I have -44 VDC where should be -40 VDC according to schematic.
    All other voltages raised as well.
    Bias voltage is -22 VDC
    433 VDC on pin 3 of output tube.
    But 396 VDC on pin 8 (lower then it should be).
    Can I go with these voltages?
     
  19. kind

    kind New Member

    Messages:
    7
    I found a strange thing while testing the amp.
    One speaker has constant continuous hum. But when I touch any pin on output tube with probe the hum abruptly disapeares.
    After about 30 seconds it gradually comes back.
    Touch again and it's gone etc.
    What on earth is going on out there?

    I should change caps ASAP.
    Spent some time looking for russian K40Y-9. Didn't find any around.
    Which caps are good with Scotts?

    There are .25 mF 200 VDC polar caps. Are they electrolytes?
    If they are not - can I use non-polar instead?
     
    Last edited: Jun 16, 2018

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