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Scott 299D

Discussion in 'Tube Audio' started by NoTransistors, Sep 12, 2018.

  1. NoTransistors

    NoTransistors Dual Turntables Super-Restorer Subscriber

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    Please forgive my ignorance here. I am more adept at mechanical things than electronics.

    My Scott still has zero hum or hiss, even on 'phono', so I am fortunate here. More quiet than even my very quiet Carver transistor integrated amp. Lately, the amp has been sounding a bit unimpressive, and easily clipped. So, I put an auto-ranging meter on the bias. Bias, either left or right, was down to 27 Ma, from what used to be 35 Ma. As bias dropped evenly on both sides, it is not a 7591. I guess. The quad of 7591 is a set of GE coin base [NOS] that I have been enjoying for I guess the last two or three years. Readjusted to 35 each side and all sounds great. Great is an understatement.
    I leave the 6U8A in place, and adjust with tonearm off the record, as any input makes the meter bounce around.
    So, is something bad going on in my amp, and am I adjusting correctly? I know that the 6U8A need to come out for 'balance' adjustment. But that I did not fool with this time.

    And I just retested bias. The left, which was 1st side I had adjusted, was 32 Ma. Did adjusting the right screw up the left? Let us see if it remains stable while I cook and eat dinner.

    Interesting edit: I have the instructions for the LK-72B [basically the twin] and it states states that while the 6U8[A] are out, as you just set the balance, set the bias to 70 Ma. Guess that means 35 Ma. But I still value your opinions on what happened to bias in the several months since I last fooled with it.. I will pull the tubes now and see if the meter shows something else.

    Thank You
     
    Last edited: Sep 12, 2018

     

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  2. NoTransistors

    NoTransistors Dual Turntables Super-Restorer Subscriber

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    Worthy of new post. Phase splitters out.................Bias jumps to over 40. Reset it down to 35. Now to put them back and take a listen. Hot Hot Hot tubes.

    So, oddly, the amp still seems to sound good. Reading is 30 Ma each, with tubes back in. This is about three above from when it was sounding bad. Seems I had forgotten that I need to pull the tubes. I will see how it sounds a few minutes after a cold start. If bad like before, problem lies elsewhere. If sound is good, I will assume that the bit more of bias was enough to satisfy the output tubes.

    What you say?
     
    Last edited: Sep 12, 2018
  3. primosounds

    primosounds SE KT120 w/ 6J5G drivers. Subscriber

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    Back in the day your amp was born 7591 were around 10 bucks and being made by the hundreds if not thousands a day. Consequently, the amps were biased quite close to the tube limit. So, it is prudent , unless you have a big stash, to back it off a little to make them GE last a few more hours before replacement.
     
  4. NoTransistors

    NoTransistors Dual Turntables Super-Restorer Subscriber

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    Back it off too much, and the sound gets ruined. Would 33 Ma do, or is it that still too toasty?
    Thanks.
     
  5. primosounds

    primosounds SE KT120 w/ 6J5G drivers. Subscriber

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    You can use your own taste or ears to decide how close to the stock voltage or lower than original values. It is just a matter of balancing the tube life vs sound quality.
     
  6. NoTransistors

    NoTransistors Dual Turntables Super-Restorer Subscriber

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    :jump:

    I have wondered how the Eastern-European after-markets sound, as eventually I just might need a set. I recall that the biggest complaint about these, after the manufacturers got the innards right, was the actual size of the glass envelope. Rather than being similar to the 6V6GT, it was more like a 6L6. My amp is designed with the tubes behind the transformers, where there is more room than on earlier Scotts. And there is no wood enclosure. Tubes in the 21st century. Who would have thunk?
    In the recent Star Trek movie that reintroduced the Kahn character [no one can ever replace Ricardo Montalban], Captain Kirk is shown in bed with two alien woman, and music is coming from a Vinyl/Tube system.
     
    Last edited: Sep 13, 2018

     

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  7. NoTransistors

    NoTransistors Dual Turntables Super-Restorer Subscriber

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    Been tinkering and listening.
    The amp may work fine with bias backed off a bit, but unless set within one Ma of the recommended 35 Ma, the imaging magic simply disappears. And this is far from subtle..
    This amp still amazes me. Wipes the floor with my [similar] Fisher X-202-B.
     
  8. triode17

    triode17 Super Member

    Messages:
    1,308
    Why are you pulling the driver tube to set the Bias? Don't. You only pull it to set the DC Balance, which is hard to do even using a scope or a meter. I have the LK72A, and have installed individual 10 Ohm bias setting resistors on each tube. This way, I can get a true DC Balance.
    I looked at the sch. and found they run the 7591 at 79% of its max. dissipation. To me , that's a bit hot. I prefer to use the 70 $% standard to start with, then I usually tune it to 75% to get a bit of extra Bass bloom. 79% is bloomy sounding to me.
    BTW, I also have a Fisher X100-2 and its no where near as good sounding as the Scotts.
     
  9. NoTransistors

    NoTransistors Dual Turntables Super-Restorer Subscriber

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    20180914_141603.jpg This is a cell-phone pic of the LK-72B centerfold. This page is for instrument alignment. It also has instructions elsewhere for the person who assembled it. Those are for someone without a meter, and use the two RCA sockets and the slide switch on the chassis top for bias and balance adjustment. I do not use this silliness.
    I adjust balance with the 6U8A tubes out. I listen with headphones and adjust for minimal hum/odd noise. And while the splitters are out, set bias.
    I can be wrong in interpreting the adjustments done with instruments page. This is why I post. To learn.

    I hope you can read the page.

    Fisher was only a name. Just like Bose. Waiting for the bullets to begin to fly.
     
    Last edited: Sep 14, 2018
  10. Patrice B

    Patrice B AK Subscriber Subscriber

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    No need to remove the 6U8's to set bias, I concur with triode17. And maybe your 7591's are tired? I had one RCA pulling 25 ma where it was at 32-33 two or three years before: shot!
     
  11. NoTransistors

    NoTransistors Dual Turntables Super-Restorer Subscriber

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    Interesting.

    I have one NEW, UNUSED quad of GE 7591A. I can see if the draw is more. You guys are good.
     

     

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  12. Patrice B

    Patrice B AK Subscriber Subscriber

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    Do you have the original bias scheme or there are 10 ohms 1/4w added resistors tied to the cathodes? In my LK72A, I've installed 4 matched 10 ohms resistors and added two bias pots. This way, it is easy (relatively speaking as I have to remove the bottom cover, some installs test points on the top) to check and set bias to around 32 ma.
     
  13. NoTransistors

    NoTransistors Dual Turntables Super-Restorer Subscriber

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    The circuitry is unmolested. I put two bias test points on the rear of the front-end shield for easy access.
     
  14. Patrice B

    Patrice B AK Subscriber Subscriber

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    Ok, looking at schematic, there are two test point so you have to measure (example) 64 ma on one test point to assume that maybe there is 32 ma on each tube? Correct? And it's why you have to adjust balance by ear, right? I know that I repeat myself but by adding 10 ohms resistors from cathodes to ground, you would not have to set bias balance by ear. You'd do all of it with a simple DVM.

    BTW, really love my two Dual idlers, a 1019 restored by a former Dual assistant service manager here in Canada and a 1215...!

    Patrice
     
  15. NoTransistors

    NoTransistors Dual Turntables Super-Restorer Subscriber

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    Well this is maddening. Pulled the splitters just to compare with last readings.
    Both sides were set at 34.
    With NEW tuves, left jumped to 38. Right to 42. Should have gotten more when I bought these two sets.
    Any reading is for both tubes on that side. So the readings are each in the 30s. A 32 reading is the sum of the two tubes divided by two, i guess.
     
  16. NoTransistors

    NoTransistors Dual Turntables Super-Restorer Subscriber

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    BTW, with the new tubes, I can crank it.

    Bias has remained 33 each side for the last two L.P.s. Splitters in place. Still out on why you two guys both disagree with the Scott instructions.
     
    Last edited: Sep 14, 2018

     

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  17. NoTransistors

    NoTransistors Dual Turntables Super-Restorer Subscriber

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    Perhaps I should get a set of the new Tung-Sol @ $33 a tube before Trump and Putin have a falling-out or one of them gets pregnant or something.
     
  18. gkargreen

    gkargreen Active Member

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    Hi NoTransistors, perhaps I can be of help. I would concur with the above suggestions, if you can put a 10 1/4watt resistor in each leg of the output tubes, you will get an accurate measure of the current drawn by each tube. I don't know if this model has both bias output and DC balance, the DC balance balances the amount of bias applied to each tube in a pair, the bias provides the amount of bias voltage needed for the tubes, Scott was pretty good about providing both controls which many other manufacturers did not. Unless your output tubes are dynamically balanced, ie., balanced to the current drawn from each, you will get problems, which is great for Scotts since they allow you to use unbalanced (to a degree) tubes and just adjust the cathode current for each to match up. And yes, you shouldn't need to remove the phase splitters to do this, however, I have found on many of the Scotts I have worked on that the phase splitter resistors (and probably the caps as well, but I always replaced the caps from the phase splitters to the output tubes) tend to go out of value and give bad voltage readings wrt the phase splitter tubes, so I would check the voltages around the phase splitters while you got a meter out just to see. As far as new 7591, what ever you do, STAY AWAY FROM THE JJ 7591 TUBES!!! They are poorly manufactured and often will blow out/short out within a week of use, the EH and Tung-sols are much better and safer to run. There are a number of things that you can do to make this amp safety-proof as well with a CL-80 in the primary of the power tranny, and screen safety resistors, I don't restore any of these without those safety resistors. If you have any questions, feel free to ask, I have restored a fair number of Scotts, right now I have a 340A and a 399, although I am running a different setup currently.
     
  19. NoTransistors

    NoTransistors Dual Turntables Super-Restorer Subscriber

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    Going to dinner and after read and re-read above, juicy post. Too bad about JJ Electronic. They seemed to show great potential when first putting themselves 'out there'.
     
  20. NoTransistors

    NoTransistors Dual Turntables Super-Restorer Subscriber

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    Still have not eaten dinner.
    Firstly, I adjusted balance. All tubes in place, input selector not on phono, as the hiss would hide what I need to hear, and headphones on and volume cranked. Amazingly, the two chassis balance pots wound up in virtually the same position.Means these new tubes are close in specs.
    On to the bias. BOTH chassis pots are in the same position for the same 32.5 Ma bias. Nice.
    Then while the volume was cranked, I put selector to phono. One side had no 'rushing' noise, the other had plenty. Took out the 4 Telefunkens, replaced them with green-print Baldwin 12AX7A. Now a bit of rushing sound on both with volume cranked. These Baldwins sound different from the Telefunkens. And have more gain, so a bit of hum when cranked, but only on phono, of course.

    No wonder why people like transistors. Half the fidelity but less fuss.
     
    Last edited: Sep 14, 2018

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