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Scott Stereomaster distortion Type 340 Tuner/Amp

Discussion in 'Tube Audio' started by LUKE RANDALL, Jun 2, 2018.

  1. LUKE RANDALL

    LUKE RANDALL New Member

    Messages:
    18
    Location:
    Rhode Island
    Is there anyone out there that knows about Scott amplifiers? I have a Scott Stereomaster Model 340 Tuner/amplifier serial number- 202267. I can not find the circuit diagram for this particular unit. I have found many versions but none have matched mine. I am having trouble with distortion. I have replaced all the coupling capacitors, all the electrolytic, new tubes in power, rectifier and preamp. The problem seems to be in the phase inverter stage with the 6U8 tubes but because i can't find the proper suit diagram I can't figure out what is wrong. My technician gave up on this project and it is now over my head as well. Is there anyone out there that can help? This unit sounds so distorted that it is not possible to listen to it.

    Luke Randall
     

     

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  2. Dave451

    Dave451 AK Subscriber Subscriber

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    Luke,
    Welcome to AK! Sorry about the issues with your 340 receiver. I have restored a couple of 340B's, which are the next evolution of the 340. Here's a link to the 340 schematic from the H H Scott website (kept going by some outstanding AK'ers, I believe):

    http://hhscott.com/pdf/340A.pdf

    It's not unusual that a given unit will not exactly match a given available schematic, as Scott often did make some changes during production. However, the website archive says that the 340 was in production for little over a year before the 340B was introduced, so I would imagine major changes were unlikely. In any case, here are a couple of questions to try and help figure this out:

    1. Is the distortion in all the audio sources (eg. Aux input, tuner, phono, etc)?
    2. Is it for both channels, or just one?
    3. If you suspect the driver/phase inverter, have you tried new tubes in that position (or swapping if it's just one channel)?
    4. What leads you to believe it's the PI that is causing the issue?
    5. Has the unit been modified in any way? If so, pictures of the chassis underneath might help diagnose it. By modified, I mean changes beyond just one-for-one like substitution of components, for example replacing a 0.1 uF 600V coupler with a newer one of same value. I'm talking about things like bypassing the loudness contour controls, or something where the component values or the circuit have been changed.
    6. What is different in your unit compared to the schematics you've seen (particularly the one at the link above)?
    7. Was the distortion always there, or did it follow some work that was done? If so, what work?
    8. Can you describe the distortion you are hearing? Is it a buzz/hum, or is the sound of music obviously distorted?

    Knowing more about these things might help figure something out.
    Dave
     
  3. gadget73

    gadget73 junk junkie Subscriber

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    Also worth a thought, if the distortion is on all inputs, have you tried bypassing the preamp and feeding signal direct into the power amp? The easiest way to do this would be to pull V2 and V102, and feed input into the non-grounded end of the volume control. At that point all there is to the circuit is a volume pot and a power amp. If the distortion stops, the issue is in the preamp. If not, you've got something in the power amp.
     
  4. LUKE RANDALL

    LUKE RANDALL New Member

    Messages:
    18
    Location:
    Rhode Island
    Hi Dave- Thank you for responding. The reason I know it is in the PI stage is because my technician said the signal is clean until it gets to that stage. I will answer some of you questions: Yes distortion is in all input stages. Yes it is in both channels. Yes I have put new 6u8 tubes in that stage. It looks like someone has definitely done things to this unit. I have put all new coupling caps in. The power supply has new electrolytic, The distortion was there when I got the unit. I put new tubes in and it was still presents. I then re cap-ed it. And still problems. The distortion is a common audio signal distortion, at low volumes it is really bad and gets a little better at higher volume but it the type of distortion that makes you want to turn the unit off and walk away. It is not a hum or power supply or grounding type of interference. As far as comparing the circuit, this link you sent me is the one i was using. I found it to be a little different and my technician said it is not the right one so he can not trace out the problem. I think he gets frustrated easily and gives up and thinks all these old units are junk anyway. He is knowledgable and does good work but will throw his hands up when ever he doesn't feel like continuing to solve a problem. I guess i might have to go in and check every resistor in that section. I am not that good at trouble shooting but I have built a bunch of amps (six) from scratch. When things go wrong I easily get confused. Any suggestions you may have after reading these answers to your questions will be helpful.
     
  5. primosounds

    primosounds Powered with pure tube sounds. Subscriber

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    I believe this unit is fixed bias. If the bias voltage is too negative then the performance of the PI stage is very non linear. Have you made sure that the bias voltage is correct?
     
  6. Dave451

    Dave451 AK Subscriber Subscriber

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    Primo,
    That is a great suggestion. Luke, can you check the bias voltages on the output tubes and let us know what you find? You would need to check the center (wiper) of the DC balance pots for -40V (the approximate value shown on the schematic). You won't be able to measure the tube cathode current draw directly, as the cathodes are grounded in the 340. Also, are there any modifications around the bias power supply, particularly the bridge rectifier that powers the front end tube filaments and the bias (negative) voltage for the output tubes through the DC balance pots.
     

     

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  7. LUKE RANDALL

    LUKE RANDALL New Member

    Messages:
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    Location:
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    Yes- I am aware of that fixed bias and it is right around -40v. Someone has done things to this circuit.....Although the problem is not in the output stage, I found a 330K resistor cut off on all four power tubes. I am going to record the resistor values off of the pins on the 6U8 tubes and see if someone can tell me if they are correct. The schematic posted on the HH Scott website is not the same schematic as my unit. I don't know if there is anyway to correlated serial numbers to schematics but that is what my technician said I need to do: serial number- 202267




    I did check that . Please see the above note to Primo sounds.
     
  8. Dave451

    Dave451 AK Subscriber Subscriber

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    Yep, sounds like someone had been messing around under the hood! Checking around the PI/Driver 6U8 tubes is a good idea--checking voltages would be a good first step. The missing 330K resistor is puzzling and alarming--these are the grid resistors for the output tubes and are quite important in the functioning of the output stage tubes. Were other resistors added in their place? If so, what are the resistance values? Completely missing, or just re-routed?

    If these resistors are missing, it could account for your problem.

    Posting a couple of pictures of the wiring around the 6u8's and the 7591 output tubes would help run things down. Dave
     
    Last edited: Jun 4, 2018
  9. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    The 330K resistors are part of the DC Balancing circuit. Removing them will effectively place the full negative bias voltage on all four output tubes. This will likely drive them nearly into cut-off, particularly if the tubes are original American pieces. This would cause distortion at all volume levels, and particularly so at low volume levels.

    Dave
     
  10. Dave451

    Dave451 AK Subscriber Subscriber

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    That's pretty much what I thought, Dave--thanks for weighing in with a clear explanation. Why would someone remove these? Anyway, Luke, the absence of these resistors is critical if they are indeed completely missing and would likely be the problem, not the phase inverter (unless those have been modified as well).
     
  11. primosounds

    primosounds Powered with pure tube sounds. Subscriber

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    Putting back the 330k resistors is a must (you can probably go to a lower value like 300k). Then you need to know what the output tube cathode current is. While your schematic is slightly different, since your unit has the tube rectifiers? If the PI tubes are the same , 6u8, the difference on that tube should be minor and not overly affect the performance of your amp vs. the SS version of the 340. So you should make a mod to put a 10 or 1 ohm resistor from cathode to ground of all the 7591 power tubes. Then you can measure and adjust the negative bias voltage to your needs.
     

     

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  12. LUKE RANDALL

    LUKE RANDALL New Member

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    P1100327.jpg here is a photo of the resistor cut on pin 5 of the power tube. It looks like a 680k resistor is there under it. I will post a picture of the 6u8 section. P1100324.jpg I checked the values on the 6u8 pins and they seemed to correlate to tHE sCHEmatic.




     
  13. LUKE RANDALL

    LUKE RANDALL New Member

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    Location:
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    Where should the 330K resistors be connected to? Please see the above photo....there is a resistor still on pin 5 ... 680K that connects to two more in parallel then to one side of the bias balance pot. (i think that is what that is.)
     
  14. LUKE RANDALL

    LUKE RANDALL New Member

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    Location:
    Rhode Island


    What should the bias voltage be on that pin5?
     
  15. LUKE RANDALL

    LUKE RANDALL New Member

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    Location:
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    I talked to my technician and he said the voltage was around -40 at the balance pot. I will check it tomorrow on Pin 5 of the power tube and see what's there..
     
  16. primosounds

    primosounds Powered with pure tube sounds. Subscriber

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    the 330k should be from the grid of the 7591 to ground. It looks like there is a 680k resistor taking its place. I think that value is too high for fixed bias and the 330k should be used. It would be best to use new resistors. It is not a good procedure to just cut a resistor and leave it unless you were rebuilding and wanted a reference. Which when you replaced the old resistor should be removed. The fixed bias voltage on the grid should be around-40v so that is ok. Your problems might be from the 680k resistor being too high for the circuit.
     

     

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  17. LUKE RANDALL

    LUKE RANDALL New Member

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    I see a 1 K resistor from the grid then to the 330K then to pin 5 (cathode) and to ground. I think you might have solved my problem....I am hoping. I will remove the 680K and put the 330K back . Thanks
     
  18. Dave451

    Dave451 AK Subscriber Subscriber

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    That's the way to do it. Agree the 680K is way too high and, as Dave G said earlier, this would contribute to excessive negative voltage on the 7591 grids and putting the tubes into a very high distortion area of their operating range. Some have mentioned mods you can do (such as cathode resistors) to improve operation, but the thing to do right now is to see if you can restore the 340 to proper operation. I'm not sure what the person who substituted the 680K resistors was thinking, but it was a bad move. The pictures were very helpful!
     
  19. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    Those 680K resistors look original. I would consider that the 330K and 680K resistors were originally factory installed, and wired in parallel, to effectively produce a 222K resistor. Scott did stuff like this when they did not provide a bias control -- just a DC Balance control, which is in fact what the control pictured is. I'm betting that the tubes were running too hot, so the 330K resistors were clipped. Now, they're running way too cool. Notice that the 270K grid return resistors have a 100K resistor in parallel with them as well. This all smacks exactly of what Scott had to do when they had no bias control, so they continually tweaked the fixed resistances for the tubes they had. Crazy stuff, but the way Scott operated. Short of installing a actual bias controls and cathode current monitoring resistors, you will end up forever finagling the value of those resistors to produce a proper current flow. But none of that can happen until some cathode current monitoring resistors are installed, which should be the first order of business. You can installed the original resistor values back where they were originally located -- but that only works in you have original line voltages -- which you don't -- and original tubes, which I doubt, and if you do, are likely well worn. Ergo, the installation of current monitoring resistors are priority one which will then allow you to "see" what you're doing with all the grid bias voltage work.

    Short of any component values that may be out of spec, I don't see any issue with the driver tube/phase inverter stage. But there are definitely problems with the output stage and the control of its quiescent current setting.

    I hope this helps!

    Dave
     
  20. Dave451

    Dave451 AK Subscriber Subscriber

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    Fascinating stuff, Dave. I have never seen this type of "we reserve the right to make modifications" type of work in any of the Scotts I've worked on, but there it is. Shows the value of pictures when viewed by the informed on these units! Luke, this may be why your tech was frustrated and was looking for a schematic that matched what he actually saw; I doubt such a schematic exists given what Dave G is saying.

    I have always installed 10 ohm resistors between output tube cathode and chassis ground in order to monitor tube current (cathode current) draw. If you do this, you won't be working in the dark on what the tubes are doing. Getting the grid resistors straight can tell you right away whether this is the cause of the distortion, as we all suspect, but adding the cathode resistors will help you toward a long term fix.

    Adding to what Dave said, replacing the 330K grid resistors with 200k to 220K resistors is a common mod in some Scott and Fisher 7591 output stages to cool off operation of the 7591's. As he notes, this may in fact be what they did by paralleling the 680K resistor with the 330K's in your unit to obtain 222k. Clipping the 330K was the wrong direction to go, unfortunately.

    Looking forward to what you come up with on this one!
     

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