Scott Stereomaster distortion Type 340 Tuner/Amp

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

  1. primosounds

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

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    OK, so, you need to set up away to get the current or voltage measuring resistors on the cathode. It is hard to see where the cathode is grounded but you need to get 4 matched 10 or 1 ohm resistors and put them between the ground and the cathode. If you mount 4 new 2 place solder lugs where the original 1 place lug is now you can add the 10 0hm resistor to the new lug and run the ground to the lug. If you mount 4 probe test points and run a wire from the cathode to the test point you can check the cathode bias from the top of the chassis. Just a suggestion if it did not occur to you. If the original resistors are all off specs and you need to replace them then doing this mod will be easier.
     

     

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  2. Dave451

    Dave451 AK Subscriber Subscriber

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    Couldn't he use the same grounding points as for the 330K/680K resistors? They are grounded in this model.

    EDIT: The 330K/680K are grounded at the cathode pin (5) which is itself grounded to the chassis, presumably by a jumper. Sorry for the mis-read of the picture. dave
     
    Last edited: Jun 5, 2018
  3. primosounds

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

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    It looks like those resistors are grounded at the screw mount. This is not an ideal situation. It would be better to make a ground that ties into the PS ground. So, if you mount a 2 place solder lug, the 2nd lug would be a ground. then you can have a ground point next to all the 7591 sockets that can be linked and tied into the PS ground.
     
  4. LUKE RANDALL

    LUKE RANDALL New Member

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    Wow- Thant gives me a lot to think about. I think Primo sounds mentioned putting a resistor in to measure bias voltage. How do I do this again?
     
  5. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    Unfortunately, Scott did not make it easy to install current monitoring resistors, as in most of their designs, they simply grounded the cathode terminal (pin 5), and then because of their daisy-chain grounding system, then used pin 5 as a ground point for other resistors in the circuit as your unit shows. If you remove all the existing connections to pin 5 of each tube to allow for installation of the current monitoring resistors, it leaves you no place to make the needed ground connections other than an "air" connection, which is both poor practice, and also doesn't look very tidy.

    The best solution I've found for years now is to run a piece of #12 or #14 gauge wire stripped out of a piece of Romex wire. Leave the insulation on it, but cut it away at the points where you need to remake ground connections at. Ground the new ground buss at the end where the ground circuit originally entered the output tube ground daisy chain, and the other end can either carry on to other ground circuits as needed, or simply be terminated to form an end point if that's what the original daisy chain ground did. To illustrate what I'm describing, I've included a pic of the underside of my own Scott 208 Basic Power Amplifier, which posed the same wiring issues when I went to install current monitoring resistors in it, and this was the solution I used when installing the resistors in that unit. You can see it as the black lead running the length of the output tube array. The current monitoring resistors are the blue resistors running from the new ground buss to each output tube socket.

    I hope this helps!

    Dave

    SAM_2259.JPG
     
  6. LUKE RANDALL

    LUKE RANDALL New Member

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    What purpose do the current monitoring resistors server? I know this is a dumb question.....but how much current am I supposed to see there? Is there an optimum current? Do I then add or subtract resistors at the grid to achieve this current? I understand the bias balance control. I am not a pro at this type of work. Should I use a 1 or 10 ohm ? It looks like you used a 2 watt size. I like how neat all your wiring is.
     

     

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  7. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    Luke -- Hardly a dumb question at all!

    A basic function of vacuum tube operation is that the negative voltage that appears at the "control grid" (pin #6 in this case), controls the amount of current the tube can pass from its cathode element (pin #5). There are other factors that control this as well, but for the purposes of this discussion, they are not important. All this business of the 330K and 680K resistors and whether they should or should not be connected and what their value ideally should be is all a secondary effort to achieve the primary goal: controlling the current flow though the tube to a target value, and causing all four output tubes to operate at this target value at the same time. The current through the tube flows from ground (the negative side of the power supply) to the cathode element inside the tube, and then by way of the emission process, to the plate element, and back to the positive side of the power supply, forming a complete circuit. As designed however, there is no way to know how much current is actually flowing through each tube.

    At the time the unit was manufactured, Scott ordered tens of thousands of 7591 tubes, and they could order them with selected characteristics -- so that each tube would in fact draw about the same known current flow with a given bias voltage applied to the control grid. Tubes -- like all manufactured devices -- have physical tolerances to their construction, and these tolerances can then vary the actual characteristic from that of the designed bogey characteristic for the tube type in question. Back in the day then, having the control grid voltage set to a fixed value was no problem, because the tubes installed were chosen to operate with a selected characteristic. Today however, Scott is gone, the American tube manufacturers are gone, so that we are left (primarily) with foreign manufactured tubes -- that while pretty good -- do not have the tight tolerances that the original American pieces were built to, let alone ones chosen for a precise characteristic. Therefore, today we need to be able to measure the current that is actually flowing through the tubes -- and be able to adjust it (via the negative control grid voltage), so that the original target current flow can be achieved. So again, without the small (recommended) 10Ω resistors installed to connect your meter to, you have no idea of how much current each tube is passing; sort of like trying to hit a given speed in a car with no working speedometer. Your adjustments with your foot on the accelerator are like changing the pin #6 control grid voltage. But without a speedometer to look at, you have no idea how those changes are effectively bringing you to your target speed -- or in this case, how the changes in control grid bias voltage are bringing you to the target current flow through the tube. That's why installing the current monitoring resistors is job #1.

    The optimum target "quiescent" (no signal) current flow can vary from one application to the next, but generally, a current flow of about 32-35 mA is correct for 7591 tubes in classic designs like Scott used. Thanks for the comment on the wiring in my unit. The current monitoring resistors I used are .5 watt devices with a tolerance of 1%. Using highly accurate high quality resistors ensures that the current you measure will be accurate, and that the resistor value will remain stable as the unit heats from cold turn on temps, to full operating temperature.

    Dave
     
  8. primosounds

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

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    It is somewhat unfortunate for you that you bought a fixed bias amp since it is a more complex type of tube amp. The fixed bias or negative voltage circuit adds another power supply which needs to be adjusted for the proper operation and sound quality. This doesn't help your situation but i just wanted to add some context to your situation and choices.
    There are different ways to add the necessary off socket ground and you have to determine which is best for you and your skill level, available parts, and other considerations. You main concern should be not to create a layout which will lead to a short.
    You can run a ground bus, or you can do nested ground where you have a solder lug in a convenient location and run a ground wire to that lug. Then you tie all the lugs to your star ground. Whichever way you decide the end result is about the same.
    Regarding layout of components, this is a matter of patience and practice, and also the use of lots of wire insulation to prevent shorts.
     
  9. LUKE RANDALL

    LUKE RANDALL New Member

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    OK-
    Ok Thanks Dave!., Here is one more basic question: Once I have installed a 10 ohm resistor from the cathode to ground, how do I measure the current? Do i measure the voltage across the resistor and use ohms's law to get the amount of current flow or do I hook up my meter in the current measuring mode? I will let you know how this works out. I appreciate all your help. Luke
     
  10. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    The first way: Hook your DC voltmeter across each resistor (black to ground, red to the cathode pin), and read the voltage. Take whatever the voltage is, and divide it by 10. The result will be the current flowing through the resistor in AMPS. Or, take the reading and multiply it by 100 and the result will be the current flowing in MILLIAMPS.

    Dave
     
  11. LUKE RANDALL

    LUKE RANDALL New Member

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    Thanks Dave. I thought it was Ohm's law.
     

     

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  12. LUKE RANDALL

    LUKE RANDALL New Member

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    Hi Dave- I am wondering if I can add the 10 ohm resistor from cathode to ground or do I need to disconnect ground from the cathode then put the resistor between ground and cathode? There are a lot of wires going to that ground on each tube. I jumped the cut of 330K resistors back on (effectively getting 222K because the 330K is in parallel with the 680K) and I am going to try amp tonight to see if the distortion is still present. I am a little leary about tackling the current measuring resistor installations. I am also wondering the best way to measure the DC balance between the two tubes, I think I am looking for the same bias voltage on the grid of each tube while the amp is running. . Is that correct?
     
  13. gadget73

    gadget73 junk junkie Subscriber

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    You would need to disconnect the ground from the cathode, the resistor goes between.

    The DC balance really is easiest set using the current monitoring resistors that aren't there, but a second best is adjusting for minimum hum at the speaker outputs. The grid voltage is not really important, equal current flow is. Equal current in each tube will minimize hum.

    alternative option, they do sell bias monitor gizmos that sit between the tube and the tube socket and bring out a pair of leads for your meter. If these are built in a sane manner, it should have the 10 ohm resistor inside the gizmo. At that point you can just use two (or four) of those to adjust bias and balance on the tubes, then remove the adapters until you need them again. I've never personally used or owned a set, so I have no idea which ones are decent.
     
  14. LUKE RANDALL

    LUKE RANDALL New Member

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    Thanks. I balanced the grid voltage but what you are saying is that that is not the goal it is current flow that needs to be equal.. In a perfect world one would think with the same tube installed and the same grid and plate voltage that you would have equal current.
     
  15. LUKE RANDALL

    LUKE RANDALL New Member

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    I am sad to report that the distortion is still hugely present even after reconnecting the 330K resistors. It is very distorted and low levels and then sounds like the tubes are starting to motorboat at about half volume. I checked the plate voltage and it is at 499-500 and I checked grid voltage and that is -30 at the pin. I have not installed current monitoring resistors yet. I was hoping to get it to function moderately well and then tackle that project. Going back to my original dialog, the fact that my technician said the problem is in the 6u8 section, I am wondering what you think my next approach to solving this problem is? I am almost ready to give up on this unit.
     
  16. gadget73

    gadget73 junk junkie Subscriber

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    Unfortunately due to manufacturing variables, that just doesn't happen. It should be pretty close if the tubes are matched but its unlikely to be exactly perfectly equal. Mismatched tubes you may as well throw a dart at the wall for results, they're all over the place. I have an old production set of 4 Sylvania 7868 tubes in something that are so different I can't really even get them to play well with each other.

    Are the voltages in the 6U8 section close to matching the schematic? Its a fairly simple inverter circuit but if the voltages are too far off, especially at pins 1, 8 and 9 it won't act right.
     

     

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  17. LUKE RANDALL

    LUKE RANDALL New Member

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    Thanks for your response. What should the voltages be on 6U8 pin 1, 8, and 9? I have not checked. I am going on the word of my technician...he says he put a scope on the input stage and as soon as he hit the 6U8 stage it went crazy. Clean prior to that.
     
  18. gadget73

    gadget73 junk junkie Subscriber

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    Schematic says 185 on pin 1, 80 on pin 8, 60 on pin 9. Grossly far off that will upset the phase inverter balance considerably and that will produce less than ideal results. Pin 1 is also the plate of the pentode side, if thats not somewhere close to right it might point to an issue on that half of the tube.
     
  19. GordonW

    GordonW Speakerfixer Subscriber

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    Have the screen resistors on the 6U8s (connected to pin 3), been checked to see if they're within spec?

    I see LOTS of driver pentode screen resistors that are out of spec (too much resistance- sometimes, almost a complete open circuit)- which reduces the screen voltage, to the point where the plate voltage goes up so much, that the tube loses virtually all of its effective voltage swing.

    One way to check- most 6U8s will have about 25 to 50v on the screen (pin 3), when they're working properly. If it's not, then definitely check those resistors.

    Also, -30V on the grids of the 7591 output tubes seems extreme. Most of the time, it's going to be more like -20V, plus or minus a volt or two, to get the tubes conducting in the proper idle current range (somewhere in the range of 30-35ma per tube, or so). I'd check that, too. It would be a good time to put in the 10 ohm cathode resistors- once you do that, you can find out what bias voltage is needed, to get the output tubes in that proper idle current range.

    Regards,
    Gordon.
     
  20. LUKE RANDALL

    LUKE RANDALL New Member

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    HI- I checked the voltages on these pins. They are all way off. Pin 1 measured 301V. pin 8 measured 34 v. pin 9 measured 24 v. pin 3 was close to the schematic measuring at 51V.......The 6U8 tubes are new....and i was having the problem with the old tubes that were in there too so I don't think it is the tube malfunctioning. The resistors around these points all measured close to their ratings. Where would all that extra voltage on pin 1 be coming from? What should I do now?
     

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