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Picked up a Rough KX-100 Tube Amp - Will need some guidance

Discussion in 'Fisher' started by Tim D, Apr 5, 2017.

  1. Tim D

    Tim D AK Subscriber Subscriber

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    I've done an IBAM from captouch's thread on his Fisher KX-100. I used his diagram from post 12 in this thread.

    http://audiokarma.org/forums/index.php?threads/fisher-kx-100-x-100-b-restoration.572641/

    I'll post pics of my installation and my parts list later, but I do have a some questions of the experts.

    The voltage across R71 (1.8K ohm) is only 34V which is the voltage across the 4 small signal tube heaters wired in series. The diagram is showing it should be 42V so this is way low for the 4 series heaters for the small signal tubes. Where should I start looking? The cathode currents are too low maybe? That leads to my next question.

    With the 10 ohm cathode resistors what current should I be aiming for? (For the moment I have them all set around 34ma. Should I crank up the cathode currents more to bring that voltage up across R71?)
     
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  2. larryderouin

    larryderouin Turn it UP, POP? PLLUUEEEZZZZZEE Subscriber

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    Now I'm in a quandry with regard to max grid resistor value on 7591/7868/6gm5 tube family and the above link has me baffled.
    This is from the RCA Books on the tubes.
    MAX Grid Value is 300K in FIXED BIAS.
    Max Grid Value is 1 megohm in CATHODE BIAS.
    captouch changed his values on his KX-100(which is normally Cathode bias) from 330K to 220k based on what's normally done on fixed bias. I didn't catch it at the time and the unit was modified to a cathode bias with individual tube adjustment. NOW here's where my quandry comes in. Is this now a fixed bias, or is it a cathode bias for the purposes of changing or not changing the cathode grid return resistors. I've usually mentioned in amps where the owner was going to change them that the change was not necessary for the above reasons (grid values vs. bias type) but if someone was going to add individual pots so the cathode current could be adjusted, should the Grid resistors and the output coupling caps be changed to 220k and 0.068uf?????????? And why or why not?

    Larry
     
  3. larryderouin

    larryderouin Turn it UP, POP? PLLUUEEEZZZZZEE Subscriber

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    Tim. Bring the cathode voltage up to 42V @ pin 3 of the tube (cathode). If you have the IBAM working you'll have to adjust each pot in turn. Try 2ma up each and check the voltage until you have 42v across the bias tube heaters. Once you have that done read across each 10 ohm cathode resistor to tell what each is doing. Do your conversions to ma and see what's happening with the tubes. You may have to make adjustments to the pots until the cathode ma are equal AND the aggregate tube voltage is approx 42v.
     
    Last edited: Apr 7, 2018
  4. Tim D

    Tim D AK Subscriber Subscriber

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    Ok Larry. Doing it in steps......

    At first about 38ma on each I was getting 39V across R71, so it is coming up. Man this thing is squirrely. I've had to go over it several times to get them to match. Each change affects the others, but it's helping.

    Finally at about 40ma plus or minus 1ma and its' now at 43V across R71, so that was my issue. This IBAM seems to be ok so far. I'll play it for a while and see what it sounds like.

    Are 40ma typical current draws on 7868's in these Fishers or is that high?
     
    Last edited: Apr 7, 2018
  5. larryderouin

    larryderouin Turn it UP, POP? PLLUUEEEZZZZZEE Subscriber

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    7868's probably. Dave is the real expert on the integrated's. I would think that cathode bias runs them really hot(meaning at the edge of the envelope). So go no higher in voltage than the manual says. You could go up to 44 but I'd cut it off at 43. I run my X-101-B (7591's) at 43V and doesn't hurt them(I'm getting ready to re-socket the outputs and haven't added cathode resistors yet.)
     
  6. Tim D

    Tim D AK Subscriber Subscriber

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    Ok. Here is the IBAM for this one and the parts list from DK based on captouch's diagram.

    CF12JT220KCT-ND RES 220K OHM 1/2W 5% AXIAL (Need QTY 4)
    3352T-203LF-ND POT 20K OHM THUMBWHEEL CERM ST (Need QTY 4)
    1189-1660-1-ND CAP ALUM 22UF 20% 50V RADIAL (Need QTY 4)
    BC3092-ND CAP FILM 0.1UF 5% 1.6KVDC AXIAL (Need QTY 4)
    SBB1602-1-ND BREADBOARD GEN PURPOSE (NPTH) (Need QTY 4)
    PPC820W-1CT-ND RES 820 OHM 1W 5% AXIAL (Need QTY 1)
    BC4724CT-ND RES 3.3K OHM 5% 1W AXIAL (Need QTY 1, but you could move and re-use existing R69 if you have a KX-100)
    1.8KW-10-ND RES 1.8K OHM 10W 5% AXIAL (Need QTY 1)

    Here is the pic of the board after build and install. The board is only about 1 inch by 2 inches - very small. I installed it with 3M double sided adhesive pads on the underside near where the bias pot would be if this had been a Fisher X100B.

    IMG_0368.JPG

    Here are the new 0.1uF coupling caps that go with the changes. See white tube caps (four).

    IMG_0369.JPG
     

     

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  7. Tim D

    Tim D AK Subscriber Subscriber

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    Larry,

    I was hoping someone like Dave would chime in about your question regarding the 220K resistors and the 0.1uF coupling caps. I want to do the right thing. I've got the 0.1uF caps in place, but I have no problem changing this combination if it is the wrong thing to do. I will say though that at least it biased out to 40-41ma and about 43V across the small signal heaters. Just not sure about the time constant change for the 220K ohm and 0.1uF cap. It sounds pretty good through my bench test speakers (on the dinning room table of course.)

    Also, I guess I have to read up on fixed vs. cathode bias. I really don't know the difference. I'm learning and this is fun!
     
    Last edited: Apr 9, 2018
  8. larryderouin

    larryderouin Turn it UP, POP? PLLUUEEEZZZZZEE Subscriber

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    At this point it won't hurt anything. Back when I started this stuff the change from 220K and .1uf was a standard change for fixed bias (like what's in the 400, 500c,800c) due to the early 7591/7868 tube designs (they couldn't handle the power) , but the time constant changed a bit from the original that the 330K/.047uf had. At that time .068uf and .082uf caps weren't plentiful and hard to find. About the time they came out in force, (4-5 years ago) Dave did some calculations and determined that the best results on a change with the 220K was a .068uf vs. the .1uf. The time constant based on 220K is actually .071uf or .072uf which with a .068uf would get you closest to the 330k/.047uf time constant.

    You actually can't hear the change but it'll show up on a scope. What I'm confused about is the individual tube Bias controls on a cathode bias. Do you leave the 330K/.047uf as you would a standard cathode bias, (based on the fact that the tube manufacturers determined that 1megohm was the max for cathode bias) or do you treat it as a fixed bias (and make the change to 220K/.068uf). The more I think about it the more I'm convinced that leaving the values alone on cathode bias is correct. BUT! I want an engineer's point of view to back me up on it. And Dave's that Engineer.

    Dave's on hiatus until the end of the week (he got married on Saturday).
     
  9. Tim D

    Tim D AK Subscriber Subscriber

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    Well congrates to Dave!!!!! So the resident genius has gone off and got himself hitched.

    :beerchug:

    My stuff can wait. I'm painting the rest of the trannies and I've got a replacement treble pot on the way anyway (the old one had a dead spot.)
     
  10. Tim D

    Tim D AK Subscriber Subscriber

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    While waiting and hoping Dave will comment regarding the change in coupling caps (to 0.1uF) and grid resistors (to 220K) as mentioned in Larry's post I continued with other work. I've got the other trannies painted and re-installed. I applied a coat of wax to the chassis and I received the replacement treble pot. It's the treble pot though that has me puzzled. It has a dead spot or so I thought. I pulled the old one out and found there is nothing wrong with it. So I have some questions that I hope someone can help me with.

    The pot was showing a dead spot in the last clockwise position. I'd say that is in the 4 to 5 o'clock positions. The right channel would entirely cut out when I hit 4 o'clock. My first reaction that there is something wrong with the pot appears to be wrong. I pulled it and checked it with my multimeter and the resistance is smooth on the wiper as measured from both ends for the full rotation of the pot. Now I'm left wondering what is causing the right channel to cut out when it is rotated near its' max position? Could I be driving something to an electrical cutoff such as the pre-amp tube or could a cap be breaking down under a load at that position? I've measured the resistors across the whole unit and the ones near the preamp tube appear to be in tolerance. I measured some of the caps such as the coupling caps (and I have replaced some) and they appear to be fine. The voltages are a bit low on the B+ on the small signal tubes. I have an entire new set of small signal tubes on order. I even swapped the right and left channel tubes and the problem is still there.

    Thoughts? Suggestions?
     
  11. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    Sounds like it might be a good time for a refresh on exactly what "Fixed" bias and "Cathode" bias means, as that is at the heart of the question regarding the discussion on maximum allowable grid #1 (control grid) DC resistance. First however, a couple of basic points to understand:

    1. The term "fixed" bias does NOT simply mean that the bias voltage is not adjustable. It has nothing to do with adjustment capability, as both a fixed bias output stage and a cathode bias output stage can be designed to allow for adjustment of the bias voltage, or not.

    2. In all traditional vacuum tubes operating in traditional output stage circuits (as is surely the case at hand), the control grid is always operated at a voltage level that is negative with respect to that of the cathode. It doesn't matter what the cathode voltage is -- the control grid must operate at a more negative potential than that of the cathode. The cathode could be (as a hypothetical example) at +100 vdc above ground, and the control grid could be operating at +50 vdc above ground. In our hypothetical example, this is just fine, as relative to the cathode, the grid is operating at -50 vdc. Or, the cathodes could be connected to ground, and the grid supplied with -50 vdc from a special negative voltage power supply. Either way, in both cases, the control grid is operating at a potential that is 50 volts below that of the cathode which makes our hypothetical tube in our hypothetical circuit very happy.

    This is inherent with the way a vacuum tube works. Opposite charges attract, but like charges repel. In the tube, electrons emitted from the cathode are ultimately connected to the negative side of the B+ power supply, while the plate ultimately connects to the positive side of it. Since these are opposite charges, there would be a very strong electron flow through the tube unless there is something there to throttle the flow. Left to operate in this condition for very long at all, and the tube would "red-plate" and be destroyed. The throttle is the control grid. By making it operate at a potential that is less than that of the cathode, the negative charge on the control grid acts to repel the electrons from the cathode, and therefore controls the flow. If the control grid operates at the same potential as the cathode, then maximum flow is achieved, which is called saturation. If the control grid is operated with enough negative potential below the cathode, then all electron flow is stopped, producing a condition known as cutoff. Normally, the quiescent (no signal) bias voltage is set so that the tube continuously conducts a target current level somewhere between the saturation and cutoff points, with the AC signal -- when applied -- causing the control grid potential to then "swing" between saturation and cutoff. Little swing = little output from the tube. Maximum swing = maximum power output. Swing beyond saturation and cutoff = distortion. There are a lot of details that determine where the optimum quiescent bias point, maximum power output, and safe operating conditions all reside. But for the purposes of this discussion, those are not important.

    With that out of the way then, we can get back to the two biasing methods known as Fixed and Cathode bias. They are defined as:

    1. Fixed bias is so defined as a circuit where the bias voltage applied to the tube is NOT affected by the current flowing through the tube. This type of design is typified by Fisher 400, 500B/C, 800B/C receivers and 202 series integrated amplifiers. In these designs, the cathode element is virtually grounded, while a negative voltage is applied to the control grids from a separate negative DC supply. As such, because the bias voltage is supplied from a separate power source, it is considered to be a "fixed" source. It remains constant regardless of changing current flow through the tube as created by the application of an AC signal applied to the control grid.

    2. Cathode bias is defined as a circuit where the bias voltage applied to the tube IS affected by the current flowing through the tube. This type of design is typified by Fisher 100/101 series integrated amplifiers, and the original 600 receiver. In these designs, the cathode element is NOT virtually grounded, but is connected to ground though a significant resistance, while the control grid typically IS grounded, or nearly so. With this type of design, because of current flow through the tube and across the cathode resistance, the cathode operates at some positive potential above ground, while the control grid operates either at ground level, or some level between ground and that of the cathode. The bottom line is that the control grid is still operating at a negative potential relative to the cathode, with the size of the cathode resistance controlling the amount of bias voltage, and therefore current through the tube.

    But note however that with increasing current flow though the tube from the application of signal, the bias voltage at the cathode will obviously increase as well, since that is the point where all current flow through the tube begins from. Greater current flow through the cathode resistance means greater bias voltage applied to the tube -- which sort of works against the applied AC signal which is trying to increase current flow through the tube, doesn't it? To prevent this from happening, a large cap is connected across the cathode resistance to help minimize voltage fluctuations at the cathode due to an applied AC signal at the grid. It can be effective enough, but doesn't stop all of the fluctuations, because there is an increase in average current flow with increasing signal that the cap cannot filter out. As a result, cathode bias can never provide as much power output as a fixed bias design can, but designed properly, it can still be quite effective -- both in terms of performance, and economy of design as well. The take away salient point however is that in a cathode biased design, the bias voltage is subject to current fluctuations through the tube created by the application of signal, where as in a fixed bias design, the bias voltage remains stable regardless of changing current flow from applied signal.

    Note then that by either varying the voltage from the negative bias supply in fixed bias designs, or the cathode resistance in cathode bias designs, both designs can be made to have "adjustable" bias capabilities. The term "fixed" bias then has nothing to do with whether the bias voltage is adjustable or not. So how does this all relate to the maximum allowable DC grid #1 resistance specification? That gets covered in the next post coming up shortly.

    Dave
     

     

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  12. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    In a vacuum tube, various conditions from operating the heater at too high a voltage, to out gassing from the heating and bombardment of residual gas molecules remaining in the structures of the tube throughout its normal operation can result in an effect known as "Reverse Grid Current". It is an effect that increases with the heating of a tube, right up to the point of maximum plate dissipation, where the tube would be operating at its maximum safe temperature level.

    When a tube is operating, the effect of reverse grid current is to cause a voltage drop across the grid return resistor (330K in most Fisher products), in the same fashion that a voltage drop is caused across a cathode bias resistor. That is, the grid starts to show a DC voltage across it with a polarity such that the grid end of the grid return resistor is more positive than the other end of the resistor, which is either connected to ground in cathode biased circuits, or the negative grid supply in fixed bias circuits. The point here is that reverse grid current always acts to reduce the effective bias voltage that is being provided by the negative grid supply, or the cathode bias resistor. Reverse grid current is usually measured in micro-amps, but because the grid resistor is so large, a significant (enough) voltage can still be developed across it.

    To help protect the tube, tube manufacturers specify a maximum DC grid #1 resistance, precisely to contain the effects of reverse grid current. By establishing a maximum grid #1 resistance value, combined with the details of a tube's manufacturing process and its ratings, the effects of reverse grid current can be held to values that become rather insignificant. But why the two different values for the two different types of biasing systems?

    With cathode bias, as reverse grid current occurs, the effect is to counter act the bias produced by the cathode bias resistor, causing the tube to draw more current. But note that as the tube draws more current, then more bias is developed across the cathode bias resistor! In this way, cathode bias tends to act as a stabilizing agent, which ultimately allows for a larger grind #1 resistance to be used before the effects of reverse grid current can cause trouble for the tube.

    With fixed bias however, there is no stabilizing action as the bias voltage is fixed regardless of current flow through the tube. Therefore, as the effects of reverse grid current cause a tube operating with fixed bias to draw more current, it reduces the effective bias applied to the tube, which makes it run hotter, causing more out gassing, which causes more reverse grid current, which reduces the bias even further etc., etc., such that the tube just gets hotter and hotter until it begins to run away with itself. As a result, tubes operating in a fixed bias circuit are always specified to use a lower maximum grid #1 resistance (than in a cathode biased circuit) to curtail the effects of reverse grid current, since there is no stabilizing action provided by that type of biasing system.

    It is of some importance to note however, that as can be seen, reverse grid current is ultimately the product of heat. Therefore, the maximum grid #1 resistance values are based on the tube operating at maximum rated dissipation levels. Because there were no bias adjustments provided in any of Fisher's receivers, and coupled with their close spacing and likely operation in a console or cabinet, it is very possible that the output tubes in these units could reach maximum safe operating temps even if the non-adjustable biasing system was designed to operate a boggy tube within the safe electrical dissipation ratings for the tubes. With the added variable of modern manufactured tubes of lessor standards than the original devices were manufactured to being now added to the mix, tube distributors like Jim McShane started universally recommending that in models like Fisher that used a 330K grid return resistor, it be reduced to (typically) 220K. The thought was that it would help stabilize the tubes (new or old) by reducing the effects of reverse grid current, from a resistance value that was seen as right at or even beyond the manufacturer's recommended maximum rating. It can now be understood however, that this is only a useful modification in those Fisher models employing fixed bias. Since 330K is well below the maximum recommendation of 1 Meg for the cathode biased model that is the subject of this thread, it does little good to reduce the original value of 330K to 220K in this case. Note that it really doesn't matter whether the cathode bias is developed across a conventional resistor, tube heaters, or even an EFB cathode bias regulator. As long as the bias is created by a voltage drop produced by current flowing through the tube, it is cathode bias. But there is one last point to make.

    Note again that the effects of reverse grid current are a product of heat, with the maximum grid #1 DC resistance values based on the type of biasing system used, and operation at the maximum ratings of the tube. In fixed bias designs that operate the tubes fairly hot to begin with as Fisher did), that provide no means of adjustment, and operate the tubes within a confined environment, then reducing the value of the grid return resistors down to 220K (or so) makes perfect sense. However, for those of us that have installed cathode sampling resistors and IBAM or IBBM circuits to allow for the adjustment of each tube to a very safe and identical level of dissipation (often accepted as 80% of rating), and who ensure adequate air flow in and around the tubes, then reducing the grid #1 return resistors from 330K down to 220K again provides for little if any return on the modification.

    This particular modification of reducing the 330K grid return resistors has become so embedded in the stable of Fisher modifications, that I've rarely addressed all of this information together regarding it, and felt that it was just easier to let it stand in place. As long as the coupling caps are adjusted accordingly, there is little harm in doing it. But as can be seen here, taken in total with the other very worthwhile modifications done to allow for measurement and adjustment of the bias voltage to each tube (in both fixed and cathode biased designs), any protection gained from the reduction of these resistors is almost entirely superseded by the efforts to measure and adjust the tubes individually.

    Modifying any Fisher model to allow for measurement of current flow and its adjustment is of supreme importance in today's audio environment. For fixed bias models, it allows for keeping the tubes safely and evenly biased for maximum performance and so that the effects of reverse grid current are safely held at bay, regardless of tube origin or country of manufacture. But for cathode biased models, it is even more important because the tubes are worked even harder in these models -- threading the needle between passing enough current to light the preamp tube heaters without exceeding the dissipation ratings for the output tubes in doing so.

    I hope this helps!

    Dave
     
    Last edited: Apr 14, 2018
  13. Tim D

    Tim D AK Subscriber Subscriber

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    :lurk:

    Dave,

    That was perfect. With the preparatory reading I did this week and your two posts I now understand the biasing schemes and the intentions of these common modifications from a high level. I won't begin to say though that I am ready to create new engineered changes, but I now understand the impacts of the common changes being made in the bias circuits of the various Fishers - particularly for my KX-100.

    Thank you very very much! :)
     
    Last edited: Apr 14, 2018
  14. Tim D

    Tim D AK Subscriber Subscriber

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    Next...... I've got several small updates and a question at the end.

    I want to thank Mrs Gillespie for letting Dave come out to play for his very informative posts 171 and 172 above! Thanks Dave! Thanks Mrs Gillespie! [BTW, I don't mean to make light of Dave's effort and time. I am super appreciative.]

    I'm calling the IBAM done for now as it appears to be working well, but I have some new small signal tubes coming (from Jim McShane!) and I am replacing one of the output tubes that was drawing a more current than the others before the IBAM. I'll re-adjust the bias after the new tubes are installed and then I'll take voltage measurements throughout the unit to see how close they match what is on the Fisher KX-100 diagram. I'm hoping that the IBAM changes don't cause any concerning differences. (Part of the reason I'm changing the small signal tubes is because I have some voltage differences (low in many locations), but I also want to start with a known set of good tubes.) For the IBAM, I believe that Dave alludes to some related bias changes that captouch did in post 92 where he suggested an increase in resistance for R71 from 560Ω resistor to 1.5K at 10W. I believe captouch and I used 1.8K, but the concept is there. This was a design suggestion to balance the current draw between the OP tubes and the small signal heaters. There is apparently a dance between the two current draws that was part of a Fisher design compromise. The EFB (Enhanced Fixed Bias (TM)) would eliminate that compromise, but Dave mentioned that it is significant work. I don't mind work, but I'd also have to determine where to put the extra components and how that might make the unit appear which is a bigger item for me than the actual work involved.

    I've got the whole unit cleaned up now and the trannies are painted and reinstalled. Not much more to do on the electricals pending a final test once the tubes are installed. The only things I haven't done are to fix the headphone identity issue (I don't have headphones and the jack and associated resistors are really hard to get at.) I have not done the EFB (one of Dave's creations and mentioned in post 97). I don't want to rule it out in the future, but that is not currently in the plan. I also got a Fisher case for the unit and once I've settled everything, I'll get it installed and post some pics.

    I have one current electrical issue and that is that the treble pot exhibits a dead spot. The pot was showing a dead spot in the last clockwise position. I'd say that is in the 4 to 5 o'clock positions. The right channel would entirely cut out when I hit 4 o'clock. My first reaction that there is something wrong with the pot appears to be wrong. I pulled it and checked it with my multimeter and the resistance is smooth on the wiper as measured from both ends for the full rotation of the pot. Now I'm left wondering what is causing the right channel to cut out when it is rotated near its' max clockwise position? Could I be driving something to an electrical cutoff such as the pre-amp tube or could a cap be breaking down under a load at that position? I've measured the resistors across the whole unit and the ones near the preamp tube appear to be in tolerance. I measured some of the caps such as the coupling caps (and I have replaced some) and they appear to be fine. The voltages are a bit low on the B+ on the small signal tubes. I have an entire new set of small signal tubes on order. I even swapped the right and left channel tubes and the problem is still there.

    Thoughts? Suggestions?
     
  15. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    Tim -- If the pot measures correctly and shows no drop outs during rotation, then this being a kit, I would suspect poor lead dress and or shielding -- most like around the speaker switch/headphone jack/treble control area.

    Try this: Use a hot signal into one of the high level inputs, such as a CD player. Operate the amplifier with the volume control set to a very low level. See if in that scenario the treble control for the right channel can be rotated throughout its full rotation without any sound dropout in that channel. If so, now turn the volume control to a position you would normally set it at when you noticed the problem with the treble control. If with the volume control set to the higher setting the sound in the right channel now cuts out with the treble control fully advanced, then the problem is due to "current feedback" which can occur when the shielding is inadequate, and the circuit has been adjusted to produce a high level of HF boost. If this is happening -- and it likely is -- the amplifier for that channel is going into a high power ultrasonic oscillation that can be very damaging to the tweeters in you speakers. If this is the case, you will certainly want to get this resolved before turning the treble to full boost again -- and/or, use test speakers when checking for resolve. If this is in fact what is happening, ensure that good lead dress is present to keep any and all speaker/headphone leads in the area away from the treble control wiring. If that doesn't resolve it, it may require adding a small shield plate between the headphone jack/speaker switch wiring, and the treble control, to prevent any interaction from taking place.

    I had to do similar work recently in an X-202 so that the amplifier would fully turn down when the volume control was at minimum. It was basically the same problem: there was unintended coupling between adjacent circuits (just like you possibly have) that in this case, happened to be on either side of the volume control. The control itself was turned fully down. But the coupling between the adjacent circuits made the amplifier act as if it wasn't. The addition of two small shield plates (along with some other measures) completely resolved the issue.

    I hope this helps!

    Dave
     
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  16. Tim D

    Tim D AK Subscriber Subscriber

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    I've been silent because I've been trying to determine where to shield and how to shield without causing a short.

    While thinking about that I did some more research:
    - I put my iphone in through the Tape Monitor jacks and didn't have the problem.
    - I also don't have the problem when I turn on the Hi Filter switch.
    - In looking at the grid on V3, the voltage was going negative w.r.t ground when I turned the treble all the way up. It was a sudden change in voltage when I crossed some invisible threshold at about 4 o'clock on the treble control. It only happens on the right channel. Don't know if this is due to reverse grid flow or some other odd tube behavior.
    - Loudness switch didn't have any affect on the behavior.
    - Volume didn't have any affect on the behavior.
    - I replaced the ceramic caps (C9, C10, C11, C12, C31, and C32) around V4 and V3. I was hoping I had a leaky cap, but that didn't have any affect - but it made me feel good that I crossed that off. :D
    - Found a problem with the shielded cable to one side of the Hi Filter switch - It had a non-infinite resistance between the conductors. Created a twisted pair of wires and replaced the grounded cable. I expect that it was overheated at some point - maybe when I was replacing the ceramic caps.
    - I've gone through and tried to add some spacing near crowded wires behind the faceplate near the controls that Dave mentioned. (Maybe this is the dressing that Dave mentioned.)

    None of this seemed to matter much to the problem.
    :idea:

    While in there I fixed the headphone identity issue. I've added a second CL-80 to the other leg coming in from the wall. I've been watching my wall voltage and it is running 120-121VAC, so I figured the approx 3V drop I get from two CL-80's will allow me to avoid having to use a variac or bucking xfrmr. I'll be running the primary of the PT at 117-118VAC which I hope is safe for this old unit.

    Today I just gave up (at least for the near future) and put it all back together and slid it into the case I got for it. I've got it hooked up to my Dual 1226 w/M91 Cart and my Iphone is running in to the Aux. I hooked it up to some 1985 Heresy's that I recently picked up cheap.

    Anyway, I'm playing some blues through my iphone into the Aux input with the Heresy's and I rotate the treble switch all the way over and what the heck? The problem is gone! Well, maybe putting the control panel shield and the bottom plate back on put enough shielding in place that it did what Dave was telling me to do all along!!!! :bigok:

    Anyway, I've done a lot of the mods on this one, but I did not do the EFB or the common/ground fix to the output transformers. It started as a total wreck with rust all over, broken/missing switches, and a blown output trannie. I can't even summarize everything that was fixed or replaced. I received a lot of help and some parts from my friends here on AK to make this right.... I want to thank everyone for all their help on this one. I'm calling it done (for now). Listening to it right now and wow! Creamy is what I'd call it and it has enough grunt to make the Heresy's sound full which is saying something as they were bass shy when I played them through my Marantz.

    Here is the final product. Thanks guys!!!! You know who you are. (Special mention to Dave and Larry and Matt.)

    IMG_0426.jpg

    IMG_0425.jpg




    Now I'm on to the other ratrig - my Fisher KM-60 tuner that will eventually sit next to it. The chassis on the KM-60 is in even worse shape. If you want to compare - take a look at post #1 in this thread. Here's a glimpse of the next ratrig.

    IMG_0429.JPG
     
    Last edited: Apr 29, 2018

     

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  17. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

    Messages:
    9,782
    Location:
    Ball Ground, GA
    Tim -- Congrats on a great looking/running amp and saving it from the graveyard! When you do get to it, your latest comments again reinforce that additional tin shielding will be needed to solve the HF feedback issue.

    Good luck with it!

    Dave
     
  18. larryderouin

    larryderouin Turn it UP, POP? PLLUUEEEZZZZZEE Subscriber

    Messages:
    21,908
    Location:
    Glen Burnie Md.
    Nicely Done Tim! Time to break out the Naval Jelly or Walnut shell blaster for that KM-60. Did you ever find an assembly manual for it? As Dave noted for the X-202, the tin shielding works very well at keeping electrons and hash where it belongs. I did the shielding on my x-202 and it's now a like a tomb with no sources connected and running Heresy's. Stick yer head right up against the speaker with it all the way up and it's DEAD QUIET!.
     
  19. Tim D

    Tim D AK Subscriber Subscriber

    Messages:
    1,413
    Location:
    Melbourne, FL
    Thanks Dave.

    I have to figure out where to place the shields. That was holding me up. I'll probably open it back up to check on the voltages in a couple of weeks. I'll take another look then.

    BTW, I can't thank you enough for your help.
     
  20. Tim D

    Tim D AK Subscriber Subscriber

    Messages:
    1,413
    Location:
    Melbourne, FL
    Hi Larry,

    Yes, I have the assembly manual, but it's missing a few of the diagram pages. It has the all-important schematic though, and I think all the assembly steps are there, and I think that is all I need.

    Interestingly, the unit seems to be very silent now that it is buttoned up. I had it running yesterday for several hours - man does the grill on the case get hot. I may have to leave it off for better ventilation or get a small fan for it. I think the heat is coming from the OP tubes. The bias is good on them so I guess they just run hot.
     

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