Rx For the Magnavox 8800 Series

Discussion in 'Tube Audio' started by dcgillespie, Apr 24, 2018.

  1. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    Continuing On:

    The clipping occurs whether operating with cathode or fixed bias; the bottom half of a sine wave (1 kHz in this case) always clips early when the UL connection is made. This shot was taken as the top of the sine wave approaches the onset of clipping. However, a quick check with the pentode output stage connection shows this behavior does not occur in that mode, with clipping extremely well balanced in presentation when driven to clipping levels.

    Back in UL mode, the NFB loop was opened so that a study could be made without that corrective element in place. The clipping remained. Further study revealed that the cause of the uneven clipping was due to one of the very circuit elements initially used to enhance the performance of the unit in the first place: convert the original Paraphase Inverter design into a Floating Paraphase design. It was found that when you bring:

    1. A UL output stage,

    2. Near full power clipping, and

    3. A traditional Floating Paraphase inverter design that directly drives the output stage

    ..... All together in the same design, then early clipping on the bottom half of the waveform will result. You take out any of the three elements listed, and the clipping evens out on the tops and bottoms of the waveform. The problem was ultimately traced to the fact that a Floating Paraphase inverter does not present an equal drive impedance to the grids of the output tubes. More specifically, the impedance between the control grid of the lower output tube and ground is much lower than it is for that element in the upper tube -- this because of the NFB employed around the inverter section of the driver stage that turns the inverter into a floating type design. In pentode mode, that is not a problem. But when the UL connection is made, the lower impedance appearing at the control grid of the bottom output tube makes the screen grid more sensitive to the UL FB signal applied there, preventing that tube from clipping. In the upper tube however, the screen grid is not as sensitive to the UL signal, causing that tube to clip early relative to the bottom tube, and producing the display above.

    This is perfectly normal behavior for pentode tubes: A pentode always produce more gain to a signal applied to its control grid when the impedance of the circuit powering the screen grid is made very low -- this because the low impedance prevents what would otherwise be a NFB signal from being injected into the grid were the impedance to remain high. This is the purpose of the screen bypass cap used in many small signal tube pentode stages. The scenario here is exactly the same, except that the roles are reversed. In this case, it is a signal being injected into the screen grid, with the impedance present at the control grid affecting how sensitive the screen grid is to a signal applied to that element.

    What was determined then, is that optimum UL operation requires the drive impedance presented to the control grids to be equal, and for maximum effectiveness, as low as possible. As a quick check to see if this theory was correct then, please refer to the schematic provided for the modified amplifier when retaining use of the stock OPT. By attaching a 10 uF 450 volt cap between the bottom of the AC Balance control (where the 470K/3.3M resistors attach) and ground, the circuit immediately reverts to that of a standard Paraphase inverter design, with the AC Balance control still performing its duty quite nicely. This now creates nearly identical, albeit rather high impedance drive circuits to the output tubes, that could still be adjusted for a balanced drive. With the AC Balance control properly readjusted, that simple change resulted in this display at clipping:
    SAM_2456.JPG

    Even clipping! So for UL operation, a Floating Paraphase inverter that directly drives the output stage is a problematic way to go. But what about the Split Load (Cathodyne) inverter? It too presents unequal drive impedance from its outputs. Since Dynaco used this type of inverter in all of it's commercial designs, and had it directly drive the output stage, shouldn't it create the same problem as well? Hafler/Laurent's solution to this problem was to always use the lowest practical plate and cathode load resistor values possible for the inverter stage, so that the effective impedance presented to both output tube control grids is low enough to prevent such behavior. The result is even clipping. But there's more.

    From the data above, note that the low distortion quiescent current for UL operation came in at 46 mA per tube. But when the inverter was modified back to standard Paraphase operation, the optimum low distortion operating point was found to be 38 mA per tube, or almost exactly where the optimum pentode operating point was found to be (37 mA per tube). So the unequal drive impedance of the Floating Paraphase inverter not only upsets the dynamic balance of the output stage (at all power levels), but also upsets the optimum operating point as well. A couple of quick IM distortion tests were then run to see if they would supply further confirmation as to what was playing out. With the inverter modified to standard Paraphase operation, and the AC Balance control set for optimum, the following data was gathered:

    1. With cathode bias, 8.2 watts equiv. power output was produced at 0.75% IMD (using 250Ω cathode resistor).

    2. With fixed bias, 9.25 watts equiv. power output was produced at 0.50% IMD (38 mA quiescent current per tube).

    So more power at less distortion with lower quiescent current was developed with both forms of bias when using the lowly Paraphase inverter circuit, which further validate the loss of performance when a UL output stage is directly teamed up with a Floating Paraphase design. This surely cannot bode well for the UL 6V6/6BQ5 circuits that both Acrosound and Dynaco show in their respective transformer catalogs, nor the design in Hafler's article, as all of these are of the type of design described here. No doubt however that the act of replacing an original pentode based OPT in a suitable commercial product with a premium UL transformer (as Hafler does in his article) would still produce significant performance gains.

    But permanently reverting back to a standard Paraphase design in this scenario is not without its problems either: The adjustment of the AC balance control is quite sensitive in that mode, with even slight changes from optimum causing distortion to rise quite rapidly. Not everybody has a distortion test set handy to set such a control accurately. The topology of the ST-35 would seem to be the simplest and most effective approach then to directly driving small power tubes in UL from a phase inverter stage, without the problems noted previously. As I'm sure most will agree, such elegant solutions are a hallmark of Dynaco products.

    For this project however, changing the phase inverter circuit to accommodate UL operation might be the last straw for some folks, as it would no longer represent anything resembling a Magnavox amplifier, but simply an ST-35 built into a Magnavox chassis, using 6V6 tubes. Fair criticism. But also the best solution for those committed to quality UL operation in the Magnavox chassis. Doubling back to pentode operation then, the following data was gathered:

    1. Cathode Biased Pentode Operation with 250Ω Bias Resistor:
    THD @20 Hz = 3.5% at 9.68 watts. @1 kHz = 1.1% at 10.125 watts. @20 KHZ = 1.65% at 8.20 watts.
    IMD @ 10.34 watts equiv. power output = 2.0%

    Cathode Biased Pentode Operation with 220Ω Bias Resistor:
    THD @20 Hz = 2.8% at 9.9 watts. @1 kHz = 0.55% at 10.45 watts. @20 KHZ = 1.10% at 8.82 watts.
    IMD @ 10.60 watts equiv. power output = 1.10%

    2. Fixed Bias Operation:
    THD @20 Hz = 3.0% at 10.60 watts. @1 kHz = 0.175% at 11.88 watts. @20 KHZ = 0.61% at 11.28 watts.
    IMD @ 11.47 watts equiv. power output = 0.58%

    COMMENT: From a purely distortion standpoint, the big advantage to UL operation is THD produced on the low end at 20 Hz -- although, when pentode power output is reduced to equal that of its UL counter parts, the difference diminishes. And, at 25 Hz, pentode distortion is just under 1.0% at 11.52 watts, so distortion drops quickly as frequency rises above 20 Hz. With a power response of 10 watts RMS from 20 Hz to 20 kHz, and all distortion under 1% from 25 Hz to 20 kHz, pentode performance is hardly shabby, with many preferring the lower damping that connection provides that endeared them to the Magnavox sound in the first place. And, maintaining pentode operation of course helps to preserve some of the original Magnavox design as well.

    Of course, there are other aspects of the amplifier that are not discussed here, yet deserve a quick mention:

    1. Frequency Response: +/- 0 db from 20 Hz to 20 kHz, within in -0.5 db to 40 kHz.
    2. Stability: Will tolerate any value of capacitive only output loading without bursting into oscillation.
    3. 10 kHz square wave presentation:
    SAM_2448.JPG

    4. And the performance as presented at the 4Ω tap:
    SAM_2449.JPG
    It is performance like this that (in part) make these transformers far and away the best product available in their power and impedance class, whether for pentode or UL service. They are not inexpensive, but unlike the Raphaelite transformers, they are absolutely worth the cost of admission for the performance they provide. The performance figures quoted throughout this post are identical whether viewed at the 4Ω or 8Ω tap.

    With that then, the amplifier has been all buttoned up (for now) in pentode mode, operating with fixed bias from a small bias supply installed earlier. As such, for a quality 6V6 amplifier of conventional design -- other than operating with fixed bias and only 12 db of NFB -- its performance is quite impressive, allowing it to rise considerably from it's basement dwelling location of Magnavox designs. A schematic will be presented in due course. For now then, those who are absolutely committed to operating these transformers in UL, the best recommendation would be to simply use the ST-35 inverter and FB/stability circuits, and operate the output stage with fixed bias. The results will be excellent.

    Finally, for those interested, the following information was gathered with a 16Ω load presented to the 8Ω tap:

    THD @20 Hz = 7.9 watts at 5.9%. @25 Hz = 8.41 watts at 2.5%. @1 kHz = 8.41 watts at .32%. @20 KHZ = 9.0 watts at 0.93%.
    IMD @ 8.50 watts equiv. power output = 1.0%.

    A 10 kHz square wave presentation remains largely unchanged with a 16Ω load, as do response and stability characteristics:
    SAM_2459.JPG

    All in all, not too shabby. A few final pics are presented:

    Top side: The chassis controls now sport knobs.
    SAM_2447.JPG

    Bottom Side: You can see the small bias supply added in the upper right corner of the chassis, powered by an appropriate transformer stolen from a discarded wall wart. This of course necessitated changing out all the Bias and DC Balance Controls for them to work properly with a fixed bias design:
    SAM_2436.JPG

    And finally, a close up of the bias supply:
    SAM_2437.JPG

    For now, that brings this Magnavox saga to an end. Time to enjoy some more listening!

    Dave
     
    Last edited: Jun 14, 2018
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  2. 6DZ7

    6DZ7 Super Member

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    Is it safe to assume that the HV winding can carry the extra power ( almost double) just from an external temperature check, after moving the power from heaters and a bulb?
     
    Last edited: Jun 14, 2018
  3. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    Good question -- but I think the assumption is a pretty safe one. The math approach was primarily aimed at ensuring total primary winding consumption was not significantly different than the application the transformer was originally designed for (and it's not). But as to the HV secondary winding's capability, I think the answer to that is best supported by by the fact that HV regulation at the output of the rectifiers -- even with the output stages operating with fixed bias -- is still on the order of ~ 3.5% from quiescent conditions, to full power output in both channels -- which is actually quite remarkable. The quiescent B+ at the output of the rectifier is also a product of 1.25 times the AC voltage applied to each side of the full wave circuit as well. If the HV winding were not up to the task at hand, the resistance of a less capable winding would not allow the performance noted to be produced. Still, it's a point that needed more clarifying, so thanks for chiming in!

    Dave
     
  4. gadget73

    gadget73 junk junkie Subscriber

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    Interesting that the phase inverter impedance differences would cause a problem specifically in UL mode. I know its beyond the scope of this mod, but it makes me wonder what would happen using a FET source follower as a buffer between the inverter and the output tubes to provide matched, low impedance drive to the output tube grids. That should in theory allow for matching a floating paraphase with the UL operation since there wouldn't be any impedance mismatch presented to the output tube grids.
     
  5. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    Gadget -- Your absolutely right -- it would solve the issue in the classic way that any "buffer" circuit does -- by preventing the effects of one circuit from affecting another. I had thought of various buffering schemes as well, but also knew I'd be summarily shot for taking a Magnavox modification any further in that direction. I figure that the SS rectification of this project and the addition of EFB to the last project have pushed that envelop far enough......

    Dave
     
  6. Schmidlapper

    Schmidlapper Well-Known Member

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    I have been working along on my own 8802 in the background following Dave's floating paraphase mod except for the two balance circuits, the EFB and using some different non ultra linear OPT, and I experienced the same cutting off of lower half of sign wave as you describe. Possibly not using ultra linear may not be a cure all. I am wondering if I should go with your initial remedy of adding the cap and reverting to straight paraphase. I don't have the AC balance installed and I don't have distortion testing equipment?

    Bill
     
    Last edited: Jun 14, 2018

     

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  7. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    If you're running the output tubes in pentode mode, the screen grids and cathodes are well bypassed to ground, and the clipping is uneven, it can only be due to basically three things:

    1. There is a drive balance supplied to the output stage that does not provide for equal clipping of the tubes.

    2. The output tubes themselves are not sufficiently balanced to provide for equal clipping at that event.

    3. The output transformer is not sufficiently balanced, causing otherwise balanced plate signals to clip unevenly.

    With the UL connection, there are effectively two inputs into each output tube (screen grid and control grid). As long as the impedance at each input is the same for both tubes, then the stage will operate in balanced fashion. In pentode mode however, one input (the screen grids) are effectively grounded, leaving only one active input per tube. While an unbalanced impedance between the remaining (control grid) inputs can still cause problems in pentode mode, the degree of imbalance required to do so is far greater than when both inputs at each tube are actively being used (UL operation).

    Dave
     
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  8. Schmidlapper

    Schmidlapper Well-Known Member

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    So a similar symptom, but different cause? I did mention previously that it was coming out of the driver that way on the scope. I appreciate your clearing it up, I don't want to lose the benefits of the floating paraphase mod fixing a symptom.
     
  9. gadget73

    gadget73 junk junkie Subscriber

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    Try hanging the scope probes on both output tube grids, with the probes set to X10 to see if the grid drive is reasonably even. Possible its just an AC balance issue.
     
  10. Schmidlapper

    Schmidlapper Well-Known Member

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    I will do that check this weekend. I'm also thinking I may go ahead and incorporate the AC balance portion of the mod for this reason. I am currently getting 6.6 Watts max clean @ 1kHz into 8 ohm load with both channels driven, 30Hz to 20KHz full power band width using -14.95dB NFB, B+ is 274V and cathode current is running 36.4mA with a 220 ohm cathode resister. 7.7K OPT are recovered from a Rowe/AMI jukebox amp.
     
  11. cozido512

    cozido512 New Member

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    I find the above result a bit surprising, after all, hundreds of thousands of UL amplifiers have been built over the years, and I can't recall that anyone experienced or reported the asymmetrical clipping behavior when using the floating paraphase or the cathodyne inverter, oh well, you learn something new everyday. Thanks again for the detailed report!
     
    Last edited: Jun 15, 2018

     

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  12. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    co -- I must admit that I was surprised myself. It was yet another twist and turn that I did not foresee at the beginning of this project. What really surprised me, is that as the cause of the issue was being determined, I started scouring my resources to find any attention given to the matter of asymmetrical clipping due to unequal drive impedance from the previous stage, and could find nothing. I'm hardly saying it doesn't exist -- just that I haven't found it yet if it does.

    So then I started looking at all the commercial UL designs that came to mind. The original UL Amplifier from Halfer and Keroes employs a driver stage that would prevent such behavior. Most of the early commercial and even DIY UL were all 6L6 based, and being largely Williamson oriented, employed driver stages as well. The bigger output tubes of the mid 50's were often directly driven by a cathode coupled inverter, which would also act to prevent such behavior. When the 6BQ5/EL84 arrived, use of the 6V6 disappeared almost overnight in high quality high fidelity equipment, with the vast majority of 6BQ5/EL84 designs not even being of UL design anyway. Off the top of my head, the small Heath and Dynaco amplifiers are the only commercial UL amplifiers I can think of using these tubes -- and they both use Cathodyne inverters. To that point then, I can't immediately think of any commercial design that in fact used a floating paraphase inverter that directly drives a UL output stage -- just the circuits offered in the Acro and Dyna transformer catalogs, that would have virtually all been built in DIY fashion, and would therefore have likely if not largely lacked any ability to observe, test, or diagnose the observation. And, given how few TO-310 and A-410 transformers are in circulation today -- compared to say the TO-300, TO-330, A-430, and A-431 transformers (all of which were used in commercial products) -- not many of the small catalog designs were even built.

    Come to think of it, as I sit here typing and mulling over all of this, I can think of in fact one commercial product that does fit the description of this topic: The Acrosound Stereo 20, a mono version of which was detailed in an article entitled "Direct Coupled", as presented by Keroes in the March 1961 edition of Popular Electronics. Among it's many features, that amplifier does fit the topic of this discussion, employing a 12AX7 paraphase inverter driving an EL84/6BQ5 UL output stage. But lets face it, there were hardly many of these units in circulation to cause much of a stink about the matter, either. And, based on personal experience, these units had other far worse issues to deal with other than full power clipping, so again, the issue would have likely been masked.

    As my memory comes up to speed this morning, I found that I actually have my own supporting evidence of this behavior with this very amplifier. There are a couple of well known AKs who absolutely say grace over their Acrosound Stereo 20 amplifiers, but not too long ago, sent me one out of frustration for numerous problems they were having with anything from overheated output tubes to high distortion and instability. One of the more obscure issues to address was the fact that the top output tube always clipped early in that amplifier! (exactly as noted with this project) As evidence of that fact, I have lifted a piece of my own correspondence back to the AKers regarding my explanation of the unequal clipping noted in that design:

    "So the last performance anomaly of the design that I wanted to address (before finally getting to optimizing the HF response issues), is the fact that in the design as presented by Acro, V2 always clips first (or early) as the onset of clipping is approached. No doubt this was (in part) some of the reason for pursuing an AC Balance control. However, tests have shown that in this case, the uneven clipping is not a product of uneven drive, but of uneven impedance. Floating paraphase inverters can be adjusted for equal clipping -- which also coincides with minimum distortion as well -- rather easily with traditional designs. Of course, this design is anything but traditional, with the point being that V2 is driven by an incredibly high impedance from the starved operation of V1A. Now V1B also operates with starved operation, but: V1B does not have the additional cathode resistance of R6 to contend with, which is necessary for insertion of the NFB loop at this input. R6 supplies negative current feedback to V1A, which inherently raises its output impedance V1A over that of V1B. As well, and this is the biggest factor, V1B operates with considerable NFB around itself, which lowers its impedance quite significantly. With the impedance imbalance from the inverter then, as the output stage approaches clipping, V2 -- driven by a very high impedance -- will clip immediately at the first sign of any grid current, while V3 can be driven slightly into the first sign of grid current because of its lower drive impedance from V1B. With 25+ db of NFB in the global loop, it seeks to correct most of the effects of the imbalance, but cannot completely do so."

    As implied in the text, the owners had tried (quite possibly at my own recommendation) to install an AC balance control to the Stereo 20 circuit to mitigate the unequal clipping themselves. In my response to them, I correctly identify the unequal drive impedance as being (what I now know to be) part of the asymmetrical clipping issue, but at the time, had not made the connection that in that particular design, it was in fact the unequal drive impedance when used in conjunction with a UL output stage. The more thorough investigation I did with the Magnavox project project here unearthed the relationship of unequal drive impedance and UL operation, which really puts the mystery to bed.

    Obviously, there is more testing to be done, primarily centering on the question: how much drive imbalance can be tolerated in a UL output stage? Does it vary depending on the amount of screen FB applied (UL tap percent)? At what level of impedance does an imbalance no longer practically matter? These are things I will get to in time. At this point however, we do know that at least a Cathodyne inverter can be used with success (or at least insignificant concern) when directly driving a UL output stage (as all the Dynaco products show), and that pure pentode operation can tolerate quite a significant amount drive impedance imbalance, as the modified pentode based amplifiers offered with this project show not only equal clipping events, but also lowest THD when the drive is in fact adjusted for equal clipping. However, with the UL efforts of this project, it is worthy to note that when the AC balance control was adjust for minimum THD at a level of (say) 1 db below maximum power output, the amplifier then clearly displayed unequal clipping, and when adjusted to minimize the unequal clipping, the THD then rose considerably from the minimum distortion settings at all power levels.

    So, there is more to do and chase down on this topic. But this does seem to be an issue that (apparently) hasn't been addressed in the literature by anyone I'm aware of -- either currently or back in the day -- which is further aggravated by the fact that the number of working examples of designs subject to this concern are extremely limited. Hafler almost certainly knew about it, as his commercial designs were clearly successful, and never based on a floating paraphase design -- except for his non-commercial catalog design that was quite likely simply a carryover from his days at Acrosound. The point is, if he did know about it, he never wrote about it to my knowledge.

    Of note also, there is one last reference by Baldwin who worked for Acrosound, and published a article entitled "High-Power Performance With A Low-Power Amplifier" in the February 1958 issue of Radio-Electronics. His design also used a classic paraphase inverter that directly drives a pair of 6Y6 tubes operating in UL mode. He discusses the general use of the circuit with other output tubes, but makes no mention of any concerns for unequal clipping. Dollars to donuts his circuit displayed the same clipping characteristics -- but they are never mentioned in the article.

    Dave
     
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  13. gadget73

    gadget73 junk junkie Subscriber

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    Dave, have you ever noticed this in your modified Pilot SA-260? That would fit the bill of floating paraphase with a UL output stage, though EL34 / 6CA7 in that case.
     
  14. cozido512

    cozido512 New Member

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    The investigation continues... this is turning out to be quite a page-turner. :trebon:.

    As you know, when output clipping is approached, the output tubes' grid current begin to flow. I wonder if adding some grid stoppers might help with keeping the phase inverter's plate loads in-check a bit longer - it might be worth a shot. Just some last ditch efforts to save the floating paraphase inverter. :)
     
    Last edited: Jun 16, 2018
  15. 6DZ7

    6DZ7 Super Member

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    How much power do you really lose by just backing the volume control back down a tad away from "full power?"
     
  16. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    Gadget -- Thank-you so much for remembering another example of the scenario type being discussed! I've done so many over the years that I think I've forgotten more than I remember!

    No notation of unequal clipping was noted in my notes -- however, there are two other items of note to mention:

    1. As part of the modification, I installed an AC Balance control into that unit, where none existed before with the original design. Hummmm But more importantly,

    2. The Floating Paraphase inverter modification I devised still uses the original driver tube (12AX7), but retains the 68K plate load resistors of the original design.

    This last point may be significant in helping to establish how large these resistors can be before the imbalance becomes significant, until more formal testing can be done.

    Thanks again for bringing that unit into the discussion!

    Dave
     

     

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  17. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    co -- I'm not giving up on the floating paraphase yet, as I think there are still possibilities yet to explore, such as (for example) using a 12AT7 inverter tube, which can still show some gain, but also handle some current too. This would allow for properly lowering the plate load resistors of the inverter stage as a proposed solution. Also, when I added the fixed bias supply, I also installed 1 kΩ control grid stopper resistors at each output tube socket. I'm afraid that in this case however, their addition only makes matters worse (than better), by incrementally increasing the driving impedance seen from the floating paraphase inverter, into the output stage.

    All comments have been and are appreciated!

    Dave
     
  18. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    6 -- about a watt. But there is also the extra quiescent current required to achieve the low distortion operating point as the inverter now stands, If this were a 30 - 50 watt design, then the 1 watt just wouldn't be significant. But with only 10 watts, each watt becomes a little more personal!

    Dave
     
  19. kward

    kward AK Subscriber Subscriber

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    This is a fascinating topic. Wish I could contribute a bit more technically to it...still these ideas are going into my play books for future reference.
     
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  20. cozido512

    cozido512 New Member

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    Right, it's the high source impedance causing the problem, and not the heavy load (caused by the grid current), so the stoppers don't really help. In any case, you already tried it. But I still find it strange that when you adjusted the output for symmetrical clipping, the measured distortion went up - not down as expected. Would be interesting to find out why. Looking forward to the next installment of your report.
     

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