Rx For the Magnavox 8800 Series

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

  1. cozido512

    cozido512 New Member

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    Dave,

    I did a quick breadboard but was not able to observe the asymmetrical clipping. Since I did it in a real hurry, I may well have messed up something in the process... in any case, here are the screen shots:
    Screenshot - 6_25_2018 , 5_31_14 PM.png

    Here is the schematic I used - directly from RDH4 but with your values, obviously, the output tubes are 6V6GT's (V3 & V4) wired for UL operation. The 12AX7's (V1 & V2) are biased with 1k/100uF.
    Screenshot - 6_25_2018 , 5_03_51 PM.png

    Here is a video showing the output as it goes from unclipped to clipped.
     
    Last edited: Jun 26, 2018

     

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  2. 6DZ7

    6DZ7 Super Member

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    I went back to find the reference to this and see one in the introduction to the (unnamed Mk. III) as a new KT-88 amp, discussing the HF tailoring from the UL tap FB. Haven't found the part about balancing the PI yet.
     
  3. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    6 -- Actually, it was part of the introductory piece in the manuals for the MK II, MK III, MK IV and Stereo 70 amplifiers. But by the time the smaller 6BQ5 amplifiers came out, even though they too used the screen grid FB connection, the claim about balancing the phase inverter was no longer made. The connection was discussed, but only in its proper context of producing proper HF stability.

    co -- Thank-you for providing your information! I wish I had your computer skills used to display your results.

    The one significant thought that comes to mind between your experiments and mine, are of course the transformer used -- and most particularly, the screen tap percent. Both Keroes and Hafler were quite emphatic that the "Ultra-Linear Effect" occurs over a vary narrow range of tap placement, with the Z-565's tap placement representing the ideal position for these tubes (Z-565 tap placement is at 25% of the winding, with both 24% and 25% quoted as optimum for the tubes). With the taps of your transformer at 20% -- and differing from those of the Z-565 in the direction of pentode operation -- that basically represents a 20% deviation from those of the Z-565, and very likely could account for the difference.

    No doubt, this is almost surely tube specific as well, which has not been investigated yet. For example, Keroes' Acrosound 20-20 amplifier uses transformers that also employ screen taps at 20% (primary imp 7.2K) -- except that it notably uses 6BQ5 tubes -- tubes which have over double the Gm of 6V6 tubes -- and that amplifier does display the unequal clipping clipping characteristic, as witnessed by myself, and its owner (text of which I posted earlier). And of course, it uses a Floating Paraphase inverter design as well. Point being that with tubes of lower Gm like the 6V6, it may not take much in the way of tap deviation towards the pentode connection to prevent the effect from happening, whereas similar tubes with a higher Gm will still display it, all else being equal. In any event, I very much appreciate your taking the time to work up your experiment and contribute your findings. It helps to define the parameters of the issue. I've still got some more tests to make, the results of which I hope to publish soon.

    Dave
     
  4. cozido512

    cozido512 New Member

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    Dave - l will spend more time with the breadboard when I return from my trip, but as you stated, the differences between the OPT’s might be a contributing factor. According to Hafler & Keores’ patent:

    For tubes of the 6V6 type a screen loading value of about 5% is preferred, and for still other tube types still a different value may be optimum.’ that translates to a 22.4% turns ratio, so neither the Z-565 nor my OPT meet the spec exactly.

    More later...
     
    Last edited: Jun 25, 2018
  5. 6DZ7

    6DZ7 Super Member

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    Thanks, found it. I read that sentence in the Mk.III manual to mean there is a frequency response imbalance. Maybe along with the recognized gain imbalance from the cathode degeneration, there is also something of an FR slope along with it that could create HF instability. Thus the added FB.
     
  6. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    And then in Hafler's June 1954 article in Radio and Television Magazine, he specifically quotes 24% as the optimum value for 6V6 tubes, with that being the figure that the Acrosound TO-310 transformer is based on. We need to be careful however that we're starting to mix two separate issues: According to Hafler and Keroes, the tap percent is based on a position that retains most of the pentode connection's power output while retaining distortion levels equal to (or less than depending on what you reference) that of triode operation - thus creating the Ultra-Linear effect. In our efforts here however, we're looking at how the tap's position affects the clipping events of a UL stage when driven by a Floating Paraphase inverter -- with the events the authors noted and the performance parameters they targeted, and that which I've noted and targeted, possibly not coinciding, relative to the tap percentage used.

    Dave
     

     

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  7. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    Whoops......

    I love a good mystery, and this has become one. So despite what I should be doing with my time today, I've dug into this a little more, and found out some very interesting things -- and something I should have remembered early on....... Ah, the price of aging.........

    First, in an effort to ensure that all last minute requests for clemency were covered for the Floating Paraphase inverter, I re-configured my outboard experimental inverter board into a Cathodyne inverter, to see if that would then force equal clipping. After all, Hafler's highly successful SCA/ST-35 amplifiers use that configuration with the very transformers of this project, and I've never recalled those designs displaying uneven clipping when matched tubes are installed. If the Cathodyne configuration produces even clipping, where the Floating Paraphase does not, then the Floating Paraphase inverter's fate is sealed.

    The Cathodyne inverter was rigged up (with no NFB like before) and the amplifier driven to clipping, with the following results.....

    SAM_2463.JPG

    Well, that theory got blown to Smitherines. So the Floating Paraphase design gets a last minute reprieve. And yet, it was still complicit in the crime, because it had previously been proven that when the drive impedance levels were made equal -- be they very low, or many thousands of Ohms, the clipping evens out. That left only one last test to make: Take the 9300 out of rotation, configure one of its
    6BQ5/EFB/Z-565 output stages for UL operation, and drive it with the same Cathodyne inverter rigged up on the experimental board to see how it would perform at the onset of clipping. That produced the following results:

    SAM_2464.JPG

    Technically not perfectly equal, as the bottom crest shows a fattening of the wave at this point before flattening out with further drive, whereas the top crest tends to flatten right away when entering clipping. But the important point is that both of these events begin at the same time -- and -- it's one heck of an improvement, and close enough that if that had been the initial result, its doubtful that any of this exercise ever would have been pursued. So what the heck is going on?

    If anything, this last test proved that there was more to what was going on than just the imbalance of drive impedance presented to a UL configured output stage, as originally thought. After all, the Cathodyne inverter itself (using 22K plate and cathode loads) produces an imbalanced drive impedance from its two outputs as well -- albeit certainly less of a difference than that of the Floating Paraphase design. But that reduced inequity seemed to make no difference in the clipping results produced when driving the 8800's output stage in place of the Floating Paraphase design. And yet, it produced by comparison excellent results when driving the 9300's output stage. With the significant difference shown between the two clipping scenarios then, something else clearly had to be going on -- and it was.

    In my post #103, I muse that the issue may even be tube specific, given the significant difference in Gm produced between the two family of tubes involved. And that may in fact be involved. But of more significance is a peculiarity of the 6BQ5 family of tubes that the 6V6 family does not enjoy. I had recognized this long ago, but it got lost in the cobwebs of time with this project until this last test jogged my memory: The 6BQ5 family of tubes -- upon which virtually every small UL amplifier is based on -- will actually reach saturation current levels slightly before Ecg = 0 is reached (and BTW: the EL34 acts the very same way). That means, that the 6BQ5 can reach clipping without drawing any grid current. That then is the missing piece, because even with an unequal drive impedance presented, a UL output stage using this tube will have far less disparity in clipping between the two wave crests because no grid current is being drawn at the onset of clipping.

    On the other hand, the 6V6 family of tubes enjoys no such advantage, meaning that in that tube, the onset of clipping coincides precisely with the onset of grid current, such that when configured for UL operation, and driven with an unequal drive impedance at the control grids, then unequal clipping will be the result.

    So what has been learned from all of this? The take away points -- in no particular order -- would seem to include:

    1. Push-pull Class AB1 operation of a UL output stage causes some tube families to be very sensitive to the equity of drive impedance presented to each output tube control grid as the onset of clipping is reached. This is because the inequity of control grid impedance in an otherwise balanced stage causes the screen grid Gm of each tube to differ, causing each tube to react differently to the UL signal presented there. All else being equal, the tube with the higher control grid drive impedance will clip first before the other tube in a UL push-pull pair.

    2. In push-pull Class AB1 pentode mode, the screen grids of the output tubes are grounded with respect to AC, which doesn't make the control grids completely immune to drive impedance, but does act to balance the Gm of each tube, causing them to largely be insensitive to drive impedance at the control grids as the onset of clipping is reached.

    3. The 6BQ5/EL84 (and 6CA7/EL34) family of tubes enjoy a unique advantage when operated in UL mode, because these tube families reach saturation current slightly before Ecg = 0 is reached. Therefore, in UL mode, they are rather insensitive to the drive impedance presented to each control grid as the onset of clipping is reached.

    4. The 6V6/6L6/6550 family of tubes enjoy no such advantage, so that when operating in UL mode, precautions must be taken to ensure that the drive impedance presented to each control grid is nearly equal, thus ensuring that even clipping is achieved as the onset of that event is reached. With the larger tubes, a driver stage is often needed (or used) which will also fulfill the requirement of providing an equal drive impedance. If a phase inverter is to directly drive a UL output stage with these tube types, then a cathode-coupled type is recommended to minimize any drive impedance inequity.

    The conclusions suggest then that the 9300 series Magnavox amplifiers are a good candidate for conversion to UL operation if so desired, but that the 175, 185, and 8800 series 6V6 based amplifiers are questionable in this regard. It also suggests that all of those who (for example) casually rotate 6L6 family tube types into Dynaco tapped screen operation amplifiers designed for use with EL34 tubes (think MK II, MK IV, Stereo 70, etc) are not only hurting performance due to the reflected impedance mismatch and improperly placed screen tap this combination creates, but are also further hurting it due to the compromised performance produced by these tubes as the onset of full power output is approached in such designs.

    Finally, these findings further support the decision to finish out the project of this thread as a fixed bias pentode based amplifier.

    As always, comments are welcomed.

    Dave
     
    Last edited: Jun 26, 2018
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  8. gadget73

    gadget73 junk junkie Subscriber

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    Hm, interesting observance about the onset of clipping prior to drawing grid current with certain tube types. Just out of curiosity, does the 6CA7 behave the same way as the EL34, or is it more like the 6L6 family? I know they're usually regarded as an EL34 equivalent, but the construction is a lot closer to 6L6. Also makes me wonder what the KT77 does.
     
  9. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    I haven't tested that yet, and wondered the same thing myself -- but the RCA tube manual indicates that the effect is the same with the American equivalents of the European tubes.

    Dave
     
  10. 6DZ7

    6DZ7 Super Member

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    Cathode-coupled meaning Cathodyne...?
     
  11. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    No -- I should have been more descriptive. Cathode coupled is a descriptor often used to describe what is also commonly known as a diff-amp inverter, grounded grid inverter, Long Tail Pair, or even a Schmitt trigger. These are all describing the same inverter topology, characterized by a common cathode connection that's (usually) connected to ground by a large resistor, a large value resistor placed across the two grids, and one of those grids grounded AC wise by means of a cap connected from it to ground.

    Dave
     

     

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  12. 6DZ7

    6DZ7 Super Member

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    SsssOK. Thanks. I'm sure everyone else already had that one... (grin 'moji thingamajig)

    I had for quite a while been using some KT-66 in my ST-70 at low to medium power and never sensed anything less enjoyable compared to the 6CA7 I pulled out of it. I recently reinstalled those for grins and a "fresh" hearing and balance checks. Came away with a shrug for my meager useage.

    Have a new entry in my jargon book today, Thanks!
     
    Last edited: Jun 26, 2018
  13. 6DZ7

    6DZ7 Super Member

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    2,077
    And your projects are always a great resource and fun to follow... despite my own minimalist philosophy on modding vintage amps.
     
  14. cozido512

    cozido512 New Member

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    If the drive impedance can not be made equal, then one needs to make sure that the grid current never flow under UL operation, which can be achieved by either sacrificing the maximum Po or by putting large enough grid stoppers (at the expense of poorer HF response). Or just stick with the pentode/triode operation. ;)
     
    Last edited: Jun 27, 2018
  15. Schmidlapper

    Schmidlapper Well-Known Member

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    :thumbsup:That’s what I’m doing. Pentode.
     
  16. cozido512

    cozido512 New Member

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    While waiting around in the airport lounge, I found some relevant information on the floating paraphase inverter in Morgan Jones' book, where he discussed the need for a build-out resistor to equalize the output impedance of the inverter, it might be worth trying when I get back from the trip to see how it behaves in the UL output stage. Here is the link to Google books.
     
    Last edited: Jun 28, 2018

     

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  17. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    As promised, here is a schematic for the final version of the modified 8800, that includes the Z-565 4/8 transformer.

    Final thoughts: I know for me -- and I suspect others as well -- the 6V6 represents as much of an emotional throwback to the early days of economical high fidelity, as it does in continuing to offer good performance today. Throughout the early to mid 50s, the audio magazines of the day were filled with circuits, reviews, and projects using this tube, from the likes of Grommes, Arkay, and transformer manufacturers. In all of it however, I'm struggling to recall a commercial UL product (other than White) that operated these tubes in UL. Besides the aforementioned DIY article of Hafler, another notable DIYer of the day (Marshall) also experimented with the tubes in a tapped screen circuit -- but again, there was no discussion in his articles regarding performance as the onset of full power output was reached.

    By the mid 50s however, the EL84 was starting to make itself known, and with its power output advantage -- as well as (we now know) its advantage when operated in a tapped screen grid circuit -- the poor 6V6 fell by the wayside rather quickly (except with manufacturers like Magnavox who could scoop up the vast supplies rather cheaply), so that ultimately, there were just very few if hardly any commercial examples of equipment operating the 6V6 in UL mode -- certainly not like there were EL84/6BQ5 examples -- which themselves were fairly rare as well. But all else being equal, in pure pentode fixed bias mode (as is used here in this final draft), the 6V6 will give the newcomer tube a serious run for its money in terms of performance delivered -- and, deliver all the nostalgia that a tube that helped build America can along with it. As far as 10 watt 6V6 amplifiers go, it will be tough to surpass this one, so for those with a fond spot for the 6V6, who want maximum performance from it, this should be on your short list. That it delivers the performance it does, with just good classic design implementation, is a testimony as to just how good its Beam Power configuration really is. It may not have the Gm or the diminutive size of the EL84, and it does have a a rather sharp clip (as compared to the EL84) in the circuits discussed in this thread, but if you can see your way beyond those drawbacks through proper design, it's minimized screen grid current and reduced heater current draw represent their own advantages, which has been put to good use here. Not bad for a tube that turned 80 years old last year!

    With that then, that makes a trio of Magnavox amplifiers visited, with the bigger units significantly improved even without a transformer change. When they are changed however, it really moves them into an entirely different performance category. If you're serious about your Magnavox 8800/9300 amplifier, the Z-565 transformer is well worth the money they cost -- compared to other replacement offerings.

    These amplifiers are each unique, with each having their own character. All are extremely enjoyable to listen to. I hope they've offered all of you some ideas for your own DIY projects, and shown that with a little re-engineering of the original designs, a lot more performance can be coaxed out of these chassis.

    Happy listening!

    Dave

    Mag 8800 w: Z565.jpg

    SAM_2476.JPG
     
  18. Schmidlapper

    Schmidlapper Well-Known Member

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    C5608FF8-F5A5-48E7-821A-155B31114105.jpeg F6CD4A66-43E3-4E23-BD35-268133071A7D.jpeg
    Dave has been patiently assisting me in the background as I adapted his excellent mod into my 8802 amp. I did not add the AC bal at this time, I don’t have the test equipment necessary to do THD testing. Testing shows it exhibits good stability unloaded and with various capacitive loads. Output is 6.1 watts both channels driven into 8 ohms, w/ 16.7dB of NFB, full power band width is 28Hz to 20KHz using recovered 7.7K OPTs from a jukebox amp. Speakers I use it with are rebuilt Klipsch Chorus II with several Crites upgrades, sensitivity 101dB, so about one watt is all I normally need.

    Thanks again Dave!

    Bill
     
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  19. s-petersen

    s-petersen Scott Subscriber

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    So.... I was following along with the first schematic, but ran into a problem, one of the original opts had an open lead, so I ordered a pair of Edcore 8K 15w opts to swap in, I would like to use the last schematic without the fixed bias, and AC adjustment, I am pretty sure I can use the first schematic with the10m resistor across the 470k, and I think I would use the 1K and .1 mfd to the 6V6 grids. Do I use the 220k or 470k to ground on the junction of the 1k and .1mfd ?
    This is a 175-67 the power supply puts out about 330v I think.
     
  20. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    That will largely depend on the inductance of the Edcore transformers, and of course the phase shift produced by the cathode bypass cap if you are going to stay with cathode bias. I would use the 220K resistors to be on the safe side. But I must tell you, if you are planning to stay with cathode bias, then the OPTs really need to be 10KΩ P-P units, as 8K transformers are a little low for using cathode bias. The reason that the inverter and following stage grid return values were altered is because when the output tubes are operated with fixed bias, the use of 470K as a grid return resistance is just too high, but is fine when the output stage is operated with cathode bias.

    Dave
     

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