Rx For the Magnavox 8800 Series

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.
 
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
 
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
 
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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
 
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.
 
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.
 
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.
 
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.
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!
 
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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
 
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.
 
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.
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. :)
 
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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
 
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
 
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
 
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.
 
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.
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.
 
I am rebuilding a 175 for another AKer, The schematic in the beginning is the one I am following, without controls. In this text it says a 10 meg resistor, on the schematic a 3.3m. Near the end of the thread it mentions the 3.3m again, so I am guessing it's 3.3m. Is this still used in the non control version?
No Controls

If desired, the Bias, DC Balance, and AC Balance controls can be eliminated from this project as follows:

1. The resistors making up the Bias and DC Balance circuits can all be eliminated, and a single 220Ω 2W resistor be connected between the paralleled output tube cathodes and ground (along with the cathode bypass cap).

2. The AC Balance control can be eliminated by replacing the 430K resistor with one of two matched 470K resistors, and the existing 470K resistor with the other matched 470K resistor, that also has a 10Meg resistor connected in parallel with it. Tie the free end of the two matched 470K resistors together, and where they connect, also connect the free lead of the .047uF cap that goes to the bottom grid of the driver tube.
 
Yeah, that was written about as clear as mud, right? Sorry about that. The point I was poorly trying to make is that when the AC Balance control is included, it is executed just as the schematic shows, using a 3.3M resistor. When the control is omitted, the tap should be comprised of two matched 470K resistors, with the bottom resistor having a 10M resistor in parallel with it.

Sorry for the confusion. I hope that helps!

Dave
 
In the interest of keeping the stock look as much as possible, how would I wire up a simple single bias adjustment such as that in the modified 9300? I would like to use the stock speaker connectors for test points and install a single pot in the space where the hum pot used to be, again much like the basic modified 9300 has. I would be starting with known, well matched output tubes so the limited bias adjustment should still provide some benefit, though not as much as the full adjustments outlined by Dave.
 
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