Rx For the Magnavox 8800 Series

[dcgillespie]

Kid -- May I assume that you're looking to build the modified version using cathode bias with the original OPT? If so, then the only way that can be accomplished is to revert (nearly) back to the cathode circuit of the original design. That is, on the modified schematic, the 470K grid return resistors for the output tubes would return to ground instead of to the balancing circuit. The four cathode terminals would then be connected back together again. From the four cathodes, there would be a single 100 uF/25 volt cathode bypass cap connected to ground. From the cathodes, there would also be one side of a 25Ω 2 watt pot, with the wiper and other side of the pot tied together, and connected to a 100Ω 5 watt resistor, and the other side of the resistor being grounded. Your test points would be ground, and the junction of the pot and resistor. Adjust the pot for 16.0 vdc across the test points (or 100Ω resistor). If you want to reverse the action of the pot, simply connect the wiper of the pot to the outside terminal connected to the cathodes, rather than the resistor.

I hope this helps!

Dave

 

[cozido512]

Dave - could you please post the schematic with the floating paraphase inverter? I may have a chance to breadboard it on my bench next week, and see if I get the same result as you (with another OPT though, since I don't have a Z-565).

 

[Schmidlapper]

Dave - could you please post the schematic with the floating paraphase inverter? I may have a chance to breadboard it on my bench next week, and see if I get the same result as you (with another OPT though, since I don't have a Z-565).

Post #15, Details, the first attachment is the current schematic. It is easily overlooked because it is a plaine white thumbnail and blends in.

Bill

 

[cozido512]

Thanks, but that’s the modified schematic with the pentode output and the paraphase inverter, not the one Dave is currently working on, which has the ultralinear output and the floating paraphase inverter.

 

[dcgillespie]

co -- The pertinent changes I made to the driver for the fixed/cathode bias, pentode/UL tests were that the output tube grid return resistors were reduced to 220K (this because of operating the output stage with fixed bias), while the plate load resistors for the driver stage were reduced to 150K each to help accommodate the increased load that the reduced grid return resistors represented. The AC balance circuit remained unchanged, while the NFB resistor became 1 KΩ, with a 10 KΩ resistor paralleling the 1000 uF inverter stage cathode bypass cap -- this to accommodate both the required driver bias for its new operating conditions, and to produce approximately the same NFB level (~12 db) when using the Z-565 transformers (in pentode mode) with FB taken from the 8Ω tap. The rest of the changes were to the output stage DC Bias/Balance circuits to accommodate fixed bias operation, and to maintain proper stability and response when using the Z-565 transformer -- both of which have no significant impact on the effect noted.

Dave

 

[cozido512]

Cool, I will see if I can duplicate what you see, albeit with a different OPT...

 

[dcgillespie]

If I may, what transformer are you using and do you know what the screen tap is (% of winding)?

Dave

 

[cozido512]

It’s 4.8k, with 4 & 8 Ohms secondaries, and the screen tap at 20%. I will load the 8 Ohm tap with a 16 Ohm resistor, so Ra-a is 9.6k, not optimal, but should be close enough to see what’s going on.

 

[dcgillespie]

Agreed, although with less screen tap percentage, the effect may be somewhat reduced, but should still be enough to provide evidence of the effect over that of straight pentode operation.

 

[dcgillespie]

For those following along with the issue I've discovered wherein a Floating Paraphase Inverter produces unequal clipping when directly driving a UL output stage, I can now provide the following additional information on this subject:

1. Use of a lower impedance inverter tube such as a 12AT7 in the floating paraphase position helps matters slightly, but does not entirely eliminate the effect.

2. The application of global NFB acts to accentuate the problem, as might be expected. When the top output tube clips, the FB signal acts to correct the condition by applying more signal to that side of the push-pull circuit, in an effort to restore the missing crest of the sine wave. This of course causes the clip to be even more accentuated.

3. With no global FB connected and close observation, the onset of clipping is actually seen to occur at the same time on both crests of a sine wave, but the characteristics of the clipping on each wave crest are seen to be very different, with the clip on the bottom of the wave very pronounced, while the clipping on the top of the wave is very soft and hardly noticeable at first.

4. The drive impedance itself seems to be rather unimportant, as driving the output stage directly from a push-pull OPT (then acting as a push-pull driver transformer) results in identical clipping as that event is approached, achieved, and exceeded. However, if a conventional 12AT7 driver stage (for example) is inserted between the driver transformer and the output stage, clipping is again still seen to be identical on the tops and bottoms of the waveform. This shows that the impedance level itself is of limited importance (meaning that it can be either high or low), but that it's being equal is of significant importance.

These results would tend to doom the Floating Paraphase inverter design when it is used to directly drive a UL output stage while seeking the ultimate in UL performance. Efforts to apply FB around the top inverter section to lower its drive impedance produced limited results, as the drive impedance is in fact lowered, but the move also increases the drive load on that section as well, producing (again) imbalance.

At this point then, unless any new additional information comes along showing otherwise, a Floating Paraphase Inverter -- when used to directly drive a push-pull audio output stage -- should only be used to drive straight pentode or triode output stages, as the imbalance produced when driving a UL output stage is notable, and significant.

I welcome any information that might be provided from others doing their own tests regarding this matter.

Dave

 

[cozido512]

Thanks for sharing your results, unfortunately, I won't be able to breadboard the circuit as planned - an unexpected business trip has just popped up. But it seems the difference in the plate resistances of the floating paraphase inverter is just too great to overcome when directly driving the output tubes in UL mode, by my estimation, the upper tube has ~80k, and the bottom tube has only ~1.7k - a whopping 47x difference! Oh well, learned something new today.

 

[gadget73]

So in UL mode, is the input impedance of the 6V6 grid lower vs pentode mode and thats what causes the inverter to be unable to drive it properly? If so, does it get even lower if you triode strap a pentode or use an actual triode? Just asking for my own understanding here since its just not something I've ever seen discussed before.

 

[dcgillespie]

Agreed. But old fashioned breadboarding is just so passé anymore: Any old slab of wood will do!



As was shown earlier then, the standard Paraphase inverter (Magnavox's original design) actually provides a better -- as in equal -- drive impedance scenario when directly driving a UL output stage than the Floating Paraphase design does, and therefore makes for a "better" inverter in that instance. This is particularly true for the design of the 8800, since that unit does not include a cathode bypass cap for the upper AF Amplifier section of the inverter stage (as is used in the 175/185 chassis), which acts (among other things) to lower the output impedance of that inverter section, again creating an imbalance. As well, it was also shown that to achieve the low distortion capabilities that UL operation provides, the matter of accurate drive balance is also of some importance, which is not a strong suit of the standard Paraphase inverter design. As a result, more testing needs to be done to see if a UL output stage is any more sensitive to drive imbalance, versus that of it's plain-jane pentode mode cousin, relative to distortion generated. But resolving that issue is another question for another time, and more properly the topic of a separate thread dedicated to that issue.

Until then, if maximum performance is the goal, then UL operation of any of the popular Magnavox push-pull designs -- even with a really high quality OPT -- may just in fact represent a step backwards rather than forwards, unless the appropriate equipment is available to adjust the unit for optimum performance. For now then, in the interest of trying to keep this project even remotely close to its original form, the amplifier will be left in fixed bias pentode mode, and as such, represents a very fine 20 watt 6V6 stereo amplifier, that few other designs using this tube could even approach matching. It's a very far cry from its original humble bargin basement beginnings, and as such with some finality now, allows a proper schematic drawing to be plugged into the to-do list of things to finish up for this project.

So just when you think that every aspect of vacuum tube amplifier design is settled, something new appears -- or something that certainly seems to be new anyway since I have found no previously published information on the findings of this work -- either directly, or in passing as comments of a larger text -- discussing the principles, application, integration, and/or installation of Ultra-Linear output stages to maximum benefit in audio amplifier design work. Work will continue of course, but for now, comments are always welcomed.

Dave

 

[dcgillespie]

Gadget -- It's not the input impedance of the 6V6 when operated in UL mode that is of concern, it is the unequal drive impedance of the Floating Paraphase inverter that is at issue. As the output stage reaches the onset of clipping, it attempts to start drawing grid current. During this time, the unequal drive impedances presented to the control grids -- in conjunction with the UL signal presented at the screen grids -- causes the tubes to clip unevenly. When the screen grids are grounded with respect to AC (pentode operation), then all else being equal, the clipping then otherwise becomes equal in every aspect from both tubes.

6 -- In fact, I use that approach in the final (pentode) built of the unit, even though the UL taps are not used for their primary purpose. The effect of such feedback works to either eliminate, or greatly minimize any step networks needed, and usually installed at the plate of the AF Amplifier stage. In Hafler/Laurent's designs, they used a pentode AF Amplifier stage, whose output impedance at the plate is always very high. While step networks at that location can be very effective in achieving proper HF stability, they also tend to cause a greater than normal increase in HF THD in the finished design, due to the significant load placed on the tube. By applying some FB from the appropriate UL tap, that load is either eliminated or greatly reduced, allowing the same level of stability to be achieved, but with far less rise in 20 kHz THD in the process.

Hafler remarked that the FB applied from the UL tap worked to balance out the unequal drive characteristics of the Cathodyne phase inverter, which of course is simply not possible. But due to the uniqueness of that FB path at the time, and discussion of the Cathodyne's unequal drive characteristics being a hot design topic in the press of the day, the statement made for a great marketing ploy that time has shown he clearly used to good advantage.

Dave

 

[cozido512]

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:


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.


Here is a video showing the output as it goes from unclipped to clipped.

 

[dcgillespie]

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

 

[cozido512]

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...

 

[dcgillespie]

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

 

[dcgillespie]

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.....



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:



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

 

[gadget73]

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.