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Unity coupled circuit destroying tubes/valves?

Scifi

Scifi

Super Member
I'm putting this in the McIntosh forum because the question involves a unity coupled output stage similar to the type used in McIntosh amplifiers.

I recently got a copy of Ir. Menno van der Veen's book "Modern High-end Valve Amplifiers: Based on Toroidal Output Transformers". I paid $38.99 for the book. I don't know why they have auctions where they're selling the book for a lot higher price right now. Maybe I just bought it on time. Anyway, in chapter 19 there is a paragraph referring to a unity coupled circuit which says:

The other configurations can be implemented in a similar fashion. Notice that no connections are shown for circuit 4. When this setup was tested, it showed an imbalance between positive and negative feedback, which quickly destroyed a set of output valves. So, don't try this at home!
Click to expand...

You can see the circuit refered to here http://plitron.com/wp-content/uploads/Atcl_4.pdf
more here http://www.plitron.com/news/audio-articles-2/

Circuit 4 has unity coupled output. I'm wondering why the tubes were destroyed and how McIntosh amps can use this type of circuit without destroying the tubes. The schematic shown in Atcl_4.pdf (fig. 2) is the same one shown in chapter 19 of the book.
 
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I can only vouch for Macs application.
In the three variants I've encountered. They have no tendency to eat tubes. I'm not disputing the book. Just that different operating conditions must be met
 
Scifi -- Any design that employs NFB within the output stage that is facilitated by the OPT must have very tight coupling between the various windings employed that facilitate the feedback. If not, the negative feedback that occurs in the easy to transform audio frequency realm can quickly become positive feedback at supersonic frequencies due to the phase shift that results from the less than perfect coupling. With the amount of NFB employed in Mac type output circuits, any resulting positive feedback is very capable of driving the output stage to beyond full power output, a condition that leaves the control grids drawing significant grid current -- more than they can safely handle. This can of course destroy the output tubes.

Suffice it to say, the Plitron transformers must have significant issues with stability when used in a unity coupled design, which can only result from less than perfect coupling within the transformer at frequencies outside the audio bandwidth. With Mac's bifilar and trifilar wound OPT technology, the windings of interest (plate and cathode) are literally wound together, side by side each other as they are interleaved with the secondary windings. Such an approach makes the coupling so complete on the primary side of the transformer that such problems don't exist. Other than using a couple of RF chokes to quash any low level parasitic oscillations that might try to form, the Mac output stage is utterly stable, requiring no special hijinks or hang on Zobel networks to achieve complete stability.

I hope this helps!

Dave
 
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I don't specifically know unity amps, but generally speaking ANY positive feedback in an amplifier is bad. It does turn them into lovely oscillators though.
 
I have no wish to imply that the toroidal transformers are not high quality. But the Mac circuit will test the limits of transformer quality. There are other more common but also otherwise very high quality transformers that will not remain stable, even when simply using the standard voice coil secondary to supply CF. The Acrosound TO-330 is an example of this. Such a configuration only has (typically) 5-7% of the plate signal appearing at the cathode, whereas in the Mac configuration, 100% of the plate signal appears at the cathode.

There is no aspect of the Mac output circuit that operates with positive feedback by design. It can only creep into the circuit by way of transformer qualities that permit it. Even the screen connection is not considered positive feedback, but rather, the prevention of negative feedback. The fact that there is "too much positive feedback" when using the toroidal transformer then, can only come from phase shift produced within the transformer itself as I originally indicated. You might find the article "A new 50 - Watt Amplifier Circuit" of interest, where in Frank and Gordon lay out the details and requirements of the Unity Coupled Output Stage. It appeared in the December 1949 issue of Audio Engineering.

Dave
 
As Davie told me once any one with enough, time, experience, patience and knowledge could build an Amp as good as a 275. The question is will they? We'll they still haven't, have they. The author of this criticism of Mac power amp circuits thinks to much of himself, assuming that just anyone can produce a Mac output transformer. And because they can then there must be a fault with the Mac design not with the execution of the design. Well after 65 years of building the most dependable high performing tube amps ever built I think his hypothesis is all wet..
 
Is there any evidence to support the claim that McIntosh amplifiers have the tendency to "eat" tubes at a statistically higher rate than the competition? - For this is the first I have ever heard of it.

If credible evidence exists that conclusively demonstrates notably decreased tube life in McIntosh amplifiers, I'd be interested in hearing it.

In the absence of such, it would seem the remarks about the Unity Coupled Circuit have been quashed right out of the starting gate.
 
There isn't. This is why its so common to find vintage Mc tube amps with the stock output tube still being used in them.

OP - see post #2.
 
Despite having been in production by Plitron since,iirc,the 1980's,and despite having great ''on-paper'' specs (size,weight,bandwidth etc) and the published work of Mr. van der Veen notwithstanding, these transformers don't seem to have achieved any great success. After having witnessed and occasionally participating in several attempts to exploit the alleged superior characteristics of these units,I quickly came to the realization that not much was to be had. The projects ranged from the swapping-out of the output transformer in existing (good quality) amplifiers to completely new designs. Transformers evaluated ranged from 40 watts to 300 watts.The results were disappointing....
And just to be clear,all of the involved individuals had engineering and technical backgrounds; this was in no way an amateur-night exercise.

On the other hand,I'm sure that were McIntosh Unity-Coupled output transformers to become readily available for purchase,they would be flying out the manufacturers door!
(And hopefully in through mine!:biggrin:)

As for this design eating power tubes,I purchased an MC-240 (1965 build) in 1996. All tubes were original.I finally needed to replace the outputs this year. Enough said.
 
Mac amps run basically class B don't they? That should be pretty easy on the tubes compared to your typical AB design.
 
In fact, it is quite the opposite. While it varies somewhat due to individual tube characteristics and from model to model, the output tubes in the classic Unity Coupled designs idle at far less quiescent current than the same tubes would in a classic pentode or Ultra Linear application. For example, in my own classic MC240, the output tubes idle at typically ~ 25 ma each, total cathode current. Subtract a couple of ma for screen current, and that means that the plates are dissipating the product of 22 ma and a measured plate voltage of 460 vdc, or about 10 watts when operating from a 122 vac line. This for a tube that is rated for a plate dissipation of 30 watts. By comparison, even guitar amps employing classic pentode designs operate these tubes at a level of about 20 watts plate dissipation, with typical high quality high fidelity designs operating them at 25 watts or more. By contrast, the Mac design is very easy on the tubes, affording a very long life.

While I didn't get into it when I wrote the explanation on how the Unity Coupled output stage operates, the Mac output stage does operate -- contrary to some popular notions -- in class AB1, not pure class B. In fact, the bigger 240 and 275 designs tickle Class AB2 operation under conditions of maximum power output. To be fair however, the class A portion of the Mac design is very limited -- a product of the low quiescent current.

Dave
 
I remember when I first read van der Veens papers on toroidal outputs,and their being available from Plitron ,located right next door in Ontario.
What a concept! The premise,the math,the engineering: spot on. This was it, every tube designers dream come true.
Man,I was SO pumped. I was bouncing up and down like a kid on Christmas morning.
They say that the proof is in the pudding. Unfortunately this ''pudding'' was of the smelly brown variety....

Dave- quite right. The Macs are probably the only tube power amp in history that don't heat the house at idle. I've always liked how the MC-275 ran the cathode-follower drivers off their own winding on the output transformer. Nice design work.
 
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The toroidal designs are particularly troublesome at low frequencies with any kind of DC imbalance across the primary. They will show themselves quite quickly in that scenario.

The extra winding and the heftier tubes used for the CF drivers in the 275 allowed for a little more kick from the stage to help with the higher bias voltage used with the bigger tubes, while still operating the drivers well within their ratings by using a lower voltage source to power them. Going the other direction, in 225, they were able to dispense with the CF stage altogether......

Dave
 
And the level at which the imbalance becomes intolerable is very low indeed. Not the best scenario with tubes,which tend to vary considerably during operation as well as throughout their lives.

We did experiment with an auto-balancing circuit,which used dc voltage sampling across individual cathode resistors to effectively track the bias to maintain equilibrium,but the circuit complexity required to accommodate all of the variables was just too much effort and took up too much space. I seem to recall someone writing an article for Glass Audio outlining a similar project,but I don't remember if it was an effective design.

I've always wondered if the CF stage in the 275 was used (through it's own winding) to provide the first few milliwatts of power before the cold-biased output stage began to wake up:cool:
 
Norman Crowhurst was truly one of the greats. I have originals of the 3 magazines you've listed,so this is a real trip down memory lane.
In his circuit you can see elements of McIntosh and later Audio Research designs.His use of push-pull feedback is very cool. His ability to achieve the results he did,using the quality of transformers chosen,is an indicator of his design skills. But unless you have a bunch of mediocre transformers (with the appropriate primary impedances) that you wish to put to use,you're out of luck.
 
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