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Parts and Pieces: A True ARC Mystery

Sorry to hear that Dave. I saw the bad weather in your area.
You always hope it doesn't effect someone you know. (I guess that's selfish in a way, but truthful:dunno:)
Good luck with the recovery . Hope power's back soon.
 
Hope your back up and running soon. My brother lives in the Alpharetta area and lost power for a few days as well.
We take the power grid for granted as it seems to work with nary a glitch but when it goes down it's a real pain. Living in remote areas or on a small spur means that your last in line for repairs.

BillWojo
 
Dave, we came this close to having an outage ourselves:

tropical_storm_zeta_tree_down_across_street.jpg

This is directly across the street from our house. Three feet to the right, and it would have been goodbye power lines. I have no idea when they would have gotten to us, as we're in the very back of a neighborhood with only one entrance- at the very end of the power line. They would have done all of the bigger outages first, and we'd likely have been at the bottom of the totem pole, here...

Regards,
Gordon.
 
dcgillespie said:
I've hardly forgotten about this thread folks -- in my area of N GA, we've been without power now since this past Thursday early AM (about 3:30) due to the rain and high winds that came through, and it's not projected to be back on until Monday. We're operating on generator now (manual hookup), but as we've been gone the past three days and thought the power would have come back on quickly enough while we were gone (it did for nearby surrounding areas), we didn't think we'd come home to all the thawed food and cleanup that's now required. So, a bunch of stuff to attend to now, and I'll get back to the thread asap.

Dave
Click to expand...

:eek:, :(, :hug:
 
So on to the final analysis -- How does it sound? Part 1

Oh boy, here we go.........

When it comes to specifically the power amplifier part of a given system's sonic equation, there's not much I haven't heard in terms of tubes used and design configuration, and the various combinations thereof. I'm hardly trying to say that I've heard it all -- but I've heard an awful lot over the past 60+ years, and knowing the designs involved of the units I've listened to -- even if I didn't understand them in my younger years -- I've none the less still learned over that time how different aspects of circuit configuration can affect the sonic delivery -- and how they can all interact in various positive and negative ways together. As a partial example, some of the various design configurations and considerations include the use of Triode, Pentode, or UL output stages (or various variations there of), Williamson, Mullard, or other small signal input/driver stage configurations, vacuum tube versus SS rectifiers, active voltage regulation versus various amounts of power supply energy storage, the impact of output stage quiescent bias current, SE versus PP operation, using large amounts of NFB versus limited -- little -- or none, the use of a single versus multiple feedback loops, and the impacts of both good/bad HF/LF transient response, frequency response, and load stability -- or the lack thereof. This covers an awful lot of design/performance territory, with almost all commercial vacuum tube amplifiers touching each one of these considerations in one way or another -- which then produces a resulting audible character.

So what's the point of all of this? The point is, that as with most engineered things, there's more than one way to skin a cat (my Principle Engineer Scooter not with standing....). Various elements of the above concepts can be put together in different combinations to produce a given or standardized target result. Of course, we all have our own preference as to what that "target result" should sound like, and so the endless search to find it -- which of course is part of the fun to begin with. But when you get beyond using that largely emotional (i.e., personal preference) concept as the yardstick and use one that deals only with standardized performance capability/results, and you're asked in a blind test of otherwise identical performing amplifiers -- but of very different design -- to identify what output tube type or output stage type each has, or driver circuit type each uses or whether a global or multiple NFB loop design is employed -- then I suggest that you don't bet the farm on being able to make that judgement anywhere close to accurately based solely on audible observations -- as the great likelihood is that you'll end up with no farm. Over my life, I have participated in way too many double blind test events that tested all manner of claims about audio equipment, their components used, and design topology employed to have seen that reality first hand.

Within the realm of practicality then, what specifications make for an "ultimate" high fidelity vacuum tube amplifier -- that is, ones that define the goal or gold standard with which to judge a given design by? There are an endless number that could be cited, but staying within a practical scientific basis, they would surely include:

1. Adequate Power -- For the room size, speaker efficiency, source material, and desired listening level, available with low distortion at full rated power output across the full 20 Hz to 20 kHz audio bandwidth (i.e., an ideal power response curve).

2. Adequate Power Supply Capability -- Such that power peaks do not cause any significant droop in operating voltages to adversely affect the performance of the design.

3. Adequately Low Distortion -- Once THD and IMD falls to or below 1% at all frequencies within the power response curve, you're seriously splitting hairs to hear the audible impact of any further reductions in distortion.

4. Adequate Electrical Damping -- Low enough to control speaker motion so as to prevent a boomy bass presentation. For most speakers, an internal resistance of about .70Ω at the 8Ω tap will typically achieve this.

5. Adequate Frequency Response -- Should be absolutely flat within the full 20 Hz to 20 kHz audio bandwidth, with a smooth roll-off above and below this range.

6. Adequate HF/LF Transient Response -- Displaying rapid settling under pulsed conditions, and smooth wave-top on 10 kHz square wave displays, with a rise time of 5 µsec or less.

7. Adequate Load Stability -- With no tendency to break into any kind of outright oscillation under any type of loading condition.

These are all important specifications, validated over time, that together produce an amplifier that is about as invisible in the amplification process as is practically possible. To that point, it would surely be a tough argument to make that achieving these specifications in some way hinders the reproduction of good sound! And, they can all be achieved through any one of a number of combinations of the design concepts listed above, so that if you encounter any two (or more) different amplifiers of completely different design, but all achieving these same end-goal performance results, well again, don't bet the farm on being able to tell the difference between them based on audible observations -- all else being equal.

In my listening room, I currently have nine different amplifier/preamplifier setups, with power amplifiers that collectively represent everything from my own designs scratch built into a Budd chassis, to stock, modified, and highly modified commercial pieces. The power amplifiers all achieve (or very closely so) the 7 attributes listed when operated at normal listening levels (< 5 watts RMS/Ch), while the preamps all have very flat performance throughout the audio bandwidth when set for that operating standard, and excellent channel balance. Collectively, the power amplifiers cover pentode, UL, and triode designs, and Mullard, Williamson, and other types of front end designs. Some have regulated power supplies, some high reserve capability power supplies, some use rectifier tubes, while other use SS power supplies. Some use a global NFB loop, others use multiple loops. One uses 6550. Another KT88. Yet another 7591. A few use EL34 others EL84. One uses cathode bias, others fixed bias, and a few include EFB. The point is, there's no two that are even remotely close in design -- but blindfolded and at normal listening levels, you'd be beyond hard pressed to identify which one is actually playing, critical listening or otherwise, because they've all been appropriately addressed as needed relative to the ideal goals sought. Now as a lower powered unit is pushed to its maximum output capabilities versus another which is not, then of course differences will be heard and are obvious. But operating will within their capabilities and producing normal listening levels, don't gamble the farm.

So what's all this got to do with the D115/V70 project? Well, a lot -- because the D115 MK II as designed from the get-go meets all of the above specifications quite handily. But as noted above, so do all the other amplifiers in my listening room. So it's no big surprise then that at normal listening levels, the D115/V70 sounds very much in keeping with all the other amplifiers in my listening room as well. More specifically, it is only when a given amplifier is asked to approach to its power output limits that any audible differences between the various designs becomes apparent. Disappointing? Hardly!!! Viewed from the standpoint of Johnson's design goals and engineering prowess, and the fact that the audio circuits of the D115 MK II have not be modified in any way from the original design (except for audibly insignificant dependability measures), it continues to validate the work of my own designs, and those pieces I've modified -- and also the standards they've been modified to. From a purely subjective standpoint then, the "sound" of these specifications, the D115/V70, and all the other amplifiers in my listening room -- is something I've become accustomed to for many many years now. As a result, even with my aging ears, I'm still very sensitive to any presentation that falls short in any of the specified areas. Asking me to describe the sound of the D115/V70 then is virtually the same as asking me to describe the sound of all the other systems in my listening room. So what is that "sound"?

End Part 1
 
How Does It Sound? Part 2

I'm hardly the Stereophile dictionary of audio terms type when it comes to describing sound, as along with being so subjective in nature, the terms are also so intricately detailed that I get lost in their translation. This is no criticism of those who use such terms to define a sound to themselves for reference purposes. But it is to say that used as a language of communication to/with others, it leaves open a wide path of endless interpretations. Therefore, I chose to list the musical pieces I use for evaluation, with more general comments as to what I hear and listen for, for context.

The associated equipment used includes a basically stock Fisher 400CX-2 preamp (tweaked only to ensure proper channel balance and intended response characteristics are achieved), stock Denon DCD-1520 CD player, and stock Klipsch Cornwalls speakers (3ed version -- 101 db sensitivity), in a quiet, acousticly flat (not alive, not dead) 15X20 foot listening room with 10 foot ceiling. No power conditioners were used, and cabling was all capable, quality pieces, but not of any unique pedigree. The Cornwalls were connected to the 8Ω output taps, and the input attenuators I developed for the D115 were used and certainly needed: Without them, quiescent low level noise was unacceptable, and the volume control was below weed level. With the attenuators, the noise floor was very acceptable, as was volume control action.

The listening material used included:

1. The Rippingtons -- Various CDs:
A. Wild Card -- "Gypsey Eyes": A bass heavy track that does not have the usual detailed high frequency content that Rippington recordings typically contain. With the heavy bass content, it can easily overwhelm even power capable amplifiers whose IM distortion or overload characteristics are less than admirable. The D115 was unflappable with this recording, giving no notice to its presence in the amplification chain at all.

B. Wild Card -- "Till You Come Back to Me": Chante Moore's voice is lush, detailed, and smooth. Very listenable and completely natural sounding -- although still has a "recorded" characteristic through no fault of the D115. Still, and excellent recording for use in evaluating reproduction of the female voice.

C. Welcome To The St. James Club -- "Welcome to Th St. James Club": Typical Rippingtons recording, with a good blend of electronic and acoustic (Sax) instruments. Plenty of high frequency information and detail, but well balanced across the audio spectrum. The high frequency information is very accurately presented: very present but not shrill or glaring. A dynamically well balanced recording.

D. Curves Ahead -- "Aspen": Smooth jazz. There is plenty of space between the various instruments to enjoy each one for its own qualities.

2. Benny Andersson -- "Piano" Any piece on the CD. Andersson is the genius writer/performing part of the 70's megagroup ABBA, and every bit a concert pianist in his own right. I particularly enjoy the track "You and I" from the musical "Chess" which he wrote the score for. Without a doubt, this is the best close-mic recordings of a piano I've ever heard, capturing all the transients of the hammer strikes, resonances of the sounding board, and harmonies of the various notes. Andersson makes full use of the instrument throughout the various recordings. A recording you can truly close your eyes and hear the piano in front of you.

3. Sinatra Reprise (The Very Good Years) -- "That's Life": One of the better 60's recordings with early stereo separation techniques. It captures all the nuances of Sinatra's voice as it is pushed through out the song. Good detail to reveal range of his vocal capability.

4. Pops In Love -- "Canon" -- Superb strings recording where they all stay separated and the details of each instrument can be distinctly heard. Dynamically huge for strings.

5. Telarc Sampler 4 -- "Fanfare" -- Extraordinary recording of Brass instruments. Never harsh or shrill as brass can sometimes be. Huge dynamics with the Timpani drums. Superb recording in the tradition of virtually all Telarc offerings.

6. Michael Murray Encores -- "Toccata, Symphony V" -- Classic pipe organ piece well recorded, and played in the typical grand Murray style. Good separation of pipes, wide dynamics, wonder ambience due to the setting. The recording captures all of it.

7. Rachmaninoff Piano Concerto No. 2 -- Cecile Licad CSO -- For piano and orchastra together, you would be hard pressed to find a more realistic recording. Close your eyes and you're there.

8. Saint Saens Symphony No. 3 "Organ" Eugene Oermandy, Philadelphia Orchestra, Murray Organist -- The whole second movement. For an organ in an orchestral setting, this is the ultimate high. Huge dynamics, huge detail, Grand Canyon space. Wonderful separation of the various instruments. Unmatched Timpani ending.

9. Come On Come On -- Mary-Chapin Carpenter -- "I Am a Town" -- Mary's voice is startlingly real on this recording, making you immediately take notice of it. A real showcase for the female voice.

10. Of course if you just want to show off, there's always the cannon shots and bells of Telarc's recording of the 1812 Overature. Always difficult for any system to do true justice to, as some element if not most of the system will be challenged. The D115 as would be expected rose to the challenge as if it were no big deal for it (and it wasn't).

There are of course many more, but these are some of my favorites for critical listening to any piece of audio equipment. On my systems, for the comments given for each recording, they all present with a clarity, detail, balance, authority, and effortlessness that make long listening sessions so enjoyable. They are all simply a joy to listen to.

So what then does the D115/V70 bring to this party? Authority, and tons of it. Under quiescent and normal listening conditions, it is as docile as your favorite EL84 amplifier with virtually no intrusion into the listening area, except for it's massive size. It is completely humless on my Cornwalls, and remarkably quiet mechanically given its massive power transformer. It's noise floor is very acceptable, and as good as it can get for dual 100 watt amplifiers with an input sensitivity of just 0.6 vac. The input attenuators make the full sensitivity of the amplifier very usable with typical preamp offerings. But that authority! Emotionally, it leaves you with a feeling of unending amounts of reserve power available (because there is!). But having that feeling from an amplifier of this power level that does everything so right is special. There are surely other vacuum tube amplifiers of this power level. But usually as available power increases, detail and finesse tend to decline. Not so with this amplifier. Which brings me to a final point: With amplifiers that in a blind fold setting and normal listening conditions sound so similar -- then what is it that makes us prefer one amplifier over another? Besides the emotional feelings that music can evoke, I believe the answer to that lies in many other emotional aspects of our equipment, which include (in no particular order):

1. Use of a favorite tube. It doesn't matter why, it's just your favorite.
2. A physical look -- some pieces are timeless in appearance, some age gracefully, some look old, worn, and hopelessly outdated.
3. A favorite manufacturer -- or one you rotate to frequently.
4. A favorite or unique type of design.
5. A level of build quality.
6. Weird stuff (said affectionately) that's off the beaten high fidelity path.
7. A favorite pioneer in the industry.

I know I'm guilty of attaching myself to virtually all of these things, and I know many others are too. Want proof? Look how many AKers talk about equipment in their "rotation". Granted, in stock form and due to design goal differences and component tolerances, different amplifiers can surely sound different -- even within the same brand and model. But once a more uniform high quality level of sound has been achieved, then why all the multiple pieces of gear? I maintain that it's because once the emotional element of the music is satisfied, we're so involved with our equipment, that we seek to satisfy the emotional element of it as well. Which amplifier is better? It depends on the day, the mood you're in, and the context of the question.

The D115 surely sits at the top of its class -- along with the other amplifiers that belong there as well. Meaning it's not necessarily better at anything than its equivalent peers (except for how any of the 7 emotional qualities make it so) -- it's just different. But also a truly wonderful amplifier. About the only negative online comments I could find regarding it are that it was a little thin in the LF department. The only thing I can offer regarding that comment is the rather low input impedance of the D115 as designed: 65KΩ calculated and measured (although specified at 75KΩ) which will be challenging to any vintage preamp to drive, which will result in a reduced LF response. However, the attenuators I devised for the D115 raise the input impedance that the preamp sees to 225KΩ, which any quality vintage preamp can easily drive with a flat response over the full audio spectrum. I would ultimately offer then that other than for the low input impedance that must be appropriately accounted for when choosing a preamp to mate with this amplifier, there is absolutely no shortage of LF punch, power, drive, whatever, in the D115 at all: Bring on the cannon shots!

Finally, I have zero experience with vintage Marantz equipment. I know it is fine equipment. One AKer suggested its designer was ahead of his time in terms of addressing the transient side of an amplifier's performance. Considering the type of tubes used in the input/driver stage, that may well be true. But others of that day were credibly addressing it as well -- Hafler for one, and McIntosh for another. Within the highest tier (virtually cost no object) of vintage audio equipment however, it was Mac and Marantz who owned the market -- but of those two, Mac was the innovator with the historically significant Unity Coupled Output Stage design, that propels them even to this day. When they originally stopped producing vacuum tube equipment, ARC picked up the cost no object mantel, which then invites the inevitable comparison between the two brands.

From a sonic standpoint, once again, don't bet the farm, as my MC240 and the D115/V70 sound virtually identical on my speakers at normal listening levels -- and they should as after all, my Mac easily meets the 7 criteria for a practical ultimate vacuum tube amplifier listed earlier -- just as the D115/V70 does as well. My Mac doesn't have as much power (virtually producing half that of the D115), but at the power levels these amplifiers produce, versus the needs of the speakers and listening environment, that's basically a moot point. So looking at the other 7 emotional areas, Nos. 2, 3, 4, 5, and 7 certainly come into play with my Mac equipment -- reinforced by the annals of time, and now -- in significant part due to this project -- Nos. 2, 4, 5, and 7 come into play with ARC, with time likely adding more to that as well. However.........

This is all based on my D115/V70 as now modified -- meaning, with the dependability measures now installed. But if you include the historically documented dependability issues with this period of ARC amplifiers, then it changes the game completely. Viewed from the angle of dependability and the rate at which they chewed up output tubes, and with all else being equal, then vintage McIntosh equipment wins hands down: You bought it, finally expend the tubes over (usually) a couple of decades, changed them out, and then kept on truckin'. As well, there were no adjustments to make when the tubes were changed out, either -- and they just continued to meet their performance specifications. ARC certainly has the history of quality sound that Mac has, but at least in the 80s, couldn't come close to the dependability record that Mac established and maintained for (at least) the preceding 20 years with its vacuum tube equipment. And for such a uniquely different approach to generating quality sound that is certainly on par with Mac equipment, that's sad because operating correctly, it's darn good stuff. There was a member over on the Mac Forum who was involved with the Mac Clinics back in the day (apparently) of the D115's production, and posted that when ARC power amplifiers of that period were brought to the clinics, they had a history of not meeting their published specifications. Based on my knowledge gained through this project then, it doesn't surprise me. For my particular ARC D115/V70 however, I believe the modifications I made to it have completely resolved the dependability issues noted, so that it will continue to perform and meet its specifications without issue for many years to come.

To that point, because of the performance capability and (now) dependability I believe this D115/V70 represents, I went ahead and sought out a companion ARC preamp to go with it. Since I like to have the ability to alter the response of the amplifier as needed (tone controls), I've got to go all the way back to the ARC SP-3-A1 to get that feature, and have now received it. While I'm still learning about and getting used to it, mine is an original SP-3, that was apparently updated by ARC to the SP-3-A1 design about 20 years ago. By all accounts, it is in perfect working order, with pristine appearance, and sounds wonderful.

Other than that, I have gone ahead and added a switched AC power receptacle on top, and in the front left corner of the V70 chassis -- this to power the dual quiet muffin fans I anticipate mounting on the back side of the tube cage, so there won't be an additional power cord hanging off the back of the cage, and the amplifier can remain self contained (power wise) when the cage is installed:

SAM_3220.JPG

SAM_3221.JPG

The plug is inconspicuous enough that even with the cage removed, the plug hardly stands out as an aftermarket modification -- and -- I believe the fans just arrived! Now, to figure out how to cut two 3" holes in the cage to accommodate the fans.

And with that, it completes the long and winding saga of my Parts and Pieces ARC Mystery, coming to a conclusion and outcome I never could -- nor would have predicted when it started. But what a learning tool it has been for me! Thank-you to those who have followed along! One final pic of the project is presented to close it all out.

Dave

SAM_3223.JPG
 
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Bravo Dave! A thread well worth the read and I know a lot of effort in building up that amp from the "parts and pieces" you started with. I'm sure the new amp will bring many years of enjoyment and be a nice collector's piece as well.
 
Awesome write up Dave. Totally makes sense why properly designed amps sound the same as well despite different design executions. Great explanation.
This needs to go up into the sticky section and preserved forever.

BillWojo
 
Dave, I forgot to congratulate you on the purchase of the ARC SP-3-A1 preamp. I'm using a SP8MKII, no tone controls but I think it's more similar than different. Both use the 12AX7 tube that ARC moved away from in latter models.
Maybe when your bored someday you can give us a write up on your preamp. Like most ARC products, plenty of information on how it sounds but little information on why it sounds so good.
I had a latter model ARC preamp with the 6922 tubes but I found the sound to be more SS than tube like. Talking with "arts" on this forum I searched for a SP6 or SP8 (don't use tone controls) and couldn't be happier. Plus I like the later model aesthetics and how they have a timeless character.

BillWojo
 
When did ARC preamps move from a straight tube design to a hybrid ? The SP-3 I think is still all tube but I know basically nothing of their models after that point.
 
As I understand it -- italicized as I am hardly any fount of knowledge about the various ARC models -- The SP-3, later the SP-3A, and finally the SP-3A-1 (which is what mine is) was (for all intents and purposes) the last of the models (or basically so) of conventional configuration -- configuration meaning that besides the traditional line amp stages, it also included a phono preamp and tone controls. It was an extremely popular preamp for ARC. It uses vacuum tubes for all amplification and buffer functions, and a very rudimentary (by today's standards) regulated B+ supply, consisting of a single pass transistor and Zener string to set the base voltage. Heaters are all DC of course. For it's day, pretty heady stuff.

The next model released (as I recall) was all SS, but then they rather quickly went back to tubes for amplification, but with that, the tone controls were (I believe) forever gone. At that point then, their preamps were just a phono preamp and line stages, with volume, balance, selector, and mode function switches. Not too long after that, they even then broke apart the phono preamp and line stage, so that you had to purchase two separate pieces if you wanted both, and I believe they even at that point offered an equalizer, since none of the models included tone controls. Currently, all their control center preamps have a model number starting with "LS" which no doubt stands for Line Stage.

I think at one point, they even did much the same thing with some of their power amplifiers, having the audio circuits in one unit, and the power supply in another.

In their quest for audio perfection, these would no doubt all be logical moves for ARC to make -- further cementing their cost no object and ultimate possible perfection status.

For me however, the SP-3A-1 is really a wonderful preamp, having all the functions I need in one box, and from a circuit design standpoint, matches well with the D115's design approach of its day. It has an older look than their more recent products, but I believe still has a timeless look about it that is enduring. Some of the vintage pieces from various manufacturers didn't wear well appearance wise to my mind, but the SP-3 preamp is one that does to me, with both it, and the D115/V70 portraying a wonderful combination of "semi-modern vintage". In contrast however, the black panel version of ARC power amplifiers have little appeal to me.

Dave
 
The ARC SP6 was pretty much an all-tube design- no tone controls, but still a tube phono stage.

I would call the SP6-B the "last great classic" ARC preamp. Everything after that was mostly hybrids, and was voiced considerably differently than the SP6B and before (they went from a bit of a "warm" balance on the earlier ones, to a more "bright" and "cooler" balance). Personally- and this seemed to be shared by a lot of people back in the day- I preferred the earlier preamps- the tonal balance was easier to live with in the long term.

Regards,
Gordon.
 
don't get me wrong, I don't have any specific objection to hybrid preamps. I use one myself, I was just curious. No tone controls on mine, and sometimes they would be useful. I was listening to an album the other night that is a bit bass shy. I don't like messing with the sub adjustments to stand in for a bass control.
 
The Last Page

One detail I keep forgetting to mention -- this amp was an introduction to me in using 6DJ8 tubes in audio positions. I have no audio pieces that use these tubes -- tuners, yes, but only in RF or non-audio positions. After all, it was never intended or designed to be used in audio applications. But, it's an extremely linear tube, so by the 80s, it attracted designers of audio equipment as a result. But the great linearity comes with a price, which I certainly was aware of, which is noise. The whole issue of the turn-off thump came to light because I was in fact turning off the unit off for the purpose of trying a different 6DJ8 in the left channel -- precisely because it was noisy. I'm aware that because of the popularity that the 6DJ8 tube has amassed over the past few decades, there are now premium versions of it produced that purportedly offer low microphonic and low noise performance. But that still leaves at least all the vintage 6DJ8 examples -- and 12AT7 examples for that matter (because it uses the same type construction -- that can be unacceptably noisy. As part of this project then, instead of forever turning the amplifier on and off to try various 6DJ8 examples to find quiet ones, it prompted me to build a little unit to quickly test both sections of both of these tube types for noise, using nothing more than an audible evaluation for go-no-go decisions -- which proved by far to be the best test for making such judgement calls. Quiet ones could then easily be found for use with the amplifier, so that once the chosen examples were installed, it could be turned on with confidence of low noise, since the tubes were already prescreened for that parameter. Microphonics is important to, as overly sensitive tubes can actually pick up a high pitched whine from the fans. So, while that was a good and useful side project resulting from the main project, it is also just one more thing that back in the day (i.e., before low noise examples were available), made the ARC vrs Mac comparison tip towards the Mac side for no-hassle use. In any give lot of tubes in that day, it was more common to find a noisy 6DJ8 than not, while it was uncommon to find many noisy 12AX7 or 12AU7 tubes. In any event, I can now easily find quiet, non-microphonic examples of these tubes ahead of time, so that needless cycling of the amplifier is eliminated in accomplishing that goal.

And finally, the fans did arrive as expected, but cutting a couple of (basically) 3 inch holes in the steel tube cage to accommodate them was no walk in the park. The easiest route would have been to obtain a 3" punch -- but punch sets of that size are quite expensive ($300-$400), and typically require a small hydraulic press to get the job done with any ease. Jon suggested a nibbler, but that takes quite some time and leaves a rough edge requiring filing to smooth it all out. A saber saw couldn't be used, as it would bump into the mounting lip of the cage. I finally settled on making a wooden jig that could be clamped to the cage, that raised a saber saw just above the mounting lip, so that it could be used to cut the hole completely from the outside of the cage. The jig worked perfectly, but still, cutting a 3" hole with any accuracy using a standard size saber saw is a bit of a trick. I'm sure there are other no doubt better ways the job could have been done -- but it worked well enough to get the job done with what I had on hand, which is always a plus.

The V70 tube cage is the same size as that for the D115, but includes two fans mounted inside the cage, sucking in from the rear, and blowing out not only through the perforated cage, but also through louvers on the front side of the cage in front of each bank of output tubes. The D115 cage of course had no fans, and also no louvers on the front side of the cage. It just has the perforated top and short sides -- and needless to say, cooks in a matter of minutes. In the V70, the fans could be mounted inside the cage because it didn't use tubes for any of the input stages, and being a triode output stage, didn't use the additional error amplifier and pass regulator tubes, either. It used a SS design for the front end, which allowed for plenty of room to mount the fans inside the cage. With the D115 cage, of course there are many more tubes populating the rear of the board, which prevents mounting the fans inside the cage. Therefore, they are mounted outside the rear of the cage -- protruding just to the back edge of the chassis. As a result, while they certainly represent something new visually that (likely) no other D115 still in existence has, neither does it look like a cheap add-on aftermarket modification, either. The fans simply appear on the rear of the cage, blowing cool air into it, similar to the V70 fans that suck cool air into it. I may provide a vent in front of each bank of output tubes similar to ARC's louvers they provided on the V70 cage if I find they are needed. I have a feeling that the cooling will be perfectly adequate without them, although with them, it may allow for lower fan speeds for even quieter operation. Time will tell with that decision.

Of the four mount holes on each cage mounting lip, only the two inside holes on each lip line up with the threaded chassis holes to secure the cage. But that's OK, because the two outside holes on the rear lip are hidden under the fans anyway, and on the front lip, the lack of any ability to mount the cage via the two outside holes is of little concern. The bigger issue is that because the chassis had to be re-drilled to mount the circuit board, and because the new holes for that are so close to the old holes, it really wasn't possible to counter sink the new holes to allow the use of counter sunk screws to mount the board the way the original mounting was done. Because the new and old holes literally run into each other, any attempt to countersink the new holes would just chew up the new and old holes into one elongated hole which would look much worse than the way it does. Without countersunk holes, it meant that screws with conventional thin heads were needed and used to mount the board in place. The problem with that is that then, the tube cage won't sit flush against the top of the chassis because it rests on top of the protruding board mounting screws. Since it's doubtful that I'll actually ever mount the cage down, I simply installed small grommets into all of the lip mounting holes of the cage (except for one), so that now, the cage rests on the grommets since they protrude out further from the cage than the board mounting screws do from the chassis. The grommets also help to prevent the transfer of any mechanical noise from the cage to the amplifier chassis as well. In the one open hole, I've attached a single flying lead that terminates with a quick disconnect. It mates to a lead coming from a rear chassis bottom plate mounting screw so that when connected, the tube cover cage is firmly grounded for safety and static concerns, and to ensure the cage won't become a source of hum for the audio circuits it provides cover for, were it otherwise not grounded. That last point is even more important now that AC power fans are mounted on it, and near the input circuits. With the grounding lead connected, there is zero hum contributed to the amplifier due to the presence of the fans. If I want to mount the cage for some reason, I can still do so by passing the four screws that match up with the chassis mounting holes first through the grommets, and then into the threaded chassis mounting holes.

Of note, I tried the suggested fans that (apparently) ARC uses from the source offered earlier in this thread, but found both of them to be unacceptably noisy, making a plastic gear type sound when they operate. As a result, I returned them and purchased some AC Infinity fans that Jon found through Amazon (model LS8038A-X), which are very acceptably quiet for the application. I don't know if ARC prescreens the fans for noise, or has them prescreened before delivery, or what. But the ones I got were very quiet in terms of the air they moved, but unacceptably noisy mechanically in that process. The Infinity fans however are excellent, and do the job perfectly.

Finally now then, the amplifier is one cool cookie, regardless of operating time -- and with that, it completes the one last detail I wanted to address with the amplifier. So now, it truly is finished! I can operate the amplifier for as long as I want, and cycle it as many times as I want, without fear or concern of damage to the tubes, my speakers, or ultimately to the amplifier itself -- while audibly, it produces all the wonderful sound of which ARC intended it to. I have no idea if any of these amplifiers are still around or in use, as they are almost 40 years old now. As such, they fall into that no man's land of not old enough to really be true vintage, but old enough to not be new or recent, either. But if any are, or if any of their related siblings are, then many of the modifications offered for the D115/V70 here would be applicable to them as well, and make for a greatly improved amplifier dependability wise.

Final pics of this last effort with the amplifier are included. Time now to listen and enjoy!

Dave


Below: The Jig I made to allow the use of a saber saw to cut the holes for the fans, so the saw wouldn't run into the mounting lip of the cage. The hole in the Jig is larger than the hole outline drawn on the cage for cut out -- so I could see what I was doing. The middle and other end of the Jig was clamped to the cage so it stayed in place as the hole was cut and the saber saw moved over the top of the Jig.
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Below: The two holes are cut. They're not perfect, but the mounting of the fans covers the imperfections.
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Below: The business end of the amplifier. The two fans are certainly a visual addition to the amplifier, but otherwise, look about as stock as any such addition was going to for this amplifier. Just to the right of the 16Ω terminal for the left channel, the flying leads from the cage and bottom plate mounting screw (near the IEC line cord connector) can be seen connected together to firmly ground the cage to the chassis.
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Below: A side view showing the connection of the two fans joined into one line cord that plugs into the AC receptacle I provided for their installation. The cord is tie wrapped along the bottom of the cage. By unplugging the fan plug, and separating the two flying ground leads, the cage can then simply be lifted off of the amplifier. If you look closely, you can see that the cage does not rest on top of the elevated board mounting screws, but actually rests on the grommets installed into the holes in the cage mounting lip.
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Below: From a normal front view, the fans are not immediately apparent, but a welcomed addition that provides complete peace of mind with extended operating periods.
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