Art -- I've got a decent pic of the bottom cover of a D70, so we know the basic venting pattern, and of course I've got the chassis here that I can measure -- remembering that it is a V70 chassis, not an actual D115 chassis. This chassis is only 15 & 3/4th inches front to back, so it won't be one that fits either a D115 or a D70.
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You know, I get that ARC was all about making an absolute ton of low distortion power in their highest end units, and some speakers are in fact insanely INsensitive and do require it. But from an everyday practical standpoint (power capability, weight, size, heat production, cost of maintenance, etc.) , I think the D70 was likely the best amplifier for the needs of most folks. I mean, here I am with 101 db efficient floor speakers in a quiet 16X20 listening room. A 100 watt/channel vacuum tube amplifier in that scenario is just nuts.
I really think that once I've put this puppy through its paces, I'm going to modify it "down" into a D70 for all practical purposes -- which will be easy enough to do without any changes to the wiring:
1. Pull one push-pull pair of output tubes out of each channel.
2. Connect my 8Ω speakers to either the Com and 4Ω terminals -- OR -- the 4Ω and 16Ω terminals. The latter would allow for a common ground connection between the output connections if needed.
Besides reducing power output and the heat output from it, it would also be much kinder to the 6550 pass regulator tube. That tube as a single device is nicely suited to be the pass element in the D70 (two 6550s per channel), but is woefully inadequate if both channels of the D115 are asked to produce full sustained power output for any length of time at all. In the D70, the 6550 regulator tube is dissipating about 45 watts when both channels are delivering full sustained power output -- perfectly fine since the tube is operating in triode mode and can dissipate a total of 48 watts in that mode. But in the D115, its being asked to dissipate about 66 watts under the same condition, greatly exceeding the capability of the single 6550 pass tube provided. My band days back in the 60s instilled in me a strong belief that all circuits should be what I call "band rated" -- that is, having an ability to operate under maximum rated conditions over time without fear of overloading any part of the design -- just like a Timex watch: it can take a lickin' and keep on tickin'. Against that backdrop then, the design of the screen pass regulator in the D115 is simply inadequate. To be fair, the single 6550 tube is likely just fine for everyday use under the dynamic conditions that music produces. But ARC made a big stink out of the specs for their amps, with the very first specification at the top of the page being that it is capable of 100 watts per channel with both channels driven. I would strongly suggest that while this may be true, it wouldn't be for long given the demands placed on the single regulator tube provided.
To punctuate just how much they skimped on this portion of the design of the D115, in their larger amplifiers, they use two pass element screen grid regulator tubes in each channel for amplifiers having a push-pull-quad-parallel arrangement in each channel (four pass elements total for both channels), which works just fine in delivering full power output all day long. But the D115 falls into that no-mans-land between the D70, and D250, where it really got short changed.
There is an easy answer to this problem today however that ARC did not have at their disposal at the time: Swap the single 6550 pass regulator tube out for a KT120 tube. The KT120 has 60 watt plate and 8 watt screen dissipation ratings, that as a single pass element fits the needs of the D115 under both channel full power conditions as perfectly as the single 6550 does for the D70. So the plan is, when I run the full 100 watts/channel scenario, I'll use a KT120 pass tube, but when the unit is knocked down to D70 power levels, then a 6550 regulator tube is perfectly adequate. The KT120 does draw slightly more heater current (about 0.1 to 0.35 amp more), but this is really insignificant in the larger scheme of things, and is greatly outweighed by the bigger tube's ability to handle the full sustained 100 watt power output capability of both channels simultaneously. The KT120 when installed is equal in height to that of the tallest power supply electrolytic caps on the board, so it should fit under the cover, and would seem to be a perfect answer. Hopefully, I'll know soon enough!
Dave
