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Hi,
Has anyone here ever owned an ARC D76-A? I bought one on e-bay as an impulse buy. It arrived and appears to have original ARC-marked 6550's in it of indeterminate age. The seller claimed that it had been recapped in the last few years. Before I installed it, I had been listening to a freshly rebuilt Marantz Model 16 (solid state) for about 4 years which has such strong bass response that I unhooked my subwoofer. Before that, for about 8 years, I listened to Dynaco MkIIIs, which had somewhat tubby bass. By way of comparison, the D76-A drops off below ~50Hz, and what bass it does have is soft, mushy, and lacks any percussive attack.
If there is anyone familiar with the design, I'd be curious to know if this is an ingrained attribute due to circuit design or transformer construction, or would tube age impact bass response? There is very little info on this model on the web. I found one review that complained expressly about the bass response, but that is just one data point.
Over the last 4 years I built a line stage and a headphone amp. I was astonished by the variance in bass response from various small signal tube manufacturers, so I have to assume that power tubes could experience similar variations.
So, thought/first-hand experience on my situation, and perhaps some thoughts on power tubes known for bass response? ARC does not recommend KT88s due to observed cold-arcing.
Respectfully,
James

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Walt

I've owned a couple of ARC amps, one of them had pretty poor bass compared to my Mac-30s, but then most amps I have had did not do well in the bass department compared to the Macs. I remember when I got my Cary Rocket 88's I was put off by the lack of bass, however Cary noted in their manual that if the amp had not been powered up for a while, the bass would be lacking and that playing/breaking in the amp over a little time, like a couple of weeks, would remedy that, and indeed it did. IIRC that the D-76 is one of ARC's premier tube amps, so I would let it play in your system for a while and see if that improves things, I would hesitate to make any changes until a break-in period had been established. I would also contact ARC, they are pretty good about keeping their name as a gold standard and may be able to help as well. Hope that helps!

It is possible that the output tubes, and other tubes are weak. ARC ran their tubes quite hot and they did not last very long as a result. If the tubes are weak, the negative feedback will be reduced, causing poor speaker damping. The result could be tubby base, or weak base, depending on the characteristics of your speakers.

David

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Walt

Thanks for all the input. Considering the age of the amp and tubes, I don't think "burning in" is an issue at this point. I have spent several hours listening to it today, and in passages with a lot going on simultaneously, it seems muddy compared to the Marantz 16. Considering that the amp was designed under the concept of "high definition" (it even says it on the front ). Any thoughts?

Considering that it has (IIRC) 15 tubes, it is surprisingly quiet, but boy is it hot! I seriously don't feel comfortable storing vinyl anywhere above it!

While I feel comfortable working on tube equipment (why else would I be in this group?) I've heard about issues with traces lifting off the PCBs. I'm also not comfortable choosing coupling cap values, but then again, that hasn't stopped me from designing stuff before

BTW, any recommendations on tube testers in the vintage world?

Another thing to consider is that in my experience, many of these 6550 amps had quite a bit of glare to their sound. This could easily unbalance the sound to the point where it seems weak in the bass when the bass is not the fundamental problem.

David

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Walt

OK, talking about tube testers, you should get a mutual conductance tester, an emissions tester is basically a much less useful tube tester. In particular, that Precision is crap! I would never buy a Precision tester and the one shown may very well not test modern tubes you run across, its too old! There are several issues about tube tester to consider, the ability to weed out obviously bad tube that can damage an amp (shorted tubes) as well as gassy tubes that can also damage an amp via grid current runaway. Both type, emissions tester and mutual conductance tester will do those tests, but when it comes to trying to determine if a tube has useful life, then your only choice is a mutual conductance tester, like the Hickok 6000 (not sure what you mean about crazy prices, a 6000 can be had for around $200 if you look around). There are a number of good mutual conductance testers, including the desirable Hickok 539 B & C models, expect to pay close to $1000 for a good working model that has been calibrated. There are also the TV-7s used by many tube sellers, and the solid state B&K 747 that is also good, but has its issues as well. I use a Hickok 539A and a Western Electric 15874-l2 cardmatic that get good use. I am also in the process of restoring a pair 539B and a 539C Hickok. There are so many variables to consider with tube testers, it is most certainly NOT just buy one and away you go! If you like, you can contact me and we can talk more about it, I would also recommend Tone Lizard's website, he has a lot of info on tube testers that will help your decision process. Hope that helps.

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Walt