Audio Research VT100 MK1 and LS15 HELP!!!

It sounds like you're at a point where it needs to go in to a repair shop for a diagnosis.  If there's that much difference in sound quality and you've tried all the obvious things, it's time to get it looked at.

Same set up, I switched out the arc and use the mac1500 and has a lot of bass response especially low. Switched back to arc and it has about 60% decreased in bass. All speakers wires are all properly connected. Can someone here explain this?

https://classicreceivers.com/mcintosh-mac-1500

http://sportsbil.com/mcintosh/Receiver/MAC1500/MAC1500_ser.pdf

But what about the LS15 preamp?

You didn’t just switch out the VT100 amp.
You switched out the preamp, amp, and balanced ICs.

Just curious, what do you have the tone controls set at on the MAC receiver? Flat, ( 0 )? How about the loudness button? In, on? Out, off?



You need to find someone to come into your home to help you to resolve your problem.

Firstly, I overlooked your comments about the LS15 going into protection frequently. This could well be the result of the V3 6922 flashing/sparking. If the LS 15 doesn't require re-biasing you can easily change this tube yourself with like kind and quality. ARC is correct, this Sovtek 6922 doesn't flash on turn on. FWIW I don't think this tube needs to be re-biased. Ask ARC.

Re the loss of bass response, as stated earlier, I think it is a fundamental difference between your Mac, an old vintage tubed receiver known for overblown low frequency response, and a modern amp/preamp known for its more linear, cool, and for many, clinical response. The basic difference between the two is huge! I suspect that you have just made a fundamental beginner mistake, and we all make them, of assuming that what we hear elsewhere will be what we hear in our own homes, but to our disappointment it does not. Been there, done that!

 This is especially so when the seller has taken great care to match and set up his audio system. A good audiophile can obtain excellent results from fairly ordinary stuff because of his basic understanding of his stuff,  how it all synergizes, and the requirements of his room for proper set up.

That said, in order to optimize the bass response of your current stuff, I would run your speakers off the 4ohm taps. I would buy a SPL meter and a test disc  and insure that you speakers are not located in a bass null (and at the same time help pin down getting the best overall frequency response). I would also buy Robert Harley's book on audio for a good primer about audio. 

@newbee thanks for your explanation.  But the different is so huge.  It’s like the ARC has no bass at all.  The Mac1500 does have tone control but i flat them just to compare and it still boost a large amount of bass.  I called ARC and they said that tube 6922 doesn’t requires re-baised but that’s not normal for it to sparks.  I called Kef and they said not recommended to use the speakers in 4ohm when the speakers are rated at 8ohms.  

Assuming that the sparking is occurring most of the time (which would not be normal), rather than just briefly during the first minute after turn-on (which might be normal, as has been said), it is conceivable that both the sparking and the shut-downs are being caused by poor contact between some of the pins on that tube and its socket.

If you haven’t already done so I repeat the suggestion I and others made earlier of taking off the top cover and checking that the tubes are securely inserted in their sockets, and that they don’t seem so loose in their sockets that they come out with essentially no resistance.

When you do that, be sure to follow the cautions Jim (Jea48) stated earlier: Unplug the component from the AC, and wait for a while after doing so before touching anything inside. He suggested waiting for 30 minutes in the case of the amp; somewhat less than that would probably be ok in the case of the preamp (although keep in mind that the tubes will be hot for a while after the component is turned off). And be sure that any tubes which are lifted out of their sockets are returned to the exact same socket.

(Jim, thanks for stating those cautions, which I should have thought of saying myself in my earlier post).

We know in this case that the amp and the preamp were working before they were transported from the seller’s house to yours, at least for the amount of time you spent listening to them at his house. Which reinforces the possibility that something happened during transportation, or during setup at your house. Such as a marginal contact between the pins on a tube and its socket turning into a problem.

Good luck. Regards,
-- Al