Arc ref 75 w/ kt150s
- Thread starter CORALX8
- Start date
My last sentence in post #20 was intentional. Early in the thread, OP said he had owned plenty of tube amps, yet later he said he wasn’t familiar with tube amps; a contradiction that suggests he’s already at the edge of his experience. Given the trajectory of the thread, it looks like he’s reached the limit of what he can safely troubleshoot, and considering he’s working with an extremely nice and expensive amplifier, it would be a shame if anything were damaged. That’s exactly why I suggested pausing and talking to someone at ARC.
CORALX8
New Member
CORALX8
New Member
Ive owned a lot, however I do not work on them myself and normally I have a technician get everything ready for me; I purchased this just yesterday and its the weekend so shop is closed until tomorrow, which I suppose then I will get it checked out
CORALX8
New Member
The hum is directly from the transformers, even when not connected to speakers
whoaru99
Epic Member
Pictures please.
CORALX8
New Member
Ill take some pics tomorrow when I bring it down to the technician and he boots it up for troubleshooting, considering how worried people are that something might break I think its best I keep my hands off it for now..
Some amps are easier to deal with than others- and some have more dire consequences for mistakes during testing and such.
The ARC amps of this era are pretty complicated, and are known for some pretty severe histrionics when they're not happy- including, and not limited to, sparks, smoke, and in rare cases, fire. And they can be very expensive to fix, when things go badly wrong. Taking something like that to an experienced tech before things go awry, is a very prudent, practical decision, in many cases.
I'll say this- I've worked on and even built quite a few tube amps. And I still won't usually take on some ARC amps, personally.
Regards,
Gordon.
ARC has a very good service department since they were bought a few years ago. Amps that were no longer serviced, like my 100.2 are now being serviced in a timely manner.
I am no expert, but I thought the KT 150 issue was solved a long time ago. I thought it was okay. But it is really easy to call ARC. They will make their recommendations. If you have any doubts about the status of your amp currently, you can get on the cue while you are asking about the tubes. The cue, a few months ago, was 2 or three months.
I am no expert, but I thought the KT 150 issue was solved a long time ago. I thought it was okay. But it is really easy to call ARC. They will make their recommendations. If you have any doubts about the status of your amp currently, you can get on the cue while you are asking about the tubes. The cue, a few months ago, was 2 or three months.
CORALX8
New Member
Yes, I forgot to check this website for the past couple of days
Anyways, I did let my technician check it and he said that the kt150s were biased a bit too high but they will work fine in the ref 75 when set correctly, he redid the bias for me and told me it was safe to use
Did you ever contact Audio Research? Being a good technician and understanding circuit design are two different things. As others another member noted, and I can confirm from firsthand ownership, their amp designs can be notoriously complex, making their components a bit 'high-strung'.
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those tubes are red plating ...shut it off and get it to a tech that has great experience in Analog amps with equipment to test all of the circuits of a analog amp..
CORALX8
New Member
My tech has already resolved the overly red tubes issue, the amp is working normally now
its just the concern of whether this can work long term or not
CORALX8
New Member
not yet, im in japan at the moment, ill go visit the ARC dealer later when im back home and get some confirmation
Oh my gosh dude, I used ARC’s Contact Us form on their website and, despite sending it after hours, they replied first thing the next morning. I’m a little surprised that action had not already been taken given how easy ARC is to work with and that we are dealing with an expensive amplifier.
Bottom line - KT120s are okay. It’s a rare, acceptable substitution for an Audio Research amp. Just because a tube fits and lights up doesn’t mean it’s right for the circuit. Don’t be the person who sticks 6922s into a preamp built for 6H30s and wonders why things go sideways.
ARC’s official word is what I’m sharing here ----- better safe than sorry.
Are ya still using the same tubes ? Red Plating tube can beat them up bad.
...............
Well, I think I’m done with this thread. At this point, the OP has repeatedly avoided taking action on even the simplest recommendations offered by multiple members.
Across two full pages, people have given OP straightforward, practical guidance:
- Contact ARC directly. OP never reached out to them; I did, partly because I wanted confirmation, and partly because it was becoming obvious no action would be taken.
- Post photos of the tubes. Several members asked for this so we could see what’s actually happening. No tube photos were ever posted.
- Provide infrared temperature readings. I asked whether you had an IR thermometer because actual temps would show whether a tube is running away. No readings provided.
- Clarify the hum source. OP said it’s coming from the transformer, but no diagnostic follow‑up was shared.
- Check bias stability. Members pointed out that grossly wrong bias can cause hum and other issues. No bias measurements posted.
Early in this thread the OP claimed to have “owned plenty of tube amps,” which naturally led people to believe he had at least some practical experience. But later he admitted he has zero hands‑on experience and normally relies entirely on a technician. Those two statements don’t line up, and they explain why none of the guidance from more experienced members has been acted on.
And let’s be honest, at least we’re not dealing with one of the more difficult Audio Research amps to bias with some of the older designs were notoriously unforgiving, requiring tightly matched pairs of quads of output tubes and, in some cases, biasing input tubes. The VT100 is a model that comes to mind. The REF 75 is far more civilized in comparison, but transformer hum, possible red‑plating, and abnormal tube glow are still issues you absolutely cannot delay or ignore.
This isn’t a casual beginner amp. The REF 75 retailed for around $10K and still commands a high price on the second‑hand market. When an amp at this level starts humming, red‑plating, or glowing abnormally, the correct response is immediate action, not two pages of vague replies and missing information. Yes, Audio Research could make any repairs required; however, that work is in no way inexpensive.
At this point it’s painfully obvious OP is trying to navigate this REF 75 with none of the foundational tube‑amp experience needed to troubleshoot even basic issues. It’s the classic situation - someone jumps straight into a high‑end ARC amplifier without understanding what they’re getting into, skips every simple diagnostic step offered by people who do know what they’re doing, and then wonders why the whole thing is misbehaving.
Honestly, it’s like watching someone buy a supercar as their very first vehicle with no training, no seat time, no idea how the controls work and then acting surprised when they end up backwards in a ditch.
Given the cost of the amp and the seriousness of the symptoms, it’s genuinely baffling that the basic steps suggested were not taken. And while OP did take it to a tech, multiple members cautioned that ARC amps are complex enough that many techs don’t want to touch them.