Tube arch

Keep in mind there are many factors that can possibly affect output tube life; correct bias settings, thermal temperatures, speaker load, how hard the amplifiers are regularly driven, and yes, even the quality of the AC they're being feed. 

The getter turning brown does indicate the tube has a lot of hours. A lot. When I started playing with rectifiers and antique tube types I bought a tube tester. Best accessory ever.... Kept me from putting a bad new 5r4gy in my new 300b . Paid for itself .

Any noise or weirdness and I pull the tube and test it. My tester also has a test that suggests whether the tube will have a good life. Not foolproof but it definitely helps weed out bad tubes....

I love tubes, but they come with their fair share of maintenance.

I went to the Dark Side this week and bought a new Cyrus integrated amp. It’s for a bedroom system . It is perfectly suited for that. The best part of owning it is that I don’t have to worry about turning it on and off for brief periods of use.... something that is not good for tubes.

I am waiting on speakers for this amp so it has been in my main listening room. It is a great little amp. I have been enjoying the hell out of it with several different speakers. While it doesn’t sound like my tube amps it is a welcome addition.

It also has Auto Off so I don’t have to worry about falling asleep in front of a tube amp , Definitely a lot safer than dozing off in front of a pair of mono EL34 amps ......

I have a quad of GL KT 88 that I use as a spare set. They look old, and they are ...they have a ton of hours on them. Still sound very good , and test within 100 Micromhos of their original test data , so you never know.

Unfortunately some amps lose the bias resistor when a tube fails. My Anthem Amp 1 would do that. I replaced several in that unit.

Change the output tubes when the silver getter flash starts turning brown.

for arc tube amps and avoiding the ’destructive protection’ the op seeks to avoid, this is bad advice

In general Audio Research recommends changing/replacing output tubes in their amplifiers every 2000 hours. Assuming you have the owner’s manual, you should be able to confirm same. Another suggestion would be to periodically check the individual output tube bias.

this is good advice

i understand in the case of the op’s amplifier using sooo many output tubes, i would suggest perhaps stretching the hours to 2500, maybe 3000... but check biasing often... when certain tubes or pair/bank starts to ask for more bias, it is a sign they are getting closer to going bad

i would also advise the op to make sure the amp gets excellent ventilation -- big impact on tube life in my experience

@jjss49 +1. A tube tester is also not a bad idea.
The Ref 750 have 18 tubes KT150s and 1 6550 per amp right? That’s about $7,000 each time you re-tube both mono blocks, every 2000 hrs or so?

I have to ask. Are you sure you need all that power?

I had monster solid state amps for well over thirty years. It was clear that more power made a big difference in the overall sound quality. Overpowering was always good.

A few years ago I switched to tubes. Audio Research Reference 160m and 160s. They can be run in ultra linear mode with 140 wpc or in triode mode… 70 wpc. My Sonus Faber Amanti Traditional have an efficiency of 90db… 3db less than yours. I have found they sound better in triode mode (a touch more musical). So I run them in triode mode. There is no noticeable difference in volume between the two modes (yes, counterintuitive). I find with tubes the wpc is very little difference in sound unless you are really under powering, unlike solid state.

I listen to them in the 70db - 80db range at most usually. I had some youngsters over here to listen to them. They listened at way more… in the 90s. They sounded great… no loss of dynamics. Although I had to leave the room… I don’t like music that loud anymore. They were listening in triode mode (70wpc).

The new Audio Research Reference amps REF 80, Ref 160, and the new REF 320. Have automatic biasing. Also the tubes are slowly brought up to full power over three and a half minutes to increase tube life and prevent the power surge that can pop tubes and take out resistors. Tube life is three thousand hours… and retubing is only 8 power tubes in total.

Taken together, I would recommend you consider trading your amp for a REF 160s or REF 160m monoblocks. I have had both, the monos have a wider and deeper sound stage… the stereo has a bit greater impact.