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Old 13th April 2007, 08:47 AM   #1
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Default Leaving tube preamp always powered on.

I would appreciate any advice on leaving a tube preamp always powered on whilst minimising safety hazard and shortening of tube life.

I am thinking of an "idle" setting which leaves the preamp ready for use rather than having to go through a warmup period.

Would lower heater or HT voltage/current work?

Another application would be for preamps with line and phono stages where the phono stage is only used occasionally but is on all the time the line stage is being used. This seems to be a waste of tube life.

The preamps I am using are all small tubes - 12AX7, 6922 etc.

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Old 13th April 2007, 08:59 AM   #2
Tweeker is offline Tweeker  United States
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For longest life you want to cut HT entirely and idle the heater at 80%.
Be sure your foil hat has a good low impedance ground.
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Old 13th April 2007, 09:29 AM   #3
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Thank you Tweeker.

Another reason I want to leave the preamp on is because I use a transistor power amp, which is always on, and turning the preamp off produces a speaker destroying thump. I have to turn the power amp off, preamp off and then turn the power amp on again which is a pita.

If I cut off the HT completely would this make a thump?
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Old 13th April 2007, 04:27 PM   #4
kevinkr is offline kevinkr  United States
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It could still thump badly depending on how quickly the B+ rises at restoration of power, and how slowly it falls when it goes into standby.

I am not completely sure about this, but I think your pre-amplifier standby scenario could lead to problems of cathode interface (loss of cathode emission) with some commonly used audio tubes. Instant on tv sets in the 1960's often used tube types with somewhat improved cathode chemistry to try and reduce this problem. (IIRC many of these sets seemed to have worse tube life, and my dad, an EE educated in the tube era, disabled the feature on our set because he maintained that it shortened tube life.)


I reread your post and noted you leave your solid state amplifier on. Regardless of power consumption/component life issues, I assume it sounds better when warm? In this scenario it seems like relay muting (shunt) at the pre-amplifier output would be the best choice. It should only unmute the pre-amplifier once warmed up, and should mute the pre-amp just prior to shutoff. This is the technique I used with ss amps and tube pre-amps. Another interesting option is a transformer coupled pre-amp - this also eliminates the issue.
"To argue with a person who has renounced the use of reason is like administering medicine to the dead." - Thomas Paine
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Old 13th April 2007, 07:09 PM   #5
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......or simple input mute switch on the power amp?
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Old 19th April 2007, 11:37 AM   #6
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Thanks Kevin and billy pilgrim.

Yes, I can and probably should add the output relays. It would also be handy if I do leave it on and there is some sort of fault condition.

But I'm still interested in ways to leave a pre on without wearing all the tubes out. What about connecting it to a variac and turning the supply voltage down when not in use? This would obviously reduce both the ht and heater voltages. The pre would still need to work well enough for my wife to listen to the radio in the morning etc and then when I wanted to do some serious listening I could wind the voltage back up.

What do you think? Crazy?

By the way one of my pre's has just failed. It's an ARC SP12 and I am about to start a new thread because it has a strange arrangement of the heater supply.

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Old 19th April 2007, 12:03 PM   #7
SY is offline SY  United States
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If you have a copy of "Valve Amplifiers," there's a lot of detail on running the tubes in standby using reduced voltages; basically, you want to switch off the high voltage and run the heaters at a reduced voltage/current. The variac might be a problem since it reduces the heater voltage while keeping high voltage (albeit somewhat reduced) on the plates.

That said, I keep my preamp going 24/7 and go many years between tube changes.
"You tell me whar a man gits his corn pone, en I'll tell you what his 'pinions is."
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Old 19th April 2007, 03:20 PM   #8
Tweeker is offline Tweeker  United States
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Dont run direct heated tube filaments at less than 80%, the Miller-Larson effect occurs below this.
Be sure your foil hat has a good low impedance ground.
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Old 19th April 2007, 04:38 PM   #9
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Default Leaving my pre-amp on standby

Hi All

I am not expert by any means but I was lead to believe that if you turn the ht off and leave heaters running you are likely to poisoning the cathode, which results with it no longer working.

Another thing worth mentioning, I know so many of us and our amps use dc for the heaters, this has a tendency to shorten the valve life too, for reasons which I can not remember.

The only other thing is why do you need to leave them on anyhow, given current trends with global warming etc, you are just wasting energy and resources.

Most amps warm up within 15 minutes or so anyhow and I was lead to believe that there was no advantage in leaving them on anyhow.

Some months ago i stopped leaving my amps on and turned them off, just as well I did as the Musical Fidelity Typhoon power amp made some most interesting noises some months back.

I attributed the creakign to the normal heating up but it was not some time after switching on there was a sudden hum in the left channel and I turned round to see there was a load of smoke coming out of the amp. Of course i switched off, the whole thing was very very hot, had i not been here and had I not ceased to leave mine on standby, i would now be homeless.

Not wishing to put you off but my advise forget the idea, did you know you invalidate your household insurance by leaving it on, as you do with other items such as washing machines should something happen and you left them un-attended then you would be able to claim nothing.

Well it is only my opinion but a worthy one I feel.

See you
Steve in the U.K.
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Old 19th April 2007, 11:28 PM   #10
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I am certainly sure that leaving equipment power-on increases power bills. Personally, I'd prefer to have my power-amp switch off when not in use, and the same applies to my preamp.

What I have found best is to switch on the preamp, then go make a cup of tea, get a beer, feed the dogs, whatever. Come back in a couple of minutes and switch on the main amp, then decide which CD to play and go get it. Anyway, by the time I've done all this, my system is usually pretty warmed-up. Maybe if I allowed it to warm up for a couple of hours, I may notice a difference in sound quality, although I feel that it may be more subjective.

A second reason for powering-down equipment is that during the long Arizona summers, it is very difficult to keep the house below 80F (27C) and it can add to the heat induced depression to think of the preamp tubes adding extra heat into the room

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