Sputtering sound on start up

johnnytheg

johnnytheg

Active Member
I have a McIntosh C11 preamp and a dynaco ST70 amplifier. My procedure is start the preamp then wait about 15 seconds and then start the ST 70. Lately I've been hearing a sputtering sound when I start the amp and since I always keep the volume down on the preamp until I play music, I assumed the problem was with the amp. I tried turning the amp on alone without the preamp first and there was no noise. Then as an experiment, I turned the preamp on, again with the volume all the way down and as the preamp was heating up, i heard the sputtering sound again. I am assuming I can rule out the amp at this point. Anyone have any ideas what could be happening in the preamp? I had it bought back to factory specs last year by Audio Classics.
 
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Yes both channels. It lasts for about 10 or 15 seconds on warm up. It's quiet for all the listening time after that which is usually about two hours.
 
Is it 15-20 seconds after turning preamp on? Or sound lasts for 15-20 seconds after 15 second delay when you turn amp on?
To me it sounds like it takes a little longer for power supply to stabilize than you expect. I'd give it a little more time before turning amp on.
Does it use tube rectifier?
 
grindfix said:
Is it 15-20 seconds after turning preamp on? Or sound lasts for 15-20 seconds after 15 second delay when you turn amp on?
To me it sounds like it takes a little longer for power supply to stabilize than you expect. I'd give it a little more time before turning amp on.
Does it use tube rectifier?
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Well, as I said in the original post, I usually turn the preamp on first, wait about 15 seconds then turn on the amp and I never heard the sound before. Lately when I do that same procedure, it's about 5 seconds or so after turning on the amp. I did some experimenting and kept the preamp off and turned the amp on by itself and no sputtering sound. I then took an unusual step of turning on the amp, letting it sit for about 45 seconds to a minute and then turned on the preamp and after about 6 seconds I heard the sputtering sound which lasted for about 10 or 15 seconds then settled down and all this time the volume knob was turned all the way down.
The Dynaco does have a tube rectifier. The C11 has 6 12ax7's
 
I would never turn amp on first. Preamp may emit harsh sounds during warmup and may not be safe for speakers. I'd say try turning preamp on first, as you were doing but give it a little more time to warm up before turning amp on. Your ST-70 probably has series diodes installed on rectifier plates. Common mod to make tube rectifier life easier. It sounds like your amp comes alive quickly and catches preamp warmup cycle output.
 
Do the panel lights flicker on the preamp? If so, the power switch is arcing and needs to be replaced.
 
grindfix said:
I would never turn amp on first. Preamp may emit harsh sounds during warmup and may not be safe for speakers. I'd say try turning preamp on first, as you were doing but give it a little more time to warm up before turning amp on. Your ST-70 probably has series diodes installed on rectifier plates. Common mod to make tube rectifier life easier. It sounds like your amp comes alive quickly and catches preamp warmup cycle output.
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So you don't think this is anything to be too concerned about at the moment? I am surprised that something in the preamp warm up would make it down to the amp and out the speakers with the preamp volume turned all the way down, but then again I am a novice when it comes to electronics. I just love the sound of my tube stereo.
 
johnnytheg said:
So you don't think this is anything to be too concerned about at the moment? I am surprised that something in the preamp warm up would make it down to the amp and out the speakers with the preamp volume turned all the way down, but then again I am a novice when it comes to electronics. I just love the sound of my tube stereo.
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Volume control is in the inputs of the preamp. Preamp tube stages have high gain and can have sounds pass into amp during warmups. Volume control position has no effect on that.
If this sound only happens within 20 seconds of initial preamp turns on time, probably not an issue but worth to keep an eye on it.
 
@johnnytheg Makes sense that the noise wouldn't be affected by the volume pot. Usually volume pots change the strength of the input signal right after it enters and before it's amplified or altered in other ways. Most likely the noise arises from circuitry that's "after" the volume pot, so to speak.
 
One other thing you can try; put both the preamp and poweramp on a switched outlet strip. Leave the power switches on the amp and preamp on and turn everything on at the same time with the power strip.
 
Anything is possible but I’d think you’d have this noise at random times too while listening to music.
 
grindfix said:
Anything is possible but I’d think you’d have this noise at random times too while listening to music.
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Well, actually back on Christmas eve while I was listening to music for about 3 or so hours, I heard a pop for this first time. This is when I first noticed something was wrong. I checked all my tubes in the Dynaco and found one of the EL34's had a short so I bought new tubes. After that, I noticed the sputtering on start up which is what inspired the original post.
 
johnnytheg said:
I was thinking that very same thing. I have the deoxit here and I can certainly clean the pins and see if that does something.
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I always try the Deoxit first for most "squirrely" sounding tube issues. You'd be surprised how many times this works. And it's a cheap try, which usually does no harm. :)

Good on jacks and plugs, too! :thumbsup:
 
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