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Conrad Johnson PV2


joeward
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I wonder if someone with more tube experience than I have can answer a question for me. Does it take a few minutes for a Conrad Johnson PV2 to stabilize after flipping it on? Seems I can get some nasty feedback from it through my speakers if I turn it on immediately but if I warm it up for a few minutes before turning on my amp it's dead quiet in that regard. Sounds really good too. Just concerned their could be a problem that needs checking out. Thanks!

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39 minutes ago, joeward said:

I wonder if someone with more tube experience than I have can answer a question for me. Does it take a few minutes for a Conrad Johnson PV2 to stabilize after flipping it on? Seems I can get some nasty feedback from it through my speakers if I turn it on immediately but if I warm it up for a few minutes before turning on my amp it's dead quiet in that regard. Sounds really good too. Just concerned their could be a problem that needs checking out. Thanks!

 

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1 hour ago, jimjimbo said:

I would make sure that the volume is turned all the way down prior to power on, then just keep an eye on the tubes for a minute or so.  I normally let my tube units warm up and stabilize for about 10 to 15 minutes (both power amps and preamps).

Are you sure the volume pot is behind the tubes? Otherwise it should make no difference regarding weird noises during warming up.

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4 hours ago, joeward said:

I wonder if someone with more tube experience than I have can answer a question for me. Does it take a few minutes for a Conrad Johnson PV2 to stabilize after flipping it on? Seems I can get some nasty feedback from it through my speakers if I turn it on immediately but if I warm it up for a few minutes before turning on my amp it's dead quiet in that regard. Sounds really good too. Just concerned their could be a problem that needs checking out. Thanks!

 

That is certainly a sign of instability with the amplifier. I highly recommend getting it to an amp doctor right away before you start burning up speaker parts from oscillations.

 

You may only hear the noise during power up/down but it may also be present during listening but above the range of hearing, most oscillations I find in tube amps are between 30kHz-60kHz. Sometimes you may notice the amp sounding a little fatiguing to the ear or most likely you don't notice anything until speaker parts start to fry or eventually the issue gets so bad the amp gets further damaged.

 

Squealing during power on/off is in no way normal.

 

 

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It only happened once and that was when I had them plugged into the same surge protector......I turned them both on at the same time. Since then I've run a separate surge protector on the preamp, turn it on first, let it warm up a few minutes and then switch the amp on. Hasn't happened again.

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  On 11/16/2021 at 8:59 AM, joeward said:

"I wonder if someone with more tube experience than I have can answer a question for me. Does it take a few minutes for a Conrad Johnson PV2 to stabilize after flipping it on? Seems I can get some nasty feedback from it through my speakers if I turn it on immediately but if I warm it up for a few minutes before turning on my amp it's dead quiet in that regard. Sounds really good too. Just concerned their could be a problem that needs checking out. Thanks!"

 

"That is certainly a sign of instability with the amplifier. I highly recommend getting it to an amp doctor right away before you start burning up speaker parts from oscillations.

 

You may only hear the noise during power up/down but it may also be present during listening but above the range of hearing, most oscillations I find in tube amps are between 30kHz-60kHz. Sometimes you may notice the amp sounding a little fatiguing to the ear or most likely you don't notice anything until speaker parts start to fry or eventually the issue gets so bad the amp gets further damaged.

 

Squealing during power on/off is in no way normal."

 

 

From your description, it could be "normal"operation. When I have used tube preamps in the past to drive solid-state amps, if your turn-on sequence is not correct, it is possible to get thumbs, buzzes and noise from your system. The noise you heard may not be "feedback", but just a noise. For example, if you turn-on the amp first, then the tube preamp, you may hear something in the speaker as the preamp slowy turns on (such as the voltage across the output capacitor charging up). This could induce a slow DC-Offset into the signal chain. I have heard this before. 

 

Always turn-on the preamp first, then the amp and the opposite when you power down - amp 1st, then preamp 2nd. 

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5 hours ago, Curious_George said:

  On 11/16/2021 at 8:59 AM, joeward said:

"I wonder if someone with more tube experience than I have can answer a question for me. Does it take a few minutes for a Conrad Johnson PV2 to stabilize after flipping it on? Seems I can get some nasty feedback from it through my speakers if I turn it on immediately but if I warm it up for a few minutes before turning on my amp it's dead quiet in that regard. Sounds really good too. Just concerned their could be a problem that needs checking out. Thanks!"

 

"That is certainly a sign of instability with the amplifier. I highly recommend getting it to an amp doctor right away before you start burning up speaker parts from oscillations.

 

You may only hear the noise during power up/down but it may also be present during listening but above the range of hearing, most oscillations I find in tube amps are between 30kHz-60kHz. Sometimes you may notice the amp sounding a little fatiguing to the ear or most likely you don't notice anything until speaker parts start to fry or eventually the issue gets so bad the amp gets further damaged.

 

Squealing during power on/off is in no way normal."

 

 

From your description, it could be "normal"operation. When I have used tube preamps in the past to drive solid-state amps, if your turn-on sequence is not correct, it is possible to get thumbs, buzzes and noise from your system. The noise you heard may not be "feedback", but just a noise. For example, if you turn-on the amp first, then the tube preamp, you may hear something in the speaker as the preamp slowy turns on (such as the voltage across the output capacitor charging up). This could induce a slow DC-Offset into the signal chain. I have heard this before. 

 

Always turn-on the preamp first, then the amp and the opposite when you power down - amp 1st, then preamp 2nd. 

 

Well designed devices will mute the outputs until things have stabilized. If no muting function and say you turn on the power amp before the preamp you can certainly hear some noises as the preamp warms up. Noises like slight hum and thermal expansion of metal creaking popping as tubes heat up but the way he described the noise was "nasty feedback" which brings to my mind a squealing noise. The squealing noise is instability from positive feedback, this is a sure sign the amp is marginally stable at best during normal operation but during power up/down the instability can become so bad you have squealing from it oscillating. That is why it squeals, it is an oscillation that shouldn't be there.

 

Unless the OP wants to change his description I stand by my statements. Any squealing even during power up/down is a sign of oscillation and that even though the amp doesn't do it during normal operation for it to do it during a power cycle means that the amp is marginally stable at best. Dismiss these warning signs at your own peril, even if you had it serviced recently doesn't mean anything. I have seen multiple vintage equipment come to me for issues just like this, last one that did exactly what you describe had different style coupling capacitors installed. The preamp had feedback around the tone circuits where the capacitors were replaced and during power up/down the amp went into oscillation which it never did before and shouldn't do. The type of capacitors allowed for a slight parasitic coupling between components which decreased the phase margin and hence lowered stability. During normal operation the squeal went away but on the scope you could see an oscillation around 35kHz that changed with volume changes. Took a bit to find the issue but once the capacitors were swapped out to small film types the phase margin increased and the preamp was stable once again. Basically the squealing is from the oscillation dropping low enough in frequency to be heard by the human ear, during normal operation the oscillation frequency increased high enough to not be heard by the human ear.

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Some static type tube noise as the tubes warm up is normal. Feedback does not sound normal to me. However, you are supposed to let any tube pre warm up a bit before engaging the amp.  Obviously, with this type of sequence, it's almost impossible to tell that the pre is having problems using your ears alone since without the amp powered on you have no signal.

 

Just as a pre-caution I'd open it up and re-seat the tubes, and check all the inter-connects. If you are getting some distortion while using the volume or other, pots, I'd lube the pots (they are the things with the knobs attached).

 

My PV3 is long since gone, but given their age, problems can crop up.

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