[cjmustang69]

I'm still quite new to tubes and am hoping I can get some experienced thoughts about an issue I have this morning.  I listened to my Cary amp last evening(280SA V12) and there was no issues at all.  I put the amp in Standby until this morning.  When I placed the amp in Operate mode - all 6 of my left channel tubes indicate they are out of bias.  There is light "popping" coming through my left speaker.  I don't understand how all 6 tubes would go out of bias at once.  Is this normal - and I just need to rebias or would it indicate some other issue? 

[Quiet Earth]

Hi cjmustang69,

I don't know if I can diagnose your problem over the internet, but here are a few questions that might help us figure it out :

1. How much did the bias change from its normal value?

2. Is there still a light popping sound coming out of your speaker, or did that go away/get worse after you left the amplifier on for a while?

3. Is this the first time that you have put the amp in standby mode overnight, or do you leave it in standby mode frequently?

4. How old are the tubes? (How many hours are on the tubes)


Here are a few initial suggestions :

You could try to rebias it back to normal, and see if the bias stays put. You can also listen for the popping to go away or get worse after you rebias it.

You can turn the amplifier off, trade the left side power tubes for the right, and rebias all of them to see if the biasing problem is in the tubes or the amplifier. Also, listen for the popping sound to move to the right side.

You can turn the amplifier off, trade the small signal left and right tubes, and see if the popping sound goes to the right channel. (There is a chance the popping sound is coming from a small signal tubes and not related to the biasing.)

[cjmustang69]

Thank you for the suggestions - I realize I didn't provide much information regarding the issue.  I can answer a few of the questions you aksed:

1.  I do routinely put the amp in Standby Mode overnight on weekends.  During the week, I turn power completely off as I only listen to amp on weekends.  Until this morning - I never had an issue doing this.

2.  I had taken my amp into Cary Audio in late July of this year.  All the tubes were determined to be "good" other than 2, which were replaced at that time.  Since late July, I wouldn't believe I've put over a couple of hundred hours on tubes.

3. I don't know how much the bias changed as I only know the red LED lights indicating tubes are properly biased didn't light for any tubes on left channel this morning.  I did turn the amp back on after my initial post this morning.  I did notice the "popping" which I hear through left channel seemed to be caused by the 2nd tube(with first tube being at rear of amp).  There appeared to be "flashing" in upper portion of tube which coincided with the "popping" sound in speaker.

   I assume the "flashing" in tube indicates an issue with it.  If this is the case, would 1 Tube impact the ability of all tubes on left channel to achieve proper bias? 

    Thank you for your sugestions/help

[Quiet Earth]

.... would 1 Tube impact the ability of all tubes on left channel to achieve proper bias? 

I don't know enough about your amp (and how you bias it) to answer, but I bet if you emailed Cary they could tell you.

But before you do that, if you believe that you have identified the problem tube as #2 from the rear left , you might try exchanging it with tube #2 from the rear right to see if the problem moves over to the right side. If that's the case then maybe it is just a bad tube.

[Ericus Rex]

It's hard to diagnose over the internet, and unfortunately, I'm not familiar with that particular amp.  But here goes:

If your amp requires 6 matched tubes per side, and all 6 are biased with only one pot, then, yes, one tube going would effect the bias of the other five on the same side.  If each tube is biased individually (this is implied in your op as Cary only replaced 2 tubes and not an entire bank) then one tube going usually won't impact the bias of the others in the same channel.  This depends on the individual circuit, however, and maybe Cary used some other type of design.

Now, if a tube does go it sometimes can take bias components (often resistors) with it to the grave.  If a shared resistor is fried than obviously that would effect the bias of the other tubes in the channel.

I agree with Quiet Earth, give Cary a call and maybe they can help diagnose over the phone.

[cjmustang69]

Thank you for replies.  I did write Cary Audio today.  They suspect it is just a bad tube.  I plan to order a couple of tubes tonight.  For clarification, my amp has 1 pot for each side (6 tubes on each side) rather than bias adjustment for each tube.  Hopefully it is just a bad tube and I'll know for sure in a few days

[Ericus Rex]

Hmmm.  Your bias scheme would imply that you would have to install 6 matched tubes per channel.  Replacing a single tube would be like putting a single un-balanced tire on your car; three tires are worn evenly, one is brand new but unbalanced.  Doesn't sound like fun, does it?  Cary must know something I don't know.  Best of luck!

[cjmustang69]

Cary had replaced two tubes in August, when I had taken it in for a checkup.  My understanding is the Lp number on boxes of tubes they installed would be what I need to have for replacement of suspected bad tube.  This is my understanding at this time.  I'll know more next week as I hope to have spoken more with Cary Audio next week.

[cjmustang69]

Well, I have my Cary amp running again.  I took the tube which I suspected was issue to Cary Audio and their testing showed it had an intermittent short.  I purchased one replacement tube, replaced the left side tube fuse which had blown, biased the left channel - and amp works fine.  At some point in the near future, I plan to purchase another set of tubes and replace the EH KT88s currently in amp.  If anyone has suggestions for tubes, I would welcome thoughts.  For reference, I'm using a Audio Research SP16 for preamp and ADS 1230 speakers.

[Quiet Earth]

Glad you're back in business. 

Yay!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

[FullRangeMan]

I put the amp in Standby until this morning.

Not safe let tube amps running alone, more than afew minutes. In my area the electric company made repairs over the night sometimes, and the energy are off about 5 minutes etc...