Amp hum increases and decreases

[pstrisik]

Originally Posted by rlw3

Peter,
In my experience since the early 80s,the sparkle, magic and quietness of the 12ax7/5751 family has greatly exceeded 6922s. Currently my lp 275s sound better than my premier 12s in the areas that more power benefits. However the 275s have a a lack of sparkle which i hope will go away when i get up to 800 hours. I am now at 400 hours and if it does not improve the 6922s will be suspect.
Previously when my 6922s did sound great it was for a short time only. Roger Majeski of music reference,who I deeply respect, told me that tubes do not have an enherent quality and the sound of the tube was dependent on the circuit that it was in. I seem to be very sensitive to tube deterioration and 6922s never seem to have a long honeymoon period before the sparkle is gone and the noise picks up. If I had the $ I would probably not go for a cj gat which is 6922 based. The tubes in my act 2 series 2 are a more robust varient with the same lack of sparkle but at least they hold up and stay consistant for a long time. They are 6n30p's I believe. I apologize for any heresy I may have commited, YMMV
PS my idea of gear with a lot of sparkle would be the equipment Zestos makes.

Interesting. I have the 11A which, aside from a mysterious hum through both speakers that cropped up recently, has been very nice. It uses 5751/6FQ7. Still waiting to get my new LP125M's set up. They use M8080 (CV4058) and 6922. Maybe the same as your 275's? So we shall see!

[rlw3]

Originally Posted by pstrisik

Interesting. I have the 11A which, aside from a mysterious hum through both speakers that cropped up recently, has been very nice. It uses 5751/6FQ7. Still waiting to get my new LP125M's set up. They use M8080 (CV4058) and 6922. Maybe the same as your 275's? So we shall see!

One source of lack of happiness with the 275s may be that each amp has 4 sets of power tubes. Some AA members I have read believe that one set of power tubes sounds better/more pure than the same circuit with 2 sets of power tubes (pr12s) and by the same logic 2sets of power tubes sound better than 4 (lp275s).
Cause and effect is hard to ascertain.
My 275s use 3 6922s. Would love to know why the m8080 was used instead??

[pstrisik]

Originally Posted by rlw3

One source of lack of happiness with the 275s may be that each amp has 4 sets of power tubes. Some AA members I have read believe that one set of power tubes sounds better/more pure than the same circuit with 2 sets of power tubes (pr12s) and by the same logic 2sets of power tubes sound better than 4 (lp275s).
Cause and effect is hard to ascertain.
My 275s use 3 6922s. Would love to know why the m8080 was used instead??

Maybe an email to service@conradjohnson.com could get an answer.

[pstrisik]

Regarding my Pr11A hum....

My repaired LP125M will be back by the weekend, so I'm taking a last try at tracking down the hum before taking the 11A out of service.

The hum is a constant sound out of both speakers and is present even if the preamp is off. If I unplug the preamp interconnects from the amp, the hum decreases slightly, but is still there. I've played with all cables and power cords. Even just moving them around should create some change in the hum if the problem was cable interference, but it doesn't change at all. I tried grounding the amp to the power conditioner. I've changed power conditioners. I've plugged in the amp directly into an outlet separate from everything else (but may be on the same circuit).

When I hook up the 125M's, lack of hum will make the 11A suspect. If the hum is there, I've got some kind of ground or interference problem.

Any chance it is a tube problem? Since it is constant and in both channels, I assumed not. But I can swap out tubes to see.

[rlw3]

Are both amps plugged into the same wall outlet? Have you tried using cheater plugs to lift the ground on one or both amps?
Lifting the ground on powerful amps is not ideal. Lastly have you tried lifting the ground of everything but the amps?(much safer) Do you have anything digital on in the house that could be turned off?

[pstrisik]

Originally Posted by rlw3

Are both amps plugged into the same wall outlet? Have you tried using cheater plugs to lift the ground on one or both amps?
Lifting the ground on powerful amps is not ideal. Lastly have you tried lifting the ground of everything but the amps?(much safer) Do you have anything digital on in the house that could be turned off?

The hum is from the stereo 11A. I am waiting for the LP125M mono to return from warranty service after which I will try the amp pair.

Safe to lift the ground briefly on the amp just for a test?

Lifting ground on everything else would be quite a job. I suppose I can turn off everything except the source preamp and amp and lift ground on the first two.

I probably have about 50 digital things in the house! I can turn off everything in the immediate vicinity.

Doesn't sound like it could be tubes to you?

I'm racking my brain trying to recall what might have changed to cause this.

[Coppy]

It's probably an issue of un-equal AC between the grounds of your components. Equipment often leaks AC to its chassis or ground.

You may not want to do this but you'll have to to track down the hum. First, unplug everything from the amp, except the speakers. Turn the amp on. If the hum is still there, it's something in the amp... could be any number of things, maybe a tube but more likely a cap. You should at least try lifting the ground on the amp to see if helps. It won't hurt the amp but could be dangerous if high voltage shorts to the chassis. I have run my amps ground lifted for some time.

Okay, say the hum is gone with everything unplugged. You're going to work back from the amp. Now, plug in just the preamp. If there's hum, lift the ground on the pre and you're done. If not, plug each source in one at a time until the hum returns and then lift the ground on the offending source to kill the loop. In my case it turned out to be the Magnum Dynalab FM tuner. Now all is well.

Good luck,
Bob

[pstrisik]

Originally Posted by Coppy

It's probably an issue of un-equal AC between the grounds of your components. Equipment often leaks AC to its chassis or ground.

You may not want to do this but you'll have to to track down the hum. First, unplug everything from the amp, except the speakers. Turn the amp on. If the hum is still there, it's something in the amp... could be any number of things, maybe a tube but more likely a cap. You should at least try lifting the ground on the amp to see if helps. It won't hurt the amp but could be dangerous if high voltage shorts to the chassis. I have run my amps ground lifted for some time.

Okay, say the hum is gone with everything unplugged. You're going to work back from the amp. Now, plug in just the preamp. If there's hum, lift the ground on the pre and you're done. If not, plug each source in one at a time until the hum returns and then lift the ground on the offending source to kill the loop. In my case it turned out to be the Magnum Dynalab FM tuner. Now all is well.

Good luck,
Bob

Thanks Bob! I have tried running it with only speakers connected and hum is still there. Tonight I will try with ground lifted and won't touch the chassis! If that doesn't work, I guess I can swap out the tubes next.

[rlw3]

Are all the source components on the same circuit as the amp? Are they on the same wall outlet? If not perhaps put all source gear on a cheap power strip so that it can go on the same wall outlet, then float the ground on the power strip. I had a circuit in another room in my house where it hummed on one outlet but not if i hooked up the amp to the next outlet on the same circuit with an extension cord. It made no sense to me at all. Do you have one of those cheap tester 3 prong plugs that lights up in different ways to show if any connects inside the wall to the outlet are missing or reversed?

[pstrisik]

Originally Posted by rlw3

Are all the source components on the same circuit as the amp? Are they on the same wall outlet? If not perhaps put all source gear on a cheap power strip so that it can go on the same wall outlet, then float the ground on the power strip. I had a circuit in another room in my house where it hummed on one outlet but not if i hooked up the amp to the next outlet on the same circuit with an extension cord. It made no sense to me at all. Do you have one of those cheap tester 3 prong plugs that lights up in different ways to show if any connects inside the wall to the outlet are missing or reversed?

I've tried on a different outlet (likely same circuit). I do have one of those testers and will try that tonight.

That amp has been running fine for a year, so I'm skeptical that it is the power. Feeling like the amp itself. I'll come back with the results. Thanks for all the input.