Re biasing cj lp 275

[rlw3]

My 275 came to me with about 100 hours on it. i have put another 115 hours on it myself. i biased the amp when it arrived and another 2 times within that 115 hour span. i never had to rebias my premier 12 or either of my music reference amps which i spent 15 years with. any thoughts my friends?
is the need to rebias a sign that the input tube or pair of driver tubes need to be replaced? is it a function of the stock 6922's from cj being a little dodgy? how often should rebiasing take place? i believe the seller was honest with me about only 100 hours on the amp as that was negative for me.

[Myles B. Astor]

You may always have to ever so "slightly" rebias the amplifier but the major rebiasing should occur within the first hundred hrs or so. That said, are you looking at the bias immediately after turn on or after 30-60 mins. I also find that the bias takes a little time to stabilize after turn on.

I always turn the bias all the way down when first inserting a new set of tubes just in case and then readjust after say 30 mins of being on :( That's been true ever since my MV75s; you might just be a little more sensitive to it having just gotten the amplifiers.

[rlw3]

Myles,the bias problem was taking place after the amp had been on a couple of hours. i use your method from paragraph two on new tubes also. a couple of days i had the system on for 9 hours( with bias lights going on a day or two later). could the amp be getting a little overheated? i remember reading a hifi article maybe in the 80's that suggested amps dont like being on more than 5-6 hrs tops. is that true or was it then? my motive for writing today is to learn if bias lights going on is a function of input or driver tubes (6922s my least fav) going bad.
btw on both of the mono blocks bias light lit up in the same way.

[pstrisik]

Bias is for the output tubes only. Input tubes don't need biasing.

You probably know this... are the bias lights coming on when nothing is playing or the pre is muted? They will sometimes flicker or blink when the amp is working. CJ says that is normal.

BTW, I'm curious why you don't like 6922's. My Pr17LS is 6922 based. I don't have any experience with other tube pre's to compare. Thxs.

[rlw3]

What I am trying to ask is when input or driver tubes wear out do they cause the bias to migrate and the lights to go on. Are they interactive. In both cases the bias needed work the day after I had run the amp for 8 or 9 hrs so is it a heat issue. I have never had to rebias 3 times in 115 hrs.
My ls16,audible illusions 3a, and music reference amps either ate 6922 quickly with lots of noise or the quality went from very good to ok very quickly.
Conversely I have always had very good luck with the 12ax7 family.

[Myles B. Astor]

What I am trying to ask is when input or driver tubes wear out do they cause the bias to migrate and the lights to go on. Are they interactive. In both cases the bias needed work the day after I had run the amp for 8 or 9 hrs so is it a heat issue. I have never had to rebias 3 times in 115 hrs.
My ls16,audible illusions 3a, and music reference amps either ate 6922 quickly with lots of noise or the quality went from very good to ok very quickly.
Conversely I have always had very good luck with the 12ax7 family.

Not in my experience.

AI has always had a rep for eating tubes ergo why Art suggests only getting the tubes from them. Also see that Upscale has specific recommendation for AI preamps.

[ronenash]

Ray, one reason you may experience bias shift is if you wall power supply is drifting. I have seen voltage swing by as much as 20v on my power outlets. After I got the Torus power conditioner which regulates the output voltage I have never experienced this again.
The B+ and heater voltage for the input tubes is regulated and thus not effected by these voltage swings whereas for the output tubes its not. If one of the tubes bias light comes on just turn the pot to turn it off. Follow this for a couple of days and your bias will be set for the worst case (highest power outlet voltage).

This is the best you can do without investing in a power conditioner.

[Joe Appierto]

Along the lines of Ron's comments, I found that on the weekend and especially earlier in the day on Saturday's and Sunday's, the LEDs would be lit and stay lit until I adjusted the bias pots. Once I'd gone through the weekend, they'd remain off until the next time I changed output tubes and then the process would repeat itself.

I think I know what you mean regarding the 6DJ8 family of tubes. When brand new, and for the first 50 hours or so, they have a sparkle to their sound which seems to diminish as they burn in.

[rlw3]

Ron and Joe thanks. Ron your idea of voltage swings makes sense as my rig sounds grey during air conditioner season. I have a shunyata hydra 4 which I like that is not designed for this type of problem. Could be that the higher power 275s react differently than all my 125-140 watt amps I have had for 20 years. Several but not all bias lights went on at the same time on both amps which surprised me. I have been turning down the bias pot to turn off the light- am surprised the bias does not have to be turned up if voltage swings downward are taking place. Shows how little i know about electrical theory and practice.

[ronenash]

- am surprised the bias does not have to be turned up if voltage swings downward are taking place. Shows how little i know about electrical theory and practice.

You would never know as the lights stay off when the voltage is lower. You only have an indication when the bias is too high and the less turn on. I would not get too obsessed over this and just turn the bias down a bit so that the leds stay off. If the bias is a bit low it will not effect sound quality.

[Myles B. Astor]

You would never know as the lights stay off when the voltage is lower. You only have an indication when the bias is too high and the less turn on. I would not get too obsessed over this and just turn the bias down a bit so that the leds stay off. If the bias is a bit low it will not effect sound quality.

Yes I agree.

[rlw3]

If voltage drift is the culprit does a 20amp circuit mitigate this type of problem? Since i can listen with my cj preamp in the 30s to 40s and the music bubbles without constraint i think i have plenty juice with my 15 amp service. had to rebias again yesterday 4th time in 118 hrs

[ronenash]

Ray, a 20A circuit does not effect the voltage swings as these come from the electrical company. My LP125mSE uses 1A each at idle into 230V. The LP275 will use twice that into 230v and 4 times that into 110V which comes to 4A for each monoblock at idle. This can go as high as 6A or even higher at high volume playback. I would say a 15A circuit would just power you monoblocks without much headroom for the rest of your equipment. I would try to have a separate 15A circuit just for the power amplifiers.

When you say you had to re-bias the amplifier what do you mean? Did some of the light turn on? How did you know you had to re-bias?

[rlw3]

Ron the lights went on now 4 times in 118 hrs,both amps, usually 4 or 5 lights each. Just got a bid to put a gas stove top in my house. The electrical and gas contractor will put in a 20 amp service for less than $300 while he is here. I need to buy 2duplexes of shunyata plugs. Don't know what to expect since volume s so low I think I should have no need, but am very curious.

[ronenash]

Ray, the bias lights can stay on for as long as an hour after power up. Did you wait for them to go out?

[rlw3]

Ron, the bias light go on after the amp has been on for 1-2 hrs. i turn down the pots when i see the light on-should i leave them on? if so why? i dont think this should become an every other day occurance

[ronenash]

No, in this case you should definitely lower the bias until the lights turn off. Is this happening with all tubes? I have had one KT120 that went bad and the bias was acting crazy. I replaced the tube and all was fine again. If this always happens in the same tube they might be gassy or faulty in some other way.
What you are describing is not normal if it will continue beyond the 150 hour mark and indicates bad tubes.
How much of a bias correction do you need to perform? is it just a slight movement or something more significant?

[rlw3]

i have to turn down the pot just a little. typically 4 or 5 tubes light up on each amp equally so i think it is a general power shortage due to air conditioner season in my neighborhood. which tubes do you think would be the bad ones-the power tubes right? that is why i was hoping it might be a driver or input tube problem.

[ronenash]

Power shortage or lower voltage should have the exact opposite effect. Its when the voltage is too high that the bias lights turn on. If its the same 4-5 tubes every time then they might be bad. If this continues you will eventually be at the lower limit of the bias range and the led will still turn on. At this point you will have to replace the tubes. After 200 hours they should have stabilized.