Hi,

Soo, I got my tubes from TT (4x TT 6L6 GC, 1 TT 12ax7, 4 JJ 12ax7), done the bias mod on my 5150, threw the new tubes in measured 28mA, cranked it up to 35mA. So far so good.

My question is: I meassure 35mA when amp is turned on, not playing or playing at low volumes but when i crank it louder and play the bias current is raising up to 47-48mA pretty often.. Is this normal? do i need to lower my bias current? Is it only the idle current that counts and the current during playing loud doesnt matter?

Thanks

Hi,

yes, this is normal. Bias measurement has to be done in idle mode so your 35mA are the value you are looking for and you are within a safe range on the 5150 (should sound much better now then before). The peak you see during playing the guitar are normal and are caused by the principle of the operating amp...

cu
bluesfreak 

Ok, thank you!

Thats what I thought, just wanted to be sure 

The 5150's bias range with 450V plate voltage is 35mA-45mA.. Is there alot of difference in tube life time between 35mA and 45mA? A little more wear and tear is fine for me, but if I need to replace tubes each rehearsal I think I stay at lower current (huuuge exaggeration but you get my point).


Also I had another question, when I meassured my old tubes they ran at 15mA. Is this dues to another type of tubes or due to they were really old? Do bias current gets lower by age of tubes? I used some sovtek tubes before..


Cheers!

Hi,

you will get the best compromise between sound and lifetime when running at approx 70% of the power tubes level, so you will be save up to 38mA @ 450V.
I would recommend to dial in roughly 35mA and then fine adjust using your ears (but keep an eye on the current, do not exceed 40mA to preserve the tubes)
The 15mA you've been measuring before is normal for the 5150 as it runs ice cold with its stock settings (usually somehwat betwenn 12 and 15mA).
Back then Peavey decided to do so as they designed the amp in a way that most of its sound is generated in the PreAmp excluding the PowerAmp from influencing the sound. In my ears this does not reveal the full capability of this Amp, having the power tubes working and sweating gives it much more balls, punch and definition and it also breathes much more when driven.. but again, this is my opinion and your mileage may vary 
If you like it leave it where it is now and enjoy

cu
bluesfreak