Bias

[hbsurfn]

I have an 06 Competition and the original Mullard's had rating of lp:44, Gm:7700, the new Mullard's I just got are lp:43, Gm:7900. Do I need to re bias? What do those number mean? Is there a guide? I used to bias my Marshall with the Groove Tubes biasing kit that I still have, but I am not sure what the settings need to be, or how to adjust the bias. Never had a reason to take her apart and look inside...and really don't want to.

[bubbastain]

I just got my first Splawn (Competition) a few weeks ago. I just biased it two days ago. It was way off. There doesn't seem to be too much activity on this board, or at least not with the Competition section. You should bias the amp anytime you put new/different power tubes in. It's very easy. Buy a bias probe and cheap multimeter from Eurotubes for like $25. Read the info they have and watch the video for the "generic bias mod video(they use a JCM800)" Watch it several times. Pay attention to the details and watch it while your doing it. Or you could pay a amp tech to do it.

www.eurotubes.com/euro-Generic-Bias.htm

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[hbsurfn]

I sent an email to corporate HQ, but got no reply. Based off what I got from the internet I was shooting for 30-32mv, which was a sweet spot on my JCM900 50 watt. What's weird is, I put the new tubes in with the bias probe, and my reading was only 14.3mv, so I put the old one's back in and it was about the same. I had to do about 12 turns on the screw to get up to a reading of 31.3mv. I also tried a couple different settings on the hum pot since this amp seems to hum with my Holy Grail reverb in the loop. Hum goes away in bypass. Sounds o.k. at 31.3mv, so I will quit messing with it, but still curious why it was so low.

[Jammermatt]

Not sure what to say about that. But Scott recommends to bias between 30 and 32, so you should be alright.

[drweller]

Straight from HQ some months back:

"We recommend to bias the Comp no more than 32ma per tube. It makes no difference if you are running in 1/2 power or not on the Comp. Talk to you soon"

Look into the Weber Bias Rite (https://taweber.powweb.com/biasrite/br_page.htm)

So easy a caveman can use it.

Sockets insert between the tube/amp - get the plate voltage and ma per socket. Super easy and very safe (no probes poking at power sources).

Also, remember that location voltages/power fluxuations will affect the bias settings - over/under voltages will do strange things to your bias. Check the power source at the wall (i.e. plug in your power conditioner for a quick reading).

OP - did you mean 14.3mv or 14.3ma? Bit of a difference.

[drweller]

Also, 32ma is a starting point - put it there and then adjust to what feels/sounds best. The hotter the bias, the quicker the tube burn and the more softer/more compressed the feel. The colder the bias, the slower the tube burn and the stiffer/more responsive the feel.

[sasquatch]

I just recently bought a quad of Sino's from Scott and installed 'em in my QR and re biased. The initial readings were quite low as well (about 24mA if I remember correctly). The tubes I replaced were the original Tung Sol that shipped with the amp. I spoke with Scott before buying the tubes, and again before biasing. He did recommend an average across all four tubes of 30-32mA. I set mine up for 31.75mA and it sounds great. The inside pair were a little hotter than the outside, so they would occasionally bounce up from 32 to 33mA, then right back down. The outside pair did the same thing, they were just a little cooler than the inside pair (1-2mA). I'm attributing this to current fluctuation. Anyway... I suggest biasing closer to 32mA since the bias tends to drift downward a bit over time, but that's just my opinion. Good luck!