JJ bias concerns

  • Thread starter Thread starter John4021
  • Start date Start date
John4021

John4021

Well-known member
Wasn't sure why a head with little time on the tubes was biased so cold.
(it was 21/23mA per tube). Went to raise the bias and at max sweep they still only came up to 31mA.
Figured tubes were garbage, since I put in a quad of Wing C's and the bias has plenty of range.

Just for the hell of it, I checked my buddys JJ,,,,,same thing. Tubes were under 28mA.

Are ARS branded tubes that inconsistent? Mine are marked "42" on the top/his are "48"
(still to learn what that number means)


SO. Anyone else check there bias on there amp.(JJ or other Friedmans)
 
Ok first off was you wall voltage exactly 120v? Factory bias should be 30 to 32 ma. With a 120 volt line. Depending what number tubes are in it will depend on how much swing the bias supply will have. If you wall voltage was low you could easily have those readings. Your bias supply can be altered to get a bigger swing.
 
RACKSYSTEMS":157zip2h said:
Ok first off was you wall voltage exactly 120v? Factory bias should be 30 to 32 ma. With a 120 volt line. Depending what number tubes are in it will depend on how much swing the bias supply will have. If you wall voltage was low you could easily have those readings. Your bias supply can be altered to get a bigger swing.
Click to expand...

I have a question Dave. Don't know about most people, but my wall voltage is usually higher.. around 122 or even as high as 125v sometimes (most homes will be a little higher than 120v around the US from my experience). I've only had to rebias my BE100 once, and I did so around 31-32ma per tube.

Does it matter much if wall voltage is higher or lower than 120v when biasing the amp? And say it's 125v the day of biasing... Should you bias a little lower... or does it even matter that much? Thanks
 
Well it's all relevant. So if you wall voltage is 125 let say and you bias the amp and then have a wall voltage of say 117 then you bias will read lower. Everything in the amp is made for a 120 line, higher gets you a harsher tone for sure. Really I love around 115v, just a bit sweeter. So as this poster is saying he is getting 22 to 24 ma, I need to know what the wall voltage is because that can vary things a lot. Best thing you can ever have is a variac.
 
RACKSYSTEMS":2a8r9hat said:
Well it's all relevant. So if you wall voltage is 125 let say and you bias the amp and then have a wall voltage of say 117 then you bias will read lower. Everything in the amp is made for a 120 line, higher gets you a harsher tone for sure. Really I love around 115v, just a bit sweeter. So as this poster is saying he is getting 22 to 24 ma, I need to know what the wall voltage is because that can vary things a lot. Best thing you can ever have is a variac.
Click to expand...
Is there a variac you can recommend Dave?
 
Carol Ann amps makes a bias monitor built into his amps, you have a few leds that tell you where the bias is at any given time (red, yellow, green). I turn my amp on, it warms up for a bit then I adjust it accordingly with a small screw driver and my bias is always perfect. I wish he would start licensing this to other companies (hint hint Dave an Alan are friends !).

It is an amazing and easy thing to have on your amps and makes a huge different in feel and tone. it changes WILDLY with the clubs I play BTW and with this it is always sounding right.

Also allows for easy tube swaps since you can then bias them yourself.
 
jharpersj":1i74mzzs said:
Carol Ann amps makes a bias monitor built into his amps, you have a few leds that tell you where the bias is at any given time (red, yellow, green). I turn my amp on, it warms up for a bit then I adjust it accordingly with a small screw driver and my bias is always perfect. I wish he would start licensing this to other companies (hint hint Dave an Alan are friends !).

It is an amazing and easy thing to have on your amps and makes a huge different in feel and tone. it changes WILDLY with the clubs I play BTW and with this it is always sounding right.

Also allows for easy tube swaps since you can then bias them yourself.
Click to expand...
I do miss the auto bias feature on my Marshall YJM100. All tube amps should have something like that and hopefully it will become a more common feature in the near future.
 
I've never owned a variac besides the variac feature on my old Mesa Mark V. It's actually the only way I could stand the overdrive, but the amp was solid as a rock with a great varic.
 
I have the same problem. All are reading 21ma and with the bias adjusted to full they only go to 29ma. My house voltage changes constantly from 117-125.
 
Well we can simply change a resistor so you have a bigger range.
 
I would really hate to ship it to CA just for that. Would a higher rated quad of tubes work?
 
Really I would just switch to TAD el34bstr tubes, we are switching over at the moment. Better sounding and way more reliable.
 
Or anyone who can solder and has worked in a amp before could do it in a change the resistor in a sec.
 
Thanks Dave! Your excellent customer service is always appreciated!
 
I think I will give the TADs a shot in my JJ. Hey Dave, what MA would you recommend for the bias on those?
 
Dave,

What sound differences would I expect with the new tubes? Would it be worth changing my originals or just wait until one fails?

Thanks!
 
You must log in or register to reply here.
Back
Top