Final biasing questions...

Pierre

Pierre

Stratologist
I got a great link that explains everything I need, however there is one problem that remains. My amp doesn't have a standby switch.
Now in the normal procedure, I'd take the tubes off, turn the amp to standby and make the appropriate reads, THEN plug in the tubes and turn the amp on.
How do I get doing this with an amp without standby?

I got a little probe today that i just built that connected a 1ohm resistor across pin 8 and the ground, with reading meters, and those ****ers don't connect to my voltmeter :laugh2: So I'll have to use the pins on the voltmeter on the pins of the wires from the probe. More inaccuracies... Ah well. I'll probably give it a try tomorrow.
 
Last edited:
Re: Final biasing questions...

You really need a bias tool, which is a two sided socket with two leads that connect to a multimeter. I like the mA version a lot more than the voltage version.
Sometimes, after the amp is hot, the readings will start fluctuating. Then, it's time to turn the amp off so it can cool down, then take a final reading after you turn it back on. When you have a good stable reading of approximately 32mA for 6L6's and 38mA for EL-34's, then you can put the amp back in the cabinet.

If you amp doesn't have a little bias pot, then there's no reason to mess with it....just put in good fresh tubes and they'll probably fall somewhere in the bias range.
 
Last edited:
Re: Final biasing questions...

Oh the amp does have bias pots.
And you know what? I'm very stupid but the tubes are marked... 25.9 and 26. I don't know what value that is (they're 6V6s) but if they're the correct MA values, can't I just adjust the bias until it reads this? :smack: and I just did get a bias tool ;)
 
Re: Final biasing questions...

AH ****! Hahahha dude I just saw my typo! Sorry man, the first 'bias switch' should have been 'standby switch' Damn I'm so distracted. Edited now. Thanks!
 
Re: Final biasing questions...

Gah I found out answers... First I'll turn the amp on with all the controls down and see in which direction I have to turn the bias trim pot until I get the biggest negative current. Then I turn it off, take off the old tubes, turn the pot in this direction all the way, use the probe, take tube measurements, pin 8, 5 and 3, do the calculations, blablabla and adjust.
Yipee.
 
Re: Final biasing questions...

Wow... ok I just started.
I installed the probe, turned the amp on with all the controls at 0, and took them pin 5/ground measurement. It read - 33 and the lowest I could go on the bias trim pot was -34. Now for 6V6s I should have been able to go up to - 50 I hear... Well let's assume that is right.
I plugged in the tubes, and am now trying to read between pin 3 and ground and it's not working... The amp crackles, and the voltmeter doesn't really give me any reading. What's up with that? I'm set to 200V and it wobbles for a second then gives me 1 .
So yep, I'm a bit stuck...
 
Re: Final biasing questions...

WOWOWOWOWO okkk weird.
I DID get a reading off pin 3. 44.6 on the 500V scale.
Now I'm reading across the 1ohm resistor (on the 200mv scale) and it first told me 0.01, then I adjusted the bias to see if I could bring it up, but I got weird results. So I got the bias where it was before and now it reads 1.1 across the resistor.

What the hell? :(

Ahhh ****... It's going up. It's at 2.2 now. Which I guess is good since it's where 6V6s should be...
 
Re: Final biasing questions...

WOOT I did it! I biased it for 11W, while 6V6s run at the MOST 12W. So that's fairly hot. I'll probably dial it down to 10 later on and see what my ears tell me (...) since I don't know what a cold or hot amp sounds like. I did get much better readings towards the end, so I'm pretty happy.
 
Re: Final biasing questions...

Well that website said the max was 12... So... I don't know haha... I'll probably decrease it a notch, but since the resistor is gone I won't be able to measure it all again.
 
Re: Final biasing questions...

I don't know the max for 6V6...
The formula for biasing is the plate dissipation of the tube ( 25 watts for
a 6L6 or an EL34 ) divided by the plate voltage ( lets use 480 for this
example ) times .7 will just get you out of crossover distortion and you can
go as high as .9 percent.
Click to expand...
 
Re: Final biasing questions...

Mhmm that's not what I used. I basically just multiplied the value I got across the resistor by the value I got from pin 3 at different bias settings. Here's a good link:
http://www.duncanamps.com/technical/lvbias.html
Basically the way I see it, there are no proper balancing to do. From what I read, there was not a particular setting I had to aim. I just had to aim where I liked it, which in a way is not great because I'd have prefered a target value :(
 
Re: Final biasing questions...

Pierre said:
Mhmm that's not what I used. I basically just multiplied the value I got across the resistor by the value I got from pin 3 at different bias settings
Click to expand...
That's the same equation :p

Power [Watt] = Current [Amp.] x Voltage [Volt]

After this article 12Watts shoul be ok (it's already 90% percent of the late dissipation I think)
 
Last edited:
Re: Final biasing questions...

Ah well it worked, so all's good.

Well... worked... My sound kind of improved. But I have VERY OPEN Bass, allover the place, flabby... The distortion isn't great either. The sound is less fizzy than it used to but still a bit. I'm still waiting for 2 new preamp tubes, maybe that'll solve things. But I don't like its overall sound. I biased it colder just before playing it by the way but I didn't measure, however I must be at around 10 or 10.5.
Do you properly hear the difference between tubes biased cold or warm only when the amp is cranked up? Or at bedroom level is it possible too?
Also, how long do tubes usually take to warm up..? I usually just plug in and play (no standby switch) and the sound takes about 5 seconds to appear..
 
Re: Final biasing questions...

Cool thing Pierre! :bowdown:

What does biasing cost usually or should cost? I hope I´ll get a tube amp soon..just to know, you know? ;)
 
Re: Final biasing questions...

You should wait at least 30s before you play or raise the volume on the amp!!
The high B+ Voltage is not good for the tubes as long as the electron cloud isn't built on! The tube life is shorten!! There is another thread howto use a tube amp without Standby switch!!

Whicht amp? Which tubes did you put in? You don't like the sound? Did you liked it more with the old tubes? Remember the tubes need some time to break in!

Did you used a probe or what?
 
Re: Final biasing questions...

Yep I used a probe. Here's what I did basically...
Turned the amp off, let it cool for a night unplugged, then I plugged in the probe, turned the amp on, took the plate voltage (pin 5) and checked in which direction I had to turn the bias pot to the highest negative voltage, I turned it all the way down, I inserted the power tubes, took pin 3 reading, the reading across the resistor (so pin 8, but I had a probe in), then changed the bias and multiplied the two latest values as I changed it to get closer to where I wanted it.

Tubes are 2 of the VERRRRYYYY OLD ECC/12AX7 in slots 2 and 3, and 2 brand new JJ ECC83S (high gain) in 1 and 4, and I'm waiting for 2 Ruby (almost new) ECC83 that'll replace the older ones. The Rubys are pretty low gain tubes. Tube 1 is the preamp 1 with an extra boost, tube 2 is preamp 2, tube 3 is the phase inverter + Bass/treble boosts, 4 is the driver.
Power tubes are 2 JJ6V6s. Biased average for now.
 
You must log in or register to reply here.

Similar threads

F
Ceriatone 18W Questions
Replies
0
Views
188
fretburner
F
Securb
Cook 'em or keep 'em?
Replies
13
Views
566
Ascension
Ascension
S
Tech' ing gear or not
Replies
15
Views
259
Chistopher
C
ErikH
NAD - Boss Nextone Stage
Replies
16
Views
2K
Mincer
Mincer
8
JCM600...what I think
Replies
8
Views
472
Securb
Securb
Back
Top