Bias question

[FUSER]

Hi Guys,

I am putting this question in this section, hopefully someone can help me.

I have an Allen Accomplice that is giving me the following reading across the bias measurement jacks and the ground.

Power tube are TAD6L6s.

Mr Allen recommends 20-20 mV for 6V6 and 30-35 mV for 6L6. So unless i am taking the readings wrong, they are way off. But to my ears the amp does sound fine. Would appreciate if someone could tell me if I am using the multimeter right and if so, should I mess with the bias pot to bring it up.

Thanks in advance.

http://www.geocities.com/syedh1232001/bias.jpg

[Blue Strat]

Everything looks ok (can't see where your probes are plugged in though). What measurement were you getting on the old tubes?

It goes to show that biasing isn't an exact science and that you can achieve decent sonic results WAY below 70% max dissipation.

Try bringing them up and listen to the results.

[FUSER]

Everything looks ok (can't see where your probes are plugged in though). What measurement were you getting on the old tubes?

It goes to show that biasing isn't an exact science and that you can achieve decent sonic results WAY below 70% max dissipation.

Try bringing them up and listen to the results.

Hi BlueStrat,

In the following pic, The test points for each power tube is on the back panel. So my red probe goes to the ground and the black probe into one of the red test points. The bias adjustment pot is under the chassis.

I am not changing tubes, just thought I would measure it and see whats going on, especially as the amp never heats up as a tube amp should
(brought this up on another thread previously).

I will try and take it upto 15mV at least and then go from there.

http://www.geocities.com/syedh1232001/acc3.jpg

[bob-i]

You're checking correctly. Assuming your meter is ok your bias is fairly low. Your ears are the final judge though, try it higher and see what your ears tell you. I've found that sometimes when things sound good, it can still be better, other times things are just fine like they are.

[MarkL8]

I dont understand have you adjusted the bias adjustment pot and that is as high as it will go? What range do you get when dialing the bias pot?

[Blue Strat]

I dont understand have you adjusted the bias adjustment pot and that is as high as it will go? What range do you get when dialing the bias pot?

I think he said (or implied) that he just checked the setting of the amp, as is, out of curiosity. Apparently, it's set at about 11mA proving the point that (despite web wisdom and regurgitation) cross over distortion rarely occurs until EXTREMELY low bias current settings.

[donnyjaguar]

This can be a tell-tale sign that the screen grid resistors are whipped. Nice meter, I can spot the Canadians on here. ;)

[Blue Strat]

This can be a tell-tale sign that the screen grid resistors are whipped.

Good call, maybe. Any history of trouble or blown tubes with this amp? Are you the original owner?

[FUSER]

Good call, maybe. Any history of trouble or blown tubes with this amp? Are you the original owner?

For Blue Strat and Donny,

No history of trouble that I am aware of. I am the second owner. The amp was "like new" as described the prev owner. I will try and contact him again, though it has been sometime since i bought it.

I have takes up the bias setting to 15 mV. This has improved the headroom. I am quite sure on this.

I guess i would have to open up the chassis to view if the above component is fried.
( Read the other thread on screen grid resistors).

The amp still sounds great, is quite like a mouse and after i took it to 15mV, it is slightly hotter than before.

[JJman]

With the amp turned off you could check the bias control to see if it is near the hottest setting. Just make sure you remember it's current position and return it there before powering back up to re-adjust.

If it's already very close to the hottest setting, you may have a problem. Otherwise, you probably merely had an amp with it's bias set too cold.

Watch the tubes, in a somewhat dark room when adjusting bias. Make sure you don't set them hot enough to create any orange/red spots on the plates themselves. Blue glow near glass is ok and heaters should be "red" when amp is on.

[FUSER]

With the amp turned off you could check the bias control to see if it is near the hottest setting. Just make sure you remember it's current position and return it there before powering back up to re-adjust.

If it's already very close to the hottest setting, you may have a problem. Otherwise, you probably merely had an amp with it's bias set too cold.

Watch the tubes, in a somewhat dark room when adjusting bias. Make sure you don't set them hot enough to create any orange/red spots on the plates themselves. Blue glow near glass is ok and heaters should be "red" when amp is on.

I checked this and the bias control is way far from its max. I had to turn about 1/3 on the pot to get to the min setting, so probably confirming that it is set cold.

As I said I will increase the bias a little bit (set to 15mV right now) and play it for sometime before moving to a higher setting till I reach the recommended values.

Thanks for all your help.