Auto Bias board documentation?

Hi All,

I just picked up a Bob Latino ST120 with all the upgrades used and it has the auto bias board. It also has digital meters in place of the original from sockets which is pretty cool eye candy... that said, is there any documentation on the auto bias board? I'm showing it steady at .50, but will fluctuate up when being pushed harder. seen it as high as .57 using Sovtek 6550WE's. is that normal?

Interstingly the first time I fired it up it had a bigger fluctuation in bias reading, but I had it turned off for couple hours and turned it on now and it seems to be way more steady around .50 never going over .52

Due to the damping factor in the control loop of the AB board you will see the "bias" voltages fluctuate as music is played. This is normal as the tubes do their job.

The AB control loop time is in the order of seconds so to accurately see what standing bias current you have you need to check during times of no audio.

With respect to the meter readings you are seeing, how accurate are the meters fitted to your amp?

here's a link to the documentation . . .
http://www.wop.cz/files/AB-Q_ST-70_ST120_manual.pdf

tubes4hifi wrote:here's a link to the documentation . . .
http://www.wop.cz/files/AB-Q_ST-70_ST120_manual.pdf

Thanks!

pichacker wrote:Due to the damping factor in the control loop of the AB board you will see the "bias" voltages fluctuate as music is played. This is normal as the tubes do their job.

The AB control loop time is in the order of seconds so to accurately see what standing bias current you have you need to check during times of no audio.

With respect to the meter readings you are seeing, how accurate are the meters fitted to your amp?

Honestly I'm not sure how accurate they are. at idle, it will constantly read ~.50

I'll try to find out who installed them and which ones they used.

Kray wrote:Hi All,

I just picked up a Bob Latino ST120 with all the upgrades used and it has the auto bias board. It also has digital meters in place of the original from sockets which is pretty cool eye candy... that said, is there any documentation on the auto bias board? I'm showing it steady at .50, but will fluctuate up when being pushed harder. seen it as high as .57 using Sovtek 6550WE's. is that normal?

Interstingly the first time I fired it up it had a bigger fluctuation in bias reading, but I had it turned off for couple hours and turned it on now and it seems to be way more steady around .50 never going over .52

1. if there is no signal (music), the AB sets the bias voltage according to the bias level set by the trimmer
2. When a signal is present, the bias voltage is set to the linear portion of the tubes anode characteristic
3. Depending on the signal level, the AB adjusts that level along the linear path and tries to keep it at the lowest region of the anode current. This saves tubes.
4. With points 1 and 2, the AB WILL ALWAYS set the exact anode current in the primary winding of the output transformer.
Pavel

pavlikkkk wrote:
1. if there is no signal (music), the AB sets the bias voltage according to the bias level set by the trimmer
2. When a signal is present, the bias voltage is set to the linear portion of the tubes anode characteristic
3. Depending on the signal level, the AB adjusts that level along the linear path and tries to keep it at the lowest region of the anode current. This saves tubes.
4. With points 1 and 2, the AB WILL ALWAYS set the exact anode current in the primary winding of the output transformer.
Pavel

Thanks for the great explanation Pavel. So it would be common to see this level get up into the .70's? peak?

Kray wrote:

pavlikkkk wrote:
1. if there is no signal (music), the AB sets the bias voltage according to the bias level set by the trimmer
2. When a signal is present, the bias voltage is set to the linear portion of the tubes anode characteristic
3. Depending on the signal level, the AB adjusts that level along the linear path and tries to keep it at the lowest region of the anode current. This saves tubes.
4. With points 1 and 2, the AB WILL ALWAYS set the exact anode current in the primary winding of the output transformer.
Pavel

Thanks for the great explanation Pavel. So it would be common to see this level get up into the .70's? peak?

yes, but only for a moment, depending on the signal strength.
Pavel