845 bias question

seanrr

Well-Known Member
I have a pair of Antique Sound Labs 1006-845 DT mono block amps. They have been heavily modified by a former owner, and they sound fantastic. I have been using the 845 tubes that came with it, but I have been thinking to upgrade my tubes for awhile. Rather than dish out $800 for the ones I would really like, I took a chance on some used ones (claimed to have been used for only 10 hours) off of eBay - these are Golden Dragon Premium and look pretty much like Shunguang 845B tubes. I popped them in yesterday when they came, but I was only able to flip the amps on this morning. Without listening to the amps, I found that it was difficult to get one of the two tubes in a bias range below 800. I believe 800 is recommended for the amp, which has a digital bias meter. Dialing the bias all the way back only brought me to about 815. Does this suggest a problem with the tube? Would it be dangerous to drive them at that bias? If not, should I put both amps at the same bias (around 815)?
 
The tubes appear to be not extremely well matched, and the bias adjustments in your amps were not designed to provide that much adjustment swing. Adjust them both to the same value, as close to the manufacturer recommended setting as you can, and enjoy. You will probably no get quite as much max power output, but I doubt you will even notice.
 
The tubes appear to be not extremely well matched, and the bias adjustments in your amps were not designed to provide that much adjustment swing. Adjust them both to the same value, as close to the manufacturer recommended setting as you can, and enjoy. You will probably no get quite as much max power output, but I doubt you will even notice.

Thanks! That is very helpful.
 
I have a pair of Antique Sound Labs 1006-845 DT mono block amps. They have been heavily modified by a former owner, and they sound fantastic. I have been using the 845 tubes that came with it, but I have been thinking to upgrade my tubes for awhile. Rather than dish out $800 for the ones I would really like, I took a chance on some used ones (claimed to have been used for only 10 hours) off of eBay - these are Golden Dragon Premium and look pretty much like Shunguang 845B tubes. I popped them in yesterday when they came, but I was only able to flip the amps on this morning. Without listening to the amps, I found that it was difficult to get one of the two tubes in a bias range below 800. I believe 800 is recommended for the amp, which has a digital bias meter. Dialing the bias all the way back only brought me to about 815. Does this suggest a problem with the tube? Would it be dangerous to drive them at that bias? If not, should I put both amps at the same bias (around 815)?

I do not know how digital values related to actual current in this amplifier. But for 845 tubes idle current can be chosen in range from 75 to 105 mA. Tubes with metal plates have limit at 80 mA, and tubes with graphite plate can run all way up to 105. Best current value depends on plate voltage - the higher voltage, the lower current. You need to keep idle power at around 100 watts.
 
Thanks! That is very helpful.
Since you do not have any leeway on the bias adjustment you need to be careful that the tube doesn't start to redplate and runaway. That would make me kinda paranoid and probably start looking for a better pair of tubes.
 
I tried them again this morning, and they seemed to not have the same issue. I am assuming the digital readout of 800 refers to 80 Ma, but if there are other ASL 1006-845 owners out there with the same version, perhaps they can verify. I will keep a close watch on them. Thanks for all the input.
 
Since you do not have any leeway on the bias adjustment you need to be careful that the tube doesn't start to redplate and runaway. That would make me kinda paranoid and probably start looking for a better pair of tubes.
Graphite plates in 845 can develop dull red color while running. This is normal for them. They are very stable - once set current does not require adjustment for hundreds hours of operation. If current suddenly changed, it is likely that something else in the circuit failed, not the tube.
 
Back
Top Bottom