I hope this helps you...
Biasing the Power Tubes
As with all power amplifiers that are not self-biasing, hand biasing the twelve power tubes is essential for optimal performance. Jadis recommends that you bias the power tubes anywhere between 4 and 5 volts. Pierre has even experimented with biasing to 6 and higher.
Choosing the right bias for your musical taste is essential to getting the best from the DA-7. In my experience, the higher the bias, the richer the sound. But higher bias also means that tubes run hotter and burn out faster. It can also produce sound overly rich and full, to they extent that the amp begins to sound too sweet, warm, thick, or even syrupy (depending upon your choice of tubes).
It would be very nice if Jadis had made biasing the DA-7s twelve power tubes as simple as adjusting twelve individual settings on the top of the amp and reading the results via a built-in gauge. No such luck. Instead, Jadis diminutive bias adjustment screws are located inside the amp. You must turn off the amp, remove the protective tube cage on top, turn the amp on its side, remove the 9 screws that hold the bottom plate in place and get the plate out of the way before the bias adjustments become visible.
Once you turn the amp over, expose the adjustment screws, and turn the amp back on until it stabilizes, you must work with four adjustment screws, one for each row of three tubes, rather than twelve screws, one for each tube. Jadis forces you to bias an entire row of three tubes at a time. And while the screws are easy to turn, the tiniest movement can make a big difference in bias.
Given that no two power tubes measure exactly alike, and that bias requires readjustment as tubes age (differently), you must bias the three tubes in a given row to an average level, and recalibrate them every three or four months. To facilitate the process, you will find yourself mapping out the twelve tubes on a sheet of paper, writing down initial bias levels, and adjusting from there.
If you encounter a marked disparity in bias levels in a particular row of three tubes, you must then begin to swap tubes from row to row until the bias range in a particular row is as close as your particular assortment of twelve tubes allows. Assuming you dont burn a hand or drop a tube in the process, getting it right can be fun. Its a bit like a treasure hunt, and ideal for compulsive-obsessive personalities.
Its an especial joy when you discover that the bias of an individual tube is to some extent dependent upon the bias of the other two tubes in its row. When you move a tube that measures 5.3 volt to another row, it may suddenly measure 4.6. Hence what initially looks like a good move may not prove efficacious.
Moving power tubes between sockets and rows is not as intellectually challenging as playing chess, but it does require attention and patience. Much experimentation is required to get your twelve tubes positioned optimally. You may even have to purchase new tubes after you discover that eight tubes are easily adjustable while the four others variously measure much higher or much lower than the others. This becomes more of an issue as tubes age.
Operating a voltmeter (I use a Radio Shack) to measure the bias is easy. You put one lead on the chassis, another on the end of one of the twelve fuses (one for each power tube) easily accessible inside the amp.
Warning. Turning the bias screws is fraught with danger. Brush against the wrong bit of metal with a metal screwdriver and you may be shocked blue by 500 Volts. This may help cure your schizophrenia, but at the cost of the hair on your head or, if you dont have any . . . lets not go there.
The trick is to use a plastic screwdriver to adjust bias. Touch the wrong thing, and no harm is done.
Not so fast. Finding a plastic screwdriver in this day and age may require another little treasure hunt. It seems Radio Shack no longer stocks plastic screwdrivers. Nor do major automotive tool distributors (at least in N. California).
I spent an entire afternoon searching for a plastic screwdriver. I went to carpet stores, auto parts stores, a locksmith, hardware stores, you name it. I tried everything short of driving 45 minutes each way to a computer chain outlet that couldnt tell you what they stocked when you called them on the phone.
Finally, a local locksmith suggested I check out the old guy in the TV repair shop across the street who keeps irregular hours. Inside a virtual museum of television history, open for 60 years and stocked with more antiquated TV consoles than I wish to recount, stood a man well into his 80s who boasts that he still works 12 hours a day. Behind him, on his tool display, hung six long Zenith TV adjustment screwdrivers in their original packaging. One had a zip code for the Zenith address; one other was manufactured before zip codes came on the scene. I bought one of each size available. One fit the Jadis. I am still alive to tell the tale.
Models and Distribution
Biasing the Power Tubes
As with all power amplifiers that are not self-biasing, hand biasing the twelve power tubes is essential for optimal performance. Jadis recommends that you bias the power tubes anywhere between 4 and 5 volts. Pierre has even experimented with biasing to 6 and higher.
Choosing the right bias for your musical taste is essential to getting the best from the DA-7. In my experience, the higher the bias, the richer the sound. But higher bias also means that tubes run hotter and burn out faster. It can also produce sound overly rich and full, to they extent that the amp begins to sound too sweet, warm, thick, or even syrupy (depending upon your choice of tubes).
It would be very nice if Jadis had made biasing the DA-7s twelve power tubes as simple as adjusting twelve individual settings on the top of the amp and reading the results via a built-in gauge. No such luck. Instead, Jadis diminutive bias adjustment screws are located inside the amp. You must turn off the amp, remove the protective tube cage on top, turn the amp on its side, remove the 9 screws that hold the bottom plate in place and get the plate out of the way before the bias adjustments become visible.
Once you turn the amp over, expose the adjustment screws, and turn the amp back on until it stabilizes, you must work with four adjustment screws, one for each row of three tubes, rather than twelve screws, one for each tube. Jadis forces you to bias an entire row of three tubes at a time. And while the screws are easy to turn, the tiniest movement can make a big difference in bias.
Given that no two power tubes measure exactly alike, and that bias requires readjustment as tubes age (differently), you must bias the three tubes in a given row to an average level, and recalibrate them every three or four months. To facilitate the process, you will find yourself mapping out the twelve tubes on a sheet of paper, writing down initial bias levels, and adjusting from there.
If you encounter a marked disparity in bias levels in a particular row of three tubes, you must then begin to swap tubes from row to row until the bias range in a particular row is as close as your particular assortment of twelve tubes allows. Assuming you dont burn a hand or drop a tube in the process, getting it right can be fun. Its a bit like a treasure hunt, and ideal for compulsive-obsessive personalities.
Its an especial joy when you discover that the bias of an individual tube is to some extent dependent upon the bias of the other two tubes in its row. When you move a tube that measures 5.3 volt to another row, it may suddenly measure 4.6. Hence what initially looks like a good move may not prove efficacious.
Moving power tubes between sockets and rows is not as intellectually challenging as playing chess, but it does require attention and patience. Much experimentation is required to get your twelve tubes positioned optimally. You may even have to purchase new tubes after you discover that eight tubes are easily adjustable while the four others variously measure much higher or much lower than the others. This becomes more of an issue as tubes age.
Operating a voltmeter (I use a Radio Shack) to measure the bias is easy. You put one lead on the chassis, another on the end of one of the twelve fuses (one for each power tube) easily accessible inside the amp.
Warning. Turning the bias screws is fraught with danger. Brush against the wrong bit of metal with a metal screwdriver and you may be shocked blue by 500 Volts. This may help cure your schizophrenia, but at the cost of the hair on your head or, if you dont have any . . . lets not go there.
The trick is to use a plastic screwdriver to adjust bias. Touch the wrong thing, and no harm is done.
Not so fast. Finding a plastic screwdriver in this day and age may require another little treasure hunt. It seems Radio Shack no longer stocks plastic screwdrivers. Nor do major automotive tool distributors (at least in N. California).
I spent an entire afternoon searching for a plastic screwdriver. I went to carpet stores, auto parts stores, a locksmith, hardware stores, you name it. I tried everything short of driving 45 minutes each way to a computer chain outlet that couldnt tell you what they stocked when you called them on the phone.
Finally, a local locksmith suggested I check out the old guy in the TV repair shop across the street who keeps irregular hours. Inside a virtual museum of television history, open for 60 years and stocked with more antiquated TV consoles than I wish to recount, stood a man well into his 80s who boasts that he still works 12 hours a day. Behind him, on his tool display, hung six long Zenith TV adjustment screwdrivers in their original packaging. One had a zip code for the Zenith address; one other was manufactured before zip codes came on the scene. I bought one of each size available. One fit the Jadis. I am still alive to tell the tale.
Models and Distribution
