Flyers99 11/14/2017 6:37 AM
Crate GT-100H Tube Biasing
Hello all, new to the forum. I own a GT-100H Stealth Tube head and I tried searching for my amp on your forum but didn't find anything. I bought new EL34 tubes to put into my Crate GT-100H Stealth amp I've had since the mid 90's but I cannot find the bias adjustment on the circuit board. Anyone have any experience with these? I called Crate (now Loud Tech Inc I guess) and they couldn't help me. I was referred to a local service center and they couldn't (or wouldn't) give me any info either. Anyone know how to bias these tubes on this amp? I have uploaded the manual and schematic and they do not show the bias points (or I can't find them?). The manual says these can ONLY be done by an authorized service center. Anyone know the trick to biasing the tubes here? Any help is greatly appreciated.
 
J M Fahey 11/14/2017 7:28 AM
There is a bias pot called AP1 .
That said, all they specify is that bias is set to -45V with no further explanation.
Specifically there are no "current measuring" test points, so I *guess* :
1) Factory loads these amps with matched tubes, so they set bias once and leave it there.
Supposedly Service Techs replace tubes and readjust bias as needed, so they donīt feed the need to help end users do it.
By the way, what thousands of Manufacturers did along the last 60 years; user accessible bias test points are an "extra feature" added to a few amplifiers, not a standard by any means.
2) again guess Service Techs start with the suggested -45V setting (FWIW Fender specified -52V , period, on many of their amps, and that with 6L6), and if needed fine tune bias using an oscilloscope, not exactly an user friendly system.
Personally I would check -45V is available at the junction R75/R76 and touch preset only if not.

If you want a finer , more "modern" adjustment , you will need to add cathode current sensing resistors and use them .
Personally, unless amp hums with all controls set to zero, one or more tubes get red hot or output is weak and nthin, I would leave as is.
Or get a scope and use that procedure.
Or have a Tech do it for you.