Bamadawg
Member
Sorry for the newbish question. I have used tube gear over the years but have a lil Stromberg-Carlson EL84/7189A power amp coming that allows bias voltage and bias balance tweaking. Amps I have used were self-biasing I guess because I never had to adjust biasing. The amp was refreshed by Bruce Brown @ Vintage Tube Audio and he provided instructions to set the voltage and balance. I have the SAMS sheet that also provides adjustment instructions but they are a little different than Bruce provided. I have to admit that I am rather dyslexic when it comes to reading, but once I see something done... I never forget it. I just want to make sure I use my Fluke in the proper way to adjust the voltage and balance.
I think the way Bruce is having voltage adjusted is for a reduction in voltage due to modern lines voltage vs what was used in 1960 when the amp was designed. He has a target of -16v thru -22v with a balance of zero. The original instructions said to target the voltage to +.3v which I semi understand in the A/B amp typology to minimize distortion (I know there is another term, DUH moment). Bruce added new 10k pots which I am guessing are the pots that will determine bias voltage. His instructions are a bit different than the original. I will post the way he explained it. I fully understand the part about clipping alligators on the stand-up pins located behind the power tubes and adjusting the balance controls to achieve zero as the balance reading, or as close to it as possible. The other part to set voltage is a little unclear to me. I am not clear as to the 'pin probe wire' he references and if I should use a alligator lead to go from the + probe to that pin before I put the probe into the gray jacks of each channel to adjust for the -16v thru -22v.
Jus want to wrap my head around it before it gets here. A pic the seller had posted here before he had it rebuilt is included.
Many thanks in advance...
Nate
Instructions:
Here is the procedure for setting the bias and bias balance:
1- Turn meter on to the 20 volt DC range (left of off position)
2- Clip the black lead to the to the chassis (gnd)
3- Clip the red alligator clip to the pin probe wire
4- Insert the red lead in either of the gray pin jacks on top of the amp and
turn the pot closest to it to get -16 to -22 volts (see note above)
5- Repeat with the other channel
6- Now disconnect the red and black leads from the chassis and pin and
connect the alligator clips to the small ring terminals behind the
output tubes. (Be careful not to ground them and be careful of the
tubes they get hot)
7- Now adjust the bias balance control for a reading as close to 0 as you
can get. (Plus or minus up to 0.5 to 1 volt is OK)
8- Repeat with other channel
9- Disconnect meter, turn off, turn up the gain and enjoy!
I think the way Bruce is having voltage adjusted is for a reduction in voltage due to modern lines voltage vs what was used in 1960 when the amp was designed. He has a target of -16v thru -22v with a balance of zero. The original instructions said to target the voltage to +.3v which I semi understand in the A/B amp typology to minimize distortion (I know there is another term, DUH moment). Bruce added new 10k pots which I am guessing are the pots that will determine bias voltage. His instructions are a bit different than the original. I will post the way he explained it. I fully understand the part about clipping alligators on the stand-up pins located behind the power tubes and adjusting the balance controls to achieve zero as the balance reading, or as close to it as possible. The other part to set voltage is a little unclear to me. I am not clear as to the 'pin probe wire' he references and if I should use a alligator lead to go from the + probe to that pin before I put the probe into the gray jacks of each channel to adjust for the -16v thru -22v.
Jus want to wrap my head around it before it gets here. A pic the seller had posted here before he had it rebuilt is included.
Many thanks in advance...
Nate
Instructions:
Here is the procedure for setting the bias and bias balance:
1- Turn meter on to the 20 volt DC range (left of off position)
2- Clip the black lead to the to the chassis (gnd)
3- Clip the red alligator clip to the pin probe wire
4- Insert the red lead in either of the gray pin jacks on top of the amp and
turn the pot closest to it to get -16 to -22 volts (see note above)
5- Repeat with the other channel
6- Now disconnect the red and black leads from the chassis and pin and
connect the alligator clips to the small ring terminals behind the
output tubes. (Be careful not to ground them and be careful of the
tubes they get hot)
7- Now adjust the bias balance control for a reading as close to 0 as you
can get. (Plus or minus up to 0.5 to 1 volt is OK)
8- Repeat with other channel
9- Disconnect meter, turn off, turn up the gain and enjoy!
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