thefragger
Certified Crazy.
Using V=IR, I think you mean a 1k resistor... ?
0.23V = 0.023A * 10R
*edit* my bad! completely misread what you meant!
It could be because the smaller the measurement gets, the less accurate the meter acts... ?
Scott 222C
- Thread starter thefragger
- Start date
Using V=IR, I think you mean a 1k resistor... ?
0.23V = 0.023A * 10R
*edit* my bad! completely misread what you meant!
It could be because the smaller the measurement gets, the less accurate the meter acts... ?
NOSValves
Super Member
Boy that is close in fact with the current set opf output tubes it's probably just fine.
For future use of output tuebs they may require less negative voltage to bias up I'd change the resistor to ground value to 10K ...I bet that give you about -10/-11 to -21 which is just great.
Get ready to fire it up and rock out
Oh once you get the tubes biased nice and even the .23V on the 10 ohm cathode rsistors report the B+ power supply voltages... I bet they are still a bit high but not enough to worry about.
when you fire it up do it first at about 70 Vac just to be sure all your changes worked. Let it warm up and check the bias current and use some cheap speakers and a music source and see if it plays music at 70V then slowly ramp it up to 117V.
If your home has high wall voltage it would be advisable to figure a way to knock it down to 120V or lower. A nice used ISOTAP does a nice job for just one vintage amplifier. You need one rated at 400VA. They can be source pretty cheap via ebay.
For future use of output tuebs they may require less negative voltage to bias up I'd change the resistor to ground value to 10K ...I bet that give you about -10/-11 to -21 which is just great.
Get ready to fire it up and rock out
Oh once you get the tubes biased nice and even the .23V on the 10 ohm cathode rsistors report the B+ power supply voltages... I bet they are still a bit high but not enough to worry about.
when you fire it up do it first at about 70 Vac just to be sure all your changes worked. Let it warm up and check the bias current and use some cheap speakers and a music source and see if it plays music at 70V then slowly ramp it up to 117V.
If your home has high wall voltage it would be advisable to figure a way to knock it down to 120V or lower. A nice used ISOTAP does a nice job for just one vintage amplifier. You need one rated at 400VA. They can be source pretty cheap via ebay.
NOSValves
Super Member
Pretty simple the lower the voltage the more inaccurate most affordable meters are at reading it! It's really not confusing. Just drop the decimal point and add mA to the number. Couldn't be more simple... In fact with a 1 ohm you have more zeroes and a decimal points to deal with.
thefragger
Certified Crazy.
Alright, so now in order to correctly bias this, I want the two sets of tubes each in their own channel and not mixed? (ie. chan1: 13 + 13 and chan2: 15 + 15, not 13 + 15 and 13 + 15)
I basically balance the control (pin 2) voltage and start playing with the negative bias pot until I measure 0.23V across that 10ohm resistor?
Holy moley I'm excited!!
I basically balance the control (pin 2) voltage and start playing with the negative bias pot until I measure 0.23V across that 10ohm resistor?
Holy moley I'm excited!!
NOSValves
Super Member
Yea I guess I should give you the biasing proceedure 
Yea run the 2 15 Ma in one channel and the two 13mA in the other. It really doesn't matter all that much. You could throw any of the tubes you have in it and most likely get them all the bias.
So before you turn the amp on balance the voltages to the grids and then adjust it to -17 volts with the new bias controls in both channels. Fire up the amp and check the bias of all four tubes to make sure everything is working as it should. if you were firing it up at full voltage I would just check to see that no tubes are above the .23V point and let it warm up and play music for a 1/2 hour or so. Then turn music down and do the following. Below is my standard instruction I send to my customers for Scott model 222C. I include a detailed picture with the written instructions. DO NOT SET THE BIAS TO .23V at 70 VAC it would be extremely high when you reach full AC voltage
You need a descent multi meter with varying scales for DC voltage to properly adjust the bias. Digital is preferred with varying scales.
Set your Meter to its lowest DC voltage setting 1VDC or above. Connect Neg probe (black) to one of the test points for the channel to be balanced and the positive probe (red) to the other (doesn't matter which one goes to which) adjust the DC balance for that channel to lowest possible reading or zero. Then repeat for the other channel. Now move your neg. probe (black) to anywhere on the metal chassis (ground) put positive lead to either test point for the channel to have its bias voltage adjusted. Set the meter to 1VDC or above. Set the bias voltage pot for that channel to read .220 then do the same to the other channel. When adjusting these voltages for one channel this will also effect the voltage on the other channel since they share the same voltage source so keep repeating until both channels read .220, go back checking balance as you go. You can play with the bias setting from .22 to .250 maximum but the higher you go the hotter the amp will run and the more stress your putting on the tubes and transformers. In fact if you don't listen at higher volumes set it to .22 VDC your tubes will love you for it!
This should be done every few months or when new output tubes are installed. Also with brand new tubes it should be done every 2 weeks for the first month until they burn in and stabilize.
Yea run the 2 15 Ma in one channel and the two 13mA in the other. It really doesn't matter all that much. You could throw any of the tubes you have in it and most likely get them all the bias.
So before you turn the amp on balance the voltages to the grids and then adjust it to -17 volts with the new bias controls in both channels. Fire up the amp and check the bias of all four tubes to make sure everything is working as it should. if you were firing it up at full voltage I would just check to see that no tubes are above the .23V point and let it warm up and play music for a 1/2 hour or so. Then turn music down and do the following. Below is my standard instruction I send to my customers for Scott model 222C. I include a detailed picture with the written instructions. DO NOT SET THE BIAS TO .23V at 70 VAC it would be extremely high when you reach full AC voltage
You need a descent multi meter with varying scales for DC voltage to properly adjust the bias. Digital is preferred with varying scales.
Set your Meter to its lowest DC voltage setting 1VDC or above. Connect Neg probe (black) to one of the test points for the channel to be balanced and the positive probe (red) to the other (doesn't matter which one goes to which) adjust the DC balance for that channel to lowest possible reading or zero. Then repeat for the other channel. Now move your neg. probe (black) to anywhere on the metal chassis (ground) put positive lead to either test point for the channel to have its bias voltage adjusted. Set the meter to 1VDC or above. Set the bias voltage pot for that channel to read .220 then do the same to the other channel. When adjusting these voltages for one channel this will also effect the voltage on the other channel since they share the same voltage source so keep repeating until both channels read .220, go back checking balance as you go. You can play with the bias setting from .22 to .250 maximum but the higher you go the hotter the amp will run and the more stress your putting on the tubes and transformers. In fact if you don't listen at higher volumes set it to .22 VDC your tubes will love you for it!
This should be done every few months or when new output tubes are installed. Also with brand new tubes it should be done every 2 weeks for the first month until they burn in and stabilize.
thefragger
Certified Crazy.
Excellent--thanks Craig for the procedure!
I swapped out the 15k for a 10k and the new(old) ranges are as follows;
tubes 1-2: -12.21V(-13.26V) to -23.40V(-21.20V)
tubes 3-4: -11.28V(-12.47V) to -23.40V(-21.15V)
I can measure the B+ from between ground and the OPT C/T, right?
I swapped out the 15k for a 10k and the new(old) ranges are as follows;
tubes 1-2: -12.21V(-13.26V) to -23.40V(-21.20V)
tubes 3-4: -11.28V(-12.47V) to -23.40V(-21.15V)
I can measure the B+ from between ground and the OPT C/T, right?
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thefragger
Certified Crazy.
WOO!
Gorgeous, beautiful sound! Got everything biased up to 23mA--those big meters are on sale again so I plan to nab a couple more, multiple meters is definitely a plus! I had to go back and fourth between both pairs because when one side was biased up the other would drift low, but they're both good now!
I AM EXCITE!
Now I'm letting everything cool down so I can put the faceplate and bottom shield back on. This one's a wrap!
Gorgeous, beautiful sound! Got everything biased up to 23mA--those big meters are on sale again so I plan to nab a couple more, multiple meters is definitely a plus! I had to go back and fourth between both pairs because when one side was biased up the other would drift low, but they're both good now!
I AM EXCITE!
Now I'm letting everything cool down so I can put the faceplate and bottom shield back on. This one's a wrap!
NOSValves
Super Member
Good Job! I like a success story! The back and forth is entirely normal. When you have 4 individual controls the back and forth is relentless.. since you have to do it with 4. Did you recheck the balance? It usually drifts a bit as you raise the current. Which should give you an example why I stated most affordable tube testers are useless for matching tubes.
thefragger
Certified Crazy.
I kept checking the bias back and forth until it stopped moving.
The meters are on sale right now at Canadian Tire for $18 from their regular $65. Now sure if you've got that store down in the US though... :scratch2: I can ship you a couple if you want?
I cannot begin to thank you enough, Craig! I really really appreciate all the help and direction that you provided me with!!
The meters are on sale right now at Canadian Tire for $18 from their regular $65. Now sure if you've got that store down in the US though... :scratch2: I can ship you a couple if you want?
I cannot begin to thank you enough, Craig! I really really appreciate all the help and direction that you provided me with!!
NOSValves
Super Member
Well so much for that idea! I did not realize you were in Canada. I won't be in Canada until May for my yearly Northern Ontario Fishing Vacation. Been doing that since I was a little kid, another great habit I picked up from my Dad.
No problem I enjoy helping out now and then. Escpecially when the arm chair guy's stay out of the way
. I really have little pateince for spending time debunking internet forum arm chair experts opinion's.
Craig
No problem I enjoy helping out now and then. Escpecially when the arm chair guy's stay out of the way
. I really have little pateince for spending time debunking internet forum arm chair experts opinion's. Craig
thefragger
Certified Crazy.
Oh, also my B+ at 117V input was measured at 438V after biasing the tubes--a little bit higher than spec, but I don't see it being a problem. :thmbsp:
thefragger
Certified Crazy.
It sounds even better with RCA 12AX7s compared to the Sovteks I was testing with! I was literally floored--Israel Kamakawiwoʻole jumped right out of my speakers and was singing for me!! NO LIE!
I'm very excited!
I'm very excited!
tube-a-lou
Addicted Member
Glad to hear you got her done. I bet it sounds wonderful. Once you get the sound
of these going you can't stop. I was reading the thread and between you and Graig
it was really informative. Enjoy it.
Tube
P.S. Don't forget pictures of the finished Scott
of these going you can't stop. I was reading the thread and between you and Graig
it was really informative. Enjoy it.
Tube
P.S. Don't forget pictures of the finished Scott
thefragger
Certified Crazy.
Here's a bunch of photos, detailing the rats-nest I added with the help of Craig :thmbsp:
Working on the wood case is the next part!
Working on the wood case is the next part!
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tube-a-lou
Addicted Member
It Looks Really Nice, enjoy it, I'm enjoying mine.
Tube
Tube
thefragger
Certified Crazy.
Here's some photos of the firebottles;
And then that dinky 1A fuse in the VARIC popped again, but she sounded great for the hour or so she was playing for!
I need a more permanent solution for stepping the voltage down!
And then that dinky 1A fuse in the VARIC popped again, but she sounded great for the hour or so she was playing for!
I need a more permanent solution for stepping the voltage down!
thefragger
Certified Crazy.
Thank you
Quick question--is it normal for that cap in the centre of all the tubes to get moderately warm? I think it's okay since it's surrounded by tubes, but just wanted to ask.
Quick question--is it normal for that cap in the centre of all the tubes to get moderately warm? I think it's okay since it's surrounded by tubes, but just wanted to ask.