soundbound
Well-Known Member
Will someone please reply or send me the instructions to how I adjust my Scott 299C two Bias pots and two DC Balance pots, or where I can get the instructions?
Thank you.
Thank you.
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How do I adjust my Scott 299C Bias and DC Balance pots?
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petes'picks
Super Member
soundbound
Well-Known Member
Thank you! That's the DC Balance pots adjustments and schematics. Although it states: (C) Remove the phase inverter tube 7199 of the output stage under test. Mine are 7591 output tubes, so does that make those the wrong instructions for mine? There are different versions of the 299C.
If that works for mine, all I need now is the Bias pots adjustments instructions, because my version has both.
Two photos of it included.
Attachments
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SB -- It is hard to tell from your pics: Follow the wiring from pin #5 of each output tube. Do the these pins connected together on the tubes in each channel, and then go to ground through a 3.3 ohm 1 W resistor (Or-Or-Gold)? Or do these pins connect directly to ground? If the resistors are used, there would be one resistor for each channel.
Dave
Dave
petes'picks
Super Member
soundbound
Well-Known Member
Thank you petes'picks.
soundbound
Well-Known Member
petes'picks
Super Member
You got lucky, I was going through a lot of Sams I picked up this weekend and ran across this one today after seeing your post last night.
Also follow the advice of Dave, he knows his stuff.
Also follow the advice of Dave, he knows his stuff.
soundbound
Well-Known Member
soundbound
Well-Known Member
All output tubes pins 5 are connected together by thin black wires. I don't see them connected to ground through a orange-orange-gold 3.3 ohm resistor.
They do not look connected directly to ground.
A better photo included with this reply. In photo you can see pin 5 on tube second from left with flash on it and all pins 5s are in same orientation. They have resistors with a yellow-purple-yellow-silver bands connected to them. Right side of resistors are soldered to pins 5. Zoom in for closer details.
Any more questions, please ask.
Attachments
SB -- I'm sorry to keep dragging this out, but it's hard to determine what I'm looking for in the pics.
The output transformers have two groups of wires coming from each of them. Within the group of three wires from each transformer, there is a red wire. It looks as if these red wires are each connecting to a 1W resistor of (again), about 3.3 ohms (Or-Or-Gold), Can you verify this fact?
The point is, that if Scott provided bias controls for each output stage, then they must have provided a way to correctly adjust it. The usual way that Scott did this on their 7591 amplifiers is to connect the cathode elements (pin #5) of the two tubes in each channel together, and ground them through a 3.3 ohm resistor, which you would take the measurement across. However, you can nearly accomplish the same thing if the same resistor is placed in series with the red lead of each output transformer. We're just trying to determine what provisions Scott provided to in fact make the adjustment with.
While you're under there, you might use your DVM to verify that there is in fact no resistance between pin #5 of the output tubes, and the chassis. The reading indicated should basically be the same as the reading produced when the DVM's leads are simply shorted together.
Let us know!
Dave
The output transformers have two groups of wires coming from each of them. Within the group of three wires from each transformer, there is a red wire. It looks as if these red wires are each connecting to a 1W resistor of (again), about 3.3 ohms (Or-Or-Gold), Can you verify this fact?
The point is, that if Scott provided bias controls for each output stage, then they must have provided a way to correctly adjust it. The usual way that Scott did this on their 7591 amplifiers is to connect the cathode elements (pin #5) of the two tubes in each channel together, and ground them through a 3.3 ohm resistor, which you would take the measurement across. However, you can nearly accomplish the same thing if the same resistor is placed in series with the red lead of each output transformer. We're just trying to determine what provisions Scott provided to in fact make the adjustment with.
While you're under there, you might use your DVM to verify that there is in fact no resistance between pin #5 of the output tubes, and the chassis. The reading indicated should basically be the same as the reading produced when the DVM's leads are simply shorted together.
Let us know!
Dave
soundbound
Well-Known Member
Dave,
No apology is necessary. I know it can be frustrating helping those who's knowledge is lacking some. You're taking your God given time to help me and I am very grateful to you and all those who ever help me out in any way! This is important to me to get right and without help I will not be able to do so.
* Yes, each red wire of the two groups of three from the two output transformers are connected to 3.3 ohm resistors. Picture included as reference.
I did not see any 3.3 ohm resistors connected directly to the output tubes very late last night, but should have looked more and seen them on the output transformers. I'm sorry about that.
I included a second photo of the area of the output tubes and transformers too, if helpful to you for any reason.
My camera is a low cost point and shoot and its photos are not the highest quality making this more difficult and frustrating for you which I'm sorry for too.
* I measured no resistance between each of the four output tubes pin's #5 and chassis ground.
Thank you very much for your continued and patient help which I'm very grateful for!
No apology is necessary. I know it can be frustrating helping those who's knowledge is lacking some. You're taking your God given time to help me and I am very grateful to you and all those who ever help me out in any way! This is important to me to get right and without help I will not be able to do so.
* Yes, each red wire of the two groups of three from the two output transformers are connected to 3.3 ohm resistors. Picture included as reference.
I did not see any 3.3 ohm resistors connected directly to the output tubes very late last night, but should have looked more and seen them on the output transformers. I'm sorry about that.
I included a second photo of the area of the output tubes and transformers too, if helpful to you for any reason.
My camera is a low cost point and shoot and its photos are not the highest quality making this more difficult and frustrating for you which I'm sorry for too.
* I measured no resistance between each of the four output tubes pin's #5 and chassis ground.
Thank you very much for your continued and patient help which I'm very grateful for!
Attachments
OK. Now we're getting somewhere.
For the record: If in fact you measured "no" resistance between pin #5 of the output tube sockets and ground, then that is technically the same as saying that these pins are connected directly to ground, as then there would be no resistance at all, but a direct connection, or dead short. The indication would be the same as if you touched the probes of your DVM directly together.
If on the other hand you got no reading at all when checking the pin #5 reading to the chassis, then that is said to be a condition of infinite resistance, just as if your probes were connected to nothing. This would be the same thing as an open circuit, meaning that no current can flow at all.
I will assume then that you got a reading between pin 5 and the chassis, that is basically the same as if you touched the two probes directly together. This test is performed with the unit turned off, and your DVM set to "Ohms".
OK. As for adjusting the bias controls, read these instructions all the way through first before attempting the adjustments. First set the Balance controls for minimum hum with the phase inverter tubes removed as previous discussed. Then:
1. Set your DVM to the lowest DC volts scale, and using clips, clip the red probe to the side of the 3.3 ohm resistor that is NOT connected to the red OPT lead, for the channel you are adjusting. Connect the black probe to the other side of the same 3.3 ohm resistor. THESE VOLTAGE POINTS REPRESENT VERY HIGH VOLTAGE WHEN THE UNIT IS ON, SO THE NEED FOR BEING DOUBLE CAREFUL IS AN UNDERSTATEMENT!! If possible, connect your meter leads BEFORE applying power to start the Balance adjustment.
2. Operate your DVM on battery power only -- do not use an AC adapter, as the black meter lead will be connected to a point in the circuit that is operating at about 450 vdc above chassis ground. Some AC adapters will break down when the DVM is connected into a circuit such high common lead voltage levels.
3. After the Balance control is set, adjust the Bias control for a reading of 0.23 vdc on your DVM.
4. Repeat the same procedure in the other channel.
There will be some interaction between the Balance and Bias controls, not only within a given channel, but between the channels as well, so it may take a number of adjustment cycles to bring the reading into compliance on both channels, when the Balance controls are set for minimum hum. The reading doesn't need to be exactly .23 vdc, and it will vary somewhat with AC line voltage and heating, etc. If the voltage is within a .22 to .24 vdc range, it is fine. The settings should be made of course with no signal applied -- which wouldn't be produced with the phase inverter tubes removed anyway. Those tubes can remain removed when the Bias control is set, since they were already removed to perform the Balance adjustment.
I hope this helps!
Dave
For the record: If in fact you measured "no" resistance between pin #5 of the output tube sockets and ground, then that is technically the same as saying that these pins are connected directly to ground, as then there would be no resistance at all, but a direct connection, or dead short. The indication would be the same as if you touched the probes of your DVM directly together.
If on the other hand you got no reading at all when checking the pin #5 reading to the chassis, then that is said to be a condition of infinite resistance, just as if your probes were connected to nothing. This would be the same thing as an open circuit, meaning that no current can flow at all.
I will assume then that you got a reading between pin 5 and the chassis, that is basically the same as if you touched the two probes directly together. This test is performed with the unit turned off, and your DVM set to "Ohms".
OK. As for adjusting the bias controls, read these instructions all the way through first before attempting the adjustments. First set the Balance controls for minimum hum with the phase inverter tubes removed as previous discussed. Then:
1. Set your DVM to the lowest DC volts scale, and using clips, clip the red probe to the side of the 3.3 ohm resistor that is NOT connected to the red OPT lead, for the channel you are adjusting. Connect the black probe to the other side of the same 3.3 ohm resistor. THESE VOLTAGE POINTS REPRESENT VERY HIGH VOLTAGE WHEN THE UNIT IS ON, SO THE NEED FOR BEING DOUBLE CAREFUL IS AN UNDERSTATEMENT!! If possible, connect your meter leads BEFORE applying power to start the Balance adjustment.
2. Operate your DVM on battery power only -- do not use an AC adapter, as the black meter lead will be connected to a point in the circuit that is operating at about 450 vdc above chassis ground. Some AC adapters will break down when the DVM is connected into a circuit such high common lead voltage levels.
3. After the Balance control is set, adjust the Bias control for a reading of 0.23 vdc on your DVM.
4. Repeat the same procedure in the other channel.
There will be some interaction between the Balance and Bias controls, not only within a given channel, but between the channels as well, so it may take a number of adjustment cycles to bring the reading into compliance on both channels, when the Balance controls are set for minimum hum. The reading doesn't need to be exactly .23 vdc, and it will vary somewhat with AC line voltage and heating, etc. If the voltage is within a .22 to .24 vdc range, it is fine. The settings should be made of course with no signal applied -- which wouldn't be produced with the phase inverter tubes removed anyway. Those tubes can remain removed when the Bias control is set, since they were already removed to perform the Balance adjustment.
I hope this helps!
Dave
soundbound
Well-Known Member
Dave,
The ohm readings I got while metering the four tubes # 5 pins to chassis were as the probes were touching showing on my Radio Shack digital multi meter as a .0 ohms reading. Open circuit shows up as 0.F M ohms display.
I have a set of meter wires with clips that function well and have been and will be using them.
I will be as careful as I can when doing all of this with the high voltages to hopefully not damage me or the 299C!
My meter only has battery operation. It has no AC adapter ability, so that problem with no arise, but thank you for informing me because I would have not thought of that and will be careful if ever using a meter with an AC adapter. I'm learning a lot. Thank you.
After reading your post I went and verified each of the four #5 pins are connected to chassis ground by using the meter's diode setting and got a tone from each attempt indicating that each of the four #5 pins are connected to chassis. It tones during closed circuit.
The settings will be adjusted with no signal applied.
I do have a set of old speakers wired to it, because I thought the output transformers may need to be loaded. I don't care if the speakers get damaged. I'd rather them get damaged than the 299C. Do I need to have a load on the output transformers at all times when it's on and even during adjustments? I've read that you should never turn on a tube component with no speakers, because that can cause damage to it from no resistance load on its outputs.
To verify, the phase inverter tubes that I remove during the adjustments are the two 6U8A tubes that are between its front four 12AX7A preamp tubes and its four 7591 output tubes?
I will do all you have instructed me to with its old tubes in to learn and see how close its adjustments are now. As you can see, its seller restored and adjusted it, so it should all measure close. I've owned it two years. Its right side plays a little weaker. Even though its seller stated its tubes have lots of life left in them, I want to put some new JJ tubes in it to see if they have it sounding better and its right side equal in volume. I've only owned a few vintage tube components, but in my few experiences new tubes always seem to make improvements. When I soon buy its new tubes, I now know how to adjust its pots.
I'll update as I go.
Thank you so very much!
The ohm readings I got while metering the four tubes # 5 pins to chassis were as the probes were touching showing on my Radio Shack digital multi meter as a .0 ohms reading. Open circuit shows up as 0.F M ohms display.
I have a set of meter wires with clips that function well and have been and will be using them.
I will be as careful as I can when doing all of this with the high voltages to hopefully not damage me or the 299C!
My meter only has battery operation. It has no AC adapter ability, so that problem with no arise, but thank you for informing me because I would have not thought of that and will be careful if ever using a meter with an AC adapter. I'm learning a lot. Thank you.
After reading your post I went and verified each of the four #5 pins are connected to chassis ground by using the meter's diode setting and got a tone from each attempt indicating that each of the four #5 pins are connected to chassis. It tones during closed circuit.
The settings will be adjusted with no signal applied.
I do have a set of old speakers wired to it, because I thought the output transformers may need to be loaded. I don't care if the speakers get damaged. I'd rather them get damaged than the 299C. Do I need to have a load on the output transformers at all times when it's on and even during adjustments? I've read that you should never turn on a tube component with no speakers, because that can cause damage to it from no resistance load on its outputs.
To verify, the phase inverter tubes that I remove during the adjustments are the two 6U8A tubes that are between its front four 12AX7A preamp tubes and its four 7591 output tubes?
I will do all you have instructed me to with its old tubes in to learn and see how close its adjustments are now. As you can see, its seller restored and adjusted it, so it should all measure close. I've owned it two years. Its right side plays a little weaker. Even though its seller stated its tubes have lots of life left in them, I want to put some new JJ tubes in it to see if they have it sounding better and its right side equal in volume. I've only owned a few vintage tube components, but in my few experiences new tubes always seem to make improvements. When I soon buy its new tubes, I now know how to adjust its pots.
I'll update as I go.
Thank you so very much!
soundbound
Well-Known Member
Dave,
You wrote, "First set the Balance controls for minimum hum." How do I set the DC Balance pots for minimum hum? Are these set to the lowest sounding hum through speakers or headphones, or is there a better way with a meter?
Is this method a good way to adjust the DC Balance pots with a meter? Setting one of the meter's clip on pin #3 ( I think pin #3 is its plate) of one of the 7591 output tubes and the other meter's clip on pin #3 of the other 7591 in its same channel and then adjusting its DC Balance pot for "0" volts DC, or as close in the millivolt range as you can get it and then adjust the same way for the other channel?
I'm sorry Dave. After reading again I'm mixed up on how to adjust the two pairs of different pots with their two different methods of adjustments.
In layman's terms please explain:
*1) Where do I put the meter's two clips while adjusting the two DC Balance pots and what voltage am I adjusting the pots to get?
*2) Where do I put the meter's two clips while adjusting the two Bias pots and what voltage am I adjusting the pots to get?
I do understand where to put the meter's two clips across the 3.3 ohm resistor while adjusting for that side. Do I put them there while adjusting the Bias pots to achieve a voltage within a .22 to .24 vdc range for that side?
Is the 3.3 ohm resistor the cathode current test point? I read you can put your red meter probe on the cathode current test point and the black probe on ground and adjust the Bias adjust pot until you reach .22 volts.
Thank you.
You wrote, "First set the Balance controls for minimum hum." How do I set the DC Balance pots for minimum hum? Are these set to the lowest sounding hum through speakers or headphones, or is there a better way with a meter?
Is this method a good way to adjust the DC Balance pots with a meter? Setting one of the meter's clip on pin #3 ( I think pin #3 is its plate) of one of the 7591 output tubes and the other meter's clip on pin #3 of the other 7591 in its same channel and then adjusting its DC Balance pot for "0" volts DC, or as close in the millivolt range as you can get it and then adjust the same way for the other channel?
I'm sorry Dave. After reading again I'm mixed up on how to adjust the two pairs of different pots with their two different methods of adjustments.
In layman's terms please explain:
*1) Where do I put the meter's two clips while adjusting the two DC Balance pots and what voltage am I adjusting the pots to get?
*2) Where do I put the meter's two clips while adjusting the two Bias pots and what voltage am I adjusting the pots to get?
I do understand where to put the meter's two clips across the 3.3 ohm resistor while adjusting for that side. Do I put them there while adjusting the Bias pots to achieve a voltage within a .22 to .24 vdc range for that side?
Is the 3.3 ohm resistor the cathode current test point? I read you can put your red meter probe on the cathode current test point and the black probe on ground and adjust the Bias adjust pot until you reach .22 volts.
Thank you.
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OK. All good questions. Here are some comments to help address your questions:
1. There are two power amplifier sections in your Scott: One for the L channel, and one for the R channel. Each power amplifier has its own distinct and separate Bias and Balance control, which is why there are four controls, two for each channel. It is understood that before starting the adjustment process, you will need to determine which Bias and Balance control serves which channel. To help you with that, use the speaker terminal strips as a guide since they are labeled as to which channel they represent. Most of the leads connected to each strip will go to an output transformer, and then each transformer will also have leads connected to a pair of output tubes. Through association, you can then identify which output tubes serve which channel. The Balance control for a given channel will be located between the two output tubes for that channel. From each Balance control, there will be at least one wire connecting between it, and a Bias control. These two controls are located together over in the power supply section near the front of the amplifier. Again through association, you can now determine which Bias control serves which Balance control, and you already know which channel each Balance control serves through the previous association. At this point then, you now know which controls are which, where they are located, and which channel they serve.
2. Each power amplifier has its own 3.3 ohm 1 watt resistor. This is the resistor connected to the red lead of each output transformer identified previously. Through association, you will now know which resistor serves which channel.
3. Each power amplifier has its own phase inverter tube. Your unit uses 6U8 tubes, with the phase inverter tube for each channel located between but offset from the two output tubes it serves. Again, through association, you now know which channel each phase inverter tube serves.
At this point then, you now know which output tubes, phase inverter tubes, Balance controls, Bias controls, and 3.3 ohm resistor serves which channel.
4. I believe you were given instructions previously in the thread to set the Balance controls, whereby you remove the phase inverter tubes, and adjust these controls for minimum hum in the speaker for the channel that each control serves. These instructions were not specifically for your unit, but will work just fine, as they are instructions that are common to many Scott models that use the general design of your model. Use them for want of anything better. Your unit does not have provisions for using your meter to adjust the Balance controls. Therefore, you will use the method of adjustment that sets these controls by adjusting them for minimum hum as directed.
When you start the adjustment process, do the Balance control adjustment first for both channels. These are only initial adjustments, as they will then be touched up throughout the entire adjustment process for all the controls. Once the initial Balance adjustments are set, then proceed on to the Bias adjustments.
5. In adjusting the Bias controls, you will need your meter to be connected to the 3.3 ohm resistor for the channel you are adjusting the Bias for. Therefore, you will need to move your connections back and forth between the two resistors, so that you are always in fact measuring the voltage for the channel you are adjusting. Alternatively, if you have or could borrow another DVM, you could set both meters up, one across each resistor, so that you wouldn't need to move the connections when using just one meter. If you do this, just be sure you are observing the correct meter for the control you are adjusting. Again, association is your friend. Connect the meter leads to each resistor as previously instructed, and adjust each Bias control for the voltage range previously given when adjusting each channel. When finished, both channels should produce a reading across its respective resistor that is within the voltage range given.
Remember that all the controls are all interactive, so recheck all of them as an adjustment is made to any of them, until all controls are at an optimum setting based on the instructions given for each set of controls.
For a person who is new to this type of equipment, the adjustment process sounds daunting enough no doubt. But after you get the hang of it, it will move easily enough so that you will become comfortable with the process. Just remember the high voltage present under the chassis when the unit is in operation!
I hope this helps!
Dave
1. There are two power amplifier sections in your Scott: One for the L channel, and one for the R channel. Each power amplifier has its own distinct and separate Bias and Balance control, which is why there are four controls, two for each channel. It is understood that before starting the adjustment process, you will need to determine which Bias and Balance control serves which channel. To help you with that, use the speaker terminal strips as a guide since they are labeled as to which channel they represent. Most of the leads connected to each strip will go to an output transformer, and then each transformer will also have leads connected to a pair of output tubes. Through association, you can then identify which output tubes serve which channel. The Balance control for a given channel will be located between the two output tubes for that channel. From each Balance control, there will be at least one wire connecting between it, and a Bias control. These two controls are located together over in the power supply section near the front of the amplifier. Again through association, you can now determine which Bias control serves which Balance control, and you already know which channel each Balance control serves through the previous association. At this point then, you now know which controls are which, where they are located, and which channel they serve.
2. Each power amplifier has its own 3.3 ohm 1 watt resistor. This is the resistor connected to the red lead of each output transformer identified previously. Through association, you will now know which resistor serves which channel.
3. Each power amplifier has its own phase inverter tube. Your unit uses 6U8 tubes, with the phase inverter tube for each channel located between but offset from the two output tubes it serves. Again, through association, you now know which channel each phase inverter tube serves.
At this point then, you now know which output tubes, phase inverter tubes, Balance controls, Bias controls, and 3.3 ohm resistor serves which channel.
4. I believe you were given instructions previously in the thread to set the Balance controls, whereby you remove the phase inverter tubes, and adjust these controls for minimum hum in the speaker for the channel that each control serves. These instructions were not specifically for your unit, but will work just fine, as they are instructions that are common to many Scott models that use the general design of your model. Use them for want of anything better. Your unit does not have provisions for using your meter to adjust the Balance controls. Therefore, you will use the method of adjustment that sets these controls by adjusting them for minimum hum as directed.
When you start the adjustment process, do the Balance control adjustment first for both channels. These are only initial adjustments, as they will then be touched up throughout the entire adjustment process for all the controls. Once the initial Balance adjustments are set, then proceed on to the Bias adjustments.
5. In adjusting the Bias controls, you will need your meter to be connected to the 3.3 ohm resistor for the channel you are adjusting the Bias for. Therefore, you will need to move your connections back and forth between the two resistors, so that you are always in fact measuring the voltage for the channel you are adjusting. Alternatively, if you have or could borrow another DVM, you could set both meters up, one across each resistor, so that you wouldn't need to move the connections when using just one meter. If you do this, just be sure you are observing the correct meter for the control you are adjusting. Again, association is your friend. Connect the meter leads to each resistor as previously instructed, and adjust each Bias control for the voltage range previously given when adjusting each channel. When finished, both channels should produce a reading across its respective resistor that is within the voltage range given.
Remember that all the controls are all interactive, so recheck all of them as an adjustment is made to any of them, until all controls are at an optimum setting based on the instructions given for each set of controls.
For a person who is new to this type of equipment, the adjustment process sounds daunting enough no doubt. But after you get the hang of it, it will move easily enough so that you will become comfortable with the process. Just remember the high voltage present under the chassis when the unit is in operation!
I hope this helps!
Dave
soundbound
Well-Known Member
Thank you Dave!
I should be able to adjust them now. I adjust the DC Balance pot to lowest audible hum output, then Bias pot to 0.23 vdc across the 3.3 ohm resistor for each channel, and then back and forth until best results. Correct?
Since you adjust the DC Balance pots to lowest audible hum output, do they have anything in common with the hum pots adjustments you see on other tube components?
I'm also wiring some test points up and placing them on top its chassis for continued future easier accessible adjustments.
I did add a couple more questions to my previous post not realizing you'd already replied to it on page two. If you want, can you reread it and answer the added questions too?
Thank you again Dave! I really appreciate your many very informative well written replies and patience!
I should be able to adjust them now. I adjust the DC Balance pot to lowest audible hum output, then Bias pot to 0.23 vdc across the 3.3 ohm resistor for each channel, and then back and forth until best results. Correct?
Since you adjust the DC Balance pots to lowest audible hum output, do they have anything in common with the hum pots adjustments you see on other tube components?
I'm also wiring some test points up and placing them on top its chassis for continued future easier accessible adjustments.
I did add a couple more questions to my previous post not realizing you'd already replied to it on page two. If you want, can you reread it and answer the added questions too?
Thank you again Dave! I really appreciate your many very informative well written replies and patience!
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You can certainly adjust the balance controls by connecting your DVM between pin #3 of each output tube in a given channel, and adjusting for a 0 vdc reading. But that method depends of both halves of the primary winding in the output transformer having the same DC resistance, which they often do not. However, if each half is within 10% of each other, then the approach will generally work well. If they are off more than that, then balancing for minimum hum will typically produce a more accurate setting.
Dave
Dave
soundbound
Well-Known Member
Thank you Dave.
N
noisehead
Guest
My thanks as well, Dave Gillespie. After searching for this procedure all evening, finally found it here.
