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#1
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Stromberg Carlson Stereo 8 - Balance setting
I have an old integrated amp and I'd like to set the balance of the
output stage(s). I think that I have to measure the voltage across a particular resistor, and rotate the pot until it reaches zero. Is anyone familiar with how to correctly set it? Does anyone know if the schematic available on-line? They stuck a copy to the inside surface of the chassis base plate, which is kind of inconvenient. thanks Mitc h |
#2
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"mitch" wrote in message news:UtIzd.18083$rL3.984@trnddc03... I have an old integrated amp and I'd like to set the balance of the output stage(s). I think that I have to measure the voltage across a particular resistor, and rotate the pot until it reaches zero. Is anyone familiar with how to correctly set it? Does anyone know if the schematic available on-line? They stuck a copy to the inside surface of the chassis base plate, which is kind of inconvenient. No, but if you unscrew the bottom plate & put it on a scanner or a Xerox machine, you'll have the schematics. It's actually lucky to get gear with schematics in it, 'coz you never run into "but wait, R1706 is *not* 220k!" - it's always the right revision. Anyhow, don't you usually have the cover off anyway, when you're working on the thing? -dim thanks Mitc h |
#3
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shiva wrote:
"mitch" wrote in message news:UtIzd.18083$rL3.984@trnddc03... I have an old integrated amp and I'd like to set the balance of the output stage(s). I think that I have to measure the voltage across a particular resistor, and rotate the pot until it reaches zero. Is anyone familiar with how to correctly set it? Does anyone know if the schematic available on-line? They stuck a copy to the inside surface of the chassis base plate, which is kind of inconvenient. No, but if you unscrew the bottom plate & put it on a scanner or a Xerox machine, you'll have the schematics. It's actually lucky to get gear with schematics in it, 'coz you never run into "but wait, R1706 is *not* 220k!" - it's always the right revision. Anyhow, don't you usually have the cover off anyway, when you're working on the thing? -dim thanks Mitc h Yea, I guess you're right. thanks Mitch |
#4
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"mitch" wrote
I have an old integrated amp and I'd like to set the balance of the output stage(s). I think that I have to measure the voltage across a particular resistor, and rotate the pot until it reaches zero. Is anyone familiar with how to correctly set it? No, but if there are two output valves in PP, and there are resistors of same value between cathodes and ground, then set the DC balance pot such that there is 0V between cathodes. That ensures equal current through each output valve. cheers, Ian |
#5
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Hi,
There are two balance adjustments you can make to a push pull power amp circuit. One is the more common DC balance. If you have separate cathode resistors for each tube, just set the balance pots so the voltage drop across each resistor is the same. If there are no resistors, what you do is connect an AC volt meter across the speaker terminals and adjust the balance pot till you get the lowest AC volt reading. This will be where the tubes balance and where you get the least hum. The other balance commonly found is called AC balance. Here your goal is to have each half of the phase inverter producing the same voltage. So, you run a sine or square wave into the amplifier's input, then turn up the level and just measure each half of the phase splitter's output. Actually the best place to measure this is at the control grids of the output tubes. Adjust the phase inverter balance pot till both voltages are the same. The last adjustment is bias. Again, if the amp has a cathode resistor, you can measure the voltage and calculate the current using ohm's law. I find this more accurate then shunting a miliamp meter across a low value resistor. If you don't have cathode resistors, you can install a set of 10 ohm resistors between the cathode and ground. This is assuming the amp is fixed bias. Bill Bittle Archangele sound: Offering service for all makes of tube hifi and guitar amplifiers. |
#6
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Wbittle wrote:
Hi, There are two balance adjustments you can make to a push pull power amp circuit. One is the more common DC balance. If you have separate cathode resistors for each tube, just set the balance pots so the voltage drop across each resistor is the same. If there are no resistors, what you do is connect an AC volt meter across the speaker terminals and adjust the balance pot till you get the lowest AC volt reading. This will be where the tubes balance and where you get the least hum. The other balance commonly found is called AC balance. Here your goal is to have each half of the phase inverter producing the same voltage. So, you run a sine or square wave into the amplifier's input, then turn up the level and just measure each half of the phase splitter's output. Actually the best place to measure this is at the control grids of the output tubes. Adjust the phase inverter balance pot till both voltages are the same. The last adjustment is bias. Again, if the amp has a cathode resistor, you can measure the voltage and calculate the current using ohm's law. I find this more accurate then shunting a miliamp meter across a low value resistor. If you don't have cathode resistors, you can install a set of 10 ohm resistors between the cathode and ground. This is assuming the amp is fixed bias. Bill Bittle Archangele sound: Offering service for all makes of tube hifi and guitar amplifiers. Thanks for your help. How do I know what the current through the cathode should be? Is that something I can get from a spec sheet for the tube? It uses 7355's. Mitch |
#7
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Thanks for your help. How do I know what the current through the cathode should be? Is that something I can get from a spec sheet for the tube? It uses 7355's. Mitch Hi Mitch, If your plate voltage is 400 volts, you should bias these tubes at 25 ma per tube or 50 ma per pair. You can go as high as 60 ma per pair (30 ma per tube). But, it is best to find out what the amplifier's manufacturer recomends. Failing that, the specs I mentioned should work. This tube is usually used in amps producing 25 - 35 watts per channel. Hope this helps. Bill Bittle |
#8
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Mitch,
Just search for the specs online using google. Lots of folks have the tube specs online now adays. Scott mitch wrote: Wbittle wrote: Hi, There are two balance adjustments you can make to a push pull power amp circuit. One is the more common DC balance. If you have separate cathode resistors for each tube, just set the balance pots so the voltage drop across each resistor is the same. If there are no resistors, what you do is connect an AC volt meter across the speaker terminals and adjust the balance pot till you get the lowest AC volt reading. This will be where the tubes balance and where you get the least hum. The other balance commonly found is called AC balance. Here your goal is to have each half of the phase inverter producing the same voltage. So, you run a sine or square wave into the amplifier's input, then turn up the level and just measure each half of the phase splitter's output. Actually the best place to measure this is at the control grids of the output tubes. Adjust the phase inverter balance pot till both voltages are the same. The last adjustment is bias. Again, if the amp has a cathode resistor, you can measure the voltage and calculate the current using ohm's law. I find this more accurate then shunting a miliamp meter across a low value resistor. If you don't have cathode resistors, you can install a set of 10 ohm resistors between the cathode and ground. This is assuming the amp is fixed bias. Bill Bittle Archangele sound: Offering service for all makes of tube hifi and guitar amplifiers. Thanks for your help. How do I know what the current through the cathode should be? Is that something I can get from a spec sheet for the tube? It uses 7355's. Mitch |
#9
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scottp wrote:
Mitch, Just search for the specs online using google. Lots of folks have the tube specs online now adays. Scott mitch wrote: Wbittle wrote: Hi, There are two balance adjustments you can make to a push pull power amp circuit. One is the more common DC balance. If you have separate cathode resistors for each tube, just set the balance pots so the voltage drop across each resistor is the same. If there are no resistors, what you do is connect an AC volt meter across the speaker terminals and adjust the balance pot till you get the lowest AC volt reading. This will be where the tubes balance and where you get the least hum. The other balance commonly found is called AC balance. Here your goal is to have each half of the phase inverter producing the same voltage. So, you run a sine or square wave into the amplifier's input, then turn up the level and just measure each half of the phase splitter's output. Actually the best place to measure this is at the control grids of the output tubes. Adjust the phase inverter balance pot till both voltages are the same. The last adjustment is bias. Again, if the amp has a cathode resistor, you can measure the voltage and calculate the current using ohm's law. I find this more accurate then shunting a miliamp meter across a low value resistor. If you don't have cathode resistors, you can install a set of 10 ohm resistors between the cathode and ground. This is assuming the amp is fixed bias. Bill Bittle Archangele sound: Offering service for all makes of tube hifi and guitar amplifiers. Thanks for your help. How do I know what the current through the cathode should be? Is that something I can get from a spec sheet for the tube? It uses 7355's. Mitch Thanks for the help. Mitch |
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