I am building a 5 tube low power am transmitter for my home so I can listen to my favorite music on my restored radios. The design is based on the "Goldberg" 4 tube transmitter with the addition of a two channel audio mixer stage.
The power supply design requires a 240Vct power transformer. There are several positive and negative DC outputs required. In an attempt to be thrifty, I would like to use a 700Vct XFMR that I have on hand. What I am planning to do is reduce the secondaries through voltage dividers prior to the full wave bridge rectifier. I prototyped this on the bench using 10W wire wound resistors and it works fine. The resistors get a bit warm but nothing alarming.

Does anyone see any problems with this approach?

Gary

Gary, I can't completely visualize your circuit, but something sounds peculiar to me. When you say that your xfmr is 700V-CT, is that 350-0-350?

And the schematic calls for a 240V-CT? Would that be 120-0-120? What DC B+ voltage are you needing?

Doug, you are correct.

:Gary, I can't completely visualize your circuit, but something sounds peculiar to me. When you say that your xfmr is 700V-CT, is that 350-0-350?
:
:And the schematic calls for a 240V-CT? Would that be 120-0-120? What DC B+ voltage are you needing?

Sorry Doug, forgot to answer one of your questions.

The design require (+) and (-) 150V.

Gary

:Gary, I can't completely visualize your circuit, but something sounds peculiar to me. When you say that your xfmr is 700V-CT, is that 350-0-350?
:
:And the schematic calls for a 240V-CT? Would that be 120-0-120? What DC B+ voltage are you needing?

:The design require (+) and (-) 150V.
:
:Gary

Gary, I'm still a little unsure of what we're talking about here. For a plate supply, you'd need a positive DC source, but not a negative?

You're proposing to use a 350-0-350 CT xfmr, through a voltage divider, and thence to a bridge rectifier? But, I don't think a bridge rectifier needs a CT xfmr. So couldn't you use just use one leg of the xfmr secondary? If the xfmr has enough capacity in one leg, then that would get the voltage down by 50% without any dropping resistors.

To drop the voltage more than the 50%, you could consider a voltage divider on the primary side instead of the secondary. Depending on what other windings are on the xfmr, maybe you could try a bucking scheme?

The thing I don't like about a voltage divider in a power ckt is that if the wrong resistor opens up, then the output voltage will increase, possibly enough to be a problem.

Negative and positive are all from perspective, of course, but a power supply that has say 150 volts negative as well as positive usually has a common point somewhere in the resistor network that is seen as zero volts. In reality, if a power supply has a total of 350 volts DC, then the zero volts (negative) is down at the center tap (regarding AC full wave rectification). Some radios tap higher up and call this zero volts, though, and then call the center tap some negative value--whatever this is with regard to the zero point. The Philco 60 is a prime example of this. Instead of having simply a zero volts point that everything rises positively above (like in most AC-DC radios), the zero volts point is actually taken where the output tube cathode takes its current from. This connects to the chassis as well. Any negative bias required is taken before the choke coil which connects to this zero volts point, and anything taken prior to this (such as directly from the transformer center tap) is labeled some negative figure even though in reality the center tap is the zero volts point. Am I making sense? Probably not, but positive and negative is all a matter of perspective.

Another prime example is the RCA 630TS chassis. Look at this schematic. Zero volts is where the chassis connects to even though this makes the chassis more positive than the transformer center tap. You can also say, though, that the transformer is 100 volts negative with respect to the chassis, which is how it is labeled within the schematic. All 100 volt negative bias is then taken at a point 100 volts negative with respect to the chassis. I guess you could say that zero volts reference point is usually where the chassis connects to with respect to all other voltages.

Other examples of where this is done: Crosley 1117, Airline 14WG-806, etc.

Normally in AC-DC radios, the chassis, when connected directly to the line, is the zero volts point, and often there isn't anything less negative than this, especially when the chassis is connected directly to the line. Everything is taken from this point. The cathode of the output tube is seen as positive with respect to the chassis.

Again, it is all a matter of perspective when referencing positive and negative voltages. A voltage is positive or negative with respect to other voltages. A zero volts point is established as a reference point. Otherwise all numbers become meaningless.

Thomas

Thomas,

This particular supply uses a full wave bridge rectifier with the center-tap of the XFMR connected to chassis. This allows for a (+) supply referenced to chassis as well as a (-) supply which is also referenced to chassis.

To reduce the XFMR seconday voltage I am using a resistor in series with each secondary, prior to the bridge rectifier.

The required DC outputs are (+) and (-) 150V at about 20ma.

My original thought was to use a voltage divider to reduce the XFMR secondary but Doug's comment about an open resistor made me re-think the design.

Gary

:Negative and positive are all from perspective, of course, but a power supply that has say 150 volts negative as well as positive usually has a common point somewhere in the resistor network that is seen as zero volts. In reality, if a power supply has a total of 350 volts DC, then the zero volts (negative) is down at the center tap (regarding AC full wave rectification). Some radios tap higher up and call this zero volts, though, and then call the center tap some negative value--whatever this is with regard to the zero point. The Philco 60 is a prime example of this. Instead of having simply a zero volts point that everything rises positively above (like in most AC-DC radios), the zero volts point is actually taken where the output tube cathode takes its current from. This connects to the chassis as well. Any negative bias required is taken before the choke coil which connects to this zero volts point, and anything taken prior to this (such as directly from the transformer center tap) is labeled some negative figure even though in reality the center tap is the zero volts point. Am I making sense? Probably not, but positive and negative is all a matter of perspective.
:
:Another prime example is the RCA 630TS chassis. Look at this schematic. Zero volts is where the chassis connects to even though this makes the chassis more positive than the transformer center tap. You can also say, though, that the transformer is 100 volts negative with respect to the chassis, which is how it is labeled within the schematic. All 100 volt negative bias is then taken at a point 100 volts negative with respect to the chassis. I guess you could say that zero volts reference point is usually where the chassis connects to with respect to all other voltages.
:
:Other examples of where this is done: Crosley 1117, Airline 14WG-806, etc.
:
:Normally in AC-DC radios, the chassis, when connected directly to the line, is the zero volts point, and often there isn't anything less negative than this, especially when the chassis is connected directly to the line. Everything is taken from this point. The cathode of the output tube is seen as positive with respect to the chassis.
:
:Again, it is all a matter of perspective when referencing positive and negative voltages. A voltage is positive or negative with respect to other voltages. A zero volts point is established as a reference point. Otherwise all numbers become meaningless.
:
:Thomas

If you use high quality wire wound resistors, and don't cook them, they don't often open up. Lots of equipment was made this way. Corrosion is the big enemy. If you keep the device away from moisture, and you use it a lot, you won't have corrosion problems.

In any case, an unregulated supply, as all old radios use, can cause trouble. If your output tubes get weak, all of the other tubes in the radio get overloaded, because the total radio load is what determines the voltage after the field coil. Good design, maintainence, and use prevent much trouble.

Thomas

Probably easier to use only one secondary and a bridge rectifier, and two 50-100K resistors, each parallel with a filter cap, in series across rectifier output. Then you can use the "center tap" as a floating ground to get dual polarity. This is the same as most of the power supplies used in modern transistor amps, similar to the first circuit in this page:
http://sound.westhost.com/project43.htm

then you can add whatever dropping resistors and extra filter caps to get the correct voltage and desired ripple level.

:
:This particular supply uses a full wave bridge rectifier with the center-tap of the XFMR connected to chassis. This allows for a (+) supply referenced to chassis as well as a (-) supply which is also referenced to chassis.
:
:To reduce the XFMR seconday voltage I am using a resistor in series with each secondary, prior to the bridge rectifier.
:
:The required DC outputs are (+) and (-) 150V at about 20ma.
:
:My original thought was to use a voltage divider to reduce the XFMR secondary but Doug's comment about an open resistor made me re-think the design.
:
:Gary

Totally appreciate what you're doing and why and am sure you'll get, with thehelp of the experts on this forum, help getting the voltages correct. I just wanted to put another small spin on this. From the sounds of the unit you're building to transmit broadcast AM it may be fairly potent in it's output. This of course depends on Final output amp and antenna.

One small word of caution. The FCC frowns especially heavy on transmissions in this band that are higher than 100 mW. That's because, depending on on all the above plus atmospheric conditions, these transmissions can travel quite extensively. They do listen. I was caught and was only putting out 500 mWs on a short longwire in my attic. Not trying to discourage you, just be aware of the regs on transmitting, especially in this band.

:I am building a 5 tube low power am transmitter for my home so I can listen to my favorite music on my restored radios. The design is based on the "Goldberg" 4 tube transmitter with the addition of a two channel audio mixer stage.
:The power supply design requires a 240Vct power transformer. There are several positive and negative DC outputs required. In an attempt to be thrifty, I would like to use a 700Vct XFMR that I have on hand. What I am planning to do is reduce the secondaries through voltage dividers prior to the full wave bridge rectifier. I prototyped this on the bench using 10W wire wound resistors and it works fine. The resistors get a bit warm but nothing alarming.
:
:Does anyone see any problems with this approach?
:
:Gary

Rick,

Thanks for the heads up. This transmitter should be less than 100mW so I should be ok.
By the way, do you know what happens if you are caught transmittting above the limit?

Gary

:Totally appreciate what you're doing and why and am sure you'll get, with thehelp of the experts on this forum, help getting the voltages correct. I just wanted to put another small spin on this. From the sounds of the unit you're building to transmit broadcast AM it may be fairly potent in it's output. This of course depends on Final output amp and antenna.
:
:One small word of caution. The FCC frowns especially heavy on transmissions in this band that are higher than 100 mW. That's because, depending on on all the above plus atmospheric conditions, these transmissions can travel quite extensively. They do listen. I was caught and was only putting out 500 mWs on a short longwire in my attic. Not trying to discourage you, just be aware of the regs on transmitting, especially in this band.
:
:
:
:
::I am building a 5 tube low power am transmitter for my home so I can listen to my favorite music on my restored radios. The design is based on the "Goldberg" 4 tube transmitter with the addition of a two channel audio mixer stage.
::The power supply design requires a 240Vct power transformer. There are several positive and negative DC outputs required. In an attempt to be thrifty, I would like to use a 700Vct XFMR that I have on hand. What I am planning to do is reduce the secondaries through voltage dividers prior to the full wave bridge rectifier. I prototyped this on the bench using 10W wire wound resistors and it works fine. The resistors get a bit warm but nothing alarming.
::
::Does anyone see any problems with this approach?
::
::Gary

Hi Gary,

Well there are two main gates as to how hard the FCC comes down. First is how bad the interference was and how many folks complained (probably not high in the AM band anymore). Second was whether you were actually trying to interfere (pirate broadcast). If you're under the legal power limit chances are you'll never hear from them. Just wanted to insure you weren't schleping a watt or two to your local antenna.

By the by, if they do decide any of the above, the fines are in the thousands of bucks plus confiscation of ALL of your equipment.

Rick

:Rick,
:
:Thanks for the heads up. This transmitter should be less than 100mW so I should be ok.
:By the way, do you know what happens if you are caught transmittting above the limit?
:
:Gary
:
::Totally appreciate what you're doing and why and am sure you'll get, with thehelp of the experts on this forum, help getting the voltages correct. I just wanted to put another small spin on this. From the sounds of the unit you're building to transmit broadcast AM it may be fairly potent in it's output. This of course depends on Final output amp and antenna.
::
::One small word of caution. The FCC frowns especially heavy on transmissions in this band that are higher than 100 mW. That's because, depending on on all the above plus atmospheric conditions, these transmissions can travel quite extensively. They do listen. I was caught and was only putting out 500 mWs on a short longwire in my attic. Not trying to discourage you, just be aware of the regs on transmitting, especially in this band.
::
::
::
::
:::I am building a 5 tube low power am transmitter for my home so I can listen to my favorite music on my restored radios. The design is based on the "Goldberg" 4 tube transmitter with the addition of a two channel audio mixer stage.
:::The power supply design requires a 240Vct power transformer. There are several positive and negative DC outputs required. In an attempt to be thrifty, I would like to use a 700Vct XFMR that I have on hand. What I am planning to do is reduce the secondaries through voltage dividers prior to the full wave bridge rectifier. I prototyped this on the bench using 10W wire wound resistors and it works fine. The resistors get a bit warm but nothing alarming.
:::
:::Does anyone see any problems with this approach?
:::
:::Gary

Too bad the antique radio community isn't a bit larger and more powerful. It'd be nice to kick all of the obnoxious AM broadcasting off the air and put some good music back on. AM around Milwaukee SUCKS! I really don't use my radios anymore. Thankfully I have over a thousand 78s. Maybe I should get some money (like that's going to happen) and make my own radio station. I could purchase CDs of my music, and what there isn't in CDs, I could make of my own. I'd like to play the records directly, but I think that this would wear them out. Well, it's a dream at this point in time.

Thomas

:I am building a 5 tube low power am transmitter for my home so I can listen to my favorite music on my restored radios. The design is based on the "Goldberg" 4 tube transmitter with the addition of a two channel audio mixer stage.
:The power supply design requires a 240Vct power transformer. There are several positive and negative DC outputs required. In an attempt to be thrifty, I would like to use a 700Vct XFMR that I have on hand. What I am planning to do is reduce the secondaries through voltage dividers prior to the full wave bridge rectifier. I prototyped this on the bench using 10W wire wound resistors and it works fine. The resistors get a bit warm but nothing alarming.
:
:Does anyone see any problems with this approach?
:
:Gary

Gary, I know you are building this as a project, but the easy way to provide mic, tape and phono/cd to your old radios is to go to Toys Are Us and buy a junior disk jocky unit for $19.95. It is battery operated, but you can change that and it is within the FCC limits of power. I have used one for over five years. Boyd