Hello all. I just came up with something new, or new to me at least. I'm sure that others have done it this way. I'm currently working on a really cool radio from a really nice guy. This radio has 201A tubes in it. I have built an A battery eliminator for it that fits neatly under the chassis. I haven't tackled the B eliminator, yet, but I want it to be out of site, too. I want to rewire the filament switch (this is a Radiola 16) so that it controls the AC current to the eliminators so that the whole thing including eliminators is switched off when I turn off the radio. This way I can use it like any other radio, not that I want to on a regular basis, as these tubes are really old and fragile. Well, considering that the 201A tubes are old and fragile and rare, and that the thoriated tungsten filaments light quite brightly, I wanted a sort of slow start device for the B eliminator. I didn't have ready access to one of those thermal resistor slow start thingies, so I came up with something else. I built the eliminator using Radio Shack's variable voltage regulator transistors (three wire square transistor-like device that can be adjusted from 1 to 35 volts using external resistors). I have some problems with these transistors, but I'll get to that later. First I'm going to describe the slow start circuit. Well, a fixed resistor goes from the output of the transistor to the ADJ terminal, which is the signal terminal--where the device senses the output voltage. Another resistor, a variable 5K unit, goes from this ADJ terminal to the negative part of the DC supply. By adjusting the variable resistor, you control how much the ADJ terminal senses the output voltage, which effectively sets the voltage to where you want it. Reducing resistance on the variable resistor shorts out to a varying degree the output voltage coming through the fixed resistor. Well, I thought that if a large enough capacitor is placed across the variable resistor, this would temporarily act as though the resistor was turned to minimum resistance, thereby reducing the output of the regulator to 1 volt. I found that a 3300 MFD unit, which is readily available at Radio Shack, gave a nice charge-up time period under the circuit circumstances. When this capacitor is placed across the variable resistor, it effectively acts as though the resistor is at minimum resistance--it shorts across the resistor. Then, as the capacitor charges up, it draws less and less current, until it draws no current at all, leaving only the resistance of the variable resistor. When you turn off the power supply with the supply connected to the tube filaments, the capacitor almost imediately discharges and is ready to do the slow start thing all over again. With this set-up, I can turn on the radio at any time, even right after turning it off, and the tubes will gradually come to full brightness. The main voltage increase is over about a 3 second period. It takes about an additional 2 seconds to go from 5 to 6 volts, and the last .2 volts takes an additional 2 seconds. From then on it maintains a perfect 6 volts until the set is again turned off. In this way I am able to soft start my filaments any time I turn on the radio, but the soft start isn't too slow. The only problem with this is that you can never adjust the voltage down while the supply is running (a warning to any of you who build this supply with the slow start feature). With such a large capacitor across the variable resistor, if you reduce the resistance, you effectively short out the large charge held in the capacitor. In order to adjust voltage, you must start at the minimum with the supply off and discharged (short out output leads momentarily). Turn on the supply and gradually increase voltage until it is where you want it to be. It is wise to connect a meter to the supply, and to disconnect the tubes. In this way you can read the voltage without the risk of blowing the tubes from too high a voltage. This supply will maintain perfect voltage both loaded and unloaded. Adjust very slowly, as the capicitor will make voltage increases sluggish. Once the capacitor is charged up at the required voltage setting, though, the regulator maintains its normal perfect voltage at all times. Lights (and toasters) can be turned on and off in the room with no changes in output voltage. Tubes can be removed and replaced in the radio, and yet the voltage remains perfectly on the mark where you set it. The only time the capacitor slows down voltage increases is when the supply is first turned on or when the voltage is adjusted.

Now, my problem with this supply, which I had even before I devised the slow start feature, is that each of the regulators is rated to pass 1.5 amperes at up to 35 volts. The supply is supplied through a full wave rectifier, and is supplied from a 1.5 ampere 12 volt transformer. Adequate filtering is provided by me both before and after the regulator. I use two 3300 MFD units both before and after the regulator, so the DC should be fairly smooth. With 6 201A tubes, exactly 1.5 amperes is being drawn. However, when using only one regulator, the regulator gets incredibly hot and shuts off due to the internal thermal protection which is built into the unit. I then wired three of them in parallel. They can run longer, but soon shut down due to thermal overload. There is mention in the literature on the back of each regulator package of the need for heat sinking. I put heat sinks on all of them, and now they can run indefinitely, but they still get incredibly hot. They smell like a computer after it's been on for a while. Now I'm passing the recommended amount through them, and with 3 in parallel, I'd think that they could handle this, as each is rated for 1.5 amperes, and I'm passing 1.5 amperes through 3 of them in parallel. Is this okay? I think that such hot temperatures with solid state circuitry would be detrimental to longevity. If any of you solid state guys have answers to this, I'd be glad to know them. Otherwise, if that's the way it's supposed to run, the slow start feature should make this one nice A battery eliminator for everyone, especially those with thoriated tungsten tubes. I would just hate to see one of those regulators short out and blow my tubes, though.

Thomas

The idea is rather clever Thomas. Do you care about the slew rate of the regulator? If not then this will be fine, but won't that cap make the regulator unable to see variations in load anymore to be able to compensate promptly?
If so... here's another thought... admittedly a quick one...you might be able to add a self-latching shunt or better yet an un-latching relay that will remove the capacitor from the circuit after you reach a certain desired threshold. That may do everythin you want... it will by default be included in the circuit... the as the voltage hit the desired level, the relay ativates into a latch state removing itself. so the the regulator is back to normal. When the power is off the relay goes back to default "connect" status.
What do you think?
peter

Hopefully these suggestions will help. One thing that is often overlooked in the three terminal regulators is wattage. If you have too much input output differential voltage the regulator will run hot. After rectification and filtering your raw input voltage will be around 16 volts. You have 11 volts * 1.5 amps = 16.5 watts to dissapate as heat. If you can reduce that differential without going below the minimum differential of about 2 volts, you would be better off.

Regulator case style: TO-3 case style generally has a higher wattage rating than TO-220. If you can get TO-3 case style you would be better off.

Balancing resistors: When you put solid state components in parallel, they will not always draw equal current. Often one will hog most of the current. Use .1 ohm (that is right .1ohm) resistors on the output terminals.

Bypassing: Use tantalum capacitors as close to the input and output terminals on the regulator as possible. Even with brute force filtering these regulators can oscillate and they will fry tubes if the do.

Crowbar protection: Find a overvoltage protection circuit and use it. Better a toasted power supply than toasted 201a's.

LM-150,250,350: This a 3 amp version

Boost transistor: Consider using a boost transistor instead of extra regulators. Most data sheets show this configuration. I have made a 13.8 volt 25 amp power supply by using 5 boost transistor and one three terminal regulator. If you use a boost circuit mount the three terminal regulator on its' own heat sink so it will not drift because of the boost transistors heat.

Big heat sink: Bigger better. Use thermal grease even on metal to metal mounting arrangements.

This is not a difinitive list but I hope it helps.
MRO

Thomas:
I have no knowledge previously about the 201A tube specs.
But...
According to the historical artical below it says that the 201A tube had the "new style" 3.3volt 60ma filaments.
Is that true? ...and if so why are we talking .250ma each at 5 volts?

http://www.moah.org/education/transHist1.html

Date 1923:
Westinghouse develops, and RCA manufactures, the WD-11, a tube with a lowered filament power of 0.25 amp at 1.1 volts, using the Wehnell oxice-coated filament. The UV199 "peanut" tubes followed, offering an even more efficient filament (0.06 amp at 3.3 volts), bringing about a great saving of battery power. This same-style filament was installed in the 201 tube, which was renamed the 201A and given a new lease on life as a result of the more efficient use of filament, or "A" battery power.

Wow, you guys both are a help. Regarding what Peter said, the capacitor doesn't seem to affect voltage regulation. When it charges up, it basically puts no load on the circuit, so it's out of the circuit. It has no more resistance, so to speak, than the resistor which it is in parallel with. It actually never has resistance, but the initial charge up acts as though the variable resistor was reduced in resistance and then gradually brought up, which allows the regulator to increase voltage. Once the capacitor is charged, I can remove tubes and put them in, and turn on toasters, and everything, and the voltage remains at exactly 6 volts at the output of the regulator. I can even short out the regulator momentarily, and then the voltage jumps right back up to 6 volts. I'd have to draw you a circuit of the regulator to show why this is so, but it's pretty conventional, so I bet you can picture it.

Regarding what Mark said, you're right about the regulators not being perfectly matched. I kind of figured that this would happen. It starts out that one gets really hot and another gets less hot. The third is only warm. Then, as things progress, they all get pretty hot, but the initially warm one doesn't get as hot as the rest. I'll have to come up with a different circuit. Using high power transistors and zener diodes sounds best instead of relying on poorly manufactured/rated regulators. I do use grease between the regulators and the heat sinks. I have noticed that the transistors in the voltage regulator which I built for the generator in my car do not get that hot. They are high current type transistors (I forgot the exact type). The field coil draws 2 amperes. When the battery is low for any reason, the transistors are feeding the field coil constantly until the time comes for them to taper off the voltage. I've been using this regulator for 2 years now with no troubles, and again, the transistors don't get that hot.

....So, it seems like you both have some very good ideas for me to experiment with. I wonder how many circuit boards I will go through before I get the right circuit. Right now I have my A battery eliminator powering a simple #44 bulb, and one of the regulators is still getting hot. What pieces of crap! None of them get hot when the supply is unloaded, so there definitely isn't a short in the supply.

Thomas

Regarding 201A tubes, it says "same style" filament. I think this might simply mean that the same type of material was used for 201A tubes. As far as I know, 201 tubes definitely have 5 volt filaments in all. Norm just wrote me...he's in Africa or something...kind of cool. He said that the original 201 tubes simply had tungsten filaments which had to be heated white hot in order to emit a good supply of electrons. Then thoriated tungsten filaments were invented which would emit plenty of electrons from a cooler filament. These tubes were numbered 201A instead of simply 201. ...So, perhaps they mentioned that the same type of filament was used in 201A tubes, but perhaps they forgot to mention the voltage difference. The 201A tubes in the Radiola 16 glow fairly normally at 5 volts, though, so it seems to me. I use 6 volts at the wiring, which is sent through the "automatic adjusting" resistor, which effectively reduces the voltage to a sort-of regulated 5 volts. The resistor is supposed to compensate for load variations due to the operation of the rheostat for the RF tubes, and for variations in battery current, etc.

Thomas

Thomas:

BTW...
Here's a cheap great thermistor.
for only about $1.42
It starts out at 220 ohms when cold the reduces itself to a few ohms as the current thru it warms it up slowly... 5 - 15 seconds or so I think.
It can handle 2 amps.
I bought 10 and use them in tube sets with tubes I am afraid to loose.
http://newarkinone.com/

SL15 22102
AMETHERM NTC Thermistor Resistance:220Ohm; Thermistor Tolerance:+/- 25%; Steady State Current:2A; Dissipation Constant:18mW/C; Leaded Process Compatible:Yes; RoHS Compliant:Yes; Mounting Type:Through Hole; Terminal Type:Radial Leaded

Thomas,

The capacitor does affect load regulation. It is not out of the circuit when charged. It acts like a battery attached to the adj terminal. When the load suddenly increases, the output of the regulator drops a little bit, the adj input doesn't see this drop for a long time because the big cap has to discharge before the drop appears at the adj terminal.

For this application it is probably OK, but a rapidly changing load would present a problem.

:Wow, you guys both are a help. Regarding what Peter said, the capacitor doesn't seem to affect voltage regulation. When it charges up, it basically puts no load on the circuit, so it's out of the circuit. It has no more resistance, so to speak, than the resistor which it is in parallel with. It actually never has resistance, but the initial charge up acts as though the variable resistor was reduced in resistance and then gradually brought up, which allows the regulator to increase voltage. Once the capacitor is charged, I can remove tubes and put them in, and turn on toasters, and everything, and the voltage remains at exactly 6 volts at the output of the regulator. I can even short out the regulator momentarily, and then the voltage jumps right back up to 6 volts. I'd have to draw you a circuit of the regulator to show why this is so, but it's pretty conventional, so I bet you can picture it.
:
:Regarding what Mark said, you're right about the regulators not being perfectly matched. I kind of figured that this would happen. It starts out that one gets really hot and another gets less hot. The third is only warm. Then, as things progress, they all get pretty hot, but the initially warm one doesn't get as hot as the rest. I'll have to come up with a different circuit. Using high power transistors and zener diodes sounds best instead of relying on poorly manufactured/rated regulators. I do use grease between the regulators and the heat sinks. I have noticed that the transistors in the voltage regulator which I built for the generator in my car do not get that hot. They are high current type transistors (I forgot the exact type). The field coil draws 2 amperes. When the battery is low for any reason, the transistors are feeding the field coil constantly until the time comes for them to taper off the voltage. I've been using this regulator for 2 years now with no troubles, and again, the transistors don't get that hot.
:
:....So, it seems like you both have some very good ideas for me to experiment with. I wonder how many circuit boards I will go through before I get the right circuit. Right now I have my A battery eliminator powering a simple #44 bulb, and one of the regulators is still getting hot. What pieces of crap! None of them get hot when the supply is unloaded, so there definitely isn't a short in the supply.
:
:Thomas

:Hopefully these suggestions will help. One thing that is often overlooked in the three terminal regulators is wattage. If you have too much input output differential voltage the regulator will run hot. After rectification and filtering your raw input voltage will be around 16 volts. You have 11 volts * 1.5 amps = 16.5 watts to dissapate as heat. If you can reduce that differential without going below the minimum differential of about 2 volts, you would be better off.
:
:Regulator case style: TO-3 case style generally has a higher wattage rating than TO-220. If you can get TO-3 case style you would be better off.
:
:Balancing resistors: When you put solid state components in parallel, they will not always draw equal current. Often one will hog most of the current. Use .1 ohm (that is right .1ohm) resistors on the output terminals.
:
:Bypassing: Use tantalum capacitors as close to the input and output terminals on the regulator as possible. Even with brute force filtering these regulators can oscillate and they will fry tubes if the do.
:
:Crowbar protection: Find a overvoltage protection circuit and use it. Better a toasted power supply than toasted 201a's.
:
:LM-150,250,350: This a 3 amp version
:
:Boost transistor: Consider using a boost transistor instead of extra regulators. Most data sheets show this configuration. I have made a 13.8 volt 25 amp power supply by using 5 boost transistor and one three terminal regulator. If you use a boost circuit mount the three terminal regulator on its' own heat sink so it will not drift because of the boost transistors heat.
:
:Big heat sink: Bigger better. Use thermal grease even on metal to metal mounting arrangements.
:
:This is not a difinitive list but I hope it helps.
:MRO

Mark,

You are "right on." Paralleling regulators is always a problem. Unlikely they will share equally. Much better to use an external transistor driven by one regulator. All the regulator data sheets show circuits for this hook-up. My favorite is an LM317 driving a 2N3055. Reducing the raw input voltage would help a lot. You don't need 16 volts coming into the regulator. 5 Volts would be more than adequate. The regulator must dissipate any excess voltage as heat.

TO-220 cases will handle only a watt or so without a heatsink. Even with a good heatsink, I wouldn't push more than 10 Watts. Also, 3300 mfd. caps maybe a bit too low. Use some tantalum in parallel with the electrolytics. Tantalum has lower internal impedance and will prevent oscillation.

The slow start circuit is interesting, but probably not vital. It will reduce the response time for the regulator to compensate for line or load voltage fluctuation, but that's probably not a major issue.

Regards,
Rich
(formerly applications engineering mgr. at Siliconix)

Thomas, I'm anxious to see the final design you come up with. A built-in battery eliminator would make these TRF sets much more user-friendly for non-techno-geeks.

As far as the soft-start feature, I personally do not think this is a very necessary feature. Just switching on the battery eliminator should provide a softer start than switching a 6-V battery across the "A" supply, which is how the set was originally operated.

Further, the three RF filaments are attenuated by the volume control, which will seldom be full-blast at startup and the detector's filament voltage is permanently dropped. Yes, the two audio tube filaments run at full voltage.

I've found '01As to be quite rugged and long-lived. I've rejuvenated weak '01As by running their filaments at 6.3V or even 7.5V for hours on end - and I've never had a filament blow during the exercise.

For your C- supply, I'd consider a 3.7-V lithium battery.

To Rich and Doug: I really desire the slow start feature. Even though these tubes may be rugged, I want to give them extra protection. They aren't extremely common anymore. The slow start feature which I devised takes only about 3 seconds to reach near full power, and takes an additional 2 or 3 seconds to go through the last .5 volts. It only takes an extra condenser to achieve this, so I'm going to stick with it. The bulbs in my mother's chandelier usually last 10 years or more because it's on a twist knob type dimmer (with switch included in the twisting action as opposed to push-to-light). I just like this idea a lot. I did not build this feature into my regulator with any intention of improving the regulator's performance, or for its longevity or protection.

Regarding why I chose 12 volts as my input voltage as opposed to 6.3, which could be possible if I make use of the transformer's center tap and a more traditional full wave rectifier (two diode) circuit, is because I felt that filtering would be better if I started with a higher voltage. This would allow the regulator to pass only what it needed. The electrolytics on the primary side could then charge up fully, or at least above 6 volts, which would reduce hum. Perhaps I am wrong on this? With choke filtering it is usually desirable to have a much higher voltage than necessary prior to the choke. If you all feel that my regulator will run cooler, and without hum, if I feed it with 6.3 volts AC instead of 12.6 volts AC (rectified and then filtered), I will change the circuit to suit this.

Thomas

The 6.3 volt arrangement with two diodes and center tap will work better. After rectification and filtering you should have about 8.2 volts input to the regulator. This should allow for enough regulator head room. You should only need to dissipate 4.8 watts as opposed to over 16 watts. With your large capacitors, your ripple voltage will be very low and will not dip below the regulator minimum voltage differential. You should see very little ripple if any at the output of the regulator. Your transformer will also be able to handle more current with this arrangement. The calculations for this are: (6.3*1.414)-.7 = 8.2
MRO

Well, folks, don't know what I did wrong last night, but I temporarily fed in a nice heavy current 6.3 volts AC, but the output of the regulator was not able to be brought up to 6 volts under the load imposed by the tubes. I don't think that electrolytics are able to maintain a full charge when loaded down that much. The voltage after the rectifier was definitely not 8 volts with the tubes connected. I'm going to have to play around with this and other ideas, and see what's going on.

Thomas

Well I use the standard power supply calculation but again theory and real life didn't jive. I pieced together the raw end of the supply and found that it only gives me 7.5 volts and about 500mv rms ripple when loaded with 1.5 amps and two 2200 mfd in parallel. My transformer gives about 7 vac loaded each leg though rated at 12.6 center tapped. Not enough headroom for the three terminal regulator. Sorry for all the bother.
MRO

Tis alright. I also rewired the regulator so that it controlled a transistor. I forgot the exact number of the transistor, but it ended with 3055. This transistor is supposed to be able to handle 10 amperes. It gets incredibly hot, though. I think I miswired the sensing circuit or something, as the voltage is stable with input voltage changes, but is not stable with output load variations. Perhaps a rewire would cure both problems. I don't know. I'm going to have to give this some thought. Another thought was to use 3 regulators again, and have a separate adjustment for each, and then adjust each on its own so that they all conduct exactly the same. This way they'll be fairly balanced. Whether or not they'll stay balanced is another thing to think about. AES's kit sounds nice now, even though it costs a bit more.

Thomas

2N3055 will handle 10 amps if you solder it to an ice cube. It is rated for gain at 4 amps. You will need a heatsink of good size to get the temp down. Depending on the raw input voltage, you may need to disspipate lots of watts in the regulator transistor. Multiply the load current times the differential voltage (Dc input minus regulated output voltage); that's the power that is heating your transistor. Example: if you have 10 volts DC on the input and 2. 5 volts at the output, your differential is 7.5 volts. At 4 amps of load current, that makes 30 watts to dissipate in the transistor... that's a big heatsinking requirement.

Rich

:Tis alright. I also rewired the regulator so that it controlled a transistor. I forgot the exact number of the transistor, but it ended with 3055. This transistor is supposed to be able to handle 10 amperes. It gets incredibly hot, though. I think I miswired the sensing circuit or something, as the voltage is stable with input voltage changes, but is not stable with output load variations. Perhaps a rewire would cure both problems. I don't know. I'm going to have to give this some thought. Another thought was to use 3 regulators again, and have a separate adjustment for each, and then adjust each on its own so that they all conduct exactly the same. This way they'll be fairly balanced. Whether or not they'll stay balanced is another thing to think about. AES's kit sounds nice now, even though it costs a bit more.
:
:Thomas