I have two old Motorola TV's and when turned on some of the the series wired tubes really glow (almost like a lite bulb), then as they warm up they go to their proper brillance.They are the right tubes and the sets work fine,but I'm afraid of burning out the 7jp4 heaters.Is there some thing I can wire in series with the filaments to give them a "soft start" ? Thank you---Chuck<P>------------------<BR>

Chuck,<P>Great question! I am similarly worried about the 7JP4 filament in my Halli T-54. I was thinking of wiring a pair of series'd back-to-back zener diodes across the filament wiring to the CRT but haven't actually tried it yet.<P>Anyone else have suggestions?<P>Personally I think anyone that designs a series string, non-power-xfmer tube set belongs in the same jail cell as the fellow that invented the slotted screw, and all the production managers of companies that still perpetuate them! Off the soap box now. <IMG SRC="http://www.antiqueradios.com/forums/wink.gif"><P>Rob<P>------------------<BR>

Does your TV have a thremistor in series with filaments? A thermistor has high resistance when cold. As it heats resistance goes down. Using a thremistor will cause a TV to take longer to come on.<P>Most series wired TV's from 1950's used thermistors. <P>------------------<BR>Norm

Someone else here once suggested this for the series string heaters that I'm going to do with a few of my sets. Remove the CRT heater from the string and replace with a resistor. Then use a seperate 6.3 volt transformer for the CRT heater. At least that way the picture tube doesn't see a turn on surge. <P>I'm also going to experiment with NTC thermistors for a soft start. Anybody have any suggestions on size and a good place to get them.<P>Paul<P>------------------<BR>

Try Digi-Key, digikey.com . Thermistors are in their catalog. <P>------------------<BR>Norm

Many people install a small 6 v filament transformer under the chassis to power the 7JP4 and put an appropriate resistor in its place in the series string.<P>------------------<BR>

Try this gentlemen:<BR>Add a 1 amp 1Kv prv diode, such as an ECG125, in series with the filament string. You may have to reduce the dropping resistor to between 20 and 50 ohms (depending on the number of tubes and the amount of line voltage to be "wasted"). Trial and error or Ohm's law will reveal the exact value. The set will warm up slower, but all of the tubes will last alot longer. <P>------------------<BR>

Just a note regarding series-string heaters. The effect you notice is the reason why "A" series tubes were developed (e.g. 12AU7A, rather than 12AU7) These tubes have an 11 second controlled warm up time for use in series-string heater sets. They minimize (but do not totally eliminate) the "heater flareup" problem. However, vacuum tubes are designed quite differently from light bulbs. They are designed to take more than 2 1/2 times their rated heater voltage. That said, it is still wise to limit the voltage to the 7JP4. A diode in series with the heater string is not going to do it. It will limit the effective voltage to the heaters to 1/2 of the usual voltage. To observe this, put a diode in series with a 60 watt bulb. The brightness will be MUCH less; probably even less than half of the normal brightness. The tubes will last almost forever, but the set will perform VERY poorly, if at all.<P> I like the idea of using two 6.1 Volt Zener diodes in opposite directions across the 7JP4 heater. But remember that they will have to carry a substantial amount of power at startup, and SOME current during normal operation as well. Size them accordingly.<P> As for the heaters on picture tubes, remember that the accepted method of extending the life of a picture tube was to use a "Picture Tube Brightener." All THAT is is a small transformer designed to raise the heater voltage of a picture tube by around 33 to 50%. Yes, tube life is shortened *slightly*, but a Brightener is a "last resort" type fix. (The tube was close to a dud anyway, right?)<P> <P>------------------<BR>

A couple of points;<BR>If you use diode in series with the heater string the effective voltage won't be halved, it will be root 2 (0.707) or equivalent to about 85 volts supply. There's a risk of cathode poisoning if underunning the heaters like this.<BR>The heaters run on AC, if you place back to back zenners across the heater they will need to be at least 9.1 volt to allow for the peak voltage.<BR>The current surge is due to the low cold heater resistance and will occur even when they are run from a low voltage transformer. However there's less risk of uneven voltage distribution before they have warmed up.<BR>The best way of controlling the surge is by using a NTC thermistor as suggested. They get hot so mount them like any other power resistor where the heat won't be a problem and they'll have mains voltage too so not where they're a shock hazard.<BR>When the ENIAC computer was operating tube reliability was a major problem just from the sheer number (18,000) used. Any failure stopped it until repaired. All tubes that failed were investigated to establish how they failed (including taking them apart) and statistics of their failure determined to try and reduce the risk. The type of heater wire construction (looped or twisted) had a major effect on failure rate. Slow heater warm up was tried to reduce failure due to surge currents but no change in failure rate was found. I still shudder at some of the flare from the heaters at turn on but it doesn't seem to be such a problem in actual operation. <BR>Don Black.<P>------------------<BR>

Back in my early tinkering days, I used to put a device called a Surgistor, made by Wuerth Enterprises, in series with the AC input This was a simple bi-metal strip switch that kicked on after a series resistor heated it and let the tubes warm up slowly. Worked great! They also put the same device in a little metal box with a line cord and outlet and called it a tube saver. I bought a few NOS Surgistors on eBay a while ago, but haven't seen them commercially advertised for years.<BR>David<P>------------------<BR>

On the Motorolas' I noticed the 6AL5 is usally the tube that flares up, very brightly as Chuck noted.<P>Well at least they are cheaper than 7JP4s <IMG SRC="http://www.antiqueradios.com/forums/wink.gif"><P>Eric<P>------------------<BR>

Of course, Don is absolutely correct on all counts. In my (silly) attempt to simplify, I left out the "numbers" and ignored them when suggesting the 6.1 Volt zener. That said, the end result is still the same. The set will probably not perform well on 85 volts, nor will the 7JP4 perform very well using 6.1 Volt zeners!<P>One point: I'm not quite certain that the correct term is "thermistor." As I recall, a thermistor is a device which changes resistance according to the temperature it is exposed to. What we are talking about here is a device which changes resistance according to the current flowing through it. The higher the current, the more the resistance increases. Unfortunately, I forget the correct NAME for this silly device. As I recall, original auto-degaussing circuits used a combination of a Voltage Dependent Resistor and this thing, in order to get the job done. The effect is to cause the initial surge current to go to the degaussing coils and then gradually be diverted to the rectifiers in the B+ section. Perhaps the parts could be scrounged from an un-restorable chassis. Again, I have forgotten more than I used to know about these things, so I leave it to others to help with the details. Just food for thought! (lol)<BR><P>------------------<BR>

Mouser has Keystone current limiters that the R-390 guys use for a "soft start" to prevent switch welding. These should work well on TV's.<P>URL is:<BR> <A HREF="http://www.mouser.com/index.cfm?handler=listproduct&categoryid=21&lst_pcode=5273" TARGET=_blank>http://www.mouser.com/index.cfm?handler=listproduct&categoryid=21&lst_pcode=5273</A> <P><BR>Scott<BR><P>------------------<BR>

Hi Bill, There are often a lot of different names used. I think thermistor is OK, it depends on the current heating the thermistor to cause its resistance to fall. The degausing resistor is sometimes called a posistor, it's a thermistor but has positive temperature coefficient to turn the current off. The surge limiter has negative coefficient to turn it on, opposite effect. Perhaps a negistor (ouch). A varistor usually means a voltage dependant resistor. How many different names can you think of for a line output transformer? LOPT, EHT transformer, Flyback, Horizontal output. I'm sure I've missed quite a few. The surge is worse where the tubes total voltage is close to the mains voltage. Where there is a large ballast resistor it helps limit the surge.<BR>Don Black. <P>------------------<BR>

I used a thermistor in my Pilot TV and it worked fine, no flareups of any tubes.<BR>I also placed a 6.3 volt filament transformer for the CRT.<BR>I also used a themistor in series with the filament string on my Motorola VT-71, but have not added a filament transformer for the CRT.<BR>Worked fine for me.<BR>====================================<BR>Bill<P>------------------<BR>

Hi Bill--The model I want to protect is a Motorola 7vt2.Looking at the heater schematic both filament strings go through the 7jp4 to ground. What type thermsister did you use and where did you place it? I looked at the Mouser site and I'm lost. Thank you---Chuck<P>------------------<BR>

Hi Chuck,<BR> I got mine from Mouser, I forgot the part #, someone here on this forum told me quite awhile ago about it, could have Alan D. or Norm L. I am not sure who it was, so please don't quote me on this... <IMG SRC="http://www.antiqueradios.com/forums/smile.gif"><BR>What you want is one that as about 120 ohms cold and around 10 ohms or less hot.<BR>I am not familar with that model Motorola, but I think if you have two seperite filament strings, you can use one thermistor in series with each string.<BR>You should have no tube flareups with that thermistor.<BR>====================================<BR>Bill<P><BR>------------------<BR>

Page listing all Inrush current limiters at Mouser: <A HREF="http://www.mouser.com/catalog/cat_610/396.pdf" TARGET=_blank>http://www.mouser.com/catalog/cat_610/396.pdf</A> <P>Mouser part #527-CL80, I have used on boatanchors and it works well. It is 47 ohm rated to 3 amp. It should certainly help on your TV. <BR>If you want to try something with more resistance, use Mouser #527-CL90 which is 120ohm rated at 2 amps. Heck, for $2.00 each, buy both and let us know which works best so we can get them for all our sets. <P>Scott<P><P>------------------<BR>

Hi - I have a couple of comments and suggestions on this topic. While it's true that using a diode won't help the power surge problem, I think you're missing a valuable point here: one diode in each leg of the filament circuit can eliminate the need for the filament ballast resistors in these Motorola sets!<P>About 2/3 of these TV's that I've seen have burned out ballast tubes anyway, and instead of replacing with a big high power resistor,<BR>I have just used a diode in each leg instead.<P>Since the voltage is being rectified and not<BR>filtered, the POWER being sent to the<BR>filaments should be cut in half with a diode, and this is what counts in heating them up.<BR>Since the ballast tubes drop about 1/2<BR>of the AC voltage in these sets, it just<BR>about works out right. A side benefit is<BR>that the whole chassis runs a lot cooler.<P>I think the question of the VOLTAGE drop<BR>may be even trickier than what has already been discussed. The AC line is 120 volts RMS, which works out to about 340 volts peak-to-peak. When you rectify it, you will get 170 volts peak-to-peak, but the waveform is no longer a sine wave so the square-root rules kind of go out the window. <BR>I don't remember how to compute the equivalent RMS value in this case, it probably requires something ugly like integrating the waveform over one interval.<BR>I don't think it matters though, since I'm<BR>pretty sure that the power is halved.<P>I have tried this myself with a TS-18 chassis and the filaments looked like they were lighting up just about right. Unfortunately, since the waveform isn't a sine wave a typical VTVM or DVM won't be able to accurately read the voltage across each tube filament! I know they make true RMS reading meters that work regardless of the waveform, but I don't have one.<P>One last thought - those thermistors from Mouser should work to solve the current surge problem, but since they are rated at 2 or 3 Amps and each leg of the filament string only draws 600mA<BR>I don't think they will reach full temparature, and therefore they won't reach minumum resitance either. Therefore, you may need to decrease the value of the ballast resistors to compensate.<P>Hope this helps,<BR>Mark E. Thierbach<P><P>------------------<BR>

Hi Mark,<BR> I have to disagree with you one one thing.... <IMG SRC="http://www.antiqueradios.com/forums/smile.gif"><BR>I have used the thermistors on TV's and have placed an ohm meter across the thermistor (after turning OFF the TV)and only after the TV has been on awhile, and the HOT readings were around 4 to 5 ohms.<BR>This has to be done right after the set is turned off because as the thermistor cools the resistance goes up.<BR>You can make the test using test leads like I did, that way you can run the set for a few minutes, have your ohm meter ready, pull power on TV,unclip test leads from thermistor and immendeatly place the ohm meter across the hot thermistor.<BR>Believe me these things get very hot and will have to be mounted in a clear space away from other components, or even on top of the chassis.<BR>They could be mounted inside the old ballast can after you gut it out.<BR>As I mentioned, I did not use a thermistor in my Motorola VT-71, but I have used two power resistors inside the ballast for the filaments and placed the B+ resistors under chassis.<BR>In those ballasts there are four restance elements, two for the filament string, and two for the B+, at least that was the case with my set.<P>========================================<BR>Bill<BR><P>------------------<BR>