Hi everyone. I have an Admiral 7T01, Chassis 5N1 (Rider page 17-9) that I have recapped. It works great, no hum, good selection, etc. --as long as I don't turn my variac past 90 volts--. As soon as I exceed 90 volts, it blows the dial lamp (fuse) when I turn it on. Even at 90 volts and below, the lamp is real bright when first switched on. Then it dims to normal. Any ideas? I'm a newbie so any help is appreciated! Thanks.
Steve

I am not sure that this is it... but Check the rectifier make sure its wired properly also check pins 2,3,7... make sure you have continuity from 2-7 and 2-3 and 3-7

because if you have an open between 2-7 but good from 3-7 then it appears that the lamp becomes part of the heater chain.

Two things to check:
Step one.....
Since the lamp and a selected / designed tap on the 35Z5 are used in parallel tandem , sharing a load
see if possibly the tap portion is open to pin 2...or possibly opening under voltage application.

Step two......
Enact a constant DC monitoring across R9 (33 ohm) and see if that DC voltage exceeds a 4.95 voltage
level as U bring up the Variac input voltage. ( The 150 ma lamp threshold). If the 35Z5 was clean of fault then start shedding loads off the main B+ line just past R9 to find out where the current hogging is. The AF output stage, of course, being the most suspect . After that suspect some component across the main B+ being resistively or partially shorted.

73's de Edd

:Hi everyone. I have an Admiral 7T01, Chassis 5N1 (Rider page 17-9) that I have recapped. It works great, no hum, good selection, etc. --as long as I don't turn my variac past 90 volts--. As soon as I exceed 90 volts, it blows the dial lamp (fuse) when I turn it on. Even at 90 volts and below, the lamp is real bright when first switched on. Then it dims to normal. Any ideas? I'm a newbie so any help is appreciated! Thanks.
:Steve

______________________________________________________

Were all the caps and filters replaced? Are the filters the proper value? Have you checked the resistors for proper value? Have the tubes been checked for shorts?

From your description there is an excessive B+ load which could be caused by the items I mentioned or a wiring error, recheck your wiring.

Dave

I replaced all the caps and filters and checked all the resistors - R11 was missing when I got the radio, so I installed one (hopefully in the proper location.) All tubes checked OK. I will go through everything again and get back to you.

The 35Z5 is notorious for having a blown pilot lamp filament tap causing your symptom. The set will work and the rectifier is fine otherwise.

Only remedy is to move the tube to a set that would not need this pilot lamp tap.

See my Service Tip 40 here for more info: http://www.philcorepairbench.com/tips/svctip40.htm

That's right! It's the 35Z5. If you don't have a spare tube at hand, temporarily patch across pins 2 and 3 with a 1 watt 40 ohm resistor.

In the future, never operate the radio with a burned out pilot lamp. Someone might have done this prior to your owning the set. This stresses the lamp tap of the 35Z5 (part of heater between pins 2 and 3). If you always keep the pilot lamp in good order, the lamp tap of the 35Z5 will probably be the last thing to fail on your next tube.

T.

Hi Chuck:
I read your 35z5 pilot lamp tip and had already been relatively familiar with the causes and so I followed it well.
However at the risk of seeming a little dumb .. I didn't follow how you arrived at:
""The value of resistor "R" is 300 ohms [5 watts] when a #40 or #47 pilot lamp is used. Should the receiver not use any pilot lamp then a resistor of 800 ohms [10 watts] must be shunted across the tapped section.""

Could you please elaborate on how/why the wattage and particular R values were selected?

:Hi Chuck:
:I read your 35z5 pilot lamp tip and had already been relatively familiar with the causes and so I followed it well.
:However at the risk of seeming a little dumb .. I didn't follow how you arrived at:
: ""The value of resistor "R" is 300 ohms [5 watts] when a #40 or #47 pilot lamp is used. Should the receiver not use any pilot lamp then a resistor of 800 ohms [10 watts] must be shunted across the tapped section.""
:
:Could you please elaborate on how/why the wattage and particular R values were selected?
:

:Hi Chuck:
:I read your 35z5 pilot lamp tip and had already been relatively familiar with the causes and so I followed it well.
:However at the risk of seeming a little dumb .. I didn't follow how you arrived at:
: ""The value of resistor "R" is 300 ohms [5 watts] when a #40 or #47 pilot lamp is used. Should the receiver not use any pilot lamp then a resistor of 800 ohms [10 watts] must be shunted across the tapped section.""
:
:Could you please elaborate on how/why the wattage and particular R values were selected?
:

I didn't do any of the "math." The total series-string heater current was used in the calculations, no doubt. The Tip No. 40 is a re-print.

Here's the info from the article header:
Re-printed from Radio Circuit Hints, Volume One, pp. 56-60. Compiled and published by Sylvania Electric Products Inc., Copyright 1943, First Edition

Chuck

:Hi everyone. I have an Admiral 7T01, Chassis 5N1 (Rider page 17-9) that I have recapped. It works great, no hum, good selection, etc. --as long as I don't turn my variac past 90 volts--. As soon as I exceed 90 volts, it blows the dial lamp (fuse) when I turn it on. Even at 90 volts and below, the lamp is real bright when first switched on. Then it dims to normal. Any ideas? I'm a newbie so any help is appreciated! Thanks.
:Steve

Steve,

Check how the pilot lamp is wired in. It should be between pins 2 and 3 of the rectifer socket. Looking at the bottom of the tube socket pin 1 is the first pin to the left of the index of the tube and count clockwise 1 thru 8. Look close while counting as some pins of the socket may have been omitted, but you can see where the pin would have been.

Radiodoc

Doc... "to the left of the index?"
I think you meant to the right when looking at the bottom right?

:Doc... "to the left of the index?"
:I think you meant to the right when looking at the bottom right?

Peter,

Thanks. I was in a hurry when I made the post. I forgot to mention the index pointed at 6 o'clock.

Radiodoc

lol... Doc:
You are quite right of course.
.. BUT...now I feel so dumb.
Y'know I NEVER even ever considered anything ....didn't even enter my mind that the index could EVER be at anything other than 12 o'clock.... now don't I feel dumb.. really dumb... because I would never have thought of that.. Why??...lol

:lol... Doc:
:You are quite right of course.
:.. BUT...now I feel so dumb.
: Y'know I NEVER even ever considered anything ....didn't even enter my mind that the index could EVER be at anything other than 12 o'clock.... now don't I feel dumb.. really dumb... because I would never have thought of that.. Why??...lol
:

Peter,

I over the years just got used to looking at the socket with the index in the view from China.

Radiodoc

Thanks everyone. It looks like the filament tap is the problem. What I still don't understand is why a lamp will work up to about 90V (and sometimes to 120V if I increase it slow enough) if the tap is open. Could it be broken and only open when hit with inrush current?
Steve

:Thanks everyone. It looks like the filament tap is the problem. What I still don't understand is why a lamp will work up to about 90V (and sometimes to 120V if I increase it slow enough) if the tap is open. Could it be broken and only open when hit with inrush current?
:Steve
Sorry - didn't finish before posting. I think I understand the Service Tip from Chuck, but I'm not sure what it means in my situation. Can I install a new lamp along with a 300 ohm resistor and continue to use this tube without damaging it (the tube) further - or blowing the lamp? Or is this only to be used with a "good" tube? Thanks again to all.
Steve

Depends on which pins are open
Can you tell us where you have contiuity and where not?

If you have an open from pin 2 to pin 7 and 3... but you DO have continuity from pin 3 to 7.... then it is better not to use the tube. Because the filament string starts at pin 2 and should exit on pin 7... but if pin 2 is open then the total filament current will enter the lamp at pin 2 and continue through the lamp to pin 3 and then on throuch the 2nd part of the heater only then out pin 7 and onward to all the other heaters.
The lamp cannot handle all that especially when the filaments are cold...
Plust the 35Z5 would only be operating on part of it's heater.

But... so anyway.. which pins are open?

Pins 2-3 and 2-7 are open with continuity between 3-7.
Steve

:Pins 2-3 and 2-7 are open with continuity between 3-7.
:Steve

I just explained how you can use your current tube. Use a 40 ohm resistor across 2 and 3.

Seems strange that the reprint suggests a 300 ohm resistor with a pilot lamp, and an 800 ohm resistor without the lamp. Details seem to have been left out. If the radio was wired in the same manner as if it would have a pilot lamp, but doesn't, the lamp tap of the 35Z5 would burn out quite rapidly (within a few months of regular usage...according to personal experience). However, radios that don't have pilot lamps are never wired with the plate current being drawn through the lamp tap. Instead the plate is either connected directly to the line cord through a resistor, or the line cord is connected to pin 3, and the plate to pin 2, which puts plate current through the lamp tap, but no other current flows through it. It serves purely as a fuse. In these two situations, the lamp tap will be no more likely to burn out than any other filament in the radio. No additional protection is needed. In the former situation, the lamp tap will glow at normal brilliance, just like the rest of the tube's heater. In the second situation, the tap portion of the heater will light very little, if at all, and will probably never burn out unless a short occurs within the radio.

The 300 ohm resistor is a fair idea for protecting the lamp tap in radios equipped with pilot lamps. However, to-day we have another device that'll do a far better job. It's called a Zener diode. Place a 6.8 or 7.0 volt zener across the lamp tap and the voltage across that point will never get above the zener's rating. Place the diode so that it regulates in the direction of plate current flow (both AC and DC flow through the lamp tap). In one direction a zener passes current like any diode, and in the other it regulates. If you place it so that it passes current freely when the plate is conducting, it'll cancel out the plate current's effect on the pilot lamp. This is why it is necessary to orient the diode so that plate current does not get bypassed. Orient the diode for maximum illumination.

Should the pilot lamp ever burn out, the zener will protect the lamp tap because it'll pass the excess current.

Thomas

Also, the reason why your pilot lamp, in its current situation, glows well at 90 volts, but burns out at higher voltage, is because at 90 volts the radio won't heat up enough to draw much plate current. The pilot lamp is mostly seeing heater current, which is 150 ma, the same as what the pilot lamp is rated for. However, if you heat up the tubes properly, plate current starts being drawn. The bulb now glows too bright with too much current being drawn through it.

Also, don't think that you can connect the pilot lamp directly in series with the heaters if you simply remove the plate current. It heats much faster than the heaters, and will be blown with a cold turn-on. The only reason why you are able to light it successfully is because you are bringing the radio up slowly. Try turning it on cold at 90 volts. The only solution in the 1940s was to put a resistor across it that'd protect it when the tubes are cold. This would be the lamp tap of the 35Z5. The other feature of this lamp tap as opposed to a resistor is that it significantly changes resistance as it gets warm, and has a very low resistance when cold, which further protects the pilot lamp. However, the 35Z5 lamp tap resistance prevents the pilot lamp from lighting at full brilliance. Some other current source must be drawn through the lamp in order to give it full current. This is where the plate current comes in. This clever set-up allows pilot lamp protection when cold (better than anything else they could come up with at the time), and full current when it's safe to allow full current. A problem with this system is that the radio's electrolytics do surge the pilot lamp a bit when the radio is turned off and right back on, but it isn't usually enough to burn out the pilot lamp.

Again, placing a zener diode across the pilot lamp is the modern way to afford complete protection to both the lamp and the lamp tap. You could technically have the zener do all the work on its own, but you shouldn't. The 35Z5 lamp tap will afford extra protection to both the zener and the pilot lamp. All three will make a nice set-up.

Thomas

I have to add something to the zener suggestion. This protection will work superbly. However, I just did some experimentation and found that one zener won't work properly because of the rectification action of the diode. Current flowing in one direction will be completely blocked out to the pilot lamp (will bypass the lamp through the zener) because zeners pass like a regular diode in one direction, and do the regulated passing in the other. However, if you wire two diodes back to back (one out of polarity to the other--for instance, face the black lines towards eachother), you will secure perfect results. As I said before, a zener passes like a regular diode in one direction, but only passes in the other direction after a certain voltage is reached. Two zeners wired back to back will work like this: one may pass all current, but the other in series with it but in opposite polarity will only pass current if it is above the zener level. The same holds true when current reverses direction, as it does with alternating current. With this set-up, current will only be passed around the pilot lamp when it is above the level you wish to maintain, no matter which way the current is flowing (since we are dealing with alternating current).

Also, I have been using 6.8 volt diodes since I have many at hand. The brilliance is quite acceptable, but you may have more satisfactory results with 7.0 to 7.2 volt diodes.

Thomas

THis idea can also be used with much success without utilizing plate current or the 35Z5 lamp tap in the pilot lamp circuit. You can now add pilot lamps to any radio by simply placing the assembly in series with the filament string. Use two diodes back to back in parallel with a pilot lamp. This can be used in any AC/DC radio.

Thomas

Many thanks, Thomas.
Steve

Also, when using this technique with radios that utilize .3 ampere tubes (25Z5, 25L6, etc.), use a #44 bulb so that the diodes don't have to take most of the current. The diodes should not get too warm, but if they do, they can be paralleled. Make sure that the paralleling diodes face the same direction (polarity). If both of the diodes originally selected have the marked ends facing eachother, the paralleling diodes must as well.

Thomas

Thanks to everyone. I think I'm beginning to see the picture here. I will try these ideas and let you know the outcome. Again, many thanks to all.
Steve

Y'know it's kinda funny...I enetered the first reply suggesting the solution:
....and after about 23 entries later here... the solution turned out exactly that.. lol

Sorry Peter, just takes me a while to put all the pieces together. Thanks for your help. It is much appreciated.
Steve

Yep, you were right! I was going to say that you were the one with the right idea, many entries later some others came up with the same one.

T.