[marty]

Hi All;
Thank You Brenda Ann, So far here is what I have found so far.. This round.. I like the Thinking that Brenda is leaning toward.. Also, yes the Dial lamp is quite dirty, but untill I can dis-assemble the tuning mechanism, I can't get to it for now..
It went about 5 minutes before it popped.. I checked the middle of the Volume control both before and after it went pop.. The before I got a Humm, after Nothing..
I re-checked the Filaments on the 42 Tube before and after the pop with my meter set manually.. It reads 5.85 to 5.89 Volts on AC.. No change after the pop.. The lamp is still lit after the pop..
I have a Large Bog Bone resistor, that gets warm, not hot but warm.. it is a 32K ohm R57, it shows 89 V on one side and 290 on the other side, before the pop.. When it pops the voltages on that resistor jump up to 97 volts and 320 volts..
And what Brenda Ann says about a B+ short makes sense, just have to find what or where.. I think once we find it, it will be one of those Homer Simpson moments..
I am leaning toward a Resistor that changes value after warming up or a cold solder joint..
Unfortunately, to get to the sockets, there are two retangular plastic boards that most of the Resistors and Capacitors are mounted to, that have to be unwired to move these holding boards out of the way to get to four of the Tube sockets.. And I don't want to unwire these boards just to get to the scokets.. I guess, I will need to follow each Socket pin to a connection on one of the Boards and check voltages that way, unless anyone has another suggestion..
http://www.nostalgiaair.org/Resources/582/M0013582.htm
THANK YOU Marty

[jerryhawthorne]

Often wrong but checking the filament voltage on the output tube really does not tell you if the tube filament is opening as your measuring across a set of parallel filament lines, not just that tube. Sorry. The big jump in voltage across that resistor could be important and indicates something has quit drawing power along that line. The one thing that draws a lot is the output tube. Try a new one!
Jerry

Still nice to know if you observe the filaments of all those tubes going dead after the pop. I would guess not as you indicated the dial light stayed on.

[marty]

Hi All;
Thank You JerryH, The Dial lamp is at the end of the string.. I know that measuring a parallel string, will not show if that one tube has gone out, but It did tell me that both before and after the pop I still had voltage and so rulled out the Transformer.. I do not have any other 42 Tubes, Is there something else that I can Temporarily use to test this Theory out ?? Can I use a 41 ?? I have two of them..
THANK YOU marty

[Mondial]

If you have audio before the pop, but don't get a hum after when touching the center of the volume control, then the problem seems confined to the audio stages. My guess would be resistors 36 or 37, which are the plate resistors of the 75 tube. If one of these 70K resistors opens as it warms up, it could be the cause of your problems.

Check the plate voltage of the 75 triode section before and after the pop. If you lose plate voltage then one of the reistors is opening and that is the cause.

[marty]

Hi All;
Thank You Mondial, I will Check those resistors on this next run thru..
THANK YOU marty

[Radioroslyn]

You can switch the 41 for a 42 in this set. If your heater is going out on the 42 tube it isn't going to be drawing any current from the HV. That's what's causing the voltage to change on your large resistor.
Terry

[marty]

Hi All;
I have taken voltages on the two resistors, before the pop, I get the following voltages, 256 Volts, R36, 216 volts, R37, 180 volts to the plate of the 75 tube.. After the pop I get the following voltages, 349 volts, R36, 260 volts, R37, 179 volts to the plate 75 Tube..
I do not have another 75 tube except for the Origional Tube, which has a loose base.. I do have another 85 Tube If I can Sub that..
THANK YOU marty

[marty]

Hi All;
Thank You, RadioRoslyn, I will try the sub and let everyone know what happens..
THANK YOU marty

[marty]

Hi All;
So far with the 41 tube in place of the 42 tube, I have the following voltages, 245 volts, R36, 209 volts, R37, 179 Volts..
Its still working.. Eight minutes and counting.. Fourteen Minutes and Counting..
THANK YOU marty

[jerryhawthorne]

Marty, I'm waiting for someone to respond to your readings as it appears to me from all voltage readings except the plate of the 75 indicate a lack of load on them occurring when the pop appears and yet the 75 plate does not go higher as well. I'm confused and waiting for the smarter people to help me learn.
Regards, Jerry

Terry came in and it looks like the tube sub might have fixed it? Still confused as to why?

[marty]

Hi All;
Thank You Jerry, I agree I am confused, as I replaced the 42 tube with the 41 tube and its still working and no popping so far.. And the voltage reading I took were from the Resisitors for the Plate of the 75 Tube which I have Not replaced.. But the voltages for it look OK so far..
There does seem to be more HUMM with the 41 tube, than the 42 Tube..
It is going on 20 minutes so far..
THANK YOU Marty

[Mondial]

Yes, it appears that after the pop, there is a loss of load current from the 42 output, which causes the B+ voltage to rise. The 75 plate voltage does not change because the loss of load causes the bias on the 75 grid to drop, compensating for the rise in B+ and keeping the plate voltage relatively constant.

Remember that the bias votages for the radio are obtained from the voltage drop across the wirewound bias resistor in the negative return of the supply. If the supply current drops, then the bias voltage will also will drop, causing a corresponding increase in plate current.

It appears that the 75 plate resistors are fine. The problem seems to be loss of plate current of the 42, which looks like a bad tube.

[jerryhawthorne]

Well, once we both have the answer, it has certainly been a learning experience. I will check my stash and see if I have a working 42 tube. If I do and you need it (no charge) I will zap it off to you. Fond memories of living in Boulder in the mid 50s. I'm certain the city has changed a lot.
PM me if you need one and I will let you know what I find. I do think I have at least one.
Regards, Jerry

[jerryhawthorne]

Mondial, thank you for that explanation you were typing it while I was wondering. It certainly helps explain something that was not intuitive to me (as many things are not). Onward and upward Marty.
Jerry

[marty]

Hi All;
Can I keep the 41 tube in here for very long ??
And THANK EVERYONE for their awsome Help..
THANK YOU Marty