JB-150 Tube heating issue help!

[443]

Okay, so I got my supposed "JB12" in the mail today. Turns out it's a JB150. The older one with the 6AQ5A keying tube. Had an RCA 8417 tube in it. After repairing a few broken solder connections inside and replacing the keying diode (1N60P, was 1N34A, and was broken) I talked on it for a while, all seemed well. I noticed that at some point the 8417 was glowing pretty red hot, I unkeyed it went down, it never glowed hot again. Output dropped from 100 watts to 60 watts.

So I thought maybe the tube was just bad, so I pulled a 8417 from my JB-200A and put it in there, keyed right up, 125-150 watts. So I think it's okay, I keyed it a few more times, noticed the tube glowing really hot on one side only, unkeyed, tube still glowing... Finally reached over flip the power switch off, glowing goes away immeadiately, let it sit for a buit turn it back on, everything is cool, sitting there idling, haven't keyed it up yet, wait about 2-3 minutes, no problem. THEN I key it up, and it works like normal. I unkey. I start to notice the glowing in the tube after I have unkeyed it is getting brighter and brighter, and I am not even keyed up... So I turn it off before it boils the tube. What the heck is going on here? Something sticking with the relay? Some kind of oscillation?

Help. I don't want to destroy my good 8417 tubes, so it's turned off till I figure this out...

 

[443]

Update. Sat and watched it for about 5 mins without taking it off standby, keying up, or anything, and the tube started glowing on it's own, turned it off, right back on, starts immeadiately glowing real hot again, turn it off for a few mins turn it back on, and it takes about 3-5 mins again before it starts glowing hot again, never keying it up, never taking it off standby. So I shut it off again, may take it apart again later and see if something else came loose or is touching something else or see if I can find something that looks bad/odd. Any suggestions what could be going on with this thing?

 

[443]

Schematic for the exact amp I have attached.

 

[443]

Trying to figure out the schematic, it looks like the Bias is fed to the 8417 or at least changes voltage when you key up (T and R markings on schematic where it says "Bias" -32(T) -16 (R). Could the bias not be turning off, a problem with the relay? The relay inside looks raggedy, like it really could use a new relay, so I am wondering if that has something to do with it....

 

[unit_399]

443 -

the JB-150 was originally sold as a receive boost amplifier (this is why the schematic says JB-150A R/B.) to get around the FCC ban on 11 meter linears. There was conversion info to linear amp included in the owner's manual. I think I have an old owner's manual in my shop, but I'll have to hunt it up. Anyway... maybe the conversion wasn't done right/completely on your unit. If I can find the conversion info, I will post it in the next day or so.

- 399

 

[443]

443 -

the JB-150 was originally sold as a receive boost amplifier (this is why the schematic says JB-150A R/B.) to get around the FCC ban on 11 meter linears. There was conversion info to linear amp included in the owner's manual. I think I have an old owner's manual in my shop, but I'll have to hunt it up. Anyway... maybe the conversion wasn't done right/completely on your unit. If I can find the conversion info, I will post it in the next day or so.

- 399

Well, I have gone over it pretty well, maybe a few things were touching, or the tube wasn't making good contact in the socket or something, because now it seems fine. But yeah, please do post the info for me so I can check that.

 

[fnkycoldmedina]

the tube should glow blue when you talk on it if all is well. maybe just the tube socket pins werent making good contact if its working good now.

 

[443]

Okay, here's an update on this.

When it starts to glow really bright (during receive not transmit) I key up, and it stops getting brighter. I unkey and it goes right back to getting progressively hotter and hotter glowing red, until I key up again.

So what is going on with that? Should I just completely disconnect the wire for the "receive mode" bias? Doesn't it only need bias on transmit anyway?

Help! I really like this little amp, it performs nicely for a really tiny 1 tuber, but I can't have it burning up $50 tubes during receive...

 

[443]

If the relay contacts were crappy, and it was losing connection on the recieve bias, would that cause it to do this? It's the center set of pins on the relay that switch the biasing, I can try cleaning them, but honestly the relay looks like it's been through WW2 and then some.... Maybe I should just replace it. Anyone know what relay would be a good replacement for this one?

 

[paws264]

When that relay ( the Antenna transfer relay) is normally closed (in receive) , there should be -17 vdc on pin five of the tube. When the amp is in transmit, the diagraphm says that it should have -32 volts on pin 5 of the tube.

Verify that you have the correct voltages on the different poles of the relay; these voltages are coming from the power supply section and, they are controlled by zener diodes.

I would suspect that something is amiss with the bias voltage in standby if the tube is getting reg in standby.

.

 

[443]

One thing I noticed was that someone has been fooling around inside this thing before, looks like they used some crappy lead free solder or some pipe solder or something really lumpy and chunky when you heat it up... I resoldered everything in the bias circuit, going to try it again. So I assume it DOES need the -16 volts or whatever bias voltage in receive/standby mode? Seems like it would only need it in transmit mode... But none theless I re-attached the wire to the relay for the standby bias. Cleaning the contacts on the relay now, will fire it up and check the voltage shortly on those pins (carefully yikes I don't like HV, and it's in the area...).

 

[443]

It looks like there is 2 diodes, and 2 resistors in that bias circuit.

GE 5ZD15
GE 5ZD22
220K resistor
100K resistor

The resistors tested good, the diodes passed the diode test function on the fluke, but I am wondering if one of those (probably the 15V) is leaking... I am assuming 5ZD15 = 15 volt antique zener diode and 5ZD22 = 22 volt antique zener diode.... Geez. It's funny how stuff (parts) have changed since then...

 

[443]

When that relay ( the Antenna transfer relay) is normally closed (in receive) , there should be -17 vdc on pin five of the tube. When the amp is in transmit, the diagraphm says that it should have -32 volts on pin 5 of the tube..

This is in reference to "chassis ground" I assume?

 

[443]

Bias voltage in receive starts out at about -23.5VDC and after a few mins when I start to see the tube getting red hot, it has dropped to about -15VDC.

You can watch it progressively drop.

What should I do? How many amps are those zener diodes that are in there now? I have tons of new zener diodes here, they are all the 1 amp variety, glass type, but none bigger, these in this amp look big, however that may just be because they are old. I am assuming the Zener diodes are the problem.

 

[443]

I am assuming the ones in this amp (zener diodes) are 5 watt, they are pretty big, at least 3 times the size of the zener diodes I have here, which are not 1.0A as I stated earlier, I meant 1W(watt). So I don't know if that's big enough for the bias current, I did not measure it yet. Will check it.

I found where I can order 5W 15V and 22V zeners, I can have them here on Friday. Let me know if you guys think that's my problem or not, will check this again later tonight or in the morning. Thanks for the help BTW.