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Power tube heaters

No Treble

No Treble

Oct 26, 2016
125
209
1994
#1
Not sure if this should go here, but I didn't see a more appropriate place for it.

Lastnight, I was rocking out, and suddenly, a slow fade of volume occured, until no volume at all was heard anymore. Maybe 10 seconds from nice to dead. Looking at the amp from behind (Ampeg V-4BH), all the dang tube heaters were OFF! AAARRGGHH! I opened her up today, and I don't see anything blatantly suspicuous, until I saw a swelled capacitor. Are there any "domed-top" capacitors in use? Seems like it has to be that, but since I can't confirm, I figured I'd ask. Can't seem to find it on the wiring diagram, and I have questions. What would make this happen? All 4 power tube heaters were off, so it's fairly easy to rule them out. It has to be a common source from somewhere else.

Any ideas?
Thanks in advance.
 
#2
You know what, thick-headed guy (me) found it. On the schematic, it's "C17". It's on the midrange tone selector control circuit, though. Seems like that shouldn't be related, even if it is a problem. All the fuses look ok, so I dunno. Still searching...
 
#3
Most heaters run off ac, straight from a tap on the power transformer, with no capacitors involved. What makes you think they’re off?

A capacitor with a dome on the end does indicate trouble - most likely in a high voltage supply.
 
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#4
micguy said:
Most heaters run off ac, straight from a tap on the power transformer, with no capacitors involved. What makes you think they’re off?

A capacitor with a dome on the end does indicate trouble - most likely in a high voltage supply.
Click to expand...
Well, when I look at it powered up, they glow so beautifully, but when it quit last night, they were not glowing with their typical splendor. Says to me, dang heaters are off!

Researching caps now. Hopefully, I can find one. I've had it opened a few months back, and I don't remember seeing that swelled cap.
 
#7
No Treble said:
Not sure if this should go here, but I didn't see a more appropriate place for it.

Lastnight, I was rocking out, and suddenly, a slow fade of volume occured, until no volume at all was heard anymore. Maybe 10 seconds from nice to dead. Looking at the amp from behind (Ampeg V-4BH), all the dang tube heaters were OFF! AAARRGGHH! I opened her up today, and I don't see anything blatantly suspicuous, until I saw a swelled capacitor. Are there any "domed-top" capacitors in use? Seems like it has to be that, but since I can't confirm, I figured I'd ask. Can't seem to find it on the wiring diagram, and I have questions. What would make this happen? All 4 power tube heaters were off, so it's fairly easy to rule them out. It has to be a common source from somewhere else.

Any ideas?
Thanks in advance.
Click to expand...


I don't have the schematic for that amp, but unless the heaters are DC they should not be connected to a capacitor. It's more common to run preamp tubes on DC since output tubes tend to require quite a bit of current. A swelled capacitor sounds ominous. They can explode with significant force and leave behind a stinky mess. I suggest taking the amp to a good tech.
 
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#11
micguy said:
Most amps have 1 circuit for the heaters on all the tubes - if the heater circuit failed, the preamp tube heaters would go out. Are the preamp tubes glowing?
Click to expand...
Well, that's a big nothing, over. No heaters on any tubes glowing. Won't give me a red "standby" LED, either. No output. That seems like a problem with the power transformer, then. Dang it.
 
#16
The cap is totally unrelated to your tube heaters. It's quite possible there's nothing wrong with the cap either, especially if it has a Lexan cap on the top. A little bulge of the Lexan is entirely normal.

Here's where understanding the circuit and systematic troubleshooting and measurements beats guessing by a mile.

Looking at the schematic, a couple of simple measurements should quickly narrow down the culprit. I can guarantee that it's not a cap that's causing your symptoms.
 
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#18
I only saw the swelled cap, and suspected something. I know they aren't normally like that, but after looking at the schematic, it shows that cap in the tone control circuit. Yes, most likely unrelated, and who knows if it's normal for them to appear swelled.That said, I am trying to find the power source for the tube heaters...
 
#19
No Treble said:
Well, that's a big nothing, over. No heaters on any tubes glowing. Won't give me a red "standby" LED, either. No output. That seems like a problem with the power transformer, then. Dang it.
Click to expand...


I had something pretty similar happen to an amp. The heater circuit was fused, and the fuses and fuse holders were getting hot enough enough to melt solder and char the circuit board. The heaters stopped working when the solder dripped out of the joint that connected the fuse holder to the circuit board. It's possible your transformer is shot, but also possible there is a broken connection somewhere.

If you know how to safely take measurements with a multi-meter, measure the transformer heater taps. The voltage should be around 6.5V AC. If you don't know which taps are the heater taps, you are getting into dangerous territory as the taps for the plate supply will have some serious voltage.

If the amp has swollen capacitors it really should not be powered on, especially with the capacitors exposed where an explosion could hurt you.

No Treble said:
This one. Still following the current path. Looks like the heaters are jumpered to H1 and H2, but I am still flipping through the other 2 pages to put it all together.
Click to expand...

Not always easy to track down the heaters. The preamp schematic shows a DC heater supply. I believe the yellow, yellow, green taps on the bottom right side of the power transformers are the heater taps. Notice the 10A fuse. This is probably where the open circuit occurred. FYI A bad tube can short between the heaters and cathode. One potential reason for the failure.
 
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#20
First check the back of the amp. Ensure that the speaker impedance switch is set properly. Reset it just to be sure that it isn't stuck between settings.

Make sure that all the tubes are properly seated in their sockets.

There is a C17 power supply electrolytic capacitor, node C, 47uF @450V. I suspect that this is the one you have identified as having a problem.

Post an image of the cap we can see the extent of the swelling. Capacitors do fail, when they do they can swell.

Re-seat the connectors in the amp. All the tubes should be labelled and removed. The amp should be turned on in playing mode, standby off. The heater and power supply voltages should be checked with a meter and compared to the references on the schematic.
 
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