HUM preamp heater short?

lowell · 11-09-2009, 06:29 PM

got a twin reverb, master volume, center tapped power transformer. A really loud hum kicks in randomly. I thought it was a power amp/supply issue, but just pulled PI tube and it went away. anyone dealt w/ this issue? does it sound right that an internal tube heater short would cause a REALLY loud hum but not blow the fuse? FYI the power transformer gets REALLY hot if amp is left on w/ the hum for too long. I realized this the first time it happened and have been shutting it off immediately when it happens from then on. Filter caps have been replaced a couple months before this issue.

MWJB · 11-09-2009, 07:39 PM

The PT doesn't get warm when there is no hum?

Sounds like a noisy preamp tube, put the PI back in but pull everything else...does it cure the hum? If so replace V5...and so on...

Check for preamp sockets that have lost their tension, turn everything full up & rock the preamp tubes in their sockets (no sheilds on for this). Retension/replace any loose sockets.

Jazz P Bass · 11-09-2009, 08:03 PM

That hot tranny is a good clue that something is pulling power.
I would pull the output tubes first.
A plate to heater short could be your problem.
Think about it. The amplifier is outputting a tone.
How does a tube amp output a tone?
The output tubes modulate the B+ voltage into the output transformer.
If the AC heater voltage (or some other anomalous voltage trickery) is "riding" on the B+, presto, a tone is output.
Something is amiss.
The trick is to find it.
Happy Hunting.

lowell · 11-09-2009, 08:05 PM

should've mentioned I tried new power tubes and still was issue. replaced PI and it's been on for 45 minutes w/ no problem. I wiggled all the other preamp tubes and seems fine. Will leave it on all day and keep testing.

Enzo · 11-10-2009, 09:00 AM

Certainly the heaters can short to something without blowing a fuse. And that would likely cause hum wherever it might have been.

11-09-2009, 06:29 PM	#1
lowell Senior Member Join Date: Jun 2007 Posts: 1,121	HUM preamp heater short? got a twin reverb, master volume, center tapped power transformer. A really loud hum kicks in randomly. I thought it was a power amp/supply issue, but just pulled PI tube and it went away. anyone dealt w/ this issue? does it sound right that an internal tube heater short would cause a REALLY loud hum but not blow the fuse? FYI the power transformer gets REALLY hot if amp is left on w/ the hum for too long. I realized this the first time it happened and have been shutting it off immediately when it happens from then on. Filter caps have been replaced a couple months before this issue.

11-09-2009, 08:03 PM	#3
Jazz P Bass Senior Member Join Date: Dec 2008 Location: Wernersville, PA Posts: 739	Hot Power Transformer That hot tranny is a good clue that something is pulling power. I would pull the output tubes first. A plate to heater short could be your problem. Think about it. The amplifier is outputting a tone. How does a tube amp output a tone? The output tubes modulate the B+ voltage into the output transformer. If the AC heater voltage (or some other anomalous voltage trickery) is "riding" on the B+, presto, a tone is output. Something is amiss. The trick is to find it. Happy Hunting.

11-10-2009, 09:00 AM	#5
Enzo Senior Member Join Date: May 2006 Location: Lansing, Michigan, USA Posts: 10,332	Certainly the heaters can short to something without blowing a fuse. And that would likely cause hum wherever it might have been. __________________ Education is what you're left with after you have forgotten what you have learned.

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11-09-2009, 07:39 PM	#2
MWJB Old Timer Join Date: Jun 2006 Posts: 1,924	The PT doesn't get warm when there is no hum? Sounds like a noisy preamp tube, put the PI back in but pull everything else...does it cure the hum? If so replace V5...and so on... Check for preamp sockets that have lost their tension, turn everything full up & rock the preamp tubes in their sockets (no sheilds on for this). Retension/replace any loose sockets.

11-09-2009, 08:05 PM	#4
lowell Senior Member Join Date: Jun 2007 Posts: 1,121	should've mentioned I tried new power tubes and still was issue. replaced PI and it's been on for 45 minutes w/ no problem. I wiggled all the other preamp tubes and seems fine. Will leave it on all day and keep testing.

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