Hum/Static/Aural Assault from out of nowhere...

[dehughes]

Yeah, another troubleshooting thread (wish I was all-knowing):

My amp just decided to start making this very loud hissy/squacky/static-y noise (think bad ground + the squawky noise you get when probing in the signal path with a DMM) literally out of the blue (okay, preceded by a slight "pop" about a minute before the downpour of noise), and after swapping out ALL tubes with KNOWN good ones, it's still at it. I started measuring voltages and poking around at solder joints and came across about 44vDC on the PI grids, and about 68vDC on the cathode. The PI setup is a (relatively) typical long tail pair...1M to grids, 820 to cathode, tail being a 47k to ground. PI plate voltage is in the 220vDC range. Power section is a 6V6 cathode biased pair. Tube recto.

When last I measured the cathodes of the PI they were in the 54-56vDC range so maybe this points to something being up with the PI? I'm sure stumped...

WHAT'S MORE, when I put the DMM lead on the cap to the PI input grid, the noise almost all but goes away (can just hear it faintly in the background). Something's just not right... This amp has been fine for weeks (about a month or two old....just finished the build...), and then this? All I did today was swap out tubes to try and find some that suited this amp's inherent tonality, so this seems to be a surprise with no apparent causality.

[Structo]

Interesting, I wonder if the cap went bad?

[dehughes]

Interesting, I wonder if the cap went bad?

I was thinking that...but I wouldn't know what a bad cap would look like. I suppose I could test for DC voltage on the NON-PI side of the cap....I'll have to do that tonight or tomorrow... It's times like these when I wish I had more test equipment and troubleshooting skill.

Amps are like kids....most anybody can make one, but few know how to keep them in line.

[LOUDthud]

Probing the grids of a long tailed PI does not give reliable readings because the meter changes the operation of the circuit. Measure across the 1 Meg resistors to look for leakage.

Does the volume control affect the noise? Noise originates before the volume control if it does. Does pulling any one tube make the noise go away? Noise originates at that stage or before if it does.

[dehughes]

OKAY, GET THIS!

First of all...thanks guys for your help. So greatly appreciated...

I was going through the amp measuring voltages, etc.., to see if I could find out what the problem might be. Well, I was measuring the plates of the recto tube and found that one side was measuring about 340vAC and the other side was switching between 120 to 133vAC. Huh.

So, I took the tube out and measured the voltages. Same deal...

Then I put in three different rectifiers (Chinese 5AR4, JJ 5AR4, JAN 5Y3, and Weber Copper Cap). Same deal....one side about 200v higher than the other.

THEN, when I had the Chinese recto in there, for a brief second the noise went away! I checked the DMM and the voltage had risen to about 290v on the "bad" side....it had quickly risen about 150v in a couple of seconds.

THEN, as I watched, the voltage dropped back down to about 130v and the NOISE CAME BACK!

So, do I have a bad PT??? I'm beginning to think so... It has always hummed (both audibly in terms of a rattly sound, and in terms of hum induced into the circuit), but it always worked. I've never come across this before, so I'm hoping you all can steer me in the right direction. I don't have a scope, but I do have a good DMM and have been reading through Darr's troubleshooting chapters.

Thanks!

[LOUDthud]

You may have found the problem. Try measuring the resistance from each rectifier plate to ground with the power off. There is a small chance you could locate a broken wire or bad solder connection inside the transformer if you take the end plates off. Otherwise, replace the transformer.

[dehughes]

You may have found the problem. Try measuring the resistance from each rectifier plate to ground with the power off. There is a small chance you could locate a broken wire or bad solder connection inside the transformer if you take the end plates off. Otherwise, replace the transformer.

Will do. Thanks!

Okay, here's what I found:

With the amp off, the "good" plate measures 127.7 ohms to ground, whereas the "bad" plate measures infinite resistance. Even at the 20M setting on my DMM, I'm still getting infinite resistance to ground.

What does this indicate?

[Andy Le Blanc]

Open winding, 127.7 is the secondary dc resistance, the large meg. reading is the insulation.

[dehughes]

Open winding, 127.7 is the secondary dc resistance, the large meg. reading is the insulation.

Dah! Oh well....at least I figured it out. Thanks dudes. Much appreciated. I'll post back once a resolution is in hand...

[rawnster]

Hey David, what's the latest here? Get it fixed?

[dehughes]

Hey David, what's the latest here? Get it fixed?

Received the replacement PT from MM (brand spanking new, as opposed to the "new" one that broke...made in 2004....), and sold that without testing it in the amp because I have a PT from Allen Amps arriving tomorrow which will put out about 20v less on the B+ winding, which should drive my 6V6s softer...just the way I like it.

I SHOULD be able to get it installed by tomorrow or Wednesday, but we'll see. Either way, it needs to be finished by the 27th!!!