PSE Hum with no tubes in

[glassesnvests]

Hey guys, head scratcher here.
I've been looking at my friend's Bad Cat Hot Alley Cat, a 7 watt two channel Parallel Single ended el84 kind of beast, and it's got an issue.
It hums. Loud. There is a loud hum at start up, then it fades a little, then gains in volume as the amp warms up. Barely any signal through, and hum is not affected by volume, tone, master volume, or the presence or absence of a guitar plugged in.
So far I've:
Replaced/checked all power supply resistors and caps
Replaced/checked for shorts in power supply and diodes and power and pre tube socket pins
Replaced tubes
Somehow got Bad Cat to give me a new OT, installed it, same problem.
 
Did I mention that the symptoms don't change/go away when there are no tubes in the amp?


I'm at the end of my troubleshooting list, and am needing experienced guys to help.
Thanks!
PS Bad Cat's emailed me to say that they don't have the schematic for the Alley Cat anymore. So no schem. I can get pics up if I need to.

[PRR]

Power supply negative lead. Where does it join the rest of the circuitry?

[glassesnvests]

It hooks into a star ground on a pt lug. All ground points in the amp come to this lug. Preamp, power cathodes ground here, along with speaker jack ground wire.  Other side of the PT is the diode rectifier ground that goes straight to the 3 prong ground.

[hesamadman]

It hooks into a star ground on a pt lug. All ground points in the amp come to this lug. Preamp, power cathodes ground here, along with speaker jack ground wire.  Other side of the PT is the diode rectifier ground that goes straight to the 3 prong ground.

I know this might be a tough rout to go since its probably circuit board wiring but maybe trace down the circuit and disconnect between the power amp and preamp...... install an input jack via alligator clips to the input of power amp and maybe see if you have the noise in the output section. Ive had a lot of luck troubleshooting this way. Narrows it down quite a bit anyway.

[VMS]

I think the rectifier ground should connect directly to the negative side of the first filter cap, because this is probably the noisiest part of the power supply.

[glassesnvests]

I know this might be a tough rout to go since its probably circuit board wiring but maybe trace down the circuit and disconnect between the power amp and preamp...... install an input jack via alligator clips to the input of power amp and maybe see if you have the noise in the output section. Ive had a lot of luck troubleshooting this way. Narrows it down quite a bit anyway.

Actually it's pretty neatly point to point on turret strips. Not a lot going on in the amp, the most complex thing is the channel switching relay.


The problem most certainly is in the output or power section since the hum happens with no power tubes in at all, right? the owner said that he had it in an iso cab backstage and it had gotten pretty hot, after which it started developing problems.

[eleventeen]

If the OT is a standup type, can you remove two mounting screws, suck up some wire slack in the leads, and rotate the thing? (or, the PT, for that matter)


I am not clear if this was a kit/factory built thing.


Maybe you have coupling between the two trannies. Yes, far fetched, even more so in a fact-built something. But if it *is* happening and you don't positively remove the possibility, certainly nothing else you do is going to fix it. 

[John]

it had gotten pretty hot, after which it started developing problems.

If it only got the bad hum after it got hot, you almost have to think bad component, which you've already checked or replaced the obvious.

[clyde]

Boy would a schematic ever help.  If it does it with no tubes, then somehow power supply noise (AC) is coupling directly through the OPT to the speaker.  Or, a ground is faulty causing the same symptoms.  Very strange indeed.

[shooter]

does the hum remain constant, or does it change when you "play around inside" with power on, like hooking up meters etc.?

 

gains in volume as the amp warms up

Is this a constant?  every time you power up?
 can you measure it?
can you "feel" the hum in either tranny?

anyway, that's just more questions

[PRR]

? the rectifier ground should connect directly to the negative side of the first filter cap

+1

[glassesnvests]

Alright, so an update:


hum stays constant whether I move the grounds around.
This is a commercial build, by Bad Cat, made in 2005. It's been working since then, until now, so I'm assuming there wasn't a layout problem to begin with.

Tried changing the orientation of the OT as a last ditch, no effect.
Disconnected B+ from the output tube socket, no surprise no hum.
Which got me inspecting closer...
To have a hum, you have to have current through the OT, which shouldn't be possible with the tubes pulled (and no FB circuit). So the current has to be coming from somewhere...


There are caps connecting pins 7 and 9 on the output tubes, which correspond to the anode and g2 if I'm correct.


Would I get this kind of loud hum at power on and through the time the amp is on if the capacitor shorted/gone bad? I'm going to replace them tonight, it just hit me while I was at work. But is this plausible? It tracks with me, since the hum is pulsed at turn on, then fades quickly, then within about 5-6 seconds is at full volume which with the B+ charge.

Thanks,
GnV

[shooter]

There are caps connecting pins 7 and 9 on the output tubes, which correspond to the anode and g2 if I'm correct

You're correct on the pins, just pull/lift the caps and see.   You're still doing the troubleshooting with all tubes out, since they don't seem to have an effect?

Did I mention that the symptoms don't change/go away when there are no tubes in the amp?

[glassesnvests]

Consider this solved! One of the caps was shorted, the casing was deformed, I'm guessing that it got too hot. The amp was in an iso box and the chassis is hung upside down so all heat goes to the chassis. Thanks for the help!