I think my MkV's hum has become louder- troubleshooting news

[Catthan]

Hi guys,

so, MkV popped fuses due to a bad power and a recto tube. Replaced those + a matching power tube (inner pair), amp works great but I think the hum when I switch on from standby has become louder.

https://soundcloud.com/catthan/boogie-hum

Hard to judge how loud the hum is but maybe if you use the tranny hum at standby as a reference..

Checked speaker cables, different cab, good preamp tubes in v1 and v7 (hum is there in all channels), loop on/ off.
Hum is not affected by cranking the output and is not louder than the guitar sound through the amp at apartment levels.
At variac mode the hum is less loud.

I also tilted the amp foreword to see whether something could be wrong like amp+cab coupling (silly ??) and it did disappear but I couldn't repeat this.

I think if it was there before I would have notice it..

Ideas?

 

[Catthan]

Ok,
No power tubes no hum (I think this is obvious?)

Then, I left the PI and 2 6l6s in and removed pre- valves one by one from 6 to 1 as they were easier to take out in that order.
Removing any of those didn't cure it..

Hum stopped only when I took the PI out (only 2 power tubes in, obvious?).

Thinking that the PI is the source I tried all the other 6 preamp valves + 4 spare I had.. No cure..

Tech time?

I hope I am not going paranoid.. Turned fan back on to compare how loud the hum is compared to the fan; It's as loud.
Not loud enough in gig volumes but I think loud enough to be heard through the PA between songs..

Can send it back for warranty work but I'm afraid of "that hum is normal" responses whereas I think it wasn't there before those power tubes went and started popping fuses...
Is it??..

 

[Authorized Boogie]

You mentioned tubes from 6 to 1. Did you swap out the 7th preamp tube??

Does the noise happen when there is no instrument connected to the amp?

Are you in close proximity to fluorescent lights, computer monitors, fans, dimmer switches, etc?

Have you tried other cables and guitars?

Have you confirmed the noise happens in multiple locations (i.e. Home, rehearsal space, friends house, work, etc)?

What wattage setting are you in?

Do you have any effects in front of the amp or in the fx loop?

Next, with the amp in the 45 watt mode - swap the inner and outer pair of power tubes. If you notice a difference it could indicate a power tube that is at fault. Only one pair of tubes is used in the 45 watt mode, so this test will isolate each pair.

One last thing is to test the amp in the 90 watt mode (or 45 watt mode, set to diode rectifier) and pull the rectifier tube out. If the noise disappers with the rectifier tube out, that could indicate a bad rectifier tube.

 

[Catthan]

Hi,
thanks!!!
Tried all those with rectifier valve out.
hum was there.
hum only went away when I took v7 out (left 2 6l6s in)
I tried about 10 other hopefully good tubes in v7 but none cured it.
The hum is there without instrument,
is insensitive to output level, loop active or not, wattage, number of power tubes,
inner or outer power tubes sockets.
At 45 & 10W the hum is less loud but still there.
At 90W it is at least as loud as the fan.
All tests done with no effects in front of the amp.

Tried different speaker cables and cabs.
All appliances and lights were off in the room.
only fridge in the kitchen.
Will try at another place tomorrow at practice.

So yeah,
the hum goes away only if I remove the v7 or the power tubes or press mute.

already talking with Kris from Boogie via email.
You guys are really good, thanks again so much!!