Buzzing/rattling power tubes

[SlasZ]

I just got a Weber MASS 1attenuator and when I play my amp with the master volume over 1-2 o'clock but with full attenuation I can hear the power tubes buzzing/rattling like when a regular lightbulb sounds when it's about to break.
It only happens when I play, not when I don't touch the guitar.
Anyone have a clue what's going on here?

[jtr]

Do you have the amp on top of the speaker bottom? If you do try taking it off .

[SlasZ]

Do you have the amp on top of the speaker bottom? If you do try taking it off .

I do but with full attenuation there's no sound coming out of the cab, so it's not vibration related.

In fact, I just discovered why it does this and it almost freaked me out
An amp tech I know also had this problem with a few Webers and when he investigated he found that due to a bad switch (Weber uses cheap MIC pots & switches) the poweramp section received over 50% more voltage than normal (as if there was no load on the amp). This is really pushing the output transformer & power tubes and you can potentially destroy them if you keep it up long enough. It also accounts for the volume drops & sucky sound quality.
I'm having the pots, switches & jacks replaced with quality stuff this weekend and I'd recommend anyone with a Weber to do the same

[toejam]

I had that happen on my THD Univalve head the other day... my EL34 power tube was buzzing at one point. I switch to a different tube in it all the time, as that's what the amp is made for (one power tube and two preamp tubes), but I never had that happen. It seemed to go away, though. There's a switch for low voltage and high voltage, which EL34s can use with either voltage, but it was only on low wattage with the built-in attenuator running. Strange.

[SlasZ]

The tech I know looked at the MASS and found out that it wasn't the switches that gave bad contact, but that for some reason when you attenuate over 50% (both knobs lower than 5), the load on the amp increases dramatically. All the electrical circuits seemed fine, except that the mass speaker was a bit dented because a switch sat in its way. At low attenuation you get about 7-9 ohms of load, but over that it increases to 20-25 ohms. This is a 3-1 mismatch with the 8 ohms of the amp & cab. Because of this the plate voltages increase to about 1500 volts causing tremendous strain on the OT and tubes. That's where the noise was coming from, the power tubes were burning out. Quite scary and strange that no one who uses Webers noticed this before as I haven't found anything on the web about this.
He's going to mail Ted about this because this can be quite devastating to a power amp

[toejam]

Damn! Weird how no one else has ever had that problem. Maybe it's just an isolated incident in your unit only?

[SlasZ]

Damn! Weird how no one else has ever had that problem. Maybe it's just an isolated incident in your unit only?

Maybe, I'm going to mail Ted Weber about and see for a replacement unit.
The mass motor (the coneless load speaker) is also a bit dented because one of the switches is in the way. Maybe they placed it wrong but I might be all units have this fault.
Anyway I won't be using it any more for bedroom playing, and they're not really usefull if you can only attenuate about 3dB which is about the only safe way to play it.. sigh...
Good thing the Bogner has a good output transformer or it would probably have been killed by that much voltage.

[jgcable]

Thats a result of over attenuation. Most people don't have that problem because most people don't use full attenuation.

[SlasZ]

Thats a result of over attenuation. Most people don't have that problem because most people don't use full attenuation.

Allthough the website says you can use it as a loadbox.
And the impedance is about 18 ohms when you attenuate at half, that's more than twice the impedance of the head...

[Spivonious]

I've heard attenuators can blow up amps. I guess this explains why!

[SlasZ]

Small update, emailed a bit with Ted and he admitted that his attenuators can do that, and he said, literally, ' If your amp can't handle a 2 to 2.5 to 1 impedance mismatch, I'd get another amp.'

[toejam]

Interesting.

[SlasZ]

Yeah so I wonder if anyone here with a Weber MASS and a multimeter could check the input impendance during various levels of attenuation?