Tube problem

[Raggi73]

Hi.
I have a Dual Rectifier 3 ch about 3 years old. I bough new mesa-tubes and these last two tubes in the amp are much bigger than the original.
The original tubes are not that much bigger than the 4 others, but these are way bigger.
Anyway. When I installed these 2 "big" tubes my amp was on and suddenly it started making allot of noise. I mean allot. I wasn't playing the amp at the time, but it was on.
Had to power the standby off and on to get it to work. I put back the originals and the amp is fine.
Faulty tubes? They are new. (excuse my English)

Any ideas.

 

[domct203]

What tubes are you referring to? The 6L6GC's, 5U4GB's, or 12AX7's.

BTW it is not a good idea to be changing tubes with the amp on (standby or not), Especially the Rectifier tubes, which is the ones I think you are talking about.

Dom

 

[Raggi73]

These are the 5U4GB's, The two Power tubes, not the 4 6L6s.
I did off course turn the amp off while changing, but the noise came while it was on (after 20 min or so) and I needed to turn it to off standby and on again to get it to work. I have put back my originals and it works fine and then turned back to the "new" and the problem came back.

 

[fluff191]

Switch the amp to diodes to see if it goes away. Sounds like you have some bad Rectifier tubes.

 

[domct203]

Let's clear a few things up first so we are using the same terms.

The 6L6GC tubes are your POWER TUBES.

The 5U4GB tubes are your RECTIFIER TUBES.

To eliminate the new Rectifier Tubes, leave your original Power Tubes in the amp and just install the Rectifier Tubes. If the noises come back you have a bad Rectifier Tube (or 2).

If there is no noise, leave the new Rectifier Tubes in and install the new Power Tubes. If the noise comes back You have a bad Power Tube (or maybe more than 1).

What brands of tubes are you using?

Dom

 

[Raggi73]

Domct203: You are right, sorry for the mixup.
I do mean the 5U4GB Rectifier tubes. My power tubes are not changed.
I use tubes bought with the Mesa Boogie brand on them. The "new" tubes are much bigger and rounder. More like a light-bulb.

The tubes are as you say probebly blown/damanged.

 

[BostonRedSox]

DOM is correct though. Although Mesa says you can change tubes in standby mode, I too would advise against it. I have had a tube break in my fingers once, and all I kept thinking was, "thank god the amp wasn't on!"

Be careful and bear in mind the high voltages you are working with.

 

[yasha]

TS, can I join in?

I am curious, I read somewhere that even when the power is off, I am still at risk of getting electrocuted. Why is that?

 

[Acesofbelkan]

Yasha, Im not amp tech but I heard the transformers hold electricity for days after you even unplug the amp. Maybe that's what youre thinking about? I'm no amp tech so let's let one of the more knowledgeable members confirm this for ya

 

[thunda1216]

it's not the transformers, it's the capacitors. they are like a glass that you fill with water. it holds the water, and dumps it all out when you turn it upside down. 600 volts or more sometimes.

 

[94Tremoverb]

Whether there's any risk or not depends on the amp's power supply design. Most high-power amps have 'stacked' first stage filter caps, with divider resistors across them. This means that when the amp is switched off, the caps will automatically drain through these resistors, and after a minute or so there is no risk (it's best to leave the standby in the 'play' position to ensure all the caps fully discharge). But be careful, because there are a few amps where this is not the case - some older 50W Marshalls for example.

It's perfectly safe to change preamp and power tubes with the amp on standby - *provided* you don't break one! - but *not* rectifier tubes - you must turn the amp off fully to change rectifiers. This is because the rectifier tubes are upstream of the standby switch and so are fully on at all times the amp is, regardless of the standby switch.

If you suspect a rectifier tube is faulty, it's safe to test each one at a time in the amp, provided you don't crank it right up. If there is a fault, it will almost certainly be just one of them - you only need to change this one. But it's not a good idea to run mismatched rectifier tubes in a Dual or Triple Rectifier, because the tube with the lower resistance will take the bulk of the current and may arc or burn out. For what it's worth, if the old ones are working you don't need to change them anyway, rectifiers either work or they don't, they don't slowly degrade in performance like the other tubes.

Switching to Silicon Diodes to eliminate the rectifier tubes will only work in a 2-channel Dual Rec - in the 3-channels, the rectifier tubes are connected whether they are in use or not - the switch just bypasses them, so the usual fault (arcing or a direct short) will still appear. In the 2-channels, it fully switches them out of the circuit.