Question about Power tubes in Dual Rectifier - Mesa Boogie

[Leooooh]

Hey,

I need some help regarding a Dual rectifier.

Client turned the amp on without plugging the cab. The veredict, one of the power tubes shorted and one wire that is connected to the power tubes board was burnt. Since I got a spare quartet of 6l6gc, I replaced them all (no Bias adjustment, since Dual Rectifiers are fixes boas), just to test the amp and soldered the burnt wire. Turn the amp on and everything seems to be fixed. Usually, turning on a tube amp with no load causes more critical damage. And I am astonished, that Mesa doesn't have flyback diodes to protect the OT. At least, I didn't see anything in my schematic.

So, here are my question. I know that the Dual rectifier is a class AB amplifier, so the power tubes works in pairs. Knowing that, can I replaced only the pair that had the tube shorted (in this case, V2 - mesa pairs V1 and V4, V2 and V3) or should I replace the whole quartet?

The tubes that are in the amp right now, are my spare tubes, that are not for sale.

Thanks a lot for the help,

Leo

[The Dude]

That's a loaded question that will get you lots of answers. In the old days, many would have just replaced the defective tube and kicked it out the door without a second thought and it would have worked fine. Then there are those who think every tube should be perfectly matched. Then there's a school of thought that if both halves of the amp are matched, it's good. IMO, there isn't a right answer.

Saying that, here is what I would do- again just my personal opinion. I would check bias on each tube and try to match them using bias current as a reference. If you can get something closely matched without replacing all of them, go for it.

[drewl]

Yeah I guess it depends on the customer and how old/good the original tubes are.

Most Mesa amps run the tubes fairly cold so they rarely fail.

[Mick Bailey]

Tube matching is one of those things that only has validity when the tubes are new. Sometimes the tubes are 'burnt in' - though this has a vague definition, sometimes the tubes are just on long enough to establish matching parameters. The whole model assumes that tubes will all age to the same degree thereafter. My observation is that often this isn't the case, particularly if the amp is run hot and hard. Those premium-matched sets can end up no better matched than random selection from a production batch. Some suppliers burn in the tubes with a decent current draw and voltage that more closely aligns to operational conditions - these will hold up much better, especially if the amp is biased on the cool side.

Consider the relationship of the tubes to the OT; you have two halves of primary winding, each side driven by a pair of tubes. The pairs of tubes are in parallel and act as though they are a single tube. So let's say you remove two tubes; to keep the amp 'balanced' the remaining two need to be either the outside or inside pair, and because they're paralleled on each side it doesn't matter which. You then have an amp with a matched pair (assuming they're still matched after years of use) So then you add another new matched pair in the remaining inside or outside locations. The amp then has a balanced load on the primary, though the new tubes may pull more or less current than the old ones.

If you do it the other way, you leave a matched pair on one half of the primary and install another matched pair on the other side, you can get a lot of hum due to the imbalance which causes loss of power supply ripple rejection. Fender use to install a bias balance control to reduce hum in some amps.

I think of the whole setup like a tug-o-war team with 2 members on each side. If I wanted both teams equally matched, do I put 2 strong guys on one side and two weak ones on the other, or have one strong and one weak guy on either side?

One thing to consider is if the original Mesa tubes are present, you stand a better chance of matching up the tubes by selecting the same tube type and colour-coded gain grouping for your replacements. You pay a hefty markup on those tubes, though.