Tube amp question

[Chief_CrazyTalk]

I should probably know this, but it's been a long time since I've tinkered with amps.

I just got my Marshall amp back out of the pawn shop. It's been gone a while, and I know it hadn't been sounding its best, and I noticed that one of the two power tubes had a white milky appearance on the top of the bottle...it's a 50W combo, so the tubes hang down from the chassis.

I did fire it up, and saw that that tube is totally dead - no light there at all. It doesn't sound so good, either. My question for the experts is, will I hurt it if I play it like this? I'm an electronics tech by trade, and have a basic understanding of electronics, and I'm not thinking it's a big deal, except for maybe impedance issues.

Your thoughts, advice, please? I'm definitely looking into replacing the pair asap...just wondering if I should abstain for now. Thanks!

[Dreamland_Rebel]

i would replace both bottles with a matched pair power tubes as soon as possible and bias them properly. You could possibly do some damage to the output transformer if you continue to run them like that.

[Cygnus X1]

Dreamland is correct but hedges.

Don't do it, get the power tubes replaced and get it rebiased.
It will damage the power stage if you try to run it single-ended.

[markD]

power tubes work in a push-pull function. BOTH need to be working or you will fuck your amp up. do not turn it back on until it is retubed with a matched pair of power tubes and properly biased.

[charvel750]

Really? I thought you could run at half the tubes? On 100 watt heads you can take 2 out and have the 2 inner running, right? So why not on a 50 watt could you not remove one?

[6string40]

I don't know shit about electronics, but from everything I've read/heard, you can run a quartet at half by removing the outer 2 tubes, but I don't believe you can run a duet with just one without damaging it. Only those amps meant to run one power tube should run with only 1 power tube. I'm sure someone that knows their shit can shed more light as to why...

[jgcable]

put new tubes in it and have it biased correctly or your next post will be..
"I just blew my amp up... what do I do"

[Cygnus X1]

It would seem so, but it isn't.
The other posts are correct.
Push-Pull, they have to work together.

Only way it might work in a 50 watt is if it was a 4 tube power setup,
and keep in mind even many 4 tube setups are not designed to run with a 2-tube "half" option.

Depends on the entire biasing scheme and they are not always designed the same.

Similar to all those speaker impedance questions...go with what it is designed for.
I don't really care what people "get away with".
I have seen too many fried amps for all those options.

[Chief_CrazyTalk]

Thank you all, for the feedback. I may have to get a new pair at GC in a pinch; are there any particular tubes to avoid?

Thanks again.

[rattlehead74]

In my opinion, any tubes that say JJ or Mesa on them.

[warlok]

Avoid JJ and Mesa?? I definitely don't agree with that. I have JJ's and hear alot of people who love Mesa's. But chief-crazytalk, you'll get everyone and their brothers opinions (and arguements normally) on tubes.. And the last time I was in guitar center, all they had in store was groove tubes. They are pretty damn expensive. You can get others online shipped cheaper and very quick. I had my full JJ set for my PV Ultra Plus in 2 days from eurotubes. Happy as a clam with them...

[rattlehead74]

Avoid JJ and Mesa?? I definitely don't agree with that. I have JJ's and hear alot of people who love Mesa's. But chief-crazytalk, you'll get everyone and their brothers opinions (and arguements normally) on tubes.. And the last time I was in guitar center, all they had in store was groove tubes. They are pretty damn expensive. You can get others online shipped cheaper and very quick. I had my full JJ set for my PV Ultra Plus in 2 days from eurotubes. Happy as a clam with them...

Your right about opinions causing arguements. Not trying to start one. I just didn't dig them in my 5150. Just ended up using what was already in the amp. Bought the JJ's from eurotube at a set with preamp tubes. Just to fizzy for me. To each his own I guess.

[warlok]

I've heard that from others too. It is all personal preference. I've seen people even argue about where to get them... lol

[6string40]

Don't go to GC...they only carry Groove Tubes...you can get better and less expensive on the web. Try here. I've always bought from them and they've been great to deal with.

[AlexL]

The white milky appearance at the top of the tube means the tubes have lost vacuum.

It should normally be silver; it is a material that absorbs remaining oxygen in the tube. If it has turned visibly white, the vacuum has failed.

From: http://www.vacuumtubes.net/How_Vacuum_Tubes_Work.htm

H. The getter

We want a good, hard vacuum inside a tube, or it will not work properly. And we want that vacuum to last as long as possible. Sometimes, very small leaks can appear in a tube envelope (often around the electrical connections in the bottom). Or, the tube may not have been fully "degassed" on the vacuum pump at the factory, so there may be some stray air inside. The "getter" is designed to remove some stray gas.

The getter in most glass tubes is a small cup or holder, containing a bit of a metal that reacts with oxygen strongly and absorbs it. (In most modern glass tubes, the getter metal is barium, which oxidizes VERY easily when it is pure.) When the tube is pumped out and sealed, the last step in processing is to "fire" the getter, producing a "getter flash" inside the tube envelope. That is the silvery patch you see on the inside of a glass tube. It is a guarantee that the tube has good vacuum. If the seal on the tube fails, the getter flash will turn white (because it turns into barium oxide).

- E.