Milky power tube...risky to even test amp?

Axe-Man

Member
Messages
7,965
I was going to audition an amp in the morning (Dual Recto) and one tube is milky as the getter has changed colour to show no vacuum.

Is it safe-ish to test it at low volumes?

The seller tested it saying it works perfectly and sound great...but...a tube has turned white.

He doesn't seem shady or is not being up front.

I'm just wanting to demo the amp and not blow it. I naturally am going to struggle to get a tech to do the tubes before Christmas so I might be kind of screwed.

Buy with blind faith as I may not be able to demo.

Demo and risk a bang...

It's a trade plus cash arrangement that probably benefits us both.
 

Avatar Tech

Member
Messages
455
Doubtful that the white tube will function at all, but depending which tube it is, certain parts of the amp that don't rely on that tube should still function. I wouldn't try playing it with a known bad power tube though.
 

pdf64

Member
Messages
9,167
Without a good vacuum, the thermionic conduction would be very unlikely to take effect, so the only current to flow will be in its heater.
It's possible that the tube may have another fault, eg a screen short, so it would be best to take it out. I don't see what harm / damage could be done from running an amp with a tube missing, whichever the affected tube slot happened to be, and with 4 power tubes / dual rectifiers, it should be possible to get it to work ok.
If the empty slot happens to be in the direct signal path of all the channels, eg the phase splitter, you may need to swap tubes around to get the amp to work ok.
Obviously though, some capability / mode / function will be missing.
 

Axe-Man

Member
Messages
7,965
I just heard from the owner and he said he played it for 15 minutes and said whilst there was a bit of background noise, he said it sounded good.

Now, I'll have no clue until tomorrow but if I can hear it and it sounds and works fine (ch changing etc) then I'll do the deal.

I have talked to my tech that will put a whole set of matching tubes through it tomorrow if it goes ok.
 
Messages
6,835
Before you go to test the amp, ask the seller which tube is milky. That tube is NOT operational. IF it is one of the 5U4's, then the amp will work without one of those. IF it is one of the power tubes, the amp will work but it will not sound as it should. IT is not the phase inverter. It is not V1.
IF it is one of the other 12AX7's, then some gain function will not work. Since he is running the amp with a 'milky tube'---which is a tube that cannot conduct current because the vacuum is gone----then you can pull that tube out of the amp for your testing. Do so and find out what function is not working...it will be a gain mode....IF teh milky tube is a 12AX7. Take a known good tube to put in that position.
At any rate, imho any such high dollar dual Rec Boogie in less than correct operational condition cannot command top dollar. IT is automatically tech time for such an amp, imho.
 

MKB

Silver Supporting Member
Messages
9,458
Hmmm, I'd never power up an amp with a tube that has lost its vacuum. Tubes are actually fairly rugged devices, so a tube would have to suffer a significant amount of trauma to get an envelope crack or lose vacuum in other ways. That trauma could damage other parts in the tube that could hurt the amp itself. The only other way I have seen tubes lose vacuum is through the use of cheap sockets, but Mesa uses really good quality parts, so that wouldn't be it. I have never seen a tube lose its vacuum any other way than some sort of physical damage (nipple gets broken off, tube dropped, pin in glass envelope bent wrong, etc). Although I have seen a tube distort its envelope when it got hot while having silicone heat sink grease on it (long story).

If the amp works with the blown tube installed, I can't imagine it not working with the bad tube removed. The tube (when blown) isn't doing anything useful in the circuit anyway, other than perhaps loading the heater winding. I'm not even sure if a heater would work without burning up if the vacuum goes.
 

walterw

Platinum Supporting Member
Messages
40,968
yeah, no.

frosted mini-wheat tube = don't turn on the amp until it's fixed, the problem may have originated in the amp not the tube.
 




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