Power tube not glowing, still seems to work

[M1dnightrider]

Hi all, hoping someone can help me out with a potential issue. I was recently gifted a pair of mullard xf2 from an old school guitar player friend of mine who very generously decided to send me a pair. I received them in the mail today, and popped them in my amp just to test. One of them is glowing brightly, and the other not glowing at all, but strangely, the amp sounds as it normally does in operation. I swapped them in the sockets and the same tube did not light up so I know it’s not the socket. The amp is a Germino Club 40, jmp 1986 circuit basically. Is it possible for this amp to run on one tube like this or is something else going on? I know sometimes preamp tubes will not have an observable glow, but have never seen a power tube do this. When I pulled them out after letting them cool for a bit the one with glow was much hotter than the other. They aren’t matched as far I know, and I have not biased my amp to work with them yet. I don’t really want to keep running them in my amp till I know because I don’t want anything to be damaged by running on one tube if it is in fact dead. Thanks

 

[Blrfl]

Get thee to a tube tester! Marshalls run push-pull and, if one of the pair is shot, it won't be good for the output transformer.

If you don't have access to a tester, check the heater resistance (pins 2 and 7) on both. I think the resistance on an EL34 should be about 4 ohms, but what you're looking for is a difference between the two. If the one that doesn't glow has a higher resistance or is open, it's had it.

If they're the same, swap them and see if the problem occurs in the same socket. If it does, your amp has a problem. It might just be that the pins on the tubes or the socket need cleaning.

 

[kbold]

What is glowing brightly? Pic's would be helpful.
I mean .... heater coils glow bright red. If one heater is not glowing, the tube won't work.
If the plates are glowing brightly ..... well, bad.
Because the fault goes with the tube swap, it indicates one of the tubes is faulty. Pic's would be helpful.

I would get a techie to look at it ... as Blrfl mentions, you may damage your output transformer (or power supply), leading to higher cost.

 

[NTC]

Your amp will sound pretty normal at low volume. Turning the amp up may sound ok, but as stated above, it isn't good for the output transformer. I had this happen in my 5E7 clone.

 

[peteb]

the other not glowing at all,

that’s not right.

are you sure you are not hearing one working tube and one not working Tube?

have you checked the bias to see if the tube that is not glowing is actually drawing current.

no heater glow?


and checking the bias won’t hurt the OT as there is no signal.

 

[Wally]

Hi all, hoping someone can help me out with a potential issue. I was recently gifted a pair of mullard xf2 from an old school guitar player friend of mine who very generously decided to send me a pair. I received them in the mail today, and popped them in my amp just to test. One of them is glowing brightly, and the other not glowing at all, but strangely, the amp sounds as it normally does in operation. I swapped them in the sockets and the same tube did not light up so I know it’s not the socket. The amp is a Germino Club 40, jmp 1986 circuit basically. Is it possible for this amp to run on one tube like this or is something else going on? I know sometimes preamp tubes will not have an observable glow, but have never seen a power tube do this. When I pulled them out after letting them cool for a bit the one with glow was much hotter than the other. They aren’t matched as far I know, and I have not biased my amp to work with them yet. I don’t really want to keep running them in my amp till I know because I don’t want anything to be damaged by running on one tube if it is in fact dead. Thanks

Fire the amp up..play a minute or two. Touch the glow8ng tube for part of a second…it will be hot. Then, touch the other tube for a part of a second….is it hot? If not, it is not functioning….bad tube.

 

[Rick Lanahan]

Sounds like a description of my brain.

 

[M1dnightrider]

Thanks all, forgot to check back to this thread for a few days after posting. Good advice all around, I have been needing to pick up/build a pair of bias probes for a while now so I’m going to go ahead and do that, and also probably post on a local forum for guitarists to see if someone might have a tube tester they’d be willing to let me use. I have also read sometimes on older tubes the pins can need a touch of solder if the joint has gone cold. It would surprise me if the tube was actually dead as it looks to be in good condition (not too worn) and he got it from a reputable dealer a while back albeit used. Will update if I find a solution

 

[Silverface]

I swapped them in the sockets and the same tube did not light up so I know it’s not the socket. The amp is a Germino Club 40, jmp 1986 circuit basically. Is it possible for this amp to run on one tube like this or is something else going on

Probably not and probably yes.

A basic emission tube tester is useful to check power tubes for shorts - but all common tube testers (except the new Orange model and similar) are useless for testing how good a power tube is or for matching, because they only use a maximum 185 volts on the plates - less than half of what most amps hit the tubes with.

You also can just stick tubes in and play - but you may cause serious problems. It's a REALLY bad idea!, especially with older tubes. It makes NO difference if the previous owner said they worked - they may have been fine in THAT amp but completely wrong in yours!

New or used power tubes in "fixed bias" amps like yours need to be 1. matched and 2. biased properly, based on voltages in the amp and based on the specifications pertaining to THOSE tubes. You can't switch models/brands or stick old tubes in without at least CHECKING the bias.

Take the amp to a professional amp tech who can check the tubes and see if they are even useful for your amp. - if so he can bias them for the best performance and style you play. It's not a DIY process unless you have the right tools, training - and know what to test.

Also - how old is the amp? Most Germinos are newer, but some are 15-20 years old. If your is it also is due for filter and bias cap replacement. They have a service life of 15-20 years, can blow without warning and if they do can burn up your power transformer - a VERY expensive repair prevented by normal maintenance.

Since you were unsure if the "glow" meant anything (and just stuck power tubes in), I assume you do not know much about amp technology or service - DO NOT open the chassis yourself if you haven't been taught all the details about amp safety - some amps can seriously injure or even KILL you when turned off and unplugged!