Deterioration Of Tubes

admin · Post by **admin** » Fri Oct 21, 2005 2:25 am

Charlie: Is it possible to talk about the deterioration of tubes in general terms? That is, do all tubes go through the same stages as they deteriorate. What causes them to deteriorate and is it an all or none process or are there tell tale signs as they are headed on their way out?

As a follow-up, short of plugging them into a tube tester, is there some practical and inexpensive way that players can test tubes periodically to avoid the surprise at the gig?

triode · Post by **triode** » Sat Oct 22, 2005 5:19 pm

You can tell if tubes are going bad by the following:

1. Reduced power output.
2. Higher than normal distorion.
3. Fuzzy or cheezy sound which should be clean
4. Amps that blow fuses can have shorted output tubes.

Visually inspecting tubes:

Reject or test if:

1. The tube rattles when you shake it.
2. The silver getter inside has turned white, which means there is not vacuum left.
3. The getter is very brown or tan colored.

Tubes tend to wear out if left on all the time, output tubes wear out super fast if biased to hot.

New tubes generally have shorter lives than good NOS stuff.

The biggest source of deterioration is heat, then vibration or a combination of both. Actually a guitar amp is a horrible environment for a vacuum tube to live in.

admin · Post by **admin** » Sun Oct 23, 2005 5:20 am

The biggest source of deterioration is heat, then vibration or a combination of both. Actually a guitar amp is a horrible environment for a vacuum tube to live in.

Is there a practical application for an exhaust fan or would this just produce interference?

I certainly notice that my Fender Twin Reverb is a little hotter when it is in the tilt back position, perhaps becuase the heat is trapped to a greater extent in this position.

soundmasterg · Post by **soundmasterg** » Sun Oct 23, 2005 3:15 pm

Peter, some amps already do have fans, like an Ampeg SVT for instance. It isn't setup to be an exhaust fan though...it is just bring cool air from outside to blow on the tubes. It certainly helps to have cool air blowing on the tubes, but depending on how it is setup, it can cause interference. Most amps don't really need a fan to perform ok as long as they have decent ventilation. Fenders are fine in this regard. The combo amps are bad sources of vibration from the speaker though. Lots of Fenders seem to do ok in regards to this, and I would guess its because they used the 6L6 and 6V6 tubes, which are more robust than EL34's or EL84's. The Vox AC30 tends to get microphonic tubes easily, and I would guess its because they use the EL84. The vintage Voxes would burst into flames sometimes too, because they didn't have enough ventilation.

doctorwho · Post by **doctorwho** » Tue Oct 25, 2005 4:28 pm

I read someplace that a simple solution for a cooling fan is to find a small 230V fan (like a computer case fan) and wire it to the 115V power line. The fan will run at half speed because the supply voltage is half its intended value, but it should still provide an adequate flow of air around the tubes, and (supposedly) will not generate noise/electrical interference.

admin · Post by **admin** » Wed Oct 26, 2005 6:36 am

Interesting fix Gary. It would seem that most amplifiers have stood the test of time and this may be another one of those situations in which, "If it isn't broken, don't fix it."

doctorwho · Post by **doctorwho** » Wed Oct 26, 2005 5:33 pm

I agree with you, Peter. I had my 1968/1969 Bandmaster Reverb for about twenty years and, although it saw intermittant use throughout its time with me, it still had the original tubes in it when I sold it and it still sounded fine. (And, of course, now I really regret having sold it!)

admin · Post by **admin** » Thu Oct 27, 2005 1:16 am

But based on only a single photo I have seen Gary, it would seem that you still have more than enough in your stable.

soundmasterg · Post by **soundmasterg** » Thu Oct 27, 2005 1:34 am

I would say that no Fender amp made would need a cooling fan, except for their 300PS and 400PS series bass amps. I'm not sure if they have one already as I've never seen one or looked closesly enough to see, but it wouldn't surprise me as they were designed by the same guy who designed the SVT. The older SVT's had an AC cooling fan, and they are noisy...so noisy that it makes the amp hard to use in a studio environment, especially after all these years. The new ones are very quiet, but I'm not sure if they are a DC brushless fan, or an AC 240 fan running on 115 a Gary mentioned. Theres a Canadian named Kevin O' Connor, who has written many good books about tube amps, and he is on record as saying something like none of the guitar amps ever made "need" a standby switch or a cooling fan until you get up around 700v or higher on the power tubes, or use transmitting tubes in an amp. I don't know if thats true or not, and I think a standby switch would certainly help a tubes lifespan, but I would agree with him as far as fans are concerned. My SVT gets a LOT hotter than your Twin Peter, but then it has 14 tubes in it too, so it needs the fan!

BTW, where has Bob Young been? He should be eating up this amp/ask Charlie section on your site.