18Watt.com

Thanks for all the tips on my tube rattling in my 18watter, from my last post. I pinned it down to my power tubes rattling. I've been running NOS Tesla's. and they probably have over 300hrs on them. I never have used a 18watt amp until I built this one (GDS) and I use it all the time . Can anybody tell me what the typical life expectancy out of a set of power tubes is ???? I,m in a working band and I use the heck out of this amp for everything. I,m just wondering if I will have to retube this thing every 6 months or so. Thanks Pigman

Pigman,

Considering how hard an 18 is on a set of EL83 tubes, you might need to replace power tubes every 6 months. I used to replace EL34s at least once a year when I was gigging a lot. EL84s in an 18 are run a lot harder than EL34s in a 50 or 100 watt.

Don't bother replacing the preamp tubes unless you have a problem with one or it's tone becomes bad. Preamp tubes will often last 10x the life of power tubes. Some even longer.

up the cathode resistor and run em a little colder.....up to next standard value

you prob won't notice the difference except you'll get a little more milage out of the tubes

if you don't want to do that, use the new, cheep, and improved disposable JJ!
just shred and flush daily

you might also move to 7189s or Sarge's Red Army Special 6P3S...(if he's got any left)

i would also put in dedicated screen resistors (470 ohm or so)

these are things I would do...others may differ in opinion

there are several ways to get the SAME tone w/o running anything over SAFE OPERATING LIMITS or exceeding MFRs RECCOMENDED RATINGS

The thing that gets me is the tone and power seems just fine, they have just slowly started to rattle more and more. Is this the norm for combo amp formats. If so I am considering building it into a head. Anybody else have this problem in thier combos?? My guess is these tubes my be fine in a head type format. Pigman

pigman,

It's funny. You can get one set of output tubes that rattle and other that doesn't. I've seen this with NOS and new production and no brand seems much better than others.

If you are using retainers on your output tubes, you might also try some padding between them and the tube, just in case they are the source of the rattle. Also, try the amp without the preamp tube covers. The springs in them can also rattle.

A head is less prone to rattles just because it is physically decoupled from the speaker. You could try shock mounting your chassis to reduce transfer and this might help, but with the tubes handing in the speaker cabinet it probably won't cure it.

herr-uuns wrote:up the cathode resistor and run em a little colder.....up to next standard value .... you prob won't notice the difference except you'll get a little more milage out of the tubes

i would also put in dedicated screen resistors (470 ohm or so)

there are several ways to get the SAME tone w/o running anything over SAFE OPERATING LIMITS or exceeding MFRs RECCOMENDED RATINGS

I agree with the Herr Doktor here. I prefer to use a 150 ohm or 180 ohm cathode resistor, and apply Jason's PSU/Screen supply mods. These reduce the screen voltage more and feed the screens via a 1k shared resistor, followed by 100 ohms at each EL84. You can see these mods incorporated in the 18W Lite IIa and Lite IIb. Your tubes will have a lot more life, and their tone will still be robust.

Zaphod, Thanks for the suggestions! I am going to reseach these mods, but could you explain to a novice what this actually does to the power tube? I assume it would change to a colder bias value and prolong the life of the tube, but in so far as the actual tube filements rattling after a period of time, I guess I don,t understand how this would effect that. I,ve been assuming that the vibration factor and nature of the combo format is the cause for a mechanical failure ie the filements starting to rattle apart. I am new to this amp building thing. Thanks Pigman

pigman,

It really wouldn't affect the rattling unless the extra heat was stressing part connections causing them to fail. That's unlikely; more likely is the speaker would cause more mechanical problems.

However, raising the value of the cathode resistor will help with tube life. I generally use 150, but I think I had one or two 20 watt amps with 180. On my own clone 18 trem I use 130, just because the originals did. But I agree, if tube life is important, you should consider it.

The screen mod is also for tube life. You can run most power tubes high on their rated plate voltages but you need to keep the screens close or below the ratings. A screen is more fragile than a plate (think about it), so this will definitely help tube life. Having said that I generally don't apply the 1K + (2x) 100 mods and haven't had much trouble.

Even in an AC30 or more tame design, EL84s only last so long. They run hotter naturally so heat = wear. But you should be able to get a year or so out of a set of new production if you use the amp regularly.

pigman wrote:...could you explain to a novice what this actually does to the power tube? ...but in so far as the actual tube filements rattling after a period of time, I guess I don't understand how this would effect that...

I was wondering if the general deterioration in the tubes' characteristics were somehow leading to them generating harmonics that resonated more with their internal mechanical parts. This is just a wild guess of course. In general the tubes will run at a somewhat cooler temperature with these mods, so that may also affect how long it takes before they get rattly. Overall, if you're using good quality tubes, you might like them to last a little longer....

Actually I am using NOS Tesla el84's, They have a nice rich sound to my ears as compared to the new set of JJ's I just put in to stop this sound, the JJ's sound good , but compared back to back with the old tesla's they seem a little sterile, yet I could live with them. The rattling as best as I am descibing this noise , may be isolated to only one tube, when this occurs I can actually touch the tube and get it to subside, I am going to get a couple more sets of the NOS Tesla's. This could be a isolated case of one tube acting up. Although I am going to do some of these mods. Pigman

here is my test score 30 hours new set of amperex half used up at 340 may vary but i dont think by much unnst knows dont need to operate beyond 300 design limits are quite enough.the tube satches if you look at it wrong! and if you insist on 340 which is the norm on the 18 prepare to prepare a tube budget.i have 20 different duets that rotate this way a coupld of years of use can be had with controls at max the new unnst model is easy on em.and what an amp.! this guy should really think about a career in electronics.lol .......i vote unnnst senior builder on the great pantheon of do it your selfers my amps are holy terrors that embarass and ashame the jj totin fool.besides i wasnt even on the harvard rowing team .lol .who sez lsd wasnt a useful drug! we found ourselves then turned and ran for our lives! and ended up here!

I think tubes tend to fail from mechanical damage before they "wear out" from actual use. Every time you pick up the amp and put it back down, the tubes take a hit. That's why you had military spec designs - B-52s - and the like - tend to rattle quite a bit.

mr frum thats a true story .on the road i lunched 2 grand worth of slvets in 7 months.16 every two weeks .why ...amps were snatched off stage while stil way hot and filiments damaged...oh thats including the pres too. i insisted on jan 12ax7s 4 got thrown out before ever usin them out of every ten .so much for not lettin them cool off.t

bad for tubes, afaik:

more current (running them harder)

more vibration, esp. when hot

more on/off cycles