Bias drifting on one tube

[Tonegeek]

The bias drifts upward about 8ma on one of the 2 EL-34s in my amp after it has been on for a while. I swapped sockets and it followed the the tube. I am thinking its time for new tubes but one question is what is causing this drift? I am guessing parts warping when heated so probably looking at replacement. Second question is should I be having to change outputs once a year? Seems like thats about all i get out of the JJ output tubes. I run 460 volts on the plates and bias at 32ma so not stressing anything that I can tell. Seems to me they should last longer. i would say i put about 6-7 hours a week on them.

[talbany]

IMO 6 or 7 hours a week for a year is pretty normal.. Kinda can't complain..
34's are prone to drift.. I have the same issue,, At first I thought it was due to the different bias supply's in the Marshalls that caused the drifting but after loading the 34's in the D-style/Fender PT tap bias I still get the drift occasionally.. Since it's one tube I would bet my left one it's a bad 34.. 34's in general are poorly constructed (Cheap Pentodes) in comparison to a beam power tubes..They don't like vibration and have burned up more screen resistors or have seen more burned up than the 6l's which rarely fry.. Just the way it is!!

The Mullard reissues were the worst!

Tony

[Tonegeek]

IMO 6 or 7 hours a week for a year is pretty normal.. Kinda can't complain..
34's are prone to drift.. I have the same issue,, At first I thought it was due to the different bias supply's in the Marshalls that caused the drifting but after loading the 34's in the D-style/Fender PT tap bias I still get the drift occasionally.. Since it's one tube I would bet my left one it's a bad 34.. 34's in general are poorly constructed (Cheap Pentodes) in comparison to a beam power tubes..They don't like vibration and have burned up more screen resistors or have seen more burned up than the 6l's which rarely fry.. Just the way it is!!

The Mullard reissues were the worst!

Tony

I am starting to think having an HV fuse is not a bad idea, especially with EL-34's. I tried some used GT 6L6s in this amp last night and prefer the 34's. I really can't complain as you say because the tone is there and 34's arent overly costly to replace. I am a bit spoiled on longevity because my FX amp which is a Twin has had the same mismatch of GE and Sylvania 6l6 tubes in it for more than 20 years! Granted they have lost some power and one is microphonic on the low end, but they still work.

[talbany]

For YEARS!! I played a 50w 2204 it was a really good sounding one too..I was working it pretty hard 3-4 shows a week 10 years.. No matter what kind of 34's I tried I would have to change them twice a year.. Most of the time they would still be operating but would start to get a little ratty around the edges.. The best sounding 34's in that amp were GT Svetlanas (I think) #7's.
I learned to live with it.

.FWIW.. The bias supply's on most Marshall's especially the newer ones can be pretty spongy and drift some when you first stick a set in.. I recommend leaving it on the bench for at least 1/2 hour w/probes on it to check the drift factor..Good Luck and Happy Easter!!

Tony

[Structo]

I have noticed that too with modern production tubes.
I don't use EL34's but with the 6L6's I have tried the first hour or so sees a bit of a drift.
Don't know what is changing in the tube that causes that but like Tony said, I usually let them burn in for a while with the meter connected to the bias test points to observe and drift before playing the amp.
Then usually a week later or so I'll meter it again to make sure it hasn't drifted too much.

I have dual bias pots on my D'Lite so I can get them pretty close.
But if later they are a few ma's off I don't worry about it.