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In Reply to: Also, how often to check bias? posted by jedinvest on March 15, 2007 at 15:27:25:
I have heard most people say 2, maybe 3 times/year. Recently I had a Mullard el34 (New Sensor-Russia) go bad which had me checking the bias every few weeks. The sound slowly got worse, so I checked it's bias to see that it ran up 15-20mA. I re-biased it a few times over the next month only to watch it finally not hold at all. The tube was only 4 months old.I bias my Triode Corporation TRV-35SE at 40mA. I know of people running their el34s at 50-60mA and getting a decent lifespan out of them(over a year).
Follow Ups:
I find it wise to check every few weeks.
I would be afraid of such a conclusion that I have to check the tubes with any frequency greater than 120 days or at all.Damn minimalist amps -- well, my next one should have a nice front-panel mounted bias meter and controls!
That when I check it usually has drifted from 1.4 to 1.2 or so. I think it is the vibration from the power tranny!
and switches to check the four tubes' plate current.
I'm constantly flipping those switches and checking the readings.
It's a sickness I Tell YOU! A SICKNESS!
You know, when you check the idel bias of a class AB amp, you are just making sure that you are not cooking the tube just sitting there. When you play music on a class AB amp, the current draw will go way up. If you are playing relatively loud music and using less than efficient speakers or ones with big impedance swings, you can ask a lot out of your tubes. The idel bias on an AB amp really does not determine tube life if it is within basic limits.
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Long Live Dr.Gizmo
I didn't know that. I always thought if the idle was set higher the tube was pushed harder when playing music as well. The harmonic richness seems to increase with a higher idle, so I guess I related that to a harder working tube.
The higher you set the standing bias, the more power in class A you can get before hitting AB output. That accounts for the better sonics. However when playing music loud, the tubes will be driven into AB and draw significant current.If you like to leave your amp on a lot without music playing, a higher idel bias will age the tubes much faster. Turn the amp on when you want music and turn it off when you are done. As the poster below says, you should get 5 years of listening to music out of a set of EL34s.
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Long Live Dr.Gizmo
I think there is simply more problems with the new production tubes than the older ones. I don't play music that loud or leave the amp on when not listening. Also, doesn't speaker efficiency come into play here as far as how hard the amp is pushed? Or even output voltage of the CD player (4v vs. the standard 2v). The volume control on my amp has never been past 10 o'clock. My personal experience has been that the tube (if not abused) has a predetermined fate from the day it was manufactured. Unfortunately I bought one that was doomed from the start.
Gents, Original UK EL34s, in vintage PP Class AB amps, lasted years and years, of hard long daily useage. Changing tubes yearly should not be acceptable !
If you set the bias right, and the tube takes out a cathode resistor, soon after, that is a bad tube, and possibly an underrated resistor, as well.
There are so many variables as to why this can happen. It might be helpful to know the circuit topology description and other voltages on the EL34s in your amps.
Class A is constant current. That means the idle current is the same as the maximum current draw. But, Class AB amps that don't cook the tubes(visible red on plates)should last years.
Maybe, these newer tube offerings are the problem. Amps bias should not waiver often, in a stable amp, with a stable tube. Perhaps, you should invest in some real British EL34s...
I'm thinking of 6L6, 6L5GC, 6550, KT66, KT77, KT88, etc.
nt
I think that is equivalent to the 6L6's, They're suppose to be rugged and I guess they were. Though, I never really warmed to the VTL 50/50 that used them.
I have a Marshall 9200, one of the only 5881/6L6 Marshall amps. I also realized about 5 years from the original 5881's. When I retubed it with 6L6GC's I noticed a much improved tone overall, however.
I almost wish that these forums would bump the latest responses "to the top" of the page, like so many other forums. This is becoming a great thread.
I have seen KT88s without any chrome flashing left, that still test and sound OK. It takes a very long time to get a KT88 that way. Likewise, some EL84s and 7189s, even some 6L6Gs...
Vintage tubes were built to last...
Concerning jedinvest's lack of love for a tube amp, perhaps it was the tube type's fault. Matching of tubes to an amp is just like matching components; finding synergy cannot often be predicted. Sometimes, two known bright components, used together, don't sound bright. Well, with tube types, it's the same, unpredictable, cause and effect. We have to try 'em and see/hear.
When you recap an amp, the caps need break-in time before we can critique them. Same thing with tubes. If you let them play music for a while, and still know that you have heard better, why stay with those tubes ? In the 6L6G family, it is known that the rugged, original British KT66s are the sweetest sounding. If an amp that uses 6L6 types can sound sweet, then, certainly, real KT66s will make that happen. Older 5881s were rarely described as sweet sounding. Detailed sounding they are. I have heard some critics applaud the newer 5881s. Certain 6L6Gs, 5932s, RCA black plate 6L6GCs are often described as sweet. Your experience with them might not be the same. Voltages and currents are important, as well as circuitry and output trannies.
For the EL34 family, the older British and Euro EL34s are the sweetest. The original, British, metal collar based EL34s are both rugged and sweeeeet. When new, these metal collared EL34s have dark flashing ! These are very long lasting tubes, but certainly at a price, and an unobtainium matched quartet. The KT77s are known as sweet.
KT88s are certainly kown as punchier and sweeter than the Tung-Sol 6550, but that does not mean which you will prefer.
I think there is a problem, a big problem, when users preconceive that a new version of an old number can sound like the old number, and last as long as the old number used to last. Reality has proven, over and over, that the more expensive older tubes, simply last longer. Most of the times, the vintage tubes sound better, as well.
Granted, there are exceptions, and some very good newer versions are available. But, please, don't even try to convince a vintage collector, that new is better...Happy Listening, with whatever tubes you choose to listen with...
My mullard 5-20s have been going for about 3 years now on the same set of EL34s - used daily. I have an old PA amp from 1960s with pushpull EL34s in. Not used so often nowadays but the valves have never been changed in the 20 years I've owned it!
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