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Topic: Power tubes dropping dead too fast???? |
Bob Carlucci
From:
Candor, New York, USA
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Posted 22 Aug 2008 11:18 am |
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Man, maybe its just me, but my tubes on all my Fender amps for the past 5 or 6 years don't last at all. I just took My 68 Showman and 70's Bassman 70 tubes to a vintage radio rebulder thats the only place I know of to test tubes anymore.. 60 miles EACH way!!
Anyway, 5 of the 6 6L6 tubes were totally wasted.. I mean fried... all the preamp tubes were fine...I can't seem to get 6 months of quiet clean operation out of todays tubes.. I have been replacing tubes in all the Fender amps I have had in recent years at a rate that is getting too expensive to handle...
I know, check caps , check this ,check that, etc etc,,,bah humbug...
This never happened years ago, unless my memory is failing me.. I used to get 6 years from a 6L6 set, not 6 months... Is it me???, or are todays tubes all junk???... The set in the Bassman 70 were Sylvania USA and were about 3 years old, but with very limited play time... amp sits for months at a time.. The Showman were Sovtek which seem to die incredibly fast.
This is an ongoing headache.
All my amps run quiet clean and loud, with no issues, but this tube replacement thing is killing me... I have been rotating between several amps to keep the tubes intact longer... btw, I have been playing more bass these days.. Will use with a bass shorten tube life??.. any ideas?... bob
_________________
I'm over the hill and hittin'rocks on the way down!
no gear list for me.. you don't have the time...... |
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Steve Alcott
From:
New York, New York, USA
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Posted 22 Aug 2008 2:22 pm
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| Do you bias the tubes when you replace them? Having incorrect bias can dramatically shorten tube life. |
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Dave Mudgett
From:
Central Pennsylvania and Gallatin, Tennessee
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Posted 22 Aug 2008 2:49 pm
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I agree on checking the bias. If the bias is hot, it can dramatically shorten tube life. Especially for loud and clean, I tend to bias power tubes a touch cool, which tends to increase tube life.
But the sad reality is that imported tubes - especially power tubes, IMHO - just aren't the same as US or West-European made tubes from 30-50 years ago. It took decades and decades of continuous refinement and strict attention to quality control to reach the state of excellence in tube manufacture from the late 50s to early 80s. Most of that manufacturing knowhow just isn't around anymore, and this is a pretty marginal industry at this point. Many reports also indicate that new tubes simply aren't getting anywhere near as good a vacuum pull - another thing that shortens life.
The other issue is that a lot of guitar amps - especially high-power Fender and Music Man amps that deliver the kind of clean power a steel player typically wants - run power tubes way over their design center voltage. This was less of an issue when those amps were made because the power tubes were way overdesigned. For example, Fender and Mesa contracted for special tubes with higher ratings specifically for this kind of problem. Good luck now - we're lucky anybody's making tubes anymore. We'd be SOL if we had to rely on US or West-Europe manufacturers, who told us 20 years ago to take a flying leap.
I think this is gonna get significantly worse as the years and decades march along. The price of NOS tubes has dramatically increased in the last 20 years, and will continue to go way up as the supply dwindles.
My opinions, of course. |
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Benton Allen
From:
Muscle Shoals, Alabama, USA
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Posted 22 Aug 2008 3:19 pm
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| Quote: |
| The set in the Bassman 70 were Sylvania USA and were about 3 years old, but with very limited play time... amp sits for months at a time.. |
Sounds like your BIAS is not correct. Those Sylvania tubes should have lasted much longer than three years under the conditions you described.
Dave is right about the current tubes being manufactured being of lower quality than the old US made tubes. I have friends that go through a set of tubes every few months. Contrast that with a Music Man HD-150 I bought new in 1984 that still has the original 4 Sylvania 6l6GC in it, and they still show 100% output.
An era gone by, I afraid.
Keep your BIAS checked!
Benton |
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Steve Alcott
From:
New York, New York, USA
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Posted 22 Aug 2008 3:24 pm
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| On the other hand, I ran a set of Svetlana EL 34s in my Dyna ST 70 for 10+ yearsn and just lost one about a month ago. I checked bias once a week. |
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Bob Carlucci
From:
Candor, New York, USA
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Posted 22 Aug 2008 4:00 pm
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No noise and no red glow from those tubes ever,,, I have seen tubes when the bias is improperly set... they run hot, and glow red.. My amps all run quiet.
One of the problems is, around here the very few guys that know how to bias an amp want to always do a cap and retube along with the bias... at $200 a pop... This may seem cynical,but buying tubes is cheaper...
I may one day put the 1 ohm resistor in the power tube grids and check current across the resistor, but I have 2 problems.. I am lazy first, and afraid of killing myself by way of accidental electrocution second  bob
_________________
I'm over the hill and hittin'rocks on the way down!
no gear list for me.. you don't have the time...... |
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Jon Light
From:
Saugerties, NY
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Posted 22 Aug 2008 4:25 pm
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If you feel like spending a bit of $$ you can get a Bias Rite from Weber and avoid the risk thing---plugs into the tube socket, between the tube and the socket.
A hot-biased tube doesn't necessarily red-plate or show anything wrong---hey, it might actually sound great--just the right growl. But there might be a bit of compromise involved between tone and economy where a colder bias sounds not quite as aggressive but sounds decent and saves many months of tube life.
I haven't been using my tube amps for a while but I was getting decent life out of my JJ 6L6's in my SF Dual Showman Reverb.
I don't have the years of experience of many of the brothers here gigging tube rigs so I don't have much to draw on---I probably would be pretty slow to recognize the gradual deterioration of my tubes so my observations may be a bit suspect.... |
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Robert Leaman
From:
Murphy, North Carolina, USA
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Posted 22 Aug 2008 5:21 pm
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When you check for grid current, I can guarantee that if you have grid current, your grid bias is way out of the ball park and/or you are grossly overdriving your tubes. There are vacuum tubes that run with grid current but not any receiving class power pentodes.
Remeber that when setting bias, it is NOT grid current that is monitored, IT IS PLATE CURRENT!
If there are 1 ohm grid resistors, they are there to suppress parasitic oscilltions, not to measure non-existent grid current.
Last edited by Robert Leaman on 23 Aug 2008 9:51 am; edited 2 times in total |
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Keith Hilton
From:
248 Laurel Road Ozark, Missouri 65721
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Posted 22 Aug 2008 5:23 pm
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Bob, check this out----- http://www.amp-fix.com/fender_twin_amp.htm
It could be your tubes are good and your bias is off. I believe jus about anyone could set the bias from these instructions. |
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Bill Hatcher
From:
Atlanta Ga. USA
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Bob Carlucci
From:
Candor, New York, USA
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Posted 23 Aug 2008 9:36 am
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Thanks guys.. been to those sites.. I'll just have to get off my ass and do it.. Been using multimeters measuring amps, ohms, volts and millivolts on automotive systems for decades, but never could find detailed info on how to bias an amp,.. one part or another always was vague.. Do I leave the 1 ohm resitors in place or remove them??.. that sort of thing... I'll get around to it .. maybe////// bob
_________________
I'm over the hill and hittin'rocks on the way down!
no gear list for me.. you don't have the time...... |
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Larry Robinson
From:
Peachtree City, Georgia, USA
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Posted 23 Aug 2008 2:11 pm
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Bob, in a Twin Reverb amp, the cathode is normally grounded. Install a one ohm 2W 5% resistor from pin 8 on the tube socket to ground. do this for each 6L6 you have. After the amp warms up, use a digital volt meter set on m/v range. Adjust the bias for 35-38 mv accross the 1 ohm resistor, this will be equal to 35-38 ma. Total current thru the tube flows through the cathode. My experience with TR is approx. 2ma screen current and 35 ma Plate current. EAsy to calculate. Measure the voltage drop accross the 470 ohm screen grid resistor. Use Ohm's law to calculate the screen current. Measure the plate voltage, multiply that by the cathode current minus the screen grid current and you will have the power disipated. I usually use 70% of the rated power output for tube, between 25-30W. I have used this method for years and it has worked for me. Using the 1 ohm resistor is just as accurate as buying one of those biasing tools for $50 plus. Using the 70% value of the rated power output depends on the B+ voltage at the plates of the 6L6's. Once you set the voltage on one tube, read the current thru each 1 ohm resistor. Swap tubes as necessary to achieve balance current thru the OT. V7&V8 are in parallel, and work in a push pull with v9&V10 which are in parallel. Swap the tubes to get a close balance in total current thru V7/V8 and V9/V10. As I said, some may disagree with my method but it has worked for me for 40 years. This setting is for static not signal input.
Regards,
Larry Robinson |
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Jim Sliff
From:
Lawndale California, USA
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Posted 23 Aug 2008 9:48 pm
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Those so-called "stock" bias settings of 30 and 60 are wrong.
They do NOT work with all tubes sold as 6L6's or 5881's. Those settings will not also be the "sweet spot 99% of the time...they're just generalities - one tube amp guy's idea of a "starting point". There's a lot more to bias setting than plugging tubes in, setting a number, and calling it a day. some 5881's will be overstressed at that setting; some NOS tubes (the Philips STR387 for example) will sound thin, needing (usually) 37ma...DEPENDING on the plate voltage, which is another critical factor in setting bias.
Sound like part of a chapter out of a Gerald Weber book, without any of the important backup information.
The "math" method is also invalid. It's "safe" but has zero to do with tone.
Bob, lastly - I'd bet your radio rebuilder is using a Hickok, B&K or similar tune tester. Great units for weeding out duds...and that's IT. They hit the plates with a whopping 160 volts or so - as opposed to the 460 of a Twin. You CAN NOT test amp tubes in a tube tester of that type reliably - and you absolutely cannot match them!
If that's what you're using for matching tubes - it's no wonder you're getting firecrackers!
Spend the money for tubes from a reliable NOS tube seller - KCA, Lord Valve, Uncle Spot...these guys have built equipment that tests amp tubes in "real world" situations.
If you continue with the method you are using I have no doubt your situation will never change.
When did you last listen to an equation?
Each tube type reacts differently at different settings, and math-based bias setting is one of the biggest fallacies around. It pretty much was dead and buried on 6-string forums 7 or 8 years ago... You CAN use math to measure plate voltage, calculate a "safe" bias range based on the tube data for whatever type you are using...but you have to set it my ear and then CHECK to ensure it's in a safe range.
_________________
No chops, but great tone
1930's/40's Rickenbacher/Rickenbacker 6&8 string lap steels
1921 Weissenborn Style 2; Hilo&Schireson hollownecks
Appalachian, Regal & Dobro squarenecks
1959 Fender 400 9+2 B6;1960's Fender 800 3+3+2; 1948 Fender Dual-8 Professional |
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Dave Mudgett
From:
Central Pennsylvania and Gallatin, Tennessee
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Posted 23 Aug 2008 10:31 pm
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I agree with Jim that it's important to listen while biasing tubes. On my old Fenders, I use the 1 ohm resistor method, squirrel away the original upper back panels, put on repros with a millivoltmeter on them, and wire them up with a switch to test each tube (with 2 power tubes) or push-pull pair (4 power tubes) separately. I test the bias every time I use the amp. OK, some of the guys I work with think that's a bit much, but I don't care.
Of course, the bias should be in a safe range. But there is no absolute "correct" number for quiescent plate current. |
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David Mason
From:
Cambridge, MD, USA
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Posted 24 Aug 2008 2:10 am
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Buy a Line 6 or Behringer amp, they're guaranteed to have "Authentic Tube Tone" and my Guitar Player magazine swears it's true!  |
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Larry Robinson
From:
Peachtree City, Georgia, USA
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Posted 24 Aug 2008 3:18 am
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| The bias current settings for 6L6GC tube will usually fall in the 30-37ma range depending on the plate voltage. I'm not sure what Jim Siff is referring to when he describes a "math" method as being invalid. Ohm's law is invalid? What is tone for one is not necessarily tone for another. You can test the balance of your OPERATING tubes by measuring the voltage accross the one ohm resistors in the cathode of the 6L6GC tubes. As Dave Mudgett writes, there is no absolute correct procedure. |
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Ken Fox
From:
Nashville GA USA
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