528hemi
338 posts
Dec 04, 2012
3:05 PM
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I am ready to pull the trigger on some matched power tubes
for my reissue bassman and the Tech suggested I go for a lower wattage matched pair as it will break up earlier?
Does this make sense? I thought breakup had to do with where you biased the tubes?
If you take 2 pairs of the same tubes and 1 set is a lower rating( not really sure what this means) and bias the same, is there a difference?
I know Groove tubes has some rating? Is this important?
Can someone explain this.
Thank You,
528hemi
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Rick Davis
968 posts
Dec 04, 2012
3:30 PM
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LOL! I think I'll wait for Mark to weigh in on this one. I have my theories but I am not sure they are correct.
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-Rick Davis
The Blues Harp Amps Blog
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tmf714
1391 posts
Dec 04, 2012
3:37 PM
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here you go:
http://www.guitaramplifierblueprinting.com/Rating.html
I would not go too crazy with this,unless you plan on installing a Hoffman or point-to- point kit.
There is only so much you can ask form a PC board before other components start to fail from the load placed on them.
I assume you plan on gigging this amp? It then becomes a reliablilty issue as well.
Last Edited by on Dec 04, 2012 3:38 PM
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5F6H
1441 posts
Dec 04, 2012
4:02 PM
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New matched tubes are matched by plate current (mA), to within +/-5mA (at approx. 30mA) typically for 5881/6L6 in a bassman style amp. They might be matched closer than this but as long as they are within 5mA of each other the vendor has no genuine reason to match them any closer than that, or to replace them. Vendors often supply transconductance figures too.
If you get 2 matched pairs, one at 35mA the other at 15mA and rebias either pair to the same point, then in effect, they are the same (within reason, accepting normal manufacturing tolerances).
The GT grading system & grading by vendors is based on testing plate current in a test rig & grading accordingly. This allows them to suggest a range, or a pair that will drop into amps that are a known quantity, without rebiasing, you might see things like "Selected Blues Deluxe Tube Set". If you buy a "30mA" pair of tubes, they will only run at 30mA if the voltages in your amp match the test rig they were graded on. The amp dictates the current, not the tube.
Grading is not directly related to the wattage of the tube (a nominal design parameter), a "hotter" pair will dissipate more Wattage at idle (no signal) but this has little relevance to the wattage rating of the tube itself, or the amp. E.g. a 25W tube can idle at 8W or 17W in a 45W amp and still perform acceptably. Fixed bias 5881/6L6 amps never idle their tubes at max W rating because it kills them when you start to play.
Perhaps your tech means he will fit a pair that idle at lower bias current (lower idle W dissipation)?
I do not recommend that he install any tubes with a wattage rating of less than 23W each (23W vintage Tung Sol 5881 are the lowest W rating I would use, but you would know if he meant these, by the number of "0"s following the price!), most current production tubes are at least 25W irrespective of their designation (5881/6L6GC etc).
Certainly do not fit any old 19W 6L6G tubes to this amp.
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Last Edited by on Dec 05, 2012 3:26 AM
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528hemi
339 posts
Dec 04, 2012
4:10 PM
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In a amp with a given bias, a higher GT rated tube will have a higher idle dissipation if no adjustments are made.
So from the link. It sounds to me that for a fixed bias amp, you can choose certain tubes that will change the bias thus break up sooner or later.
I wonder if you take thier number 1 rated tube if you can actually bias it up to say 40ma with a bias pot or it reaches a lower max?
This aspect can be used to fine tune amps such as older tweed era Fender amps, Mesa Boogie Amps, Hiwatt,
Orange, etc. If you find as an example that a #5 tube has an idle of 26mA and you wish something closer to 30mA
you may want to try a #7 rating.
528hemi
Last Edited by on Dec 04, 2012 4:16 PM
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5F6H
1442 posts
Dec 04, 2012
4:16 PM
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@528Hemi - "I wonder if you take there number 1 rated tube if you can actually bias it up to say 40 wit ha bias pot or it reaches a lower max?" With the Kendrick style bias pot (replacing R41 with a 50K cermet pot) you can bias any 5881/6L6 so hot that it will fry! The grading system is more aimed at amps that do not have adjustable fixed bias, your pot makes the grading largely redundant.
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528hemi
340 posts
Dec 04, 2012
4:19 PM
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Gotcha....makes sense.
I am going to go for the Tung-Sol 5881 reissues. I will compare those to the Stock groove tubes fender branded originals.
OK I think I am done for awhile with these tube questions. LOL
528hemi
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JD Hoskins
143 posts
Dec 04, 2012
5:10 PM
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Mark, I'm concerned, as you know I have mounds of respect for your opinions and expertise.
I've been running vintage TungSol's in my Bassman amps (both my RI, that I no longer have and my Clark and another hand wired by a local guy) for years and have built up a stash that should allow my son's to probably continue. What's the problem I should be expecting?
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5F6H
1443 posts
Dec 05, 2012
3:24 AM
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Hi JD, Vintage New Jersey Tung Sols are rated at 23W per tube, they're pretty sturdy therefore will take regular bias currents in a RI bassman (which can run higher voltages than an original amp), certanly in an original (30-35mA depending on plate voltage, or 16W idle as a *safe* limit). These are the tubes that were originally fitted to 5F6A Bassmans and early brown blonde amps (as I'm sure you well know), there have been concerns voiced on the interweb by folks who point out the 400v limit on the datasheet, but this is a conservative rating, Fender fitted them to amps that ran well over 400v from day one.
In short, your biggest concern is your friends asking to look at your collection of 5881s and then inadvertantly wandering off without replacing them! ;-)
It's really the older tubes with a lower W rating than 23W that I was suggesting it wasn't wise to run in this amp.
Regards, Mark.
I have edited my earlier post, as I realised (on review) it could be read more than one way.
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Last Edited by on Dec 05, 2012 3:28 AM
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JD Hoskins
146 posts
Dec 05, 2012
7:43 AM
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thanks Mark, I knew the tubes had been holding up very well, but thought maybe I should be concerned about the health of my amps. Soon to be a fixed income senior and don't need unexpected expense.
now I gotta go count tubes :^)
Last Edited by on Dec 05, 2012 7:45 AM
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