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Old 20th February 2009, 04:57 PM   #1
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Default 6l6 Grid resistors??

Why do the majority of guitar amp manufactures ignore the Grid 1 resistance spec of a 6l6??? Mesa dual rectifier uses 220K bias feed into 2 6l6's with 1.5k on each grid1.


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Old 21st February 2009, 09:33 AM   #2
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^^

Probably using cathode bias. The grid resistor limit is less important with cathode bias than with fixed bias. Also, transistors have ratings, tubes have guidelines.
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Old 21st February 2009, 03:17 PM   #3
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No, there is no cathode bias on these amps. All fixed bias. Good point about the guidelines though.....

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Old 21st February 2009, 08:55 PM   #4
kevinkr is offline kevinkr  United States
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And Mesa amps have some history of cooking their output tubes under some conditions...
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Old 22nd February 2009, 03:20 AM   #5
nhuwar is offline nhuwar  United States
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I think this is funny as Mesa's are some of the most expensive amps on the market. I'm amazed at how the output power rating on guitar amp are usually way higher then the normal output power rated on the spec sheet per a particular configuration.

IE I have seen some amps stating that they have an output power of 200 watts for a pair of kt88's in push pull.


I don't know if this is just marketing or pushing the crap out of the output tubes .
All I know is when I decided to look at some amps to copy for a guitar amp I was left a little confused at how they are coming up with there ratings.

Ah just my 2 cents

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Old 22nd February 2009, 03:30 AM   #6
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Quote:
Originally posted by kevinkr
And Mesa amps have some history of cooking their output tubes under some conditions...
Their narrow minded approach to fixed bias is likely a good reason why:
http://www.mesaboogie.com/US/Smith/biasadjust.html

More comments on this here .

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Old 22nd February 2009, 11:01 AM   #7
SY is offline SY  United States
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From Mesa:

Quote:
Some newer amps have LED indicators connected to the circuit which will turn on when the right threshold of current flow has been reached. This is an improvement, and almost worthy if you're willing to except resistors and lights added into your amplifier's audio path -- which we aren't.
Besides badly needing a copywriter and a proofreader, they also need to think through some cost accounting issues. They're trying to save a resistor, a voltage test point, and a pot per tube (or per pair). Nothing more, nothing less.

I would personally not buy their products.
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Old 22nd February 2009, 04:18 PM   #8
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I have personally owned a mesa triple rectifier and have played many other mesa products. While I agree that they are severely overpriced, I have found that the quality is absolutely stunning.

As for the power ratings, I haven't come across any that were fudged. Triple rec 6x6l6GC=150W dual rec 4x6l6GC=100W Thats only 25W for a tube rated at 30W.

Mesa has a valid and good argument on the biasing topic...............really only to sell their relabeled Chinese tubes........


Again, It is not just mesa that I notice this. Randall, Soldano, Peavey and a whole slew of manufacturers do it almost always 220K feeding 1.5k-2.5K grid stoppers per pair.


one of my favorite sites out there..

www.schematicheaven.com


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Old 22nd February 2009, 08:46 PM   #9
twystd is offline twystd  United States
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As for the power ratings, I haven't come across any that were fudged. Triple rec 6x6l6GC=150W dual rec 4x6l6GC=100W Thats only 25W for a tube rated at 30W.



Dude, the 6L6GC is rated for 30 watts dissipation. There is a big difference between watts of plate dissipation, and actual output power. Even in class AB, I doubt you could get much more than an actual 15 watts at the most per tube, without killing them. Then there is the problem of keeping them in balance enough, with the parallel PP topology, to get the most out of each tube. Put them on the bench and see what they'll really do.

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Old 23rd February 2009, 02:58 AM   #10
kevinkr is offline kevinkr  United States
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Quote:
Originally posted by twystd



Dude, the 6L6GC is rated for 30 watts dissipation. There is a big difference between watts of plate dissipation, and actual output power. Even in class AB, I doubt you could get much more than an actual 15 watts at the most per tube, without killing them. Then there is the problem of keeping them in balance enough, with the parallel PP topology, to get the most out of each tube. Put them on the bench and see what they'll really do.

twystd
All true, but in AB2 a quartet of vintage U.S. made 6L6GC will do a bona fide 100Wrms, and no they probably don't last that long.. (I can vouch for the hundred watts, tube life is a different matter.)
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