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PostPosted: Tue May 24, 2005 9:52 am 
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Mark I

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I was trying my amp out with EL34's the other day, so I had the amps bias switch set over to "el-34," and of course when I put my 6L6's back in I forgot to change the bias back to 6L6.Now, leaving the bias set for el34s made my amp sound a lot better, more dynamic and punchy. My question is- how long could my tubes withstand that kind of inaccurate voltage. Is it something I should worry about?

I remember reading somewhere that mesa's are biased a little cold and flipping the amp over to the EL34 mode while 6L6's are in there, warms it up a bit.

About how much voltage is being delivered to each tube and is it safe to run the amp in that mode?

I should also mention I'm running it at 50 watts, only two power tubes.


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PostPosted: Tue May 24, 2005 11:08 am 
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Mark III
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bob at eurotubes has a couple things to say about using those switches to match the bias to kt88's, his 6l6/el34 combos, or even just warming up 6l6's.

im sure this guy isnt going to risk ruining his reputation and buisness by gving out false information...

http://www.eurotubes.com/euro-n.htm

"The Dual and Triple rectifiers are set very cold and can benefit greatly from having a trim pot installed and the bias adjusted properly or if your using 6L6's then using a cooler grade and running the amp in the EL34 mode which gets the bias very close to where it should be."

now, there is the argument that you shouldnt f%&# with using differnt branded tubes or setting up the amplifier contrary to randall smiths design, but then again boogie wouldnt exist if it werent for his screwing around with fenders designs.


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PostPosted: Tue May 24, 2005 1:04 pm 
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Mark I

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Interesting. I don't really want to have a bias pot installed, but I do like the idea of getting cooler graded 6L6's and running them in the EL34 mode. When he says "cooler graded," what exactly does that mean?


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PostPosted: Wed May 25, 2005 10:16 am 
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Mark III
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when tubes are manufactures, they 'pull' a different draw from the amp. this draw it 'matched?' by the bias. many companies will test tubes to find out their draw, so they can sell them with the appropriate characteristics in mind. an example of htis would be mesa, selling tubes that fall within the set bias of their amplifiers. mesa isnt the only company with a tube tester, and if you go to someone like eurotubes, they can find you set that will either fall within that bias range, or just outside out of it, e.g. a hotter or colder tube. the difference int he draw versus the set bias will create a differnt tone...

i actually have no idea what im talking about, but this explanation seems the logical one... :wink:

Andy.


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PostPosted: Wed May 25, 2005 12:26 pm 
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Mark III
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the_meridian wrote:
I was trying my amp out with EL34's the other day, so I had the amps bias switch set over to "el-34," and of course when I put my 6L6's back in I forgot to change the bias back to 6L6.Now, leaving the bias set for el34s made my amp sound a lot better, more dynamic and punchy. My question is- how long could my tubes withstand that kind of inaccurate voltage. Is it something I should worry about?

I remember reading somewhere that mesa's are biased a little cold and flipping the amp over to the EL34 mode while 6L6's are in there, warms it up a bit.

About how much voltage is being delivered to each tube and is it safe to run the amp in that mode?

I should also mention I'm running it at 50 watts, only two power tubes.


I've heard that they are biased really cold too. I haven't heard of running in EL-34 mode with 6L6's though. I'm running EL-34's and THD Yellowjackets in mine, and to me it sounds a lot nicer. I am however considering having the pot added the back, along with some way to quickly and easily check the bias with a multimeter.

BTW, How many rectifier tubes are you running?

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PostPosted: Thu May 26, 2005 9:53 am 
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Mark III

Joined: Sun Jan 23, 2005 9:54 pm
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Here is how everything is pretty much:

First off, the EL34/6L6 bias switch basically allows throws 1 - 47K resistor in the bias circuit for the 6L6GC bias mode. When you slide it over to the EL34 position, it will throw another 47K resistor in parallel to the already existing 47K resistor. Then that is followed by another "fixed" resistor which also sets the negative grid voltage. This last fixed resistor is normally where a "variable resistor" (aka Bias Pot) would be in the bias circuit.

The bias is set fairly cold on the Rectos. EL34's are typically below 50% plate dissipation at Idle. 6L6GC's are usually in or around the 40% plate dissipation area at Idle. Both of those figures are based off of the Bold/Silicon Diode selection, which is where you will find the highest Cathode Current readings (Bias or Bias Current). Each time you swap a mode the plate voltage will lower and the bias current will lower because you aren't changing the negative grid voltage. Also, the Bold/Spongy switch is a variac switch. Bold is full power and Spongy will lower the plate voltage. The rectifier selection will also raise or lower the plate voltage as well. The Silicon Diode rectifiers will yield higher plate voltages and the Tube Rectifiers will yield lower plate voltages.

Here are some ballpark areas of where the plate voltage is both bias modes with the amp's overall mode shown. These aren't exactly correct for every amp because each amp is different and power tubes have different natural plate current draws.

In the figures below S=Spongy, B=Bold, T=Tube Rectifiers, SD=Silicon Diode Rectifiers, the 3 digit numbers are the plate voltage in that paritular mode.

S/T (mode) - 355 to 375V(Plate voltage) - 12 mA(bias current) - 6L6 mode
S/SD - 390 to 400 -18 - 6L6
B/T - 410 to 430 - 16 - 6L6
B/SD - 460 to 470 - 25 - 6L6

EL34's are pretty much in the same ballpark, but the plate voltages will be on the higher side.

That is pretty much where the "Mesa spec" tubes fall.

Now, if you throw a 6L6GC or any tube in the EL34 bias mode, the bias current will be much higher. 6L6GC's draw more natural plate current than EL34's. If you load "Mesa spec" tubes into the circuit, they will be running way too hot in the Bold/Silicon Diode mode. I've seen them draw as much as 55 to 60 mA. The Bold/Tube mode can be safe. Spongy/Silicon Diode is usually in the same ball park as the Bold/Tube mode, but Bold/Tube can sometimes run fairly hot as well. Spongy/Tube is still safe, but you may not notice much of a difference from the stock sound.

JJ 6L6GC's typically don't draw a whole lot of natural plate current. A lot of folks throw in the term that they have a "soft vacuum" and they do. You can get a set of these that will bias up in the same range as a very hot EL34. Bob @ Eurotubes specializes in matching these types of tubes.

Anyways, I'm tired of typing. When the bias is hotter, the dynamics of the amp will pick up greatly, but I wouldn't throw KT88's in a Recto because they aren't designed to handle those types of tubes.

Dale


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PostPosted: Thu May 26, 2005 9:56 am 
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Mark III

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musicbox wrote:
when tubes are manufactures, they 'pull' a different draw from the amp. this draw it 'matched?' by the bias. many companies will test tubes to find out their draw, so they can sell them with the appropriate characteristics in mind. an example of htis would be mesa, selling tubes that fall within the set bias of their amplifiers. mesa isnt the only company with a tube tester, and if you go to someone like eurotubes, they can find you set that will either fall within that bias range, or just outside out of it, e.g. a hotter or colder tube. the difference int he draw versus the set bias will create a differnt tone...

i actually have no idea what im talking about, but this explanation seems the logical one... :wink:

Andy.


You pretty much hit it. Good tube vendors can supply you with power tubes that fall within the "Mesa spec" range. These tubes are typically used by all amp manufacturers of high gain amplifiers. These tubes have the best blend of headroom and touch sensetivity.

Dale


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PostPosted: Thu May 26, 2005 10:00 am 
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Mark III

Joined: Sun Jan 23, 2005 9:54 pm
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Location: Louisiana
the_meridian wrote:
Interesting. I don't really want to have a bias pot installed, but I do like the idea of getting cooler graded 6L6's and running them in the EL34 mode. When he says "cooler graded," what exactly does that mean?


It means that they don't have a lot of natural plate current draw. When you load a power tube into a circuit and make no changes to the bias circuit's resistance, a cooler tube will bias up cooler and a hotter tube will bias up hotter.

A lot of folks tend to overlook this when selecting power tubes for their amplifiers. The tone will be different from what they were used to hearing and techs might have a problem with biasing them up properly. A hotter tube, a "Mesa Spec" tube and a cooler tube will all sound different even if they are biased up the same.

Dale


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PostPosted: Thu May 26, 2005 10:05 am 
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Mark III

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musicbox wrote:

now, there is the argument that you shouldnt f%&# with using differnt branded tubes or setting up the amplifier contrary to randall smiths design, but then again boogie wouldnt exist if it werent for his screwing around with fenders designs.


Yeah, you can look at buying tubes from Mesa two different ways:

1. You can say it's a convenience because you don't have to worry with biasing an amplifier.

2. You can say that it's a marketing ploy for Mesa to make more money. If you don't use "Mesa Tubes", it will void your warranty. There are vendors that test as well, if not better, than Mesa who can supply you with tubes that fall within this same range.

As far as running KT88's, these amps will run fine for a little while, but then bad things start to happen. They do sound good in a Recto, but at the same time the transformers in a Mesa Recto can't handle them.

Dale


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PostPosted: Thu May 26, 2005 5:13 pm 
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Mark III
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aha! i was hoping somone who actually knew somethign would reply... thanks for the great response!!!


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PostPosted: Tue Jul 26, 2005 7:12 am 
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Mark II

Joined: Thu Mar 03, 2005 3:30 am
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Location: California
so is anyone actually running 6l6gc in el34 mode?

I take it from the settings someone gave on the rectifer that since i have a series 2 Rectoverb i could not try this wihtout killing my amp. I've got SED 6l6s from doug.

Reason i ask is that when i first got the amp it came with stock mesa tubes and the amp was extremely hot, and the tubes were full of bright blue gas. these new tubes just have a light orange glow.

What do you guys think?


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PostPosted: Tue Jul 26, 2005 8:17 am 
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Mark III

Joined: Sat Mar 26, 2005 9:17 am
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any time a tube goes blue or purplish, it means the tubes are leaking and
letting air in, and it obviously means they need to be replaced.
normal tubes glow orange or light red and pairs should always be pretty
close to the same color. i had one of my 84's die on my and i knew right
away because it was a way different shade of orange than the other one.
(it sounded awful too, heh).

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PostPosted: Thu Nov 30, 2017 11:30 am 
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Bottle Rocket

Joined: Thu Nov 30, 2017 11:25 am
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I know that Im resurrecting this from the dead but it looks like Kim Thayil is using 6L6 tubes on the EL34 bias setting.

look at 28:47 in video.

https://www.youtube.com/watch?v=uEl7u9Z3lK0&t=442s

Anyone have any insight on this?

Thanks,

Doc


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PostPosted: Thu Nov 30, 2017 10:38 pm 
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They look like EL34s.

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