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Need some help buying tubes..


Andrewsonfire

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Im about to order tubes for my Peavey VK112.. and it needs three 12ax7's and two 6L6GC's .. I picked them out but need to know if im not making some stupid mistake like buying the wrong size or something.. this is my first dealing with replacing tubes.

 

i was going to get

6L6GC

http://www.tubedepot.com/eh-6l6.html

12ax7's

http://www.tubedepot.com/jj-ecc83.html

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You don't have to bias the VK112.

 

Here's a few helpful links for the VK amps:

 

http://ultimate-guitar-valveking.wikispaces.com/message/list/home

 

http://www.ultimate-guitar.com/forum/showthread.php?t=969296&page=1&pp=20

 

I the JJs aren't that expensive, so you can try them and see what they sound like, although I don't particularly like them. A suggestion would be to put a 12AT7 in V2 to smooth out the gain a little. That's what I did with mine and am happy (NOS JAN Phillips 12AT7 to be exact).

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It is a self-biasing amp. You do not have to re-bias. Just be sure to get a matched set of power tubes. As long as you get 6L6 power tubes you are good to go. Do not use EL34 or EL84 power tubes.

 

 

What exactly does "self-biasing" mean? I have a Mesa Boogie MKIIB and it is a self biasing amp. I haven't replaced the tubes since I have had the amp. The thing still cooks with some type of 6L6s and I believe Mesa Boogie preamp tubes. I figure it is probably worth replacing them sooner rather than later.

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Technical definition: A grid bias provided automatically by the resistor in the cathode or grid circuit of an electron tube; the resulting voltage drop across the resistor serves as the grid bias. Also known as automatic C bias; automatic grid bias.

 

Practical definition: You do not have to adjust anything when you change power tubes since the amp is designed to work with one kind of tubes only. Usually found on lower powered amps, 50 watts or less.

 

This means you simply pop a matched set of new tubes in and play.

 

Pre-amp tubes are all self biasing. You never bias them, even on an amp where you have to bias new power tubes.

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Biasing is sort of like setting the idle on a car. But some amps don't need it.

 

Anyway pulling the old tubes out and putting the new ones in is kind of tricky. You have to pull a little to each side and in all directions. And putting them in is similar. Anyway make sure the amp is unplugged while you change them.

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Uggg.

 

There are two different ways companies bias their amps.

 

Fixed Biased and Cathode Biased.

 

First of all, the Valve king is FIXED biased. It is more commonly refered to as non-adjustbale fixed bais...which is common in Peavey (and the same with Mesa). none of these amps self bias, none measure the tube dissapation and adjust the bias to correct for plate voltage. All Peavey and Mesa do is pick a bias resistor that falls somewhere in the middle of the road and assume most tubes will fall within this range..."one size fits most." However, this is highly inaccurate. While all tubes can operate safely in this range, many tubes will run very cold. If you want the tubes to run optimum, you either have to get tubes that have been tested for the amp's specific setup (like Mesa, Eurotubes and Doug's tubes does) or go in there and tweak the bias. Peavey and Mesa use this system because don't want to deal with warranty and service calls from people who don't know what the hell they are doing with an adjustable fixed bias setup (a la Fender, Marshall, etc., where you can adjust the bias with a pot in the chassis) and can screw it up/.

 

Cathode biasing, which is the process Bro Blue described, is often (ina ccurately) called self-biasing, but it does offer a much, much larger bias range, so it's generally always plug and play with one specific tube type (but may have to be changed for other tube types or changes in B+ voltages). The Valve King is NOT cathode biased (though mesa offers a few cathode-biased amps).

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And BE CAREFUL. Even when the amp is off and unplugged it can still have enough juice to kill you. Yes, you read that correctly. Not hurt, but kill you.

 

Wha? :eek: Somebody tell me more about that, please. I have a new set of tubes arriving this week - young family with another child on the way. Should I add to my life insurance policy?

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I would recommend a 12AT7 for the Phase Inverter slot too, instead of a 12AX7

 

 

Better to use a 5751 if you want to do that.

 

The 12AT7 has different plate resistance compared to a 12AX7. It won't be biased right in a circuit design and will run very cold, giving significantly less than it's 60mu gain. Despite the best efforts by the non-taube-savvy public, 12AX7's, 12AT7's, 12AU7's, and 12AY7's were never intended to be interechangeable; the 5751 is the only tube specificially designed to be a direct lower-gain replacement for a 12AX7. Not saying it will hurt the tube, but without tweaks to the circuit, you're just not running some of those tubes optimumly.

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Better to use a 5751 if you want to do that.


The 12AT7 has different plate resistance compared to a 12AX7. It won't be biased right in a circuit design and will run very cold, giving
significantly
less than it's 60mu gain. Despite the best efforts by the non-taube-savvy public, 12AX7's, 12AT7's, 12AU7's, and 12AY7's were never intended to be interechangeable; the 5751 is the only tube specificially designed to be a direct lower-gain replacement for a 12AX7. Not saying it will hurt the tube, but without tweaks to the circuit, you're just not running some of those tubes optimumly.

 

 

No. They work great in the PI slot, it's the other slots that the 5751 may be better...

See this: http://verosurfcam.com/Phaseinverter.pdf

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