# Biasing KT77's?



## FILTHnFEAR (Nov 18, 2013)

Will switching from EL34's to KT77's without biasing cause damage to my amp or tubes? I've read that the 77's need to be biased hotter than 34's, and I will have that done, but probably not till later this week. Tubes will be here tomorrow and I'm very impatient. I just wanna know if damage can be caused without biasing first. I'm guessing no, but not really sure here.

Thanks


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## vick1000 (Nov 18, 2013)

KT77s bias just like EL34s. But as with any tube change, you need to check and rebias the amp, as even the same brand and model tubes will bias differently in every amp, thus the reason for matching tubes.


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## Wrecklyss (Nov 19, 2013)

While i don't think you would hurt the tubes on a cold bias, the bias does need to be matched to the tubes. KT77s can be biased to the same level as EL34s but they can take a hotter bias too. 

Would you get away with it for only one week without biasing? Probably, but you will get the most out of your new tubes (and be sure not to shorten life or cause other problems) with a proper bias.


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## petereanima (Nov 19, 2013)

Don't make the istake and think something like "Oh, my EL34s are biased at 35mA, so if I now plug the KT77s in, they will also run straight away at 35mA" - it doesn't work that way. Depending on the circuitry of your amp, depending on what the KT77 draws, how it's burned in etc.etc. it might very well also be that if you just plug-and-play, they run on 150mA. Or 5mA.


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## sage (Nov 19, 2013)

Yeah, I would just wait and let the amp tech perform the swap. It's worth the $40.


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## vick1000 (Nov 20, 2013)

sage said:


> Yeah, I would just wait and let the amp tech perform the swap. It's worth the $40.


 
Totally NOT worth $40. It's as easy as changing strings on your guitar, you just need the right tools and a little knowledge.


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## Wrecklyss (Nov 20, 2013)

vick1000 said:


> Totally NOT worth $40. It's as easy as changing strings on your guitar, you just need the right tools and a little knowledge.



The right tools are more than $40 though. The first time i biased my own amp i blew two Radio Shack multi meters. The only one i found that could handle the job was a Fluke--not $40.

For 4 power tubes though, the rest is easy enough to build. You basically need 4 alligator clamps, 4 one foot sections of 14ga insulated wire, some solder, heat shrink tubing, and two 1 ohm 10 watt resistors. Solder a one foot length of wire to each end of the resistor, insulate with heat shrink tubing, fix an alligator clip to each end (you need two of these).

The easiest way to do it at home is the standby switch ("Carvin method"). There are quite a few sites that explain this step by step as well as youtube videos showing the procedure. Just be very careful not to accidentally touch the wrong thing as this can damage the tubes, your amp, or yourself. 

Also, KT77s can take a hotter bias than an EL34. Look up the specs from the manufacturer for the range they can operate in. Maxing them out will shorten their life, but running them at 75-80% will give you enhanced sonic qualities over EL34s with a good service life (also quite a bit more power, expect about 120-140 watts out of a typical 100 watt head depending on a number of other factors). It's a good project to learn how to do at home, but a good multimeter is key.


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## vick1000 (Nov 20, 2013)

Wrecklyss said:


> The right tools are more than $40 though. The first time i biased my own amp i blew two Radio Shack multi meters. The only one i found that could handle the job was a Fluke--not $40.
> 
> For 4 power tubes though, the rest is easy enough to build. You basically need 4 alligator clamps, 4 one foot sections of 14ga insulated wire, some solder, heat shrink tubing, and two 1 ohm 10 watt resistors. Solder a one foot length of wire to each end of the resistor, insulate with heat shrink tubing, fix an alligator clip to each end (you need two of these).
> 
> ...



Most modern amps have test points, and for those that don't, there is the 20$ socket probe from Eurotubes, who also sells a $10 meter with it.

The point is, what if you don't like how the new tubes sound? Are you going to take it back everytime you want to change tubes? Spent the $40 now on the right tools, and learn to do it yourself, and save a lot more later.


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## glpg80 (Nov 20, 2013)

Wrecklyss said:


> The right tools are more than $40 though. The first time i biased my own amp i blew two Radio Shack multi meters. The only one i found that could handle the job was a Fluke--not $40.
> 
> The easiest way to do it at home is the standby switch ("Carvin method"). There are quite a few sites that explain this step by step as well as youtube videos showing the procedure.



The right tools involve knowing more about plate dissipation and what to do to counteract red plating. It also involves quiescent bias currents that drift.

It also involves math and the plate voltage of the specific amplifier. It also involves screen grid current.

It also involves knowing that your method is not a blanket solution for every amplifier out there as there are many methods for using a standby switch. With my designed amplifiers, if you shorted across the standby switch a current meter you'd potentially fry the PT if the mains fuse did not go first.

I will only say this once. If you don't know WTF you are doing in the chassis then stay out and pay someone else that does. These amplifiers are not toys and a $30 bias job is not worth potentially killing yourself. Especially with over 500V DC across hanging gator clips. I definitely do not recommend anyone ever do that. Ever. That is just asking for an accident. 500VDC is enough to cause permanent nerve damage and if it crosses your chest and your heart, has more than enough current to defibrillate your heart putting you into cardiac arrest and potentially killing you.

Do not *f*uck around with cheap "carvin method" biasing to try and save $30.


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## glpg80 (Nov 20, 2013)

vick1000 said:


> Spent the $40 now on the right tools, and learn to do it yourself, and save a lot more later.



The correct tools range from $160 meters, to knowing how to modify the circuits to measure plate current safely, the math to get the corrected value, and then knowing what this means in relation to tubes.

It can be as high as $280 for some biasing probes that don't involve circuit modification.

And before anyone tries, the bias voltage has nothing to do with the bias current. Tubes are transconductance devices meaning current is what they operate and are controlled by.


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## FILTHnFEAR (Nov 20, 2013)

Well, a quad of new JJ KT77's, just arrived at my doorstep, not 15 minutes ago. Thanks to Eurotubes and the USPS.

But I definitely do NOT know wtf I'm doing, so I will wait until I can take it to my tech to have them properly biased.

Thanks for the info/advice everyone. Much appreciated.


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## Wrecklyss (Nov 20, 2013)

To be fair, i've only used the Carvin method on Carvin amps which are known to come biased cold from the factory. My Mesa was fixed bias and i always brought my Vox to a tech.


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## vick1000 (Nov 20, 2013)

glpg80 said:


> The correct tools range from $160 meters, to knowing how to modify the circuits to measure plate current safely, the math to get the corrected value, and then knowing what this means in relation to tubes.
> 
> It can be as high as $280 for some biasing probes that don't involve circuit modification.
> 
> And before anyone tries, the bias voltage has nothing to do with the bias current. Tubes are transconductance devices meaning current is what they operate and are controlled by.




What are you talking about? We are talking a EL34 to KT77 switch. You check the bias on the EL34s, swap in the KT77s and adjust to match, or a little hotter if you want. If you want to go 6550 or KT88, sure, you need to be more deliberate.


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## glpg80 (Nov 21, 2013)

Does not matter. If you change tubes without taking into consideration plate dissipation, plate voltage, and which tube is considered the most hot, you can run the risk of red plating or having DC leakage current on the primaries of your OT causing damage. Old tubes drift bias, and to correctly bias you need to consider the bias they are currently at is not what is within specification for the new tubes or that the new tubes are not defective. Without equipment and measurement with biasing you would never know. 

Tubes are never a drop in and guess ordeal unless aftermarket design with auto-biasing allows for it.

Not all amplifiers can be biased without knowing anything at all about the tubes or the amplifier either which is my main point. Modifications are needed to allow adjustment - and/or - spend $300 on a device to let you know what you're actually adjusting.

It does not matter whether you're staying in the same family or going from beam powered to kinkless tetrode. No tube is equal. Biasing is equally important and identical for all.


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## satchisgod (Nov 21, 2013)

Just to go back to your very simple question, all you really need is to get a bias probe and multi-meter. I got mine (the cheap one) from Eurotubes delivered to Ireland. Best thing i ever bought. You can check bias on the fly. Easier than changing strings. 

It's very easy when your bias trim pot is easily adjustable, i.e. there an easy access knob for adjusting like in my Peavey JSX. Just a case of puting in the probe, putting one of the tube on it with the optherss in there spots and turning on the amp.

You only need to research on the www.eurotubes.com website. From my experience don't overcomplicate it. It's simple and it seems to do a great job in keeping everything in check.

For example, a mate of mie bought a second hand 5150 taht was retubed before the sale. It sounded like shit. So i called over and used the bias probe. The amp was reading at about 12mA, so i brought it up to about 35mA and the thing absolutley roared and came to life. 

So basically, go to eurotubes.com, do a bit of reading, buy the cheap probe and multi-meter, and from then on always be in control of your amp!


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## sage (Nov 21, 2013)

satchisgod said:


> Just to go back to your very simple question, all you really need is to get a bias probe and multi-meter. I got mine (the cheap one) from Eurotubes delivered to Ireland. Best thing i ever bought. You can check bias on the fly. Easier than changing strings.
> 
> It's very easy when your bias trim pot is easily adjustable, i.e. there an easy access knob for adjusting like in my Peavey JSX. Just a case of puting in the probe, putting one of the tube on it with the optherss in there spots and turning on the amp.



Did you find that the readings from the bias probe varied greatly from the readings from the bias test points on the JSX? I always wondered if Eurotubes' claim that the JSX test points were way off is actually true.


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## glpg80 (Nov 21, 2013)

satchisgod said:


> Just to go back to your very simple question, all you really need is to get a bias probe and multi-meter. I got mine (the cheap one) from Eurotubes delivered to Ireland. Best thing i ever bought. You can check bias on the fly. Easier than changing strings.
> 
> It's very easy when your bias trim pot is easily adjustable, i.e. there an easy access knob for adjusting like in my Peavey JSX. Just a case of puting in the probe, putting one of the tube on it with the optherss in there spots and turning on the amp.
> 
> ...



I have replaced rectifier diodes in the bias sections of three 5150's because of those cheap kits. People buy them and then play through them at the same time thinking it is alright.

If you buy the eurotubes do NOT play through the amplifier while biasing.


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## vick1000 (Nov 21, 2013)

sage said:


> Did you find that the readings from the bias probe varied greatly from the readings from the bias test points on the JSX? I always wondered if Eurotubes' claim that the JSX test points were way off is actually true.




The test points on the JSX/XXX/3120, all measure negative grid voltage, not grid output.


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## satchisgod (Nov 22, 2013)

sage said:


> Did you find that the readings from the bias probe varied greatly from the readings from the bias test points on the JSX? I always wondered if Eurotubes' claim that the JSX test points were way off is actually true.


 
Never checked that to be honest so haven't a clue. I went on the advice of Eurotubes 100% and not to use them


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## satchisgod (Nov 22, 2013)

glpg80 said:


> I have replaced rectifier diodes in the bias sections of three 5150's because of those cheap kits. People buy them and then play through them at the same time thinking it is alright.
> 
> If you buy the eurotubes do NOT play through the amplifier while biasing.


 
Interesting. Luckily i don't do that. When biasing i just make sure there is a speaker load and that is it.


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