Bias Voltage (probably a question for the RD guys)

[Gee Donner]

I'm going through a lot of different tube types in my Lynchbox. So in order to calculate the right bias voltage range, I need to know the actual plate voltage being put out from the power transformer. Maybe you can tell me?
The numbers that are written in the manual might be OK for 6L6 or EL34 tubes, but not for KT88.

I guess there is not a major difference if running it on 220V instead of 110?

 

[Gee Donner]

For you who doesn't know the theory behind biasing different tube kinds, here is a little lession on how to bias different kinds of tubes exactly in your loved RM. There is alot of ways to do it, but I like this one the most:

Different tubes kinds has different plate dissipation (AV) which is required to know to calculate the correct bias range for a specific kind of tubes. For example, the EL34's has 25 watts of dissipation per tube. Therefore in a 100w amp equipped with four EL34's you get (4x25=100w). Normally 6L6 tubes has the same dissipation unless you use a different kind of 6L6, like 6L6GC30W. The other factor you need to know is the plate voltage (AS). I don't know this yet but I hope the RD guys will answer my question above. If not I'm simply going to open my amp up, and measure it. If so I'll get back to you about it as soon as i figured it out. Now, this voltage (AS) I recommend that you write on a piece of tape and stick inside the amp somewhere, so you can read it in case you forget it. But for right now lets imagine that the AS= 463 Volts (typical Marshall range).

Normally, the biasrange used in different amps is 50%-70% of the total bias range. The higher you go the warmer and clearer sound you'll get, but shorter lifetime on the tubes. So therefore I don't recommend to exceed 70%. Over 70%, there is hardly any audible difference anyway, but sure you can try 80% as well.

Remember that inside the MTS amps, are 1 ohm resistors which lets you know the voltage by reading amps on your multimeter. That is an outcome of Ohm's Law.

Ok, knowing that we have:
AV (plate dissipation in watts) =25W
AS (plate voltage in volts) = 463V

The formula is:
Min value (in mA) = (AV * 500) / AS (500 for 50%)
Max value (in mA) = (AV * 700) / AS (700 for 70%)

This gives us:
Min value (in mA) = (25 * 500) / 463 = 26.99 mA =27 mA
Max value (in mA) = (25 * 700) / 463 = 37.79 mA =38 mA

Hope you can use this information. If you have any questions, let me know!

///Gustaf

 

[jmgman70]

500v I believe.

 

[RD/Rich]

500V is correct.

 

[okstrat]

Real quick and easy way to find out voltage on your amp is measure pin 3 on a power tube socket. There's not a 'magic' bias setting for any tube either, imho - just a limit where the tube will burn up. Obviously you want to avoid this to keep from hurting your tubes or the amp, but there's a wide range of bias that will work. Just depends on what you like.

If you can tell what a power tube 'red plating' looks like, then you can actually bias a tube amp by ear and eye... just be sure to keep a close look at your power tubes to be sure they aren't self destructing.

Pete

 

[khingpynn]

Ok strat gave some good info in that last post... I prefer to set my bias with a scope and then listen to the amp... if I'm not hearing the mojo then I tweek by ear while whatching my tubes and my meter/scope. The ear tells me what I want to hear while the tube tells me if it's happy while the meter/scope tells me the technical data in other words am I within safe operating perameter.

 

[Gee Donner]

Of course the ear plays the main roll here, but the theoretic numbers gives you a hint on how far to go.

If you bias an amp after what your ear says, then you might get the red plating syndrome later while playing. It can take up to 30 minutes while playing loud before the "red plating" appears if you don't bias the amp correctly. And especially if you use an attenuator.

///Gustaf

 

[okstrat]

Of course the ear plays the main roll here, but the theoretic numbers gives you a hint on how far to go.

If you bias an amp after what your ear says, then you might get the red plating syndrome later while playing. It can take up to 30 minutes while playing loud before the "red plating" appears if you don't bias the amp correctly. And especially if you use an attenuator.

///Gustaf

30 minutes? If that was the case, then it would take 30 minutes before you read correct measurements on the tube either if you were biasing by any other method.

I'm not understanding your attenuator comment either - you're saying the tube would be more likely to redplate if you use an attenuator. So if you set the bias 'correctly' (with whatever formula you want to use) and then used an attenuator, it would go past those settings?

Pete

 

[Gee Donner]

I think that the tricky part of the biasing process on a randall system, is that its hard to bias every tube individually by ear, like kingphynn I recommend to do it together with measure equipment. Like kingphynn uses a O-scope. A marshall for example, that uses 4 matched tubes has one bias pot. That makes it easier to hear the difference when you reach the "sweet spot". Eventhogh I prefer the randall systems a lot more than the "marshall system".

So the reason why I put this post about the formula is because I thought the info can be useful for people that, like me, have a hard time to dial in the bias by ear when you tweak each tube individually. The other reason was to show people how to figure out the right settings for their kind of tubes, the kinds that the manual doesn't determine.

On my amps that I build, I have individual matching compensation. This means that every tube can be dialed in to "match" the others. And then I have a common bias pot that acts on all 4 tubes. When I designed that circuit, I thoght it was the best of both worlds. And it is working pretty good to be honest.

And yes, the more you push the tube, the higher bias value you get. If you leave your meter in the amp while idling and then start playing. You will see how the settings change according to hot the signal is. Same phenomena.

///Gustaf

 

[Gee Donner]

Real quick and easy way to find out voltage on your amp is measure pin 3 on a power tube socket. There's not a 'magic' bias setting for any tube either, imho - just a limit where the tube will burn up. Obviously you want to avoid this to keep from hurting your tubes or the amp, but there's a wide range of bias that will work. Just depends on what you like.

If you can tell what a power tube 'red plating' looks like, then you can actually bias a tube amp by ear and eye... just be sure to keep a close look at your power tubes to be sure they aren't self destructing.

Pete

By pulling the tube out if the socket and then measure the plate voltage between pin 3 and 8 on an octal socket, you'll not get the right value. The best way to do it, is to open up the amp and measure from the backside if the socket while the tube is plugged in to it. You could also use a bias socket then you can figure it out without opening up the amp. Example 450v unloaded will be approx. 420-430v loaded.

 

[okstrat]

So to get your bias readings, you crank the amp and play it while it's on the scope? I'm not debating whether there is a limit to what the tubes can take (and it varies from tube manufacturer to tube manufacturer - a 'real' 5881 from the 50s of which I have a small stash won't take bias settings that a Sovtek 5881 will.) Going by 'numbers' only will likely get you red plating on the real deal, and they are getting insanely expensive.

Use the numbers as a starting point, and a 'don't go past this' limit. But some amps do sound better imho with less than 70% plate dissipation. Some sound better with more. If 6-12 months of great tone is the price, I don't mind watching my tubes for a bit while I'm tweaking the amp. Nice thing about how easily the bias is set in the MTS series is that you can do that while playing - it's not a hardship like some amps where you have to take the whole **** thing apart to get to a hidden bias pot and test area!

Just to make things interesting, some amps pushed the tubes beyond the recommended settings too, and if you run them more conservatively they don't sound as good... Marshall 18s and Peavey Princeton Reverbs are this way. They also both chew up tubes like candy, but what a tone!

Pete

 

[Gee Donner]

Ok, I think were almost on the same track now.

What I ment with "playing with the multimeter/O-scope hooked up to the bias", was to tell you that an idle bias setting changes as soon as you plug in your guitar and plays on it. When you stop playing, the value drops down to the setting you gave it. So if it doesn't redplate while idling, it can still start redplating while playing. Thats what I ment.

But I'm probably going to mod the bias system so i can "match up for unmatched tubes" by tweaking the bias screws on the amp. And when that matching is done, i can tweak all of them at the same time with another pot that I install into the amp. Like I said, I've done this before on my own amps and it works good. What do you think about this idea?

In the post that I wrote about biasing formulas, I wrote that max dissipation is required to know in order to calculate the bias range. So for example, if you don't know the max dissipation in watts on your specific tube (in your case 5881's from the 50's), you can't calculate the bias range in mA that you need (normally you can find the right datas on the internet). I'm not familiar with 5881's, but if they have the same max dissipation as the newer 5881's, then they have the same bias range. But probably different "sweet spots" though.

BTW, below is a link for tube datas that I like.
http://www.drtube.com/tubedata.htm

///Gustaf