Purpose of Plate Voltage Test (tube amps)

Theplate voltage can tell you what the maximun bias voltage can be. If you know that a 6L6 should put out 25 watts then you can plug it into 25w/plate voltage=bias current (mA). This tells you how hot you can set your idle current. If you bias closer to the max, then you'll get a warmer, richer sound which I think might be easier to overdrive. Biasing "colder" (like most stock amps come) will have a less pleasing tone, but the tubes will last longer, and that's why most amps come out of the factory with cold bias settings - it makes the amp to be perceived as more reliable.

Here's a good link
http://www.people.cornell.edu/pages/mt24/Amp/mm/biasmeas.html

 

Plate current biasing is for guitar players who like to guess what their amp is really doing. The ONLY way to know exactly what's going on is to bias an amp using an oscilliscope. ANY time you see "mA" in relation to bias, that's plate current biasing. Most DIY guitar amp guys swear by it, but it's a good way to fry a high powered amp.

 

Just to save you guys a few sets of tubes, you need know something. In the instance of the 6L6 (30W) to find the correct bias point, here is what you do.

Take the wattage of the tube, in this case 30W and divide this by the plate voltage which for the sake of discussion we will theorize to be say 450VDC. So with this information we determine that:

30/450=0.066666666666666666666666666666667

Multiply this number by 1000

= 66.666666666666666666666666666667 (call it 66)

This is the 100% plate dissipation limit of the tube. This IS NOT WHERE YOU WANT TO BIAS THE TUBE!! If you do bias the amp at 100% at 450VDC, this tube will start glowing red and soon fail and you'll be scratching your head wondering why it failed.

You can get away with biasing a tube at 100% dissipation if you are running in Class A which has drastically lower plate voltages (closer to 250-300 range).

However, since most tube bass amps run in Class AB, the general rule is that you should bias the amp at 70% of maximum plate dissipation, so from our previous findings take the maximum value found and take 70% of it. Example:

66*.7 = 46.2 or just 42

What does 42 mean? This is the amount of plate current per tube that the tube should idle at in order to run at 70% of max plate dissipation.

This will be a safe number ONLY for these tubes and for this plate voltage.

Here again is the formula for finding the correct setting for each tube.

(Power rating of tube) / (Plate Voltage) = X1
(X1) x (1000) = X2
(X2) x (70%) = Class AB Idle Point

These are generic instructions that should help you guys get your amps working nicely without hurting anything.

Also, here are some of the Power Ratings for some of the more common output tubes:

KT88 - 42W
6550 - 35W
KT66 - 30W
6L6 - 30W
EL34 - 25W

Hope this helps!

 

The tool that you are refering to is used to measure the CURRENT of the output tube and in order to do this, it is measuring the VOLTAGE across a 1ohm resistor that is housed in the unit. You should set your meter to read DC Voltage or mV if you have that setting.

 

Nope, however, they can run 6L6's, 6550's or KT88's

 

There are lots of "plate voltages" in a tube amp.

In the preamp, it tells you if all the tubes are approximately working OK, because the "plate supply voltage" usually is dependent on current draw through dropping resistors. Individual plate voltages, even more so, good information on performance.

In the power amp, it basically tells you if the rectifiers are working and if anything else is drastically wrong....

Otherwise, not a lot.

As far as biasing, well, that's an interesting area......

There is a range of voltages which are "acceptable" biases for a tube amp, with a range of performance associated with them.

When working with the usual guitar amp or lower powered bass amp I completely agree with PBG.... once you are "in the zone" with the bias, a 'scope is your best friend at setting a final value... with a sound test following it.

I am not fond of the idea of just calculating the bias current, although that will "work" in that it likely won't burn up the amp. It may not get you to a good point sound-wise.

I'd use that as a starting point and adjust while looking at the scope. Then check current again to be sure nothing is way off, and sound check. As tubes age, they have bias shifts, etc, or there could be another problem. You don't want to find that you were led into setting the bias way wrong by another fault.....

When you get to amps which are pushing the tube ratings, and have high plate voltages, like an SVT, your best bet is to set per the factory settings, in the way suggested.

Not that there isn't a range for them also, but its smaller, and "guessing" at a setting is not a super idea unless you like bad sound and/or buying output tubes.

If you have a lot of experience with a particular type amp, or one specific amp, you can do more adjusting to get exactly what you want for those units, but I'd not recommend that except for good techs who thoroughly understand the consequences.

 

Just to be clear, when I was refrring to burning an amp up, I was talking about amps like the SVT or Fender 400PS. Rich Koerner has the famous melted tube pics on his site to prove that plate current disappation isn't always accurate. If you have a high powered, high plate voltage amp with a dynamic drive stage, idle disappation won't hold the tubes out of runaway on large signal peaks.

It's that reason that I HATE all those plug-in bias gizmos. They give a lot of people a false sense of security. Besides, a 1 ohm resistor is a LOT cheaper.

 

And its already inside an SVT, (I don't remember if there is one in a Fender). That little bias point jack goes right to a resistor inside.

When you have a high powqered amp, with significant plate current at idle, there is another possible problem, although the effect is slight. If you DO use a plug-in gizmo, when you take it out, whatever resistor it put in is out also.

But, you biased it WITH that resistor in the cathode circuit, so there is a slight change in bias.... Current goes up a bit.

If you are "skating" already, depending on the size of that resistor, your "up a bit" might put you in a not-so-good range.

Mostly the "sensing" resistors are so small, that it isn't a problem. But with all the people making "tube stuff", they may not ALL be OK. (And, if they get the pin size wrong, could make bad contact, or stretch the socket contacts, etc, etc.)

When the resistor is built-in, there is never a problem. You measure what really is happening, and you don't mess with anything.

 

There's a great article on the Mesa website about biasing, etc. that you might want to check out.

 

JacksonAmpworks has the knowledge! I should have put a discalimer in my first post. It's been a few years since I've messed under the hood of a tube amp. And the ones I have played with were all guitar amps. I know close to nothing about bass amps and what the differences may be.

I completely forgot about biasing to 70% plate dissapation. If I was to work on my own amp, I would have fried it. Be sure to know what you're doing before you dive into any tube amp. I'll graciously leave this up the the experts now.

 

That article is crap. I've addressed it before. Rather than link, I'll just repost it.

For the benefit of the readers here, I'll use Randall Smith's own article to show why bias adjustments are necessary:

Vibration and user error is NOT the fault of the circuit. While this may justify making the control more inaccessable, it in no way demostrates a need to do away with it. Yes you can severely damage an amp by improperly setting up its bias; you can also achieve the same result from load mismatches, faulty power, incorrect connections, and dropping it.

Maybe things are different in California, but it's hard enough to get most people to pay legitimate repair charges even when they are performed for FAR less than they should be.[sarcasm] Sure, there are whole legions of techs who simply steal money from their clients.[/sarcasm] I got OUT of repairs because there is no money in it and I was tired of the hassle of deadbeats. A bias job was charged a bench fee, just like any other service that required my expertise and test equipment.

This is an outright lie. Not only do the same types of tubes bias to different voltages in a circuit, but the SAME tubes draw different amounts of bias current as they age. Cheap tubes are worse than good ones about it. Nobody's "fooling" anyone. Read the RCA Receiving Tube Manual sometime. It's where Leo Fender got most of his amp designs.

Here he backpeddles and tries to specify VOLTAGE as opposed to current, but even this is still wrong.

I (current in amperes)= E (electric potential in voltage)/ R (resistance in ohms)

Measure bias voltage without a tube present vs with one. The draw of the tube WILL lower the voltage because the bias supply attempts to maintain constant current and the impedance of the bias circuit has changed due to the presence of the tube.

That's a scary thought. How do you account for the differences in tube construction that result in different internal impedances then? Oh yeah, you have to buy HIS tubes, 'cause they're "special."

Here, he even admits what I've been asserting! And as far as being "difficult to measure" that's another lie. All you need is an oscilliscope, a voltage meter, and a test tone and you will have quantifiable measurements which show EXACTLY the effects of changing bias current. Matter of fact, that's the proper procedure for SETTING bias!


...There's more, but that should give you a good idea.

 

A quick Google search will yield several articles on biasing...each with a completely different take on biasing procedure. Here's a viewpoint that's more or less in line with PBG's preferred method (the oscilloscope):

http://www.tone-lizard.com/Biasing.htm

Here's an article with an exact opposite viewpoint:

http://www.aikenamps.com/Biasing.html

Here's Lord Valve's opinion...

http://www.duncanamps.com/technical/lvbias.html

Confused yet? How about the Eurotubes site:

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

Have fun!!!

 

With regard to bias.......

Bias voltage must vary with plate voltage, because higher voltage requires more bias for similar plate current. Most amps with a rectified bias voltage derived from the plate supply winding will do the variation relatively accurately vs the change in plate voltage with mains voltage, etc.
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Fixed bias , no adjustments.

Amp companies that have a fixed bias voltage and no pot, ALSO (usually) grade tubes. The idea is that they use a single grade of tube, which has a particular required bias voltage at the nominal plate voltage.

The grade used, which should be the middle of the distribution for lowest cost, will then bias up correctly when simply plugged in. it works pretty well, and cuts way down on labor cost to sort tubes, or set bias. It also avoids biasing errors and warranty claims. But getting the tubes is usually more expensive, maybe significantly more expensive, both for the manufacturer and the customer wanting replacements.

Older tubes may be somewhat over-biased in teh unit, since their "gain" has decreased, so you may have to replace sooner. And the "grade range" must be controlled well and be appropriate or it won't work well.

If you put in a single nice new tube of the same grade, leaving the older set's remaining tubes in place, the new one may be worked a bit harder, and could possibly fail first....or be overheated at high power, or it may be fine, depends on how "dead" your old set is.

Then again, if you put in a set that are not equivalent to the expected grade, they may bias up with too much, or not enough, plate current... burn up, or sound bad. If they are close, they probably will work OK.

If you put in a single tube of a different grade, or the set has one unmatched tube, it will likewise possibly fail, or overheat. Or it may be "lazy" and not have sufficient plate current.

It is similar to matching the tubes in a "bank", except that all the tubes must match, and they can only be from a certain range of characteristics.

I don't necessarily agree 100% that a non-adjustable fixed bias setup is bad. It can be very reliable and effective.

It does, however, limit you on replacements. You need to get ones with the same specs as the originals, in matched sets.

if you go to a third party to get them, that supplier has to know the appropriate specs. The manufacturer may not want to divulge those specs.... and the supplier may not know them.

And, in the old days, the "replacement tubes" available at the corner store (yes, it was like that) were usually all of the distribution EXCEPT the range that the manufacturers spec'd. Those were selected out and went to the manufacturers. They often were not available generally.....