# Biasing Help Needed!



## savageblue (May 18, 2010)

Recently I bought a 1979 Deluxe Reverb. I bought a new set of premium tubes from the Tube Store and a BiasPro probe. I have followed the directions but only see a voltage in the range of 6-10, with the meter on the 200mV setting. Both tubes are similar.The bias pot moves the voltage up and down but only in that range.

The amp sounds and seems to work great. As a check, I put the probe on my jcm800, which was biased by a pro, and it reads about 39 with the meter set at the 200mV range. This makes me believe both the probe and the meter are functioning properly.Also put a new battery in the meter to be sure.

I have little experience and need some help. What are these low readings indicating? When I play my guitar through DR the voltage swings all over the place, but with no signal the voltage is low. What am I not seeing?


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## WCGill (Mar 27, 2009)

I don't use these bias probe thingies to bias, but I'm sure they're just a 1 ohm resistor on the cathodes of the power tubes to convert mv to ma. What's your plate voltage? The readings are useless without knowing this unless they have some system that factors it in. 6-10 sounds low for anything.


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## savageblue (May 18, 2010)

The probe works as you describe.Along with the probe the manufacturer supplies a chart of Examples of Cathode Current Range.For typical 6v6 in Fender Deluxe chart suggests a range of 12ma to 20ma, With JJs rated a bit higher at 19ma to 24ma. I am only comparing the readings on the chart to what mine read.I don't know how to read the plate voltage and not sure about poking around inside the amp.
Do the volume controls on the amp need to be maxed while biasing?

Again, the amp sounds and plays great


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## dtsaudio (Apr 15, 2009)

Your 6V6's are running very lean. I'll make the assumption the plate voltage is the same as the schematic (415V) at the maximum current your getting (0.010ma) that's only 4.15W. The tube is rated at 14W and should probably be run at about 50% ie 7 watts. That translates to about 17ma. 
So what's happening? Either the plate voltage is low as WCGill alluded to or the tubes are running low for a given current, and you can't get the bias low enough to bring the current up.
There's a number on the Tubestore sticker. This is the current for a set (but unspecified) bias. It's possible you've got a set on the very low end.
So you've got a couple of options. Live with it. You'll do no damage (but I'd have the amp checked), or contact the Tubestore and exchange them for a hotter set.
*But *without knowing the plate voltage you are taking a shot in the dark.


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## StevieMac (Mar 4, 2006)

savageblue said:


> I have followed the directions but...When I play my guitar through DR the voltage swings all over the place, but with no signal the voltage is low. What am I not seeing?


Just a general note of caution: I wouldn't recommend playing through your amp while the bias probe is plugged into it. I believe these tools are made for measuring current & voltage _during idle_. You might void your warranty if using improperly...


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## savageblue (May 18, 2010)

Thanks for the suggestions.
Does low plate voltage suggest that it is time to recap the amp?
Does the volume control of the amp have any bearing on the test results?
I think I will study about testing and working on a running amp. The time has come. I am aware at least somewhat , of the dangers.

AS for voiding the warranty, the probe plugs into my own multimeter, so Im not too worried.


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## Wild Bill (May 3, 2006)

savageblue said:


> Thanks for the suggestions.
> Does low plate voltage suggest that it is time to recap the amp?
> Does the volume control of the amp have any bearing on the test results?
> I think I will study about testing and working on a running amp. The time has come. I am aware at least somewhat , of the dangers.
> ...


The plate voltage should be consistent until and unless the filter caps are finally toast, SB.

The volume control is irrelevant. Adjusting the bias sets the idling current through the output tubes. Sorta like setting the idle speed in a car. Too low and performance suffers. The power drops and the tone is "thin". Too high and the tubes are stressed, in extreme cases rather spectacularly! When you set the bias you do it with no signal applied. 

Don't see how warranty on the bias probe is a factor. There's not enough current flowing through a 1 ohm test resistor from even a several hundred watt output stage to blow anything, even if it is a 1/2 watt resistor. Simple Ohm's Law shows that.

I respectfully disagree with Dan of DTS Audio about the idle current settings. While his figures make perfect sense for a "hifi" application virtually all the guitar amp world has done it differently, ever since tubes were first put into guitar amp service. Leo Fender would take that 14 watt Plate dissipation figure for a 6V6GTA, divide it by the plate voltage to get the actual plate dissipation in watts and then set the bias control to put that dissipation level to 70% of that.

So if we assume a plate voltage of 415 vdc on the plates of the 6V6s, our math would show 14/415 = .034 ma., then .034 X .7 = 24 ma. 

So EACH tube should show an idle current of 24 ma.! For the most precise reading you have to measure the plate voltage. If you don't it's like a one-armed fisherman trying to show you how big was the one that got away. The answer makes no sense! However, the voltage on the schematic is close enough in most cases, although with older amps the line voltage tended to be lower than that of today. This means that the plate voltage can be a fair bit higher than shown on the schematic and so it might be a good idea to set the idle current a couple of ma lower. The "sweet spot" is not all that critical anyway and your ears will never notice any difference.

More study is ALWAYS a good idea! I have been studying for over 50 years and I still have a lot to learn. Very early in life I found out that I had two choices when working with electronics. I could spend days farting around trying to get something to work by sheer luck or I could spend half an hour with a textbook and learn how things REALLY work! Once you understand, instead of wasting days you can do things in MINUTES!

Imagine building an amp and not having memorized the colour codes that tell you the resistor values. Instead of just grabbing the right one when you need it and soldering it in you have to instead measure every damn resistor with a meter to know if it is the right value!

I just don't have that kind of patience, myself!

Wild Bill/Busen Amps


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## dtsaudio (Apr 15, 2009)

Your right Bill, I should have written *minimum* 50%. But I agree 70% is much better. In HiFi amps bias is typically 75-80% of max dissipation.


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## savageblue (May 18, 2010)

What conditions could potentially contribute to the low plate voltage?


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## dtsaudio (Apr 15, 2009)

> What conditions could potentially contribute to the low plate voltage?


Bad rectifier most likely. But have you checked the plate voltage?


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## loudtubeamps (Feb 2, 2012)

Has the amp had any modifications done to the output / bias circuits?
If U are not sure , some close up pics. posted here of the output tube section of the chassis from within would tell the tale.


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## savageblue (May 18, 2010)

I don't know if the amp as been modded. I will take some pictures if I decide to open it up.

Here's another newbie question, but it occurred to me that the readings I am taking are only on one tube at a time. Do the readings need to be added together?


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## Wild Bill (May 3, 2006)

dtsaudio said:


> Your right Bill, I should have written *minimum* 50%. But I agree 70% is much better. In HiFi amps bias is typically 75-80% of max dissipation.


You're the hifi man, Dan! After all these years of listening to Pat Travers on 11, I just can't tell anymore! LOL

Wild Bill/Busen Amps


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## Wild Bill (May 3, 2006)

savageblue said:


> I don't know if the amp as been modded. I will take some pictures if I decide to open it up.
> 
> Here's another newbie question, but it occurred to me that the readings I am taking are only on one tube at a time. Do the readings need to be added together?


Nope! Not if the Bias Probe is being used with only one tube at a time!

I'll bet 2 beer that the guys are telling you straight - someone put a "cold" set of 6V6's in there and the setting must be adjusted.

Wild Bill/Busen Amps


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## StevieMac (Mar 4, 2006)

Wild Bill said:


> Don't see how warranty on the bias probe is a factor. There's not enough current flowing through a 1 ohm test resistor from even a several hundred watt output stage to blow anything, even if it is a 1/2 watt resistor. Simple Ohm's Law shows that.


Didn't realize he was using one of those basic units. I based the comment solely on Weber's cautionary note about use of their Bias-Rite unit:

*MANUFACTURING NOTE: As Weber re-releases the Bias Rite units, we have upgraded some of the components. However, in order to remain sensitive enough to provide accurate reading, some parts are still very delicate. DO NOT attempt to play your guitar while the Bias Rite adapters are inserted.** Doing so may very well cause the current to exceed acceptable tolerance for some of the components and damage the unit. Remember, you are only using the Bias Rite to adjust the IDLE current, which means the current level when no signal is being put through the amp.* ​


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## jb welder (Sep 14, 2010)

What do you measure with the old tubes in?


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## GTmaker (Apr 24, 2006)

savageblue said:


> The probe works as you describe.Along with the probe the manufacturer supplies a chart of Examples of Cathode Current Range.For typical 6v6 in Fender Deluxe chart suggests a range of 12ma to 20ma, With JJs rated a bit higher at 19ma to 24ma. I am only comparing the readings on the chart to what mine read.I don't know how to read the plate voltage and not sure about poking around inside the amp.
> Do the volume controls on the amp need to be maxed while biasing?
> 
> Again, the amp sounds and plays great


You have all the information you need to bias your amp...
Knowing the range is the most important and you have that (12 -20mA)..

here is your problem:
You said you set your meter at 200mV.
Why are you trying to read milli Volts when you should be trying to read milli amps.

Set your meter to read milli amps and adjust your bias accordingly.

I know all this stuff about knowing your voltage is important BUT that leg work has allready been done to standard fender amps.
You have the info now go and use it. BIas withing the normal range and you will be just fine.

and thats all I have to say about that...

G.


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## jb welder (Sep 14, 2010)

GTmaker said:


> here is your problem: You said you set your meter at 200mV. Why are you trying to read milli Volts when you should be trying to read milli amps. Set your meter to read milli amps and adjust your bias accordingly.


 Sorry but that is not correct in this case. Many bias probes (such as this one) have 1 ohm resistors in the adapter sockets. The 1 ohm resistors are in series with the cathode so you can measure the voltage across the resistor. Because it is 1 ohm, the math works out so whatever you measure in millivolts is equal to the current in milliamps. This is much safer than using the current range of a multimeter. As he mentioned, he measured his other amp the same way and got proper results.


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## GTmaker (Apr 24, 2006)

jb welder said:


> Sorry but that is not correct in this case. Many bias probes (such as this one) have 1 ohm resistors in the adapter sockets. The 1 ohm resistors are in series with the cathode so you can measure the voltage across the resistor. Because it is 1 ohm, the math works out so whatever you measure in millivolts is equal to the current in milliamps. This is much safer than using the current range of a multimeter. As he mentioned, he measured his other amp the same way and got proper results.


Thanks for the clarification and as usual, I was wrong.

G.

For my interest: in the following equation
"So if we assume a plate voltage of 415 vdc on the plates of the 6V6s, our math would show 14/415 = .034 ma., then .034 X .7 = 24 ma."

What is the range of plate voltage found in Fender Deluxe Reverb amps ?


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## Wild Bill (May 3, 2006)

GTmaker said:


> What is the range of plate voltage found in Fender Deluxe Reverb amps ?



As usual GT, it all depends!

Re-issues seem pretty close to the schematic figures. It is normal for the idle current to pull the plate voltage down a bit as it is increased but not that much with a solid state rectifier and larger filter caps. With re-issues of older models the big factor seems to be the power transformers are designed for the typical wall voltage we see today, which is about 122 VAC.

Back in the late 50's and 50's it tended to be more like 115 vac. A difference of 5-7 vac multiplied up by the ratio of the windings to deliver the high voltage to feed the rectifier will raise the end result at the tube plates by maybe 25 volts of dc or more.

This isn't going to change the equation result much when you calculate the target idle current but still, I just don't feel comfortable taking the plate voltage for granted from a schematic! All kinds of things will change that voltage a little bit. More important, a BIG difference will show you that something is wrong! 

How comfortable are you with someone tuning up your car by ear, with no actual measurements? It is even more critical with electronics.

Takes me only a second to measure the plate voltage when I have the amp opened up. Then I KNOW! I take the same approach with some of these newer amps that have the idle current test points and adjustments brought out of the chassis, so that you don't have to take it out of the cabinet to bias up new tubes. I open it up anyway! It's a great time to give a quick visual inspection that nothing seems to be going "crispy" and to check the main voltage points to be sure things are in the ball park.

It's like when your car is up on a hoist for an oil change. A good mechanic will take a quick look to make sure your transmission is not threatening to fall out!

Wild Bill/Busen Amps


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## LydianGuitars (Apr 18, 2013)

GTmaker said:


> here is your problem:
> You said you set your meter at 200mV.
> Why are you trying to read milli Volts when you should be trying to read milli amps.


No, that's not the problem. He's using the cathode resistor method of biasing.

The first thing he needs to do is figure out the plate voltage, figure out what the screen current is and then, he can calculate the expected bias value for approx 65% plate dissipation.


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## GTmaker (Apr 24, 2006)

Wild Bill said:


> As usual GT, it all depends!
> 
> Re-issues seem pretty close to the schematic figures. It is normal for the idle current to pull the plate voltage down a bit as it is increased but not that much with a solid state rectifier and larger filter caps. With re-issues of older models the big factor seems to be the power transformers are designed for the typical wall voltage we see today, which is about 122 VAC.
> 
> ...


*This isn't going to change the equation result much when you calculate the target idle current but still, I just don't feel comfortable taking the plate voltage for granted from a schematic! All kinds of things will change that voltage a little bit. More important, a BIG difference will show you that something is wrong! *

Once again Bill you make a lot of sense...

I would guess that on most amps that are placed in front of you, you allready know what the normal range of plate current should be.
Checking to see if the amp is in that range to confirn normal operations makes sense to me.

G.


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