Bias Rite measurement discrepancy

fullerplast

Senior Member
Messages
6,781
I just corrected my original post. I did make a mistake in it. The resistor goes from pin 8 (cathode) to pin 1 (G3) and then continues from Pin 1 (G3) to ground. This is my homebuilt plexi 1959ish copy. Not sure if that is the best way to place the resistor, but I did copy that from some instructions I found that included a photo.
Much better.
 

fullerplast

Senior Member
Messages
6,781
I can make good music on a cheap SS amp and 90% of the people would never know the difference.
True, but that's not really the point in the Amps Technical Info forum, is it? I wouldn't talk about biasing in a songwriting forum either. The points Mike and others have made here are perfectly valid, regardless of how things were done in the past (mostly out of ignorance).

We're not talking about the music here, it's about how amps work and how things affect tone. Different forums for different topics.
 

iggs

Member
Messages
2,619
Guys back then "like me" were more worried about having an amp, guitar and playing well. This whole bias thing was never an issue until tube amps became more important than the music. I can make good music on a cheap SS amp and 90% of the people would never know the difference.
So basically what you're saying is that if your car was red-lining at 8000rpm on idle you wouldn't take it to a mechanic to adjust it? (or do it yourself)

:jo
 

Blue Strat

Senior Member
Messages
30,151
So basically what you're saying is that if your car was red-lining at 8000rpm on idle you wouldn't take it to a mechanic to adjust it? (or do it yourself)

:jo

"The best sounding amps (and best performing race cars) are the ones that are just about to blow up".
 

jetwolf

Member
Messages
36
Guys back then "like me" were more worried about having an amp, guitar and playing well. This whole bias thing was never an issue until tube amps became more important than the music. I can make good music on a cheap SS amp and 90% of the people would never know the difference.
Sure, any good musician can make even bad equipment sound good, but I certainly don't see anything wrong with musicians understanding what makes their equipment work best and what works best for them.

For me, it's been pretty fascinating to see what the amp sounds like at different bias points. I would much rather adjust the bias by what sounds good to my ear and then just checking to see if it is within the limits, than to take it to a tech and have them adjust it to either what sounds good to them or just following numbers on a sheet.

Case in point: after I began doing my own biasing a few years back, I pulled out an amp that I hadn't gigged with in quite a long time (10 years) and was thinking of selling it because I was not happy with the sound.....just too mushy in the low end. It had been biased by a local tech before I started doing it myself. When I checked it, I found that the amp was running way too hot (at least for me), over 70% dissipation. I rebiased it to my ears and checked it (55 percent happened to sound good), and it made a huge improvement, and now wouldn't think of selling it.

By the way, if you are not a tweaker, why do you bother to read the technical section?
 

Chris Scott

Member
Messages
9,095
Boy, did I stick my foot in it, or what?

FWIW, my point re. music made by players blissfully ignorant to the inner workings of their equipment was made simply as a fair counter-point, and not to suggest that striving to improve the quality of your sound (or the longevity and reliability of your amp) is not as important as making music.

...but you guys knew that, right?

Mike's point is indeed a valid one, and I'm glad he made it.
 

jetwolf

Member
Messages
36
In my case, my discrepancy definitely points to my cath resistor readings being false only when the Bias Rite heads installed, and there may be an easy explanation for that.

Ironically, I was taking the readings of the cathode with the Fluke while I was eyeballing the Bias Rite meter in realtime to avoid a possible several milliamp natural fluctuation due to inconsistent power (as mentioned before), and it was this reading of both meters in realtime that somehow caused the false numbers at Pin 8. The Bias Rite apparently always read correctly (or at least very close-within a ma or two..close enough). My later findings with Bias Rite only and then Pin 8 only (no bias rite heads) were very, very close, although I did not have time to log those figures. I feel much better; I can't stand tools that do not measure accurately or give inconsistent readings. I'm glad I found an explanation. That 10ma difference bothered me.

Also, I had asked Ted Weber if having a 1 ohm resistor installed would cause a false reading in the Bias Rite, and he replied that it would not, which turned out to be true. Apparently, it was the opposite of that: the Bias Rite caused a false reading in the cathode when read with a multimeter while the Bias Rite heads were installed. Now, I can't say if all Bias Rites will cause this or if it is just mine. I was just trying to test the accuracy of my Bias Rite. Most people, of course, would be using one method or the other and not try to read both simultaneously like I was.
 

Randy

Silver Supporting Member
Messages
3,922
Stupid question time;

Is it OK to play through the amp while you have Bias-Rite heads in place?
 

John Phillips

Member
Messages
13,040
No, often not, and this may also be the reason behind your odd readings when metering at the same time.

The trailing wires can cause the amp to become unstable - not necessarily full self-oscillation, although they can do that too, and sometimes outside the audio range so you can't hear it.

Just about any of the current-measuring methods can do this too, it's not unique to the Bias Rite, but the worst are those which measure current directly, rather than a voltage generated by the current passing through a resistor.

In any case, taking readings while playing through the amp is fairly pointless because the signal current will throw the readings all over the place. (It might be useful with a test signal, if you're concerned about maximum power in the duty cycle.)
 

Randy

Silver Supporting Member
Messages
3,922
No, often not, and this may also be the reason behind your odd readings when metering at the same time.

The trailing wires can cause the amp to become unstable - not necessarily full self-oscillation, although they can do that too, and sometimes outside the audio range so you can't hear it.

Just about any of the current-measuring methods can do this too, it's not unique to the Bias Rite, but the worst are those which measure current directly, rather than a voltage generated by the current passing through a resistor.

In any case, taking readings while playing through the amp is fairly pointless because the signal current will throw the readings all over the place. (It might be useful with a test signal, if you're concerned about maximum power in the duty cycle.)
John, were you responding to my question about playing through an amp while the Bias-Rite heads are in place, or an earlier post?
 

John Phillips

Member
Messages
13,040
Sorry, I should have quoted you - yes, I was refering to playing through the amp with the Bias Rite heads installed. Don't.
 

JackButler

Silver Supporting Member
Messages
3,822
Ok, as a fe wposts ago someone asked a similar question to what I am going to...for someone not in the know techincally and not having good techs around, and having a multitude of amps that need tubes now and later, is the Bias Rite a fairly accuarte and easy approach to use?
 

fullerplast

Senior Member
Messages
6,781
.for someone not in the know techincally and not having good techs around, and having a multitude of amps that need tubes now and later, is the Bias Rite a fairly accuarte and easy approach to use?
Yes, it's as easy as it gets. You need to be well aware of amp safety issues if you need to open the chassis to adjust the bias, and you need to know what the measurements mean (what the bias operating points should be), but other than that the Bias Rite is suitable for non-techs.
 

Blue Strat

Senior Member
Messages
30,151
Ok, as a fe wposts ago someone asked a similar question to what I am going to...for someone not in the know techincally and not having good techs around, and having a multitude of amps that need tubes now and later, is the Bias Rite a fairly accuarte and easy approach to use?
Yes, if you get a good one. There are other bias products as well.
 

KeithC

Silver Supporting Member
Messages
5,338
I just got an amp with bias points and after confirming how to check bias with my DVM I have found that the Bias Rite I have been using is within 1ma as compared to my DVM readings.
Good to know.

One thing about playing with the Bias Rite installed.
Weber said at one point that overheating might be a problem but other than that it was no problem.
Of course I know measuring with no input is the way to measure but when listening for differences in cool or hot sound wise it is safe to keep the Bias Rite hooked up and tweek your bias knob and listen.
Otherwise there is a long wait time to let the tubes and Bias Rite cool down.
But, keeping it hooked up and played through for hours might not be a good idea.


PS: I'm not disputing the much more learned John P just stating what Weber said about keeping it hooked while tweaking the bias and listening.
Don't keep it hooked up all the time as John said :AOK
 




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