Thread: when checking bias with a meter in series...

1. when checking bias with a meter in series...

....with the plate, is the current you read the correxct current or do you have to account for the screens. And if so, how? Add 5 ma, subtract? In the weber cathode bias current calculator it says "In these calculations, 5% of the cathode current is assumed to be screen current". I'm not sure whether they are saying that it's already included in the results or not. Anyways, when i checked the current with my meter in series with the plate, i got 5 less ma than the calculator. I calculated my amp's current with the sag resistor switched in and got 54 ma, but my meter showed 48.8. I then checked it with the sag resistor switched out and got 63 ma with weber's calc, got 56.8 with my meter. Both readings about 5 ma less than weber showed. So which is the correct number...what weber's calc showed or what my meter did? heres all the exact numbers...

WITH SAG RESISTOR SWITCHED IN
------------------------------
plate current: 48.8 ma
plate voltage: 346 VDC
cathode voltage: 25.5 VDC

weber calc plate current:54 ma

WITH SAG RESISTOR SWITCHED OUT
------------------------------
plate current: 56.8 ma
plate voltage: 399 VDC
cathode voltage: 29.7 VDC

weber calc plate current: 63 ma

From weber's tables i deduced that without the sag resistor switch in, the current, at least as weber shows it is right at 100%. My meter shows what would probably be around 85-90% w/o checking it exactly. I've been told it's fine to run cathode biased amps at 100% at idle, then again some people say it's nuts. I have no idea what to believ, tho my best guess is it's ok since i've used it like this for a long time with no ill effects and the tubes still sound new.

2. First question. Did you buy the bias meter, or build one? The makers of one of the bias meters have some base numbers. I ran an old fixed bias Twen at idle, and then full on with the 1000 Hz sine wave at around 500mv which is kind of hot, but pedals can put 3X that much voltage in. When I did all my calculations I got something like 18 watts and with signal 27. I have found that the plate current as measured with the meter in series is very close to what actually comes all the wat to K. I can try putting a bias pro/king/whatever on one of my tube testers, and compare its reading to that of the tester. The actual readings from the Simpson watt meter, the bias pro, and the plate voltage were really pretty much right on the money with the 1000Hz signal. How accurate the Simpson wattmeter was is a mystery, as I have never calibrated it, but the 260 it runs into is right on.

3. It's just a basic a voltage meter and i soldered the leads from plate pin and OT lead and set to read ma.

4. From your numbers, and i'm assuming your using 470R cathode resistors, it would appear that the weber calculation does include the screen current, and that this current is 5.2 and 6.2mA respectively.

5. daz,
if you put your current meter in series with the plates, you're measuring the plate current alone, and that's why you're about 5 mAmps off from the Weber calculator, which takes the screen current into account instead. Should you put your current meter in series with the cathode, you'd read the TOTAL current flowing through that tube ( which is the sum of plate and screen current ), and that's exactly the current that tube is drawing from the PS ( and your readings would match ).

Cheers

Bob

6. What's a Weber calculator? Is that what you are calling his Bias Rite? Or is it something else? Oh, I found it.

Assuming use of the Bias Rite, it even calls itself a cathode current monitor. COnsider what you are measuring. When you break into the plate lead, all you CAN measure is plate current. When you break into the cathode lead, you measure the current through the cathode. The cathode has no idea where the current goes or comes from, all it knows is what is flowing right there. The current flow from the cathode usually goes to the plate mostly and some to the screen grid.

The bias rite simply measures the cathode current, nothing more nor less. When they say that they assume 5% of the cathode current is screen current, then assume that whatever reading you get, 5% of it was from the screen.

But the calculator tells you what it does. It asks for the plate voltage, then it asks for CATHODE current. Try your example. I entered 369v in the plate volts box, and 63ma in the cathode current box, then I clicked "Calculate power." It filled in the power box with 22 watts, but note it also automatically reduced your 63ma to 60. SO If you want to know what your plate current is from a cathode current reading, you can subtract about 5% from it. If you use his calculator, just enter the numbers it asks for and the result comes out.

His 5% estimate of 3ma is not exactly your 5ma reading, but it is close enough for amp work. If you are concerned over a couple percentage points, it is easy enough to take the readings and use a pocket calculator. That is what I did below.

So whatever you measure at the plate will also be flowing through the cathode. But the screen current also flows through the cathode. SO the plate reading will be plate current. The cathode reading will be plate current and screen current added together.

I can;t get the quote thing to work now, grr...

You said:

WITH SAG RESISTOR SWITCHED OUT
------------------------------
plate current: 56.8 ma
plate voltage: 399 VDC
cathode voltage: 29.7 VDC

weber calc plate current: 63 ma

I assume you mean the Weber said 63ma of CATHODE current? In any case, you have 399v on the plate and 30v on the cathode, leaving 369v across the tube. 369v x 56.8ma = 20.9 watts. What tubes are you running? 21 watts is about right for a 6L6.

You said:

WITH SAG RESISTOR SWITCHED IN
------------------------------
plate current: 48.8 ma
plate voltage: 346 VDC
cathode voltage: 25.5 VDC

weber calc plate current:54 ma

346v on the plate less 26v on the cathode means 320v across the tube. And with 50ma plate current 320v x 49ma = 16 watts.

8. daz,
if you put your current meter in series with the plates, you're measuring the plate current alone, and that's why you're about 5 mAmps off from the Weber calculator, which takes the screen current into account instead. Should you put your current meter in series with the cathode, you'd read the TOTAL current flowing through that tube ( which is the sum of plate and screen current ), and that's exactly the current that tube is drawing from the PS ( and your readings would match
Thanks, thats what i needed to know. But a bit confusing because i was told once when i had 1 ohm resistors on the cathodes that if i measure across them to get plate current i need to add or subtract (can't recall which) screen current.

I assume you mean the Weber said 63ma of CATHODE current? In any case, you have 399v on the plate and 30v on the cathode, leaving 369v across the tube. 369v x 56.8ma = 20.9 watts. What tubes are you running? 21 watts is about right for a 6L6.
63ma is what the weber calculator showed for "Approximate plate current in each tube" when i entered the voltage drop across the cathode, the resistor value, (450R) and the plate to cathode voltage. I'm using el34's, and it showed what is 100% current according to thier scales and 25 watts. What am i missing?

SatchmoeddieII....thanks for the offer, but i'm fine with what i have. But as i mentioned to robert, when you measure across a 1 ohm resistor on the cathode, don't the results have to be compensated for by adding screen current? I was told that before.

9. I put the meter in series with the cathode. It showed about 10 MA more, tho i'm using a different meter. What worries me is this. I read before that with cathode bias the amp's max current is at adle. But i strummed a guitar thru it and the current jumped as high as 124ma. WTF? At this point i have no idea wht to think because if thats the case even if i biased it at 70% at idle it would jump well past 100%. What am i missing here?

10. Not when you are on the cathode pin. I think someone else explained that. If you like send an address, and I will be more than happy to send instructions for how to make a set of these. I had to make a second set with narow tube bases, and tube sockets so it fits the Fender stuff with those base clips. I also was kind of in a pinch with making test fixtures for Hickock 539, and KS15750 tube testers for calibration. An 11153 ohm resistor that is + or - 10 ohms is a pain to make, but it did the trick. I also needed 6K ohms + - 10 ohms, and other values at either 1% or 10%, and a dummy rectifier. The regulator section was the only real problem on those testers. My Triplet 3444 only takes two fingers, and a thumb to calibrate. I must have gotten something right, as they regularly test within 2% of eachother now. The bias pro,king dual bias etc all use a 1 ohm resistor and the 100mV DC scale of a digital volt meter, and the 1 ohm is inline with the cathode pin #8 on the EL34, 6L6, 6V6, 5881, KT66, KT77, KT88, but the EL 84 uses 9 pin miniatures. For plate voltage a lead comes off pins 3 and 8 for the common octal tubes including 6550. For 7591 pin 3 & 5 is used for voltage reading, and 5 forcurrent, but the current reads on a mV scale, and is straight across converted to mA. EL-84 mA is on pin 3, and voltage between p & K is pins 6 & 3. 6973 is a 1 ohm across socket & pin for pin 7, and voltage is between pin9 & 7. Those are about all I use myself. I can no longer find 8714s, and convert those to 6550s. These are easy to make, and work quite well. I cut up socket savers for the El-84, and 6973 fixtures. I am a bit of a Valco freak, so Supro, Valco, and Gretsch amps that use the 6973 kind of attract my interest. I saw a 6V6 McKinney go for well over 800. I was bowing out of bidding, as by then I can aford the Mec. Mag. X-fmrs. or with the 6V6 I already have some nice sets from this and that kicking around. Which Weber book are you using for his conversions, and charts? I have the dark green paperback. Measuring the cathode amperage should get the stuff from the screens too. If you want some simple plans for the bias probes feel free. You have the email, I am already inundated with SPAM, and I lost internet service for a while, so I am way behind, and rather unhappy. My cable company has been screwing us for 23 years. We have put up with crappy pictures, and really poor service when digital came in, but they are the only game in town. 256K is a little too slow for me.

11. Enzo....you were using the wrong calculator...well, either that or I was. But the one i used is under the one you used and i used it because my amp is cathode biased and the one i used says thats what it's for. I may be missing something, but it sure wouldn't be the first time.

12. Originally Posted by daz
...What worries me is this. I read before that with cathode bias the amp's max current is at adle. But i strummed a guitar thru it and the current jumped as high as 124ma. WTF? At this point i have no idea wht to think because if thats the case even if i biased it at 70% at idle it would jump well past 100%. What am i missing here?
Who said that with cathode bias the max current is "at idle" ?

The current does not change significantly between idle and playing conditions only in a class A amp, because the tube(s) are always on, but in any AB1 class amp ( most of the amps out there in the real world ) the current does indeed swing from idle, or quiescent, to a maximum. The idle current, imposed by biasing the tube(s) is there to keep the cross-over distortion away, and it's normal for the current to swing from a minimum to a maximum in any AB1 amp.

Tubes can go over their max PD provided that the AVERAGE power the plate(s) dissipate is within their limits ( they can exceed 100% during the "on" condition because they're biased under their 100% at idle )

Hope I've managed to be clear enough

Cheers

Bob

13. The current does not change significantly between idle and playing conditions only in a class A amp, because the tube(s) are always on, but in any AB1 class amp ( most of the amps out there in the real world ) the current does indeed swing from idle, or quiescent, to a maximum. The idle current, imposed by biasing the tube(s) is there to keep the cross-over distortion away, and it's normal for the current to swing from a minimum to a maximum in any AB1 amp
So it's ok that i was readung as high as 126ma when playing ? (that was with master way up)

14. 63ma is what the weber calculator showed for "Approximate plate current in each tube" when i entered the voltage drop across the cathode, the resistor value, (450R) and the plate to cathode voltage. I'm using el34's, and it showed what is 100% current according to thier scales and 25 watts. What am i missing?
Oh. I'm not doing very well here, am I?

I didn;t realize you had EL34s.

Your cathode resistor measures 450 ohms then? Or is it a 450 ohm resistor that we have not yet measured? Always check the value, because it affects your calculations. A 10% 450 ohm resistor can measure between 405 ohms and 495 ohms and be within spec.

You can measure cathode current by adding in a 1 0hm resistor and reading across it, like the bias probe adaptors do. Or you can measure voltage across the cathode resistor and calculate from Ohm's Law. It doesn;t matter, both ways work.

You are right, I was not looking far enough down the page, but the results should be similar. Looking at his calculator, enter 29.7v across 450 ohms into Ohm's Law and you get 66ma. His calculator shows 63ma for the plate. Clearly then his result has subtracted the 5% already.

When we "bias" an amp, we are setting idle current. As soon as you start playing, the amp is no longer idling. Current readings you get during play have nothing to do with setting the bias. The tube dissipation from idle is not related to power output.

15. Thanks Enzo. Then let me ask you this...63ma is from my calculations 100%. Is there no amp aside from a true class A that should be at 100% at idle? I always seem to see people saying that with cathode bias you can set bias to 100% at idle. But if thats not the case, what should i do...bias it the same as fixed, 70%? By the way, i have seperate cathode resistors per side as you apparently realized, and they are precision so they are a true 450R each.

16. When you played your guitar through your amp, you saw the cathode current rise dramatically. I bet you didn;t also check to see what the B+ was doing or the cathode voltage. Your B+ will drop as the current draw increases. This is due to transformer winding resistance, any power supply series resistance, and especially if you have your sag resistor switched in.

Increased current through the cathode resistor of course increases the voltage across it - Ohm's Law. So as the current rises, the cathode voltage rises. This tends to push the tube bias colder.

23w out of 25w is 92%. Pretty hot to me. But if your tube isn;t red plating, you might get away with that if it sounds OK.

So you can generally set the bias somewhat hotter on cathode biased amps. But 100% dissipation would generally mean you were running in class A.

17. Ok, well i guess i'll toss the 450R's and put a couple 500R's i have in. I imagine that should bring it to a pretty reliable state since i never had issue where it is now. What % would you suggest as optimal for tone w/o torturing the tubes?

18. Originally Posted by Enzo
....So you can generally set the bias somewhat hotter on cathode biased amps. But 100% dissipation would generally mean you were running in class A.
Enzo,
I agree with everything you said in your last post, but allow me to ( respectfully ) disagree about the above paragraph.

The class of operation is not to be stated in idle conditions, but rather in working conditions, e.g. with signal flowing through the tubes IMHO.

The above is easily demonstrated using the AC30 as an example. The output tubes are ( cathode ) biased at about 10VDC, the +B is about 320VDC, so 310VDC are dropping through the output tubes in idle conditions. The idle current, if memory serves me well, is about 47 mAmps. This means the tubes are biased idle at ( 310*0,047 ) = 14,57 W of plate dissipated power, which is 121 % of an EL84/6BQ5 max plate dissipation. If we look at the sole "idle current" criteria, this would classify the AC30 as a "class A" amp, and this is not the case, as we know.

The rationale for this:
If we'd look at the waveforms on the plates with a scope, we'd clearly see that there is an evident current swing between idle and working conditions, as the current swings from 47 to some 92 mAmps, and it would also be clear that the tubes are on for more than 180 degrees of the signal's period but much less than 360 degrees ( some 250 degrees on and some 110 degrees off, and this places the AC30 with no doubt among class AB1 amps ).

Take the above numbers as approximated figures ( I'm getting old and I don't remember if they're the "dead-on-real-life" values, my two neurons, the only ones left in an otherwise empty space, are on strike right now, asking for an early retirement ), though they should be accurate enough for our purposes.

In a true "class A" amp, the tubes would be conducting during the entire signal's period, the waveforms wouldn't show any cutoff, and the plate current swing would be very little (ideally zero swing-no change ).

All the above statements proof, ( I hope convincingly ) that biasing tubes at, or above 100 % is not to be considered a synonymous of being in "class A".

Cheers

Bob

19. The bottom line is that if you can get the power stage's B+ current draw to increase by applying a signal, compared to what it is at idle, then the amp is Class-AB. Class-A draws the same current at full audio output as it does at idle.

This is for unclipped output, once the power amp starts to clip all bets are off. Heck, they're all Class-C if you drive them hard enough.

Nerd disclaimer: even-order nonlinearities in the tubes can increase the supply current even in a Class-A setup. RDH4 calls this effect "rectification".

20. You are right, and i am aware of that, but I didn;t mean it that way. If you idle at any level, you are not yet operating in ANY class, since there is no signal to conduct.

What I was trying to say, and it came out wrong, was that when you see a cathode biased amp that is biased to 100% dissipation, it was PROBABLY an amp designed to run in class A. SO the bias is not the defining issue, the bias level is a clue to the design intention. Of course your good example is contrary to my point anyway, so I probably ought not to even make the observation.

I really should have made myself more clear, as I often hear people erroneously assuming that cathode bias itself makes an amp class A.

21. Originally Posted by Enzo
What I was trying to say, and it came out wrong, was that when you see a cathode biased amp that is biased to 100% dissipation, it was PROBABLY an amp designed to run in class A.
Well, that seems reasonable. After all, if the tubes were idling at 100%, and signal could increase the dissipation, it would go above 100%, which is bad.

But when we see an AC30 idling at 125%, lord knows what to think. I say Class-OMG.

22. Originally Posted by Steve Conner
....But when we see an AC30 idling at 125%, lord knows what to think. I say Class-OMG.

OMG!

Steve, what I said about Enzo as a teacher days ago applies to you as well!

It's waaay easier for people who want to learn something new if hard-to-grasp concepts are presented by a knowledgeable person in a hilarious way. ( not to mention the sheer amusement found in reading a good joke )....

You know what? I'll prepare a sticker saying "True O.M.G. class amp" and I'll stick it on my '64 AC30! I swear!

My two neurons want to thank you as they're still laughing!

Cheers

Bob

23. Ok, now that you freaking genius techs have had your little party, can i ask you something? (seriously, you guys amaze me with your knowledge...just wish i understood 1/2 of it !)
Anyways, i'm going to throw either 500R or 560R's in it today if i don't have an answer by the time i leave work. So i just want to know whether it's not a good idea to leave it at 100% and if not, what would the highest percent you think i should set it? I can get some more resistors locally with a bit of a drive, but i have a set of 500's and 560's. As i said right now it's at 100% with the sag resistor out. It's biased pretty cold when i switch it in, but i'd prefer to have it biased right w/o the sag R, as thats the feel i'd prefer if i could only have one. I like the way it sounds right now, and i haven't had any issues even with the couple rare instances the old band got together and played. But with what you guys are saying i'd rather have peace of mind if the way i'm running it really is bad.

By the way, is it just me or is the forum extremely slow? It took it a full two minutes from when i clicked reply to when the typing field actually came up.

EDIT: this is truly a letdown. I replaced the 450R per side with 560R per side, and now it's running at 85% in no sag mode. But it really lost it's edge. It just doesn't sound nearly as good. Very disappointing. But i fear going back to the 450's because thats 100%. Like i said, i never had a problem like that. But after this thread i can't help but worry that it may be the reason the first amp like this i built (now owned by a friend) blew it's output tranny. It was running 100% too.

24. Hi Again Daz,

I guess you moved directly to 560s to be on the safe side, but why don' t you give the 500s a try? When trying to find a "sweet spot" changes have to be applied gradually....

Anyway, I've got some questions for you :

1 - can you describe what changed with the sound when you switched to 560s?
( less/more lively? Less warmth? Unpleasant sound/unwanted distortion? Other changes? )

2 - Are you using matched output tubes ?

3 - Is the OT properly sized? ( do you have the OT specs ? )

Cheers

Bob

P.S. Yep, the forum looked slow in the past days, things seem to be OK now......

25. 1 - can you describe what changed with the sound when you switched to 560s?
( less/more lively? Less warmth? Unpleasant sound/unwanted distortion? Other changes? )
Thinner and a bit less harmonically rich...not as round and lively sounding. i guess i shouldnt have made it sound as bad as i did in the last post. It's not far worse. But it did lose that fine line between good sounding and exceptional.

2 - Are you using matched output tubes ?
yes. winged C's
3 - Is the OT properly sized? ( do you have the OT specs ? )
Yes. Its a 3.2k 50 watt marshall replacement type from hammond.

As for the 500R's, i DID try them before. I had the 450R's in there for many months. Then i decided i'd try running cooler after my friends amp i mentioned blew his OT. So i stuck 560R's in his and mine. Then i felt it didn't sound as good and stuch the 500's in mine. I wasn't sure at that point how far i was from the way it sounded with the 450's. then the other day after a month or so of 500R's in it i went back to the 450's, and when i did i realized how much tone i'd lost. Today when i went to the 560's i lost that tone again. 500's wont make a difference, so i know going to those isn't worth the hotter running. If i'm gonna chance running hot i may as well use the 450's and sound great. The 500's and 560's both sound about the same. but like i said, it's not a huge loss. It's just that this amp is at the point where i've had it sounding amazing, and any loss that removes that edge even tho subtle seems more than subtle, if that makes any sense.

26. Hi daz,
I already had an idea about the changes in sound you now described, but I didn't say because I didn't want to exert any influence on your judgment.

What I'm going to say now could sound like an heresy to someone ( the "matched tubes' lovers" ), so I beg your ( and the real gurus ) pardon.

I strongly suggest you to take those matched tubes out the amp, find a pair of "reasonably close" ( but not matched ) tubes and stick them in.

Why am I saying this?

Because I suspect that, by biasing the tubes colder with the 560 Ohm resistors, you are suffering a loss of even order harmonics - this makes the sound more sterile ( less warmth ).

Push pull designs like yours already have a natural tendence to cancel even order harmonics by design, and the more the two legs of the PP are symmetrical, the more this effect is pronounced. Add this to the presence of a couple of good quality "perfectly" matched tubes, and the even order harmonics are more likely to be canceled.

Biasing the tubes on the hot side helps you recover some of these "nice" harmonics, and this could explain why the tone suffers with a colder bias.

In other words, by biasing the amp hotter you take away some of the the "coldness" imposed by the output stage symmetry. Unfortunately, to find a tone you like, you have found that you have to bias the currently installed tubes at 100 %.

The reason why I'm suggesting you to used "unmatched" ( I'd better say "reasonably close", say within 5-8 mAmps ) tubes is that their "non symmetry" causes some more even harmonics to pass through the output stage and reach the speaker. At that point you' ll probably find that you can bias the tubes colder ( say around 70-80 %, 18-20 W, which is a reasonable value for almost any EL34 AB1 amp out there in the real world ) without losing too much warmth/liveliness.

Hope this helps

Best regards

Bob

27. I've been thinking about that actually because at one point i have mismatched cathode bypass caps and it sounded really good in that way. i no longer have because i had some motorboating and was told i should use much bigger caps because the 47uf and 22uf i was using were too small and the motorboating i was occasionally experiencing was due to that. So i putt 220uf's in there. But with 220's i didn't think mismatching them would matter because at that size all frequencies are likely allowed to pass. But i've been thinking i should try maybe a 100uf on one side and i may try that today.

that said, i'm of course talking about AC signal. But you are talking bias current. So if i try that, what about this idea....how about if i just put say a 600R on one side and leave the 560R on the other? If that would work as well i'd rather do that instead of always having to order tubes that aren't matched and worry about making sure just HOW mismatched they are or arent. Come to think of it, i suppose it's easy enough to try by simply adding 50 or 200R to one side. I'll try it today and let u know. Thanks.

28. I added a 68k to one side and it basically sounded like it does when i intentionally imbalance the PI by making the plates more asymmetrical. That would be that it sounds darker/muddier. It takes some highs away which is good, but it doesn't liven things up in any respect including harmonics. So thats a no go. Guess i'll just leave it as is. It's good as is, but i hate knowing i've had it sounding better but can't leave it like that.

EDIT: I tried what i had sdaid i didn't think would work, IE: AC imbalance via different sized caps. As i said it worked before but i didn't think it would work with caps as big as i had to go to rid it of that motorboating. But it did. i added a 220uf to one side making it 440uf. so 440 on one side and 220 on the other. Didn't so much thicken things up, but it took the hard edge off and added the harmonic complexity i was missing to some degree. Sound pretty darn good like this. Think i'll try a 100uf in place of the 440 tho and see how that works. In any case, things are much better like this.

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