Checking bias with 1 Ohm resistor question

[Platefire]

Just worndering before I try it to be safe. Would it be safe to use insulated alligator clips and connect one end to cathode and other end to 1 ohm resistor hanging in space and check the bias in mV with a MM that way(touching the hot MM lead to the end of the dangeling 1 ohm) without installing the resistor itself in a vintage amp? or is that a bad idea? Or does the resistor have to placed in series with the cathode resistor or ground? Platefire 

[sluckey]

Or does the resistor have to placed in series with the cathode resistor or ground?

yes

[jjasilli]

More verbosely:
This is Ohm's Law.  V_(drop) = R x I.  You are measuring the voltage drop across the resistor.  So the current of the circuit must flow through the resistor to produce the voltage drop.  The resistor cannot "hang-off" and be in an open circuit, or part of some other circuit.  It must be in series with the circuit under test. By the clever use of a 1 Ohm series resistor, the value of the voltage drop always equals the value of the current flow within the circuit being measured.

This is also Ohm's Law: 
You don't need to install a bias sense resistor in the vintage amp.  You can just as well read the voltage drop across the existing cathode resistor; then divide that by the value of the cathode resistor, to compute the value of the current.  First turn off the amp, etc., and pull the power tube(s).  Measure the actual value of the cathode resistor.  I.e., don't rely on the color bands or the value stamped on the resistor which may no longer be accurate.

If the resistor is feeding 2 tubes your voltage drop measurement will be double.  I.e., each tube will be drawing 1/2 the value you are reading (which is not strictly true unless the tubes are matched).  If you need to know the current draw of each individual tube, then 2 resistors, one for ea tube, must be used.  Hence the use of bias test sockets, so as not to disturb the existing circuitry. 

[sluckey]

First turn off the amp, etc., and pull the power tube(s).  Measure the actual value of the cathode resistor.

You don't need to pull the tubes to measure the cathode resistor. Same goes for plate, screen, or grid resistors.

[Platefire]

Thanks, seemed to easy to be workable

So if you got two 6L6's with a 150 Ohm/7 watt cathode resistor to ground.

If you were to install the 1 Ohm (I'm not on this amp) it would be installed as follows:

Cathode/1 Ohm resistor/150 Ohm 7 watt to ground?

You would measure with the MM between the 1 Ohm resistor and the 150 Ohm cathode resistor for mV?

Please clairify this for me because I keep getting confused about it and I need something I can make a copy of and use for a reference when I forget. Thanks, Platefire

[jim]

jjasilli is saying don't bother with the 1 ohm r.  Lets say there is a 35 volt drop across your 150 Ohm r.  Then by ohms law 35 divided by
150 = .23 or 23ma.  Jim

[Platefire]

Thanks Jim, and I understand what your saying, don't install a 1 Ohm in a vintage amp and use the voltage drop method to calculate bias. Yep, I got it and will use that method on this amp.

I have installed 1 Ohm resistors in my builds with the external check points and understand how to do that. I'm just trying to understand hows to scab one into and existing amp properly--even though I'm not going to install them in this old fender I'm refering to. So my question is---if I were going to install them, where would it be installed and where would you measuer at? I'm kind of fuzzy on my understanding of that and want to clear it up. Platefire

[sluckey]

Measure ACROSS each 1Ω resistor.

[Platefire]

Thanks sluckey!!!

One more question? if the cathode resitor is a 7 watt resistor that fender originally put in there, wouldn't you be defeating the purpose of the high wattage resister scabbing in a one or two watt resistor? 

[sluckey]

no. You could even use a 1/8 watt resistor.

wouldn't you be defeating the purpose of the high wattage resister

What is the purpose of using a high wattage resistor?

[Platefire]

Ok thanks. I did the cathode voltage drop measurements but the results don't sound right to me?? Maybe I did something wrong or the amp is out of whack. I do know that the bias voltage is reading about -37 and should be around -47 if I'm reading the schematic right. This Pro Reverb does not have a bias circuit like an AA165 like the tag on the inside of cab but has a bias circuit like an AB668(attached). So the bias measurement are as follows:

tube #1-Cathode voltage=8.27. Measurement at ground end of resistor 0.00. Cathode measured an even 150. 8.27 divided by 150=.055

tube #2-Cathode voltage 8.17. Measurement at ground end of resistor 0.00. Cathode measured 145.
8.17 divided by 145=.056

I know 6L6's need to be somewhere around 35 mA. Can't relate that to what I am getting? I hope thats not 55mA! Plate

[sluckey]

I know 6L6's need to be somewhere around 35 mA. Can't relate that to what I am getting?

Where'd you learn that? 35mA is kinda meaningless without knowing the plate voltage too.

55mA per tube is a very believable current for that amp. And your calculations are correct. One tube is drawing 55mA snd the other is drawing 56mA. Very close match.

[Platefire]

Both tubes are right at 454 Plate voltage. I could of swore that Doug said 35mA was a good safe setting--not too hot or cold. 25 for 6V6's and 35 for 6L6's. So with those plate voltages am I in good shape? Plate 

[sluckey]

What's the max plate dissipation?
What's 70% max plate dissipation?(or whatever %max you want to run)
What's (454-8)*.055?

Look it up. Do the math. Then YOU tell us.

[HotBluePlates]

Had a reply all ready, but will let you answer Sluckey's questions so the process will stick with you.

[Platefire]

454-8=446x.055=24.53

[phsyconoodler]

Ok.Before we go any further here,we need to know what YEAR the amp is. You say it's a Pro Reverb?
 If it is a pro reverb and it's a Silverface,then it has the bias adjust AND the cathode resistors on the cathodes. Very confusing system.
  So you have a fixed bias supply giving a negative voltage to pin 5 and the cathode resistors as well which changes everything.
  So with that combination you are getting 55ma,right? If it was just fixed bias alone,that would be too hot,but it's a combination of the two so the cathode current reads much higher than you would set it with just fixed bias alone.
  You are getting 24watts per tube,right? So 70% would be 21 watts if it was just fixed bias with a 30 watt tube.
  So because you have the combination fixed bias/cathode bias you may be ok to run them at that current level.I frankly am confused by it,other than when you do the math it's not bad.
  Now Plate,you are right when you say 35ma is a good place to bias a 6L6 FIXED BIAS Fender with about 450v on the plates,but with this amp that does not necessarily apply.
 This is why lots of people have converted the amps over to a fixed bias only system so it's not so confusing for the layman to check and maintain.
  That and blackfacing the Silverface amps. The Blackface versions were simpler and easier to maintain and figure out.
  Sluckey wants us to think for ourselves,so he rarely gives the exact answer or rather,he relies on using ohms law and common sense.He is one smart dude,let me tell you.He THINKS things through.
   Lots of us here on the forum are looking for paint-by-number answers.I am woefully guilty of that!
 

[HotBluePlates]

454-8=446x.055=24.53

Are you using 6L6GC's? If so, what percentage of maximum plate dissipation are you running your tubes at?

[HotBluePlates]

...
  So with that combination you are getting 55ma,right? If it was just fixed bias alone,that would be too hot,but it's a combination of the two so the cathode current reads much higher than you would set it with just fixed bias alone.
 ...
  So because you have the combination fixed bias/cathode bias ...

The method of providing bias is somewhat tricky, but tube current is still tube current.

So, Platefire, what does your calculated tube current about the idle dissipation of your tubes?

[phsyconoodler]

Plate wrote:

454-8=446x.055=24.53

His tubes are running at 24.53watts. So is that ok with the combination bias method or not?

[sluckey]

24.53/30 = .818 = 81.8%

You probably like to set your fixed bias amps at 70%. And you probably set your cathode biased amps 80-100%. So, 81.8% seems reasonable to me for a mixed bias amp. I'm sure we would have heard by now if the tubes were glowing red.

Just for comparison, my 5E3 is idling at 82%.

[Platefire]

Well I think the 25.53 is plate dissipation in watts. I'm still looking around trying to determine the percentage that represents. I know 70% is hot for a 6L6. 60% would be more what I would think not too close to the edge.

HotBluePlates--I'm not understanding you last question? Did you mean to say "What does calcultated tube current tell you about the dissipation of your tubes"?

Phsyco--your right--it has cathode resistors and fixed bias--strange! The SN#A 11899 I think was made around 1968/1969. The tube tag inside the amp says it's a AA165 but I think it's a combination of several different models. The bias is ab668. Yelp, sluckey is very much appreciated on my part. I need to determine, I think how watts dissipation relates to % of total dissapation. I'll do a little more research. Plate  

[Platefire]

sluckey, where did the 30 in your calculation come from???

[phsyconoodler]

I know 70% is hot for a 6L6. 60% would be more what I would think not too close to the edge.

 You are thinking FIXED bias here again.You need to expand it to include the combination fixed/cathode bias that you have in the amp.

 You likely have nothing to worry about at all.

[phsyconoodler]

The 30 is how many watts maximum a 6L6GC is rated at. That's where he took the percentage from.
You are at 24.53 watts or 81.8% of that 30 watts.
 

[sluckey]

sluckey, where did the 30 in your calculation come from???

http://tdsl.duncanamps.com/tubesearch.php

[phsyconoodler]

Pay attention class !!!!

Reminds me of when I was a mechanic and friends would ask me to help them fix their cars.I'd end up doing it all cause I couldn't stand watching them fumble around.After the umpteenth time of fixing it for them I got wise and would tell them how to do it and get them cracking open a manual so they could learn to do stuff on their own without me ending up doing it all.
  It worked for a couple of guys who took the ball and ran with it.
Funny how neither group of guys ever said thank you.The ones who didn't want to learn OR the guys who did learn.
  We need to give Sluckey a BIG thank you for his attempts to make us think for ourselves.

[Platefire]

Yep--Your tube's spec data+your tube's loaded measuremtents+ correct applied formulas=Your tube's present operating conditions

Thanks, sluckey--I copied the the tube data search to favorites

BTW-One thing that had me concerned on the bias with this amp was the negative voltage on the schematic shows it at -47 and my measurements shows it at about -37. I checked the AC from PT to bias circuit that was suppose to be 54VAC and was around 49VAC. So I was thinking maybe the fixed 2.7K dropping resistor needs to be adjusted to get the negative bias voltage in spec  I guess lesson learned is do the dissipation math for your tube and if it's in good range--all is fine.

[HotBluePlates]

HotBluePlates--I'm not understanding you last question? Did you mean to say "What does calcultated tube current tell you about the dissipation of your tubes"?

24.53/30 = .818 = 81.8%

You probably like to set your fixed bias amps at 70%.

I do know that the bias voltage is reading about -37 and should be around -47 if I'm reading the schematic right.

Sluckey showed this was 81% dissipation for a 30w 6L6GC. People often cite 70% for class AB (though designers don't necessarily adhere to that).

You said upfront you have -37v of fixed bias instead of the -47v the schematic says, and that's why you're idling a bit hot. Has nothing to do with fixed/cathode bias, but that you have less negative voltage than the schematic says you should have.

So I was thinking maybe the fixed 2.7K dropping resistor needs to be adjusted to get the negative bias voltage in spec 

Either reduce the value of the 2.7k resistor, or raise the value of the 15k (18k?) resistor from the bias balance pot to ground.

[sluckey]

Either reduce the value of the 2.7k resistor, or raise the value of the 15k (18k?) resistor from the bias balance pot to ground.

And change the bias cap too if not already done. Many times a new cap will have a greater voltage output than a dried out cap.

BTW, my vague replies were not meant to be mean spirited. I was just trying to get you to pick up a pole and try to catch one for yourself. It's fun!

[Platefire]

Believe me sluckey I didn't take it as mean spirited. One thing I realize about myself is I've come back to this same subject several times and you've helped me patiently each time but by now I should be getting a handle on this. I really do want to get to a point where I don't need to ask anybody about this and can just do it! Since I don't do this very often it becomes unfamiliar with me. What I need to do on my part is not be impatent and just throw a post up looking for quick answers but research my past history on this and refresh myself and save the questions for the hard answers that I might need after I've done all I can do.

On adjusting this bias circuit, I'll inform the customer about it to see if he wants me to spend anymore time on it working that out. It's either adjust it or keep a close eye on those plates. Thanks, Platefire

[kagliostro]

Please can someone post a link to the schematic of this amp that has both fixed and cathode bias working at the same time ?

I'm not able to find such version of the Pro Reverb (?)

Thanks

K

[sluckey]

K, scroll up to reply #10.

[kagliostro]

Thank Steve

I completely missed that link 

Franco

[sluckey]

Someone was working on one of these a few months back. Posted lots of pics. Those cathode resistors were clearly visible on the tube sockets. Bet you can find the thread if you search.

[Platefire]

Here is a picture I had that shows the cathode resistors and part of the bias circuit on the bottom.

[phsyconoodler]

I just worked on a super reverb that has the same circuit in it.I was very tempted to rip it out and make it adjustable fixed bias but I decided to listen to the amp first. Well the amp sounds awesome!
  I left it alone.
Same with my 75 Vibrolux.I was so tempted to blackface it and had the resistors all ready on the bench.At the last minute I decided to play it to get a before and after picture in my head.
   I left it alone too! It sounds delicious. My buddy's Vibrolux is blackfaced and it is killer too,just a little different. Sometimes if it ain't broke,don't fix it!
 

[PRR]

> where did the 30 in your calculation come from???

6L6GC rated max dissipation is 30 Watts.

A 6L6GC should be OK _at_ 30W for many-many hours.

There's two ways we use this.

1) Cathode-Bias. Here we usually aim for MAX safe idle dissipation because we won't get any more current on loud signals. If we use a Cathode Resistor for bias, the dissipation is fairly stable and self-correcting. It should be safe to idle at 99% of 30 Watts.

2) Fixed Bias. Here we CAN get more current on signal peaks *and* we do NOT have the self-correction of cathode-bias. If the tube goes a little gassy, current may rise uncontrollably. Since we don't need MAX idle current, and it is unsafe to get too close to the limit, we may-and-must turn-back from 100% rated dissipation. Idle diss of 20% to 80% is reasonable. The low end for soft-played battery radios, the high end for wall-plug gear where sound quality matters. If you don't have an opinion, 70% is often fine.