Need a pot size to adjust bias current

[pbman1953]

HI All,


Recently I was able to get a sextet of Philips 7581A tubes for my Fender Studio bass. They are the same specs of a 6L6 but with 5 more watts a tube. I installed them today, so with my Bias Rite, I made a quick measurement. Voltage is at 490 and the current is 59-60. I need to bring that down, they are running too hot.


What sized pot and resistor do you suggest? I'll need to install that before the balance (Output Tube Matching) pot, correct? or after?


Thanks

[stratomaster]

That's a fairly wasteful bias supply. It's essentially dropping 61v across that 2.7k 2W resistor and making lots of heat in the process. 

The thing to remember about bias supplies is the further from 0 you are on the negative side the colder the bias.  So we have a few options to get you there.  You can decrease the value of the 100ohm or 220ohm resistors.  This would put you at a more negative voltage going into the bias balance pot.

If you want to make it adjustable I'd replace the 2.7k resistor with something like a 15k 1/2W resistor and add a 10k pot wired as a variable resistor in series with 220ohm resistor.  That would make a voltage divider with the balance pot sitting between the upper and lower legs of the divider and limit the current through that bias supply to something more reasonable.

[pbman1953]

Thanks. I have a 10k pot. Would you mind making a diagram.? I'd understand that better


Thanks!

[stratomaster]

Something like this.

[pbman1953]

Thanks for that!


After far as adjustment, the more the resistence then the current will decrease , correct?

[shooter]

in fixed bias, you're adjusting VOLTAGE not current

[mresistor]

Bias supplies are supplying a voltage to the grids of the power tubes. This voltage is a "potential" as there is basically no current flowing in the circuit. It's there just to keep the fixed bias output tubes in a "quiescent" conductive state.

[pbman1953]

I had very good success.


I'm at an average of 19.... watts per side




Tubes 1-6
19.2 
21
19.5
19.2
19.4
20.4




I'm going for a middle range (60%- 70%) balance between 18-21 watts

[pbman1953]

Even though the bias came out well, I don't seem to have the same volume as the JJ6L6's that were in there. All the correct pre-amp tube type's are in the amp, too.


Any thoughts?

[stratomaster]

Your JJs were idling very high.  You've brought that down significantly along with changing to a vintage and potentially tried set of tubes with higher plate dissipation specs.  I'm not surprised you're hearing an overall volume reduction. 19W avg on a 35W tube is 55%, and it's likely much lower if you used a cathode current sense probe as that includes screens too.  Also, the plate voltage will sag much more with the higher draw from the more powerful tubes, especially as you dial up the bias.

As a quick test you can rebias your current set of tubes at 85% (or whatever the JJs were idling at) and see if the output returns to where it was with the JJs.  Don't leave it there, but at least you'll know why the amp behavior changed.  You may be happier with the tone closer to a 25W/tube idle dissipation as measured by your probe.

[pbman1953]

I had my specs from the JJ's. Around the just under 500v for all and a cathode current of 39. Actually it was 490 v  x .030=  19.2 watts .


But all along I was wrong , I saw a spec sheet and it is 35 watts , so my range should be 21- 24.5 watts. That's enough that could be the difference. I'll have to pull it out of the case


That should solve the situation


Thanks!!

[pbman1953]

Your JJs were idling very high.  You've brought that down significantly along with changing to a vintage and potentially tried set of tubes with higher plate dissipation specs.  I'm not surprised you're hearing an overall volume reduction. 19W avg on a 35W tube is 55%, and it's likely much lower if you used a cathode current sense probe as that includes screens too.  Also, the plate voltage will sag much more with the higher draw from the more powerful tubes, especially as you dial up the bias.

As a quick test you can rebias your current set of tubes at 85% (or whatever the JJs were idling at) and see if the output returns to where it was with the JJs.  Don't leave it there, but at least you'll know why the amp behavior changed.  You may be happier with the tone closer to a 25W/tube idle dissipation as measured by your probe.

The rebias helped  but what would you suggest as landing percentage? I know it's all about sound on what the bias is set at. I have a master and again to make that come out the way I want.

[Willabe]

I know it's all about sound on what the bias is set at.

Setting the bias is 1st and most importantly about not over heating the tubes, limiting their output.

Then after that you can play with the bias setting for tone. But that only goes so far, it's not magic wand. 



 

[pbman1953]

After a day I went back to test again and I need a little more balance from one side to another. The " output Tube Match" pot doesn't seem to do much from turning I'm not seeing one side going down vs. the other

[AlNewman]

After a day I went back to test again and I need a little more balance from one side to another. The " output Tube Match" pot doesn't seem to do much from turning I'm not seeing one side going down vs. the other

The output tube match setting isn't there to bias your tubes, it's meant as a fine tuning listening tool to reduce hum for best results.
If you have a serious differential between one side or the other, it would be better to switch tubes so you're pulling equal current through both sides of your ot if possible.

[Willabe]

After a day I went back to test again and I need a little more balance from one side to another. The " output Tube Match" pot doesn't seem to do much from turning I'm not seeing one side going down vs. the other.

How far apart are the 2 tubes in current?

Do you see any change in current draw when you turn the balance adjust pot? We need numbers. 

Where are they at with the balance pot turned full counter clockwise, then full clockwise?

Sometimes you need to swap the tubes in their sockets to get them closer to each other in current draw.

[pbman1953]

After a day I went back to test again and I need a little more balance from one side to another. The " output Tube Match" pot doesn't seem to do much from turning I'm not seeing one side going down vs. the other

The output tube match setting isn't there to bias your tubes, it's meant as a fine tuning listening tool to reduce hum for best results.
If you have a serious differential between one side or the other, it would be better to switch tubes so you're pulling equal current through both sides of your ot if possible.

Rotating the pot doesn't do much. A milliamp at best

There's no serious difference between the side. Before any swapping the average between the sides was a 1 watt difference. I was trying to make it equal


The tubes read like this after some swapping-


Average voltage is 480

This amp has 6 tubes

22.4
23.5
21.5

22.3
22.6
23.7


Over all now it's a 22.5 vs. 22.8 , that's as close as I'll get it

[AlNewman]

That's close enough for rock n roll.
Is that watts/tube?  After you subtract the screens, your plates are dissipating around 65% maybe?  It's up to you, but I'd probably drop that to 50-60% P.D. by increasing negative voltage at your bias pot.  6x6L6 aren't a cheap replacement.  I'd try to get the most life I could out of them.
Then adjust the output matching pot to the quietest setting.

[Willabe]

Rotating the pot doesn't do much. A milliamp at best

Somethings wrong then.

Your numbers look pretty good but if you want to be able to use the balance circuit then start measuring. Somethings off.

Take ALL the power tubes out and check for -dcv at pin 5 of each power tube socket. Use gator clips on your meter probes.

Check for -dcv on each socket pin 5, full counter clock wise, then full clock wise. How many volts (-dc) does it swing? From what to what? 

If no or very little -dcv swing, work your way backwards towards the PT acv that feeds the -bias dcv. Go backwards until you find where it stops/starts being adjustable.

I would think that if the -dcv swings down 3dcv on 1 side, then it should swing up 3dcv on the other side. Maybe -40dcv/-40dcv to -43dcv/-37dcv. 

[pbman1953]

That's close enough for rock n roll.
Is that watts/tube?  After you subtract the screens, your plates are dissipating around 65% maybe?  It's up to you, but I'd probably drop that to 50-60% P.D. by increasing negative voltage at your bias pot.  6x6L6 aren't a cheap replacement.  I'd try to get the most life I could out of them.
Then adjust the output matching pot to the quietest setting.

You may be confusing the Hum pot with the Matching pot. The matching is a balance between the sides connected to the pin 5's

[pbman1953]

Rotating the pot doesn't do much. A milliamp at best

Somethings wrong then.

Your numbers look pretty good but if you want to be able to use the balance circuit then start measuring. Somethings off.

Take ALL the power tubes out and check for -dcv at pin 5 of each power tube socket. Use gator clips on your meter probes.

Check for -dcv on each socket pin 5, full counter clock wise, then full clock wise. How many volts (-dc) does it swing? From what to what? 

If no or very little -dcv swing, work your way backwards towards the PT acv that feeds the -bias dcv. Go backwards until you find where it stops/starts being adjustable.

I would think that if the -dcv swings down 3dcv on 1 side, then it should swing up 3dcv on the other side. Maybe -40dcv/-40dcv to -43dcv/-37dcv.

I went in and there hardly to none difference between either full rotation.


Now I want you to check my connections-






First a note- the schematic shows -61v to the input of balance pot. This is now connected to the wiper of the 10k pot. The real measurement is -78. Is that a concern?


I sent 2 pictures- the 10k pot and the balance pot




10K pot picture- Notice the pinkish wire , bias feed, going to the middle leg of the pot. The yellow wire (output) goes to the input of the balance pot. The pinkish wire was original connection.




Balance pot- the yellow wire comes in from the left from the 10k pot to the input tab of the balance pot. I attached the 15k resistor to the wiper of the balance pot


Are all these connections correct?
 

[stratomaster]

What is the voltage at every lug on your balance pot?

Schematic reading low is normal and likely for 2 reasons.  Increased modern wall voltage vs when the schematic was created and the resistance that circuit now sees is much higher than stock at that point with the 15k in series with the 10k variable resistor.  That is on purpose. The original dropped 60v across 2700 ohms.  That's just under 1.5 watts without a good reason for it. 

With your new config you will drop no more than 0.4 watts across the 15k--but likely way less.  And you should be able to adjust negative bias from -47 to -78 or thereabouts.

If you measure all of the voltages at each lug of the bias balance pot that will tell us a good bit.  Measure at full CCW, centered, then full CW.

You can take the tubes out for this measurement, but I'd rather you leave them in then cool the voltage out of the 10k pot to say -65v so there's a load on the bias supply but you're in no danger of over dissipation.

[Willabe]

I'm sorry guys, I got this wrong. 

Full mod schematic drawing is in reply 5. And it was discussed early in the thread. I crossed out where I was wrong below. I'm sorry.   

I went in and there hardly to none difference between either full rotation.

Where did you go in? And did what? Measured where, measured what?   

Your not giving us detailed information.

You didn't tell us you or somebody modded the amp's balance circuit. Why did you change the balance pot 2K7 R to a 15K R?

I asked for measurements from each of the tube sockets, pin 5, FCC and FC. You reply; "I went in and there hardly to none difference between either full rotation." Tells us pretty much nothing.

Now I want you to check my connections-

We can't. We don't know what you tried to do and what you really did.
A schematic of the mod would be of great help. It can be hand drawn.

Are all these connections correct?

I have no idea from the pictures or your description. And we don't know what you or someone else tried to do.

Need a schematic of the mod you tried to do.

First a note- the schematic shows -61v to the input of balance pot. This is now connected to the wiper of the 10k pot. The real measurement is -78. Is that a concern?

More negative dcv is safer than less negative dcv.

[pbman1953]

I'll send the hand drawing for now. I'll make the measurements later


If you can try zooming in 


Hope the drawing helps



Thanks

[Willabe]

My fault, I'm sorry, I got this wrong. It's in reply 5.

Where's the rest of the circuit? We got that much from your description. Like to see the full picture.

Why did you change the balance pot 2K7 R to a 15K R?

If you can try zooming in.

Zooming in doesn't help. It's not about that. All those wires, and we can't see the full balance circuit, it's too spread out in the chassis.

[stratomaster]

Why did you change the balance pot 2K7 R to a 15K R?

That was on my recommendation.  The original config had a high wattage resistor and I found it to be unnecessarily wasteful.  He had a 10k pot on hand, so a 15k fixed reduced the current/heat through the bias supply and gave him a swing of about 40% of the input voltage. 

This likely upset the ratio of the bias balance pot.  I didn't figure it would upset it beyond the point of usefulness though.

[Willabe]

What is the voltage at every lug on your balance pot?

If you measure all of the voltages at each lug of the bias balance pot that will tell us a good bit.  Measure at full CCW, centered, then full CW.....

You can take the tubes out for this measurement, but I'd rather you leave them in then cool the voltage out of the 10k pot to say -65v so there's a load on the bias supply but you're in no danger of over dissipation.

Very surprised at this reply, it's not safe, yet. Take the measurements at pin 5 of EACH power tube socket. After this is verified, that there is sufficient -dcv at every socket AND that it doesn't drop out as you rotate the balance pot and the bias pot, then you can measure at the balance pot with tubes in.

[Willabe]

That was on my recommendation.  The original config had a high wattage resistor and I found it to be unnecessarily wasteful.  He had a 10k pot on hand, so a 15k fixed reduced the current/heat through the bias supply and gave him a swing of about 40% of the input voltage. 

This likely upset the ratio of the bias balance pot.  I didn't figure it would upset it beyond the point of usefulness though.

Ok, fine.

But after these changes he has to go in and with ALL the power tubes out, take full measurements to see if it works.   

[Willabe]

Guys, my apologies are above, reply 22 and 24. I edited the posts by putting a line through my mistakes.

I'm sorry, I got it wrong. 

[Willabe]

This likely upset the ratio of the bias balance pot.  I didn't figure it would upset it beyond the point of usefulness though.

That's ok, it was worth trying it.

Can he maybe put the 2K7 back on the balance pot and put the 15K R on the 10K bias pot 0 end lug to ground?

That way the balance circuit is untouched, just the -bias dcv feed to it is altered. That might get both pots to work then? 

Fender had a reason why they used that 2K7 to ground on the balance pot. I'd like to know how they figured that value?

[pbman1953]

What is the voltage at every lug on your balance pot?

If you measure all of the voltages at each lug of the bias balance pot that will tell us a good bit.  Measure at full CCW, centered, then full CW.....

You can take the tubes out for this measurement, but I'd rather you leave them in then cool the voltage out of the 10k pot to say -65v so there's a load on the bias supply but you're in no danger of over dissipation.

Very surprised at this reply, it's not safe, yet. Take the measurements at pin 5 of EACH power tube socket. After this is verified, that there is sufficient -dcv at every socket AND that it doesn't drop out as you rotate the balance pot and the bias pot, then you can measure at the balance pot with tubes in.

I will this later today, but since each 3 pc set is tied together at pin 5, why would there be a difference between them?

[stratomaster]

Willabe is rightfully being very cautious with respect to the bias on the amp as losing bias has serious ramifications. He's also instilling best practices with measurements.

I was less concerned that there was a problem with the bias once you provided per-tube dissipation information, and took that as evidence of a functioning bias adjustment arrangement.

[Willabe]

I will this later today, but since each 3 pc set is tied together at pin 5, why would there be a difference between them?

You still check all of them to make sure. You never know. We've seen guys have 1 tube socket on the same side not getting -bias dcv. It's rare, but it happens.

You have NOS Phillips 7581A tubes, they are hard to find and must cost a good chunk of change. Why wouldn't you be extra cautious with them?   

[Willabe]

I was less concerned that there was a problem with the bias once you provided per-tube dissipation information, and took that as evidence of a functioning bias adjustment arrangement.

Yes, true, but he was also saying turning the balance pot wasn't really doing anything. And he was asking us to look at his new wiring.

At that point, who knows what's going on.

And he's got NOS Phillips 7581A tubes, pretty rare, and big $$.

[pbman1953]

Measurements starting from the -61 line-

Connected to the 10k pot wiper-76

Output of the 10k pot-    -57

Input to the balance pot-  -57

Tube sockets- 1- 6 in volts

Full Counter Clockwise  -  Full Clockwise   

1-511  4-512                  1-512  4-510
2-513  5-512                  2-511  5-512
3-512  6-510                  3-510  6-510


Edit; Moved the FCC/FC #'s to see them better, Willabe

[Willabe]

I take it you left a decimal out?

FCC 1, -511, should be -51.1?

[Willabe]

Now with ALL the power tubes still out, please measure and post the -bias adjustment 10K pot's -dcv swing, FCC to FC.

[pbman1953]

Sorry guys I was measuring the wrong pin-

All those were pin 6
Let's try this again-

Full Counter Clockwise  -  Full Clockwise   -  all in negative
1-58.3  4-57.6               1-57.6  4-58.2
2-58.4  5-57.8               2-57.6  5-58.2
3-58.3  6-57.6               3-57.6  6-58.2

10k pot swing


-48.4  to -76.1

[AlNewman]

If I had a question, it would be 2 parts.  What is the voltage drop of your grid leak resistors with the tubes in? And what is the value of the balance potentiometer?

There are several different Fender designs using similar circuits.  Some use different value grid leaks, with higher value potentiometers.  Yours has equal grid leaks, with what looks like a 400R potentiometer.  (the schematic isn't really clear.)

Also, I'm assuming the left side of the pot on the schematic would be the hum balance setting, while the right side is the output balance setting?

[pbman1953]

If I had a question, it would be 2 parts.  What is the voltage drop of your grid leak resistors with the tubes in? And what is the value of the balance potentiometer?

There are several different Fender designs using similar circuits.  Some use different value grid leaks, with higher value potentiometers.  Yours has equal grid leaks, with what looks like a 400R potentiometer.  (the schematic isn't really clear.)

Also, I'm assuming the left side of the pot on the schematic would be the hum balance setting, while the right side is the output balance setting?

.412 on a 2k setting

"I'm assuming the left side of the pot on the schematic would be the hum balance setting, while the right side is the output balance setting?"

That's a bit confusing because there's a matching pot AND a hum pot. The Hum pot is on the power power side near the power cord and the output matcher is on the out side near the speaker jack.

Edit; tightened up the spacing. Willabe

[Willabe]

What is the voltage drop of your grid leak resistors with the tubes in?

The grid draws no current from the -bias, so there should be no voltage drop across the grid leak R. 

And what is the value of the balance potentiometer?

I'd like to know that too. Just curious.

Also, I'm assuming the left side of the pot on the schematic would be the hum balance setting, while the right side is the output balance setting?

What are you trying to say?

Are you referring to the 2 pots, 1. bias balance, 2. Hum balance pot.

Or that balance pot has 4 lugs, it's a tapped pot. So, looking at the schematic, 1 wire goes out of the pot to the left of the pot, wiper, the right side of the pot is the tap.

[Willabe]

pbman1953, could you please tighten up the spaces between lines/paragraphs?

It would be easier for us to read then. Thank you. 
 

[AlNewman]

Yeah, it's interesting.
I wonder if the 2 pots are in parallel, maybe 800R each out of circuit.
This isn't a suggestion, but say instead of changing the voltage dividers between the 2 sides, you go back to the original circuit but instead of the 2.7k resistor on the ground side, replace it with, say a 1k safety resistor to ground and a 5k pot in series.
Again, this isn't a suggestion, just an idea.

[pbman1953]

I sent the schematic, shown there is the 400 matching pot. The hum pot is also shown.

[Willabe]

Yeah, it's interesting.

I wonder if the 2 pots are in parallel, maybe 800R each out of circuit.
This isn't a suggestion, but say instead of changing the voltage dividers between the 2 sides, you go back to the original circuit but instead of the 2.7k resistor on the ground side, replace it with, say a 1k safety resistor to ground and a 5k pot in series.

Again, this isn't a suggestion, just an idea.

Please go back and read the OP, replies 1,2,3, 25 and 29.

They should help. 

[AlNewman]

What is the voltage drop of your grid leak resistors with the tubes in?

The grid draws no current from the -bias, so there should be no voltage drop across the grid leak R. 

And what is the value of the balance potentiometer?

I'd like to know that too. Just curious.

Also, I'm assuming the left side of the pot on the schematic would be the hum balance setting, while the right side is the output balance setting?

What are you trying to say?

Are you referring to the 2 pots, 1. bias balance, 2. Hum balance pot.

Or that balance pot has 4 lugs, it's a tapped pot. So, looking at the schematic, 1 wire goes out of the pot to the left of the pot, wiper, the right side of the pot is the tap.

If the grid draws no current, than how could either of those pots do anything?

We've discovered the balance pot is 400R in circuit.

Looks like there's 2 pots, although drawn as 1 in the schematic.  I wonder how they interact?

[Willabe]

I wonder if the 2 pots are in parallel, maybe 800R each out of circuit.

He added a 10K pot to adjust the -bias. So now there's 3 pots, -bias balance, Hum balance, and now the added  -bias adjust.

So which 2 pots are you referring to?

[pbman1953]

I wonder since the tubes measure so closely that I should bypass the balance pot and just use the  bias pot

[Willabe]

If the grid draws no current, than how could either of those pots do anything?

Those pots have a R to ground, so it draws current by dumping dcv to ground. But that's before the grid leak and grid stopper R's, they draw no current, so they drop no -dcv.

Read this from our friend sluckey's web site, scroll down to the page 9 on bias supplies and read the part in green;

https://www.sluckeyamps.com/misc/Amp_Scrapbook.pdf

Looks like there's 2 pots, although drawn as 1 in the schematic.  I wonder how they interact?

I don't see them drawn as 1 in any schematic. Show us which schematic please.

[Willabe]

.412 on a 2k setting

What does this mean? 2K setting? .412, is that a voltage drop?