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question on ohm's law

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Posted on April 26, 2016 at 05:31:38
vinnie2
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 If I want to reduce the B+ voltage going to the plate of a driver tube with a resistor, what value for the current do I use in ohm's law? Is it the plate current for the tube (say 6mA for a 26) or do I use some other value?
For example, I have a full B+ of 400vdc and I want it reduced to 110vdc at the plate of a 26, that's a drop of 290vdc. Plate current for a 26 is 6 mA. Is the right way to calculate R to divide 290vdc by 0.006 to get 48.8k ohms?
I ask this because what I am calculating and what I am getting when I measure it are very much different. I am getting almost no reduction in voltage when I measure it using a 48k resistor between the B+ source and the plate of the 26.
I am guessing I have something wrong in theory or practice, but I don't know what.

 

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RE: question on ohm's law, posted on April 26, 2016 at 06:01:29
euro21
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The suggestion (Ohm's law) not entirely good.

If you use tube, the first momentum to know tis tube's datasheet.

As you can see, 290V as the #26 tube's anode voltage is unrealistic (if the cathode is almost grounded).

The grid bias voltage (for example -9V) and the target anode current (6mA) specify exact (depends of the tube's condition) anode-cathode voltage (in this case 138V).

The anode resistor (as load) is minimum double of the tube's plate resistance (about 7k6), so 18-24k is enough.

If the anode resistor is 24k, the voltage over this resistor is 6mA*24k=144V

So, the calculated B+ is 138+144=282V

If your raw voltage is higher (400V), use Ohm's law for dropping resistor:
(400-282)/0.006= 19666.

Use 20k 2-5W resistor AND 22-47uF capacitor to shunt (AC grounded) 24K and 24k resistor common point.

 

RE: question on ohm's law, posted on April 26, 2016 at 06:22:58
euro21
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 20K and 24k resistor common point.

 

RE: question on ohm's law, posted on April 26, 2016 at 07:33:24
FenderLover
 Could you tap the voltage, a little further down the PSU rail? Maybe, where the voltage in closer to 300VDC? Maybe, in a bit more linear range for the tube's anode plate needs?

 

How are you setting bias voltage, and what is cathode resistance?, posted on April 26, 2016 at 08:51:30
Lew
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 One explanation for your finding is that the tube is not, in fact, pulling 6mA.

 

RE: question on ohm's law, posted on April 26, 2016 at 08:54:52
Palustris
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 We know from the tube manual that one operating point for the 26 is Ep = 135V; Ip = 5.5mA with a grid voltage of -10V. So we simply draw a load line that fits the curves which happens to be 20k ohms. We have 400V as our PS voltage so we know that we need to drop that to 240V for our plate supply. 400V - 240V = 160V so we know that we need to drop 160V for our plate supply so 160V / .0055 = 29090 ohms. The standard resistor value is 30k ohms: close enough for audio. We need a PS cap at the 240V voltage node.





 

RE: question on ohm's law, posted on April 26, 2016 at 11:39:19
vinnie2
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 I see I had the cap in the wrong place for starters. I will give this a try. Thanks.

 

RE: question on ohm's law, posted on April 26, 2016 at 11:41:29
vinnie2
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 Actually I am trying to use 110vdc on the plate, 400vdc-290vdc.

 

RE: question on ohm's law, posted on April 26, 2016 at 11:42:30
vinnie2
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 Don't think that is available, but I will check.

 

RE: question on ohm's law, posted on April 26, 2016 at 11:56:36
FenderLover
 Think that cap is important for isolating the various stages tapping off the PSU rail. I believe whether the cap is before or after the tap to your 26 is not important for the anode voltage.

 

RE: question on ohm's law, posted on April 26, 2016 at 12:04:29
vinnie2
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 Could be. Won't hurt in any case.

 

now I am really puzzled ......, posted on April 26, 2016 at 12:24:23
vinnie2
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 Set it up per your drawing and started bringing up the B+ with my variac. The voltage to the 26 plate started coming up and then after a bit it started decreasing instead. The more I turned the power up on the variac the more the B+ at the plate end of the 20K resistor went down. What would cause that?

 

RE: now I am really puzzled ......, posted on April 26, 2016 at 13:17:32
Triode_Kingdom
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 "The more I turned the power up on the variac the more the B+ at the plate end of the 20K resistor went down."

It sounds as though B+ at the anode is dropping because the filament is heating and the tube is drawing more current.


 

RE: now I am really puzzled ......, posted on April 26, 2016 at 13:50:30
Palustris
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 "The voltage to the 26 plate started coming up and then after a bit it started decreasing instead."

Yes, that's what we expect. Until the tube conducts, the plate supply voltage will be imposed on the tube's plate; as the tube starts to conduct, the the voltage on the plate will decrease until the tube reaches full conduction at which point the voltage should be the plate voltage we have computed.


 

RE: now I am really puzzled ......, posted on April 26, 2016 at 13:55:00
vinnie2
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 Oh! How 'bout that. So no chance of having a melt down if I just keep increasing the voltage up to full B+?

 

RE: question on ohm's law, posted on April 26, 2016 at 13:56:52
Palustris
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 "whether the cap is before or after the tap to your 26 is not important for the anode voltage"

The capacitor needs to be at the position shown in the drawing.

 

RE: now I am really puzzled ......, posted on April 26, 2016 at 14:01:59
Palustris
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 You won't know if your circuit works as predicted until you apply full voltage.

 

RE: now I am really puzzled ......, posted on April 26, 2016 at 14:21:37
vinnie2
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 Just got through taking it up to full mains, and the voltage never did start going back up on the 26 plate. At full mains I had a B+ of only 283vdc, and the reading at the 26 plate node (end of 20k resistor) was only 15vdc. Any ideas of where to start looking? I have checked the schematic several times and it seems put together ok, and I know it worked on the breadboard with this same psu.

 

RE: question on ohm's law, posted on April 26, 2016 at 14:22:22
vinnie2
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 It is.

 

RE: now I am really puzzled ......, posted on April 26, 2016 at 14:35:38
vinnie2
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 That's what palustris thinks too, but when I took it up to full mains the voltage did not come back up.

 

RE: now I am really puzzled ......, posted on April 26, 2016 at 15:14:17
Palustris
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 Vinnie,

You have to start to be specific:

"Just got through taking it up to full mains, and the voltage never did start going back up on the 26 plate. "

What does this mean?

"At full mains I had a B+ of only 283vdc, and the reading at the 26 plate node (end of 20k resistor) was only 15vdc."

You told us that the power supply voltage was 400V. Where are you measuring?

A voltage is a difference of potential between two points. If we don't know the two points then the statement is meaningless. The voltage on the plate of the tube is called "plate voltage" and it is relative to the cathode. If you don't use specific terms we can all agree on your statement is useless.

"Any ideas of where to start looking?"

How are you establishing bias? What is the cathode voltage relative to ground and the grid voltage relative to the cathode?



 

RE: now I am really puzzled ......, posted on April 26, 2016 at 16:07:24
vinnie2
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 Sorry, thought I was. I mentioned in the previous post that the voltage at the end of the 20k resistor started going down after turning up a
the power part way. You said that was to be expected and that it would come back up when the plate was conducting. i was telling you that it just kept going down until I reached full mains voltage with the variac. At that time I measured 283 vdc at the distribution panel for B+. That is a terminal strip with all tabs connected. If I disconnect the opt from that strip I have a B+ of 400vdc (B+ to ground), but when the opt is connect and the node for the 26 tubes is connected to B+ too the B+ drops to 283vdc (B+ to ground) and the voltage reading between ground. Bias is set by the cathode rersistors (820 ohms for 26, 1200 for 45.) All of these readings were taken with just the left channel connected to B+.
I will try going back and reading it between the plate and the cathode and see what I get. I will also measure the other two voltages you asked for.

 

more info..., posted on April 26, 2016 at 16:31:05
vinnie2
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 I went back and measured between pin 2 (plate) and pin 4 (cathode) on the 26 with the variac at full mains and got 6vdc. I also measured B+ at the wire on the opt where B+ goes into the opt. It was 280vdc between there and ground.
The cathode resistor on the 26 is 820 ohms and goes from the center wiper of the hum pot to ground.

 

Filament Emission, posted on April 26, 2016 at 21:31:58
Triode_Kingdom
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 "when I took it up to full mains the voltage did not come back up."

When you turn up the Variac, anode voltage "peaks" when you reach a point where A) a good amount of B+ from the power supply reaches the anode, but B) the filament hasn't yet heated to a point where the tube conducts well. As you continue to raise the primary voltage past that point, heater emission rules the day. B+ is increasing, but so is the tube's emission and current draw. Also, I'm pretty sure that the emission in a DHT continues to increase as filament voltage is raised. This is different than most IDHTs, where cathode emission has limits created by other factors, and will generally plateau before reaching full operating voltage.





 

RE: Filament Emission, posted on April 27, 2016 at 03:28:07
vinnie2
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 Is that in any way responsible for the low B+ numbers I am seeing?

 

Ever get this figured out? (n/t), posted on April 27, 2016 at 07:32:34
FenderLover
 mt

 

RE: now I am really puzzled ......, posted on April 27, 2016 at 08:09:07
Triode_Kingdom
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 "I will try going back and reading it between the plate and the cathode and see what I get. "

I'd be more interested in knowing the voltage between the cathode and grid. It sounds as though the tube might be turning on too hard.

 

RE: now I am really puzzled ......, posted on April 27, 2016 at 08:47:10
vinnie2
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 will do

 

RE: now I am really puzzled ......, posted on April 27, 2016 at 10:13:22
vinnie2
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 I measured -2.2vdc with the negative probe on pin 4 (cathode) and the positive probe on pin 3 (grid) of the first 26. The variac was at full mains and the B+ was reading 283vdc at the terminal strip. That is where it should be reading close to 400vdc.

 

OR, posted on April 27, 2016 at 10:15:44
Frihed89
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 12mA-(12mA/240V) * 135V = 5.5mA.

After being completely confused (for years) by how to draw load lines to find OPs, this Post + replies has finally made the basics almost clear to me.

Many thanks to you all.

 

RE: now I am really puzzled ......, posted on April 27, 2016 at 10:37:54
Palustris
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 OK, you are making progress; well done.

Now you need to do what you should have done before you started your project: draw a full schematic (all you need is a pencil and a piece of paper). Drawing a schematic does several things for you. It gives you a reference to work from; it gives you a way to record voltages and changes; but most important it will be a learning aid giving you a visual means of understanding your circuit.

So, carefully draw out your circuit including every component, their value and their connections. Energize the circuit and record the voltages at each connection. Compare those voltages with the voltages you expect according to the calculations performed when designing the circuit. Use ohm's law to determine how the circuit is working by measuring voltages across resistors and computing the current.

Once you have drawn out the schematic and entered the voltages you will have a visual aid to help you understand how the circuit works and you will have a precise means of discussing your project with others.

 

RE: OR, posted on April 27, 2016 at 10:47:53
Palustris
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 Excellent! Now practice with various tubes, loads, and operating points. Post as new topic if you get stuck.

 

RE: now I am really puzzled ......, posted on April 27, 2016 at 11:40:27
Tre'
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 " That is where it should be reading close to 400vdc."

What do you base that on?

If the power supply isn't regulated then the voltage will change.

The more current you draw, the lower the B+ voltage will be.

Like Palustris said, draw a full schematic.

That will help you, and those trying to help you, understand what's happening.

Tre'


Have Fun and Enjoy the Music
"Still Working the Problem"

 

RE: now I am really puzzled ......, posted on April 27, 2016 at 13:00:55
vinnie2
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Well I sort of did some of that a while back in the "hot time in the old town post" with this schem. Thought I needed a new thread, but maybe not a good idea. Anyway, I have not done all you suggested yet though as I have not filled in the voltages. I did not calculate the voltages when designing it as I used Drummer Willy's schem for his 26/26/45 amp and just modified it slightly here and there.
Ignore the operating point notes at the bottom of the page as they are not all correct. Still separate filament trannies for each tube though.
Is it best for me to fill in the voltages and post them in this thread, or should I start a new thread to give us a clean sheet of paper to work on?

 

RE: now I am really puzzled ......, posted on April 27, 2016 at 13:16:29
vinnie2
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 That is based on what it was delivering to the old amp it was powering. I used this psu in the brread board I did of the amp I am currently trying to build and it worked very well at that time. The B+ measured out at about 375vdc then I think.
Please see my response to Palustris above for the rest of the story.

 

RE: now I am really puzzled ......, posted on April 27, 2016 at 13:51:38
Tre'
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 You need to show a full schematic of the power supplies also.

Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"

 

RE: now I am really puzzled ......, posted on April 27, 2016 at 17:34:36
vinnie2
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 Can do. Do you think I should start a new thread with the new schems? This one is getting kind of long and rambling.......

 

RE: now I am really puzzled ......, posted on April 27, 2016 at 18:15:50
Tre'
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 That's probably a good ides.

With a complete (make it as complete as you can) schematic the people helping you have something concrete to go by and you will be able to make more sense of their suggestions.

Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"

 

RE: now I am really puzzled ......, posted on April 28, 2016 at 01:15:29
euro21
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 This schematic is inoperable!

1.)
If the second #26 grid voltage is 110V and cathode voltage is about 5V on 820R (target anode current is 6mA), the grid-catode voltage is +105V!!

The second #26 tube will destroy (grid-cathode diode will conduct several mA current).

2.)
The #45 cathode voltage is 1200R*0.034A=40.8V
If anode voltage is 223V and cathode about 41V, anode-cathode voltage is 182V.
Datasheet: 182V, 34mA -> grid-cathode voltage about -33V.
So grid will be at -8V potential.

IMHO the original schematic was AC coupled, capacitors between stages.
Grid to ground resistor shows this.

Impossible to build from well fuctioning AC coupled amplifier DC coupled ones without complete redesign.

 

Thanks for all the help folks!!, posted on April 28, 2016 at 02:13:17
vinnie2
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 I have decided to take a break while I breadboard one channel again and draw up a new and more complete schem that includes the psu.
I am also thinking I may strip the chassis and start over again with the new schem as there are some things I want to change already.
Once all that is done I will come back with a new thread and either show you the working amp (it did work before, honest) or post the new schem and ask for help debugging it.
Thanks again for all the help, I really appreciate it!

 

RE: now I am really puzzled ......, posted on April 28, 2016 at 12:29:30
RPMac
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 There are coupling caps between the stages that aren't drawn in the schematic.

I'm puzzled about how the components on the 45 cathode work. Shouldn't they be the same layout as the 26?

 

RE: now I am really puzzled ......, posted on April 28, 2016 at 14:30:01
vinnie2
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 Good catch. Boy did I ever screw up that schem. It is the same setup as the 26's, just different values. see my last post at the top of the thread for what comes next.

 

Thanks, posted on April 28, 2016 at 15:15:17
Frihed89
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 First I have to figure out "loads". I'll do some more reading and look for circuits that have all the values written in. These are often hard to find.

The math is not my problem. It's trying to line up the theory with the lingo. There's a lot of air in between I can't figure out until i stumble on something like this post.

 

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