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In Reply to: RE: now I am really puzzled ...... posted by vinnie2 on April 26, 2016 at 14:21:37
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?
Follow Ups:
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.
Edits: 04/27/16
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.
Edits: 04/26/16
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.
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?
Edits: 04/27/16 04/27/16 04/27/16
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.
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?
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.
You need to show a full schematic of the power supplies also.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Can do. Do you think I should start a new thread with the new schems? This one is getting kind of long and rambling.......
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"
"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.
--------------------------
Buy Chinese. Bury freedom.
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.
Edits: 04/27/16
" 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"
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.
will do
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