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    Building, troubleshooting and testing of these amplifiers should only be
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    the safety precautions around high voltages.

power supply problem

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ChrisA

Member
Joined 2008
chopchip said:
so i reinstalled the OT with the brown wire to B+ and the blue to the plate and the black wire on the secondary to ground and now the voltage has dropped again. darn!
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How big are those two 12V transformers you are using back to back for power? It could just be that they are under-sized for the load. Are you getting anything near mains voltage at the 120VAC side of the second transformer.

The transformer may have a fairly high series resistance and you are using two of these.
 
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chopchip

Member
Joined 2010
ChrisA said:
How big are those two 12V transformers you are using back to back for power? It could just be that they are under-sized for the load. Are you getting anything near mains voltage at the 120VAC side of the second transformer.

The transformer may have a fairly high series resistance and you are using two of these.
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im building this from a tried and true design that i found on the internet, they are 1 and 2 amps, im getting main on that side of the 2nd xformer, i think the problem is in the filament and power tube grid circuit. im going to redo it, i think i made some assumptions about where the ground goes. ill get it going in a day or two i hope.
thanks for the reply!
 
Cassiel

Cassiel

Disabled Account
Joined 2004
If you would have asked first I would have told you to avoid a fixed bias amp. Too late, now that tube it's probably no good anymore. Cathode bias is good for beginners as it protects your tube in case you do something wrong, you did something really wrong (no bias voltage) and the tube just suffered the consequences (current runaway). Try another tube just in case, if voltage doesn't drop circuit is OK, if it still drops, circuit is not OK. Recheck.
 
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chopchip

Member
Joined 2010
stalker said:
If you would have asked first I would have told you to avoid a fixed bias amp. Too late, now that tube it's probably no good anymore. Cathode bias is good for beginners as it protects your tube in case you do something wrong, you did something really wrong (no bias voltage) and the tube just suffered the consequences (current runaway). Try another tube just in case, if voltage doesn't drop circuit is OK, if it still drops, circuit is not OK. Recheck.
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i have two tubes they both do the same thing. i have checked over it many times, im quite sure i have the circuit layed out correctly. both those indicator LEDs light up correctly.

i should have taken electrical engineering
 
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chopchip

Member
Joined 2010
stalker said:
I didn't. Take it easy, things will fall into place sooner or later. No time now to help you, maybe someone will. If not, I will post tomorrow, we'll see if we can find :magnify: the problem.
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great! thanks, man!

if i yank the tube out all the voltages are correct on the caps. they are stable and the correct value. when i shove either of the two tubes into the SOCKet the voltages drop.

is it really possible that i ruined both tubes? how do you think i did that? or did i accidentaly come upon two bad tubes. they were both tested by the vendor and tested 2000 for the first section and 7000 for the second. i dunno.
 
chrish

chrish

Member
Joined 2003
Those LEDs lighting up only means that here is power to the LED. Measuring the voltages at the appropriate points in the circuit will help diagnose the problem.

You state that you have checked over the wiring and there is no problem. There is either a problem with the wiring or with one or more of the components. If both tubes are bad, it is most probably a wiring problem. Stalker has already told you that no bias will lead to a runaway on the tube - probably destroying it. If there was no bias, there was probably a wiring or other component problem.

I would suggest checking all voltages with the tubes removed. This is what you should do anyway before plugging in any tubes, especially if you have no spares. If the voltages are good, then try with the tubes in. If the voltages are wrong, don't fry your tubes, find the error and fix it.

I would also suggest placing a 1 ohm resistor between pin 3 and ground. This way you can read the voltage drop across it and using Ohms Law calculate the current flowing through the tube (use DC mV, each mV drop across the resistor is 1 mA current).

When building, you should build and check in stages so that you can find and correct errors along the way. For this design, I would have built the power supply to the filament first, check for correct voltage. If OK, then construct the bias supply. Voltage OK, then move to the back to back transformer and voltage doubler. Voltage OK - move on to signal wiring. When complete, check all voltages again to make sure you have not made any more errors. If there are some, check for wiring errors (most likely) then damaged components. After everything checks out OK, THEN place a load on the output transformer (a large 8 ohm resistor equal to the expected output power of the amp plus a safety margin or a speaker that you don't mind destroying) or you may destroy the output transformer. Only then plug the tubes in circuit, check voltages again. If OK, try signal source and speaker etc....

If you take this systematic approach you will find the source of any errors quicker and destroy less components in the process.

So, if you had 0v at the grid of the power tube for any period, you probably have destroyed both tubes.
 
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chopchip

Member
Joined 2010
chrish said:
If you take this systematic approach you will find the source of any errors quicker and destroy less components in the process.

So, if you had 0v at the grid of the power tube for any period, you probably have destroyed both tubes.
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thanks for that great advice!!


i check the voltage to the r4 470k resistor and it is about -35v but after it the voltage is down to about -3v or so. i dont understand why there is so high a value a resistor there. the tube gets quite hot very fast when i plug them in. i guess i destroyed both of them. i ordered some new ones but that will take a month before i see those and then i fear i will ruin those too!

so with no negative voltage to the grid the current of electrons through the tube is not restricted and then the plate overheats and burns out? what exactly happened?

thanks again for the advice
 
chrish

chrish

Member
Joined 2003
Some one with a little more experience may offer some advice as to the specification of the 470k resistor, I have seen this value used in this location in other designs though. There should be very little current flow through this resistor, it is providing a bias voltage only, not current, so there should not be any significant voltage drop. Using Ohm's Law, if there is a voltage drop, there is a current flow. If there is a current flow here, then there is a problem, either a wiring problem or the tube is drawing current through the grid.

The 1 ohm resistor to pin 3 should be an easy mod. If you think the tubes are destroyed anyway, you may want to quickly check the voltage drop across it to see the current flow through the tube, I think the schematic showed 35mA.

Are the voltages OK with the tubes removed?
 
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chopchip

Member
Joined 2010
chrish said:
The 1 ohm resistor to pin 3 should be an easy mod. If you think the tubes are destroyed anyway, you may want to quickly check the voltage drop across it to see the current flow through the tube, I think the schematic showed 35mA.

Are the voltages OK with the tubes removed?
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i dont have that value resistor right now. i have a variety pack ordered and should be here soon. and that is a good idea, then i can measure the current through the tube as it passes from ground up through the tube. right?

the schematic specifies a 215v B+ but the voltage doubler provides about 320v which is what i measure at pin 2 without the tube and also at pin 1 i read 290 volts. so i dont quite understand what is going on there either

thanks again!
 
Wavebourn

Wavebourn

Member
Joined 2006
wavebourn.com
Feed-through capacitor means it has one pole on a bolt, and another on a rod that goes through it. They are used to pass DC or low frequency through a hole in chassis, but ground HF.

http://www.hobbyprojects.com/electronics_component_symbols/what_is_feedthrough_capacitor.html

Do you have a DMM to check capacitors before soldering them in?

feedthrough.jpg
 
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chopchip

Member
Joined 2010
Wavebourn said:
Feed-through capacitor means it has one pole on a bolt, and another on a rod that goes through it. They are used to pass DC or low frequency through a hole in chassis, but ground HF.

Do you have a DMM to check capacitors before soldering them in?
Click to expand...

oh i c. that is what they are. i have about a hundred of them.


i have a dmm but not one that checks capacitances. i found a .015uF 250v cap from an old project.

so do you think the tubes are fried?

thanks!
 
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chopchip

Member
Joined 2010
now i get a voltage of -37 to the grid which is good. and the plate voltage is still 320 which i think is too high because the schematic says it should be 215 which i dont understand because the voltage doubler doubles the 120volts rms which comes out to this 320 volts. so i read 320v at pin 2. i tried it with a tube and got a loud screech out of the speaker and quickly unplugged it.:eek:

but i think it is going in the right direction. do you think the tube is fried still?

another liberty i took was that i used 1N4007s for all the diodes.

thanks
 
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