Hey guys, just another (amongts the meny) questions I have. Is it comon for a tube amp's power supply caps to dis-charge within seconds of turning the power switch off? From what I have become to understand (which isn't much...but I am trying :>) caps will dis-charge some power over time...but within seconds!
Rino Odorico
Rino Odorico
Hey Frank
Hey Frank, yep...seconds...450VDC to 8VDC in 20-30 seconds. Sounds strange doesn't it. Wonder if I have one or two caps drying up.
Rino Odorico
Hey Frank, yep...seconds...450VDC to 8VDC in 20-30 seconds. Sounds strange doesn't it. Wonder if I have one or two caps drying up.
Rino Odorico
CAPS
Hi,
Yes,that looks like it to me.
Any idea about the value of the resistor across those caps (you will have to unsolder it to measure) if any?
Or if you have a circuit diagram,that would help too.Sometimes they are put where you don't expect them...
Cheers,
Hi,
Yes,that looks like it to me.
Any idea about the value of the resistor across those caps (you will have to unsolder it to measure) if any?
Or if you have a circuit diagram,that would help too.Sometimes they are put where you don't expect them...
Cheers,
RE: Caps
Checked all of the resistors ( I was bored one day), they all measure up to spec. and don't look heat worn.
Don't have a scanner, can't post the wiring...have to move up to the late 80's in technology soon :>)
Is there a way to check old caps without a cap meter?
Rino Odorico
Checked all of the resistors ( I was bored one day), they all measure up to spec. and don't look heat worn.
Don't have a scanner, can't post the wiring...have to move up to the late 80's in technology soon :>)
Is there a way to check old caps without a cap meter?
Rino Odorico
Hi Rino
The discharge time of the B+ filters can be affected by several things. Do you have any voltage dividers on the B+? Remember, tubes will still conduct for a short time after the filiment supply is shut off, up to several seconds depending on the tube.
The discharge time depends on the amount C in the filter and the load. Try measuring the DC resistance from B+ to ground. The lower the resistance, the faster the discharge will be.
Remember to make sure the power supply caps are discharged before you connect your Ohm meter. 
Connect the + Ohm meter lead to the +cap terminal and the - meter lead to ground. The reading will jump to a low reading and then, as the meter charges the caps, the reading increase. When the Ohm meter reading stops changing that is the resistive load on the power supply.
It's just a good old RC time constant the determines discharge time.
Later
Bruce
The discharge time of the B+ filters can be affected by several things. Do you have any voltage dividers on the B+? Remember, tubes will still conduct for a short time after the filiment supply is shut off, up to several seconds depending on the tube.
The discharge time depends on the amount C in the filter and the load. Try measuring the DC resistance from B+ to ground. The lower the resistance, the faster the discharge will be.
Remember to make sure the power supply caps are discharged before you connect your Ohm meter. Connect the + Ohm meter lead to the +cap terminal and the - meter lead to ground. The reading will jump to a low reading and then, as the meter charges the caps, the reading increase. When the Ohm meter reading stops changing that is the resistive load on the power supply. It's just a good old RC time constant the determines discharge time.
Later
Bruce
CAPS
Hello Rhino,
Is that the Lux MQ3600 given you trouble again?
If so,I think I can find the circuit diagram on the net.
Still,Bruce's proposed method is valid so you can check that way.
One thing to keep in mind though is that if filter caps don't keep their charge they usually don't filter too well either,so you have should hear some hum on the speakers.And since you always said they were sounding fine...🙄
Cheers,
Hello Rhino,
Is that the Lux MQ3600 given you trouble again?
If so,I think I can find the circuit diagram on the net.
Still,Bruce's proposed method is valid so you can check that way.
One thing to keep in mind though is that if filter caps don't keep their charge they usually don't filter too well either,so you have should hear some hum on the speakers.And since you always said they were sounding fine...🙄
Cheers,
Hey Frank, yep your right about the MQ-360. The amp does work and sound fine (as you predicted), just posted this question because it always made me wonder. As far as the hum, it's not...quiet as a mouse. Just asking for knowledge sake. I love tinkering with electonic gear...and learning about it.
Rino Odorico
Rino Odorico
CAPS
Rhino,
Sure.
Still,don't get paranoid here please.
If it's working fine then leave well enough alone.
If it ain't broke don't fix it.(In cowboy speak.) 😉
Cheers,
Rhino,
Sure.
Still,don't get paranoid here please.
If it's working fine then leave well enough alone.
If it ain't broke don't fix it.(In cowboy speak.) 😉
Cheers,
The tubes still conduct while they are hot, and they drain away the charge. Remember the caps are pretty small (10s of uF)
You could remove them from the circuit and test them with an ESR meter if you really want to know....
My old can caps from the 70s still work fine...
You could remove them from the circuit and test them with an ESR meter if you really want to know....
My old can caps from the 70s still work fine...
Ok, have the amp in front of me. I'm testing the resistance from B+ of the cap to ground. And what I read is 3 of the 4 caps read 3 Meg Ohm each, and the 4th one reads around 18-19K. Shouldn't all the caps read the same?
Rino Odorico
Rino Odorico
CAP
Fellows,
That's not the correct way to measure a capacitor.
The component you are trying to measure must be disconnected from the B+.
Then it should be discharged and measured with a capacitance meter.
An Ohm reading will also give you an idea about it's condition.
All this under the assumption that all caps are the same in value and voltage?
Rgds,😎
Fellows,
That's not the correct way to measure a capacitor.
The component you are trying to measure must be disconnected from the B+.
Then it should be discharged and measured with a capacitance meter.
An Ohm reading will also give you an idea about it's condition.
All this under the assumption that all caps are the same in value and voltage?
Rgds,😎
Long Short
Best to put a shorting clip across the cap and leave it to fully discharge for exrended period (1 minute or more).
Because of dielectric absorbtion, a briefly shorted cap will later show a voltage across the terminals.
Even picture tubes (glass dielectric) exhibit this effect.
Eric.
Best to put a shorting clip across the cap and leave it to fully discharge for exrended period (1 minute or more).
Because of dielectric absorbtion, a briefly shorted cap will later show a voltage across the terminals.
Even picture tubes (glass dielectric) exhibit this effect.
Eric.
Hey Folks,
I know nothing of the amp in question, but I did notice with one of my PS designs that with a large inductance (5H) when the power went off, the caps discharged quite quickly. 3-6 seconds. Could it be that the collapse of the magnetic field causes a back emf that discharges the capacitors quickly? I did notice that when I changed to a 1.5H choke in the same PS the caps discharged noticably slowly. That is music kept playing for 20-30 seconds.
Just aksin.
Gabe
I know nothing of the amp in question, but I did notice with one of my PS designs that with a large inductance (5H) when the power went off, the caps discharged quite quickly. 3-6 seconds. Could it be that the collapse of the magnetic field causes a back emf that discharges the capacitors quickly? I did notice that when I changed to a 1.5H choke in the same PS the caps discharged noticably slowly. That is music kept playing for 20-30 seconds.
Just aksin.
Gabe
CAP DISCHARGE
He Gabe,
Depending on how that choke is connected in the PSU and its series resistance,maybe one will then discharge the caps faster than the other.
But a choke causing back EMF to discharge the caps at such speed?
You surely must have a very low impedance load connected such as your speakers shunting the whole circuit?
I'm no solid state expert (assuming this is what you're talking about) but I don't see that happening in tube circuit with a choke in series with the PSU B+.
I am as curious as you are here.
Cheers, 😉
He Gabe,
Depending on how that choke is connected in the PSU and its series resistance,maybe one will then discharge the caps faster than the other.
But a choke causing back EMF to discharge the caps at such speed?
You surely must have a very low impedance load connected such as your speakers shunting the whole circuit?
I'm no solid state expert (assuming this is what you're talking about) but I don't see that happening in tube circuit with a choke in series with the PSU B+.
I am as curious as you are here.
Cheers, 😉
Hey fdegrove,
It is a tube amp. Pi filter. 47µF at the rectifier (tube), 5H choke, and 80µF after. The chokes DC resistance is about 105 ohms.
The back-emf was my only explanation. The B+ is feeding four 6BQ5's in a push pull amp. The driver tubes both only draw about 1 mA (0.25 mA per triode).
Wierder things have happened! 😱
😉
Gabe
It is a tube amp. Pi filter. 47µF at the rectifier (tube), 5H choke, and 80µF after. The chokes DC resistance is about 105 ohms.
The back-emf was my only explanation. The B+ is feeding four 6BQ5's in a push pull amp. The driver tubes both only draw about 1 mA (0.25 mA per triode).
Wierder things have happened! 😱
😉
Gabe
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