• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Caps...what the...

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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
diyAudio Senior Member
Joined 2002


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...

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
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. :att'n: Remember to make sure the power supply caps are discharged before you connect your Ohm meter. :att'n: :bawling: 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.

diyAudio Senior Member
Joined 2002

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...:rolleyes:

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
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...
diyAudio Senior Member
Joined 2002


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?

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.

diyAudio Senior Member
Joined 2002

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?:xeye:

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! :eek:


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