What could be the reason for too high voltage from filter caps?

O RUde

Member
Messages
22
Hi, I wonder what could cause too high voltage from filter caps? Might be a difficult question, but if there are some general rules?
 

Vanyu

Member
Messages
739
You'll have to elaborate a little bit.

First, what amp is in question here?

Second, where are you measuring your voltages at, and which filter caps are in question? And what makes you think your voltages are too high? 400v-500v is nothing out of the ordinary at the first node. Typically these caps see whatever voltage the rectifier is putting out (which is usually really high!).

Is your amp working in its current state? Blowing fuses? Blowing tubes?
 

Tron Pesto

Member
Messages
820
And just to help you zero in on the details, capacitors themselves do not supply voltage. No voltage comes "from caps." Capacitors are passive components that affect a circuit's performance to a desired result.

In the case of filter caps, voltage/current is typically supplied through a rectifier powered by a transformer (secondary). In your search for why your voltages are too high, you have to look up the chain at what the transformer and rectifier are doing. Those two things will have the greatest effect on the voltage. Filter capacitors simply smooth the AC pulses coming from the rectifier.
 
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Blue Strat

Member
Messages
30,065
Yes, what amp? Are the power tubes installed? If not, the plate voltage will be high. What voltage are you getting and what's the voltage rating of the amp? I should have mentioned this first, but change your meter's battery first.
 

tarweed

Member
Messages
876
Hi, I wonder what could cause too high voltage from filter caps? Might be a difficult question, but if there are some general rules?
if one or more caps are blown (i.e. show as open circuit to AC), then your meter may be showing something closer to the peak (unfiltered) voltage.
Also, what else is in the filter circuit? Choke, or resistor - to make a LC or RC filter? If shorted, then the output voltage would be higher
 

PushedGlass

Member
Messages
594
Yeah - there's simply nothing to go on here. I'll just say that these power supplies are most thoroughly unregulated; they arrive at the voltages they produce as a result of how much they're loaded by the circuit in whatever state it happens to be.
 

O RUde

Member
Messages
22
Sorry about the vague kind of question. I am building a Dumble clone, and when I finished it there was sound going through, although not the right sound. Then at some point it all went dead. I have changed all the tubes and they seem to be OK, glowing as usual. Normally you can get some "scratch" sounds when you touch certain pins on the tubes, but nothing. When I measure the B+'s I seem to get a bit high voltage, about a 100 more voltage on b5 and b4 than the schematics states. It seems like the tubes doesn't get any working feed, there is only a very low hum in the speaker. This amp is difficult, bit too much I must admit.
 

Vanyu

Member
Messages
739
I hope you didn’t build the amp solely off of that layout... Do you have an actual schematic to your amp or just that layout?
 

Vanyu

Member
Messages
739
That's interesting Vanyu, why do you think that? I mostly used that layout yes? But I have the schematics as well:
https://www.dropbox.com/s/poza97suk09sdeg/183.pdf?dl=0
That’s still only the layout you posted.

The reason I make mention of the layout is because this is a quite complex build, and not referencing the actual schematic is a recipe for gremlins inside the amp. You really need to find an actual schematic for your amp so you (along with all of us) truly understand what’s happening in your amplifier. A layout should only be used as a cross reference for component placement.
 

HotBluePlates

Member
Messages
9,795
... I seem to get a bit high voltage, about a 100 more voltage on b5 and b4 than the schematics states. It seems like the tubes doesn't get any working feed, there is only a very low hum in the speaker. ...
Now that we have a schematic, it's very easy to devise a troubleshooting strategy!

Measure the voltages at pins 3 & 8 of V1 and V2. The schematic shows you should be seeing something between 1.5 to 2v at each cathode. What do you have?
 

killer blues

Member
Messages
3,044
Sorry about the vague kind of question. I am building a Dumble clone, and when I finished it there was sound going through, although not the right sound. Then at some point it all went dead. I have changed all the tubes and they seem to be OK, glowing as usual. Normally you can get some "scratch" sounds when you touch certain pins on the tubes, but nothing. When I measure the B+'s I seem to get a bit high voltage, about a 100 more voltage on b5 and b4 than the schematics states. It seems like the tubes doesn't get any working feed, there is only a very low hum in the speaker. This amp is difficult, bit too much I must admit.
Check the screen grid resistors they may be open
 




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