bad transformer tap ?

[Ang3lus]

Hey all,

I've recently came across a weird problem, I've got an amp, which has a 15vac tap (which NOW reads 12.3vAC) and is connected to a set of 4xuf4007 rectifiers to turn it into dc (i'm getting 13.10v after rectification) that goes into a 12v LM7812 regulator.

after the regulator i'm getting 11.59vDC which I think is not enough for elevated heaters and switching supplies (a set of Omron g5V-1 12vDC SPDT).

Switching is not working, getting really low volume and distortion was cut by half.

I've already tried replacing the omron switches and the problem remains, the switching problem is new but the distortion that was cut by half isn't.

has the PT gone bad ?

I remember clearly it used to be 15.2vAC and the heaters ran at 12.60vDC.

[ToneMerc]

First; LMxx regulators need to see at least 3 volts above the target output for best regulation and then the 1A capable current output is only if it's heatsinked. In this particular situation a better choice may would have been the 7810. However, I don't think you have a voltage problem, but a current problem as this relay should trigger even at 9.5V or so.

What do you know about current capability of that 15V winding?

TM

[Ang3lus]

Should be 2A, but if it's gone bad I don't know how much it is now.

is there a way to check ?

[JMFahey]

Hey all,

I've recently came across a weird problem, I've got an amp, which has a 15vac tap (which NOW reads 12.3vAC) and is connected to a set of 4xuf4007 rectifiers to turn it into dc (i'm getting 13.10v after rectification)

First red flag.
A 12VAC winding gives you roughly 16VDC .
2 possible problems:
a) your winding is way overtaxed.
What/how many/how are the filaments you are trying to feed?
b) I *hope* you have a big capacitor after that diode bridge ... at least 4700uF 25V ; 10000uF even better.
Now if you forgot it or have a woefully inadequate one there (say 100uF) 13.1 VDC looks what your meter would read with a full wave rectified ... unfiltered voltage.

that goes into a 12v LM7812 regulator.

as said above, you need at least 3VDC across the regulator (meaning you need at least 16VDC to start with) for it to work.

after the regulator i'm getting 11.59vDC

see above

which I think is not enough for elevated heaters and switching supplies (a set of Omron g5V-1 12vDC SPDT).

WHAT elevated heaters?
You don't need them if running 12VDC .
You can't elevate them if supply is +12V to ground anyway.

Switching is not working, getting really low volume and distortion was cut by half.

You have a different problem there, which you'll have to troubleshoot on its own, but unrelated to somewhat low filament voltages.

I've already tried replacing the omron switches and the problem remains, the switching problem is new but the distortion that was cut by half isn't.

See above.

has the PT gone bad ?

I remember clearly it used to be 15.2vAC and the heaters ran at 12.60vDC.

Check above suggestions.

[Ang3lus]

I fixed the channel switching problem, was the goddamn jack shorting all the time.

I still have the problem with the voltage though.

I checked the voltage at the capacitors, one side reads 6.3v and the other 6.1v

that's after rectification, the winding by itself reads 12.47v., it's like it's not rectifing the winding at all.

I got 5x12ax7 tubes for that winding and 3xomron g5v-1 12v SPDT, 2A rating on that winding, should be sufficient.

I have 4x4700uf@25v capacitors there.

I'm attaching a schematic.

[ToneMerc]

I got 5x12ax7 tubes for that winding and 3xomron g5v-1 12v SPDT, 2A rating on that winding, should be sufficient.

You are forgetting the current capability of the rectifier, you are exceeding it's rating.

TM

[Ang3lus]

should i change those out to uf5408?

[ToneMerc]

should i change those out to uf5408?

Sorry Ang to lead you astray, they are fine., I meant to say regulator, not rectifier. However, I should have looked at the schematic, as the devil is in the details. That transistor circuit on the output the LM7812 increases the net current capacity of the DC filament supply. Thus, no worries on the 1A current capability of the LM7812.

TM