5f2-a. big noise, like feedback

[pietro_moog]

hi guys
i just finished my amp.
i powered for a minute and it didn't explode. cool.
but it has this very bad noise, i don't know.. sounds like a sort of feedback to me, but i'm not sure. i didn't plug the jack, the amp was alone.
the noise didn't seem to increase with volume or change by tone pot.

any ideas?


i used 2 100Ω resistors to ground heathers to pin 8 of the power tube.
my output transformer is a Hammond 1760C, i wired like this:
primaries ==> blue (dotted in the hammond paper) to pin 3 of the power tube
red to the 16µ cap for power
secondaries ==> black to ground
yellow to signal



ops: this post needs to be relocated in the "debugging your build zone"

[pietro_moog]

i read some stuff around here, and i found out that the right term for my problem was "squealing".
i read that the main cause was the incorrect wiring of the output transformer, and mainly because often colors are inverted.

so i inverted the secondaries (the incorrect way) and now the amp seems to work correctly.

what are the odds!


BTW, this thread needs to be moved to the "debugging your build" section

[RWood]

That's strange, but I'm glad it's working for you. This might be related to how you have your NFB circuit built. If you want to find out, disconnect it and swap your secondaries back the way they were. If it sounds as it does now, the squealing is in your NFB.

On the other hand, you could just enjoy the amp as you have it! How do you like the sound?


RWood

[pietro_moog]

hi. thanks
what is the NFB?
the amp is very silent (no hum) and not so loud (which is perfect to me)
it doesn't break, this is strange
(i need to repair my guitar, i'm using my friend's one, a lemon yellow ibanez. i have a stratocaster)

i thought it would be a different amp. more noisy, more distorting.
maybe the strato would perform different..

all in all i'm happy, i need to try it deeper

[JHow]

what is the NFB?

NFB is "negative feedback", basically a portion of the output signal is fed back into a previous stage 180 deg. out of phase (hence, negative) to reduce oscillation and make the amp more stable

On your amp you should have a line running from the speaker jack "tip" back to a 22K resistor on the board, and then back to the cathode of the second half of the 12ax7. That is the nfb circuit.

You don't absolutely need it the nfb. You can experiment by unhooking the line at the jack and get a looser, grainier sound, or you can make it switchable, you can use a pot instead of the resistor - or you can keep it the way it is. If you get the transformer leads reveresed, you can get the oscillation that sounds like was the problem you had earlier in the post.

[ThermionicScott]

so i inverted the secondaries (the incorrect way) and now the amp seems to work correctly.

what are the odds!

About 50/50.

[Enzo]

It is working now, stop fixing it. Just enjoy the amp.