amp hum after mod - help!!

G

gusfinley

New member
Okay, so I modded my amp (DSL401) - adding birght switches to both of the channels, removing a unnessasary capacitor that was limiting the bass response and added pin jack to the back of the amp so I could check the bias without taking the amp apart.

So i put it all back together and fired it up - no sound - then the sound faded in and there was all this crackling and stuff and then I watched on the 1/2W screen resistor burn up!! YIKES!! I looked around and found that I hadn't connected the HT to the output transformer - it was way in the back and close enough to the place it was supposed to go that it looked cennected when I double checked.

Well, I was only running my amp with two tubes anyway, so I put the two tubes in the outer sockets. I turned it on and then I see lots of blue sparks in the tube that was in the socket when the screen resistor burned up. WOAH!! Well, I have like six spare tubes so I put in another set and everything seems ALMSOT alright....

One strange thing is that when I turn down the master volume all the way now, I still get a signal coming through the speakers..
there is also a permanant hum. It almost seems as if the master volume knob is more like a presence knob now because the volume doesn't change as much as the tone does - crank the volume and it get bassy, turn it down and it gets thin...

I am assuming that somewhow somewhere the signal from the phase inverter is sneaking past the master volume and into the grids...

one more odd thing.. I was cheking the differences in the circuitry between the two tubes. and noticed that when I connected the lead of my multimeter to pin 2 of the leftmost tube there was a loud pop. When I connected it to pin 2 of the rightmost tube there wasn't a pop. This was the same for the inner sockets that contained no tubes (including the one that had failed previously) The left one popped and the right one didn't..

anybody have any suggestion as to what might be the problem here? It is baffling me!! Thanks for any input..........
 
Thats gotta suck. I would have shit myself if I saw that.

Well, that is why even amp and guitar techs will make you sign a waver of liability. ;)


gusfinley said:
One strange thing is that when I turn down the master volume all the way now, I still get a signal coming through the speakers..
Click to expand...


I have that with my Super Reverb and my ProJr. Its very faint. Almost unnoticable. I figure its nothing.
 
Assuming that you replaced the burnt resistor, you need to go through everything that that resistor was attached to and on up the line. When 1 component burns up, it normally takes a few more with it. It isn't obvious because the others might not have completly burnt up.
You should take it to a tech, you might have screwed up the choke or one of the transformers.
 
well.....

Ah, its wasn't so bad... That's the beauty of tube amps, you screw up one socket, so you plug it into the other one, you screw up a tube so you put another one in... no biggie....

about the hum.... I think I have traced it down to somewhere in the master volume circuit. I did some poking around with a multimeter and found that the resistance from the grid resistors to ground on the leftmost cokets varies between 7-10 Mohms when I vary the Master volume. The right ones vary from 25K-200K. Since the master volume is a 200K pot, my guess is that somewhere between the bias circuit and the grid resistors there is an open circuit, bad solder joint, or something... I won't have time to track it down for a while, though... I measured the bias at the grid resistors and one side is reading -12V (left most tubes) and the other is reading -17V, so it seems it would have to be in there somewhere... perhaps the master volume knob is bad... hard to say, but I'm glad I was able to track it down.....

The good thing is that while I have it all apart I can take out the bright cap I put in because it is WAY too bright- I put in the factory spec 470pF cap - I realized yesterday that the guy I bought my amp from had already modded it to a 100pF cap, which sounds MUCH better!! No wonder people are always dissin this amp!! I can't stand the 470pF cap in there!! I'm going to install pin headers where all the "bright" cap go, then i'll be able to just plug different values of caps in and find what I like and then wire that one to the switch....

Oh, yeah... I just ordered a digital camera, so I'll put the bad tube back in my amp long enough to get one good picture of the tube sparking inside, maybe even some video.... You might all like to see that - especially if it is in someone else's amp!!

anyone else have any input?
 
Outlaws said:
I have that with my Super Reverb and my ProJr. Its very faint. Almost unnoticable. I figure its nothing.
Click to expand...

maybe my master volume pot has gone bad.... I was beginning to notice a faint volume from the channels with it turned all the way down BEFORE I started working on it.... maybe all the moving around of the PC board messe it up....
 
gusfinley said:
about the hum....

-------------------------------

maybe my master volume pot has gone bad.... I was beginning to notice a faint volume from the channels with it turned all the way down BEFORE I started working on it.... maybe all the moving around of the PC board messe it up....
Click to expand...

Is this the same? I was refering to it amplifing my guitar a hair. Not hum. I mean, they still make the normal tube "hum", but I was refering to being able to hear my guitar if I play it even when the volume is all the way down.
 
pheeww!!

For what its worth, I have isolated the problem to the master volume circuit - probally a bad solder joint created from stress placed on the pot during the modification.

YES!! You will be seeing PICs of a tube throwing sparks!! and maybe some video... as soon as my camera comes....

Outlaws said:
Is this the same? I was refering to it amplifing my guitar a hair. Not hum. I mean, they still make the normal tube "hum", but I was refering to being able to hear my guitar if I play it even when the volume is all the way down.
Click to expand...

The HUM is everpresent now (due to the "floating" grid) - ehich causes the "pops" I was hearing when i was testing it... but I do hear the guitar signal also when the volume is all the way down...
 
Hey gusfinley!

I also have a DSL401... can you tell me, did removing the cap bring more bass response on the clean channel, the OD channel, or both?

And can you tell me which cap that is and how to remove it?
 
ModestCargo said:
Hey gusfinley!

I also have a DSL401... can you tell me, did removing the cap bring more bass response on the clean channel, the OD channel, or both?

And can you tell me which cap that is and how to remove it?
Click to expand...


You might want to research this a bit more first. If you don't know how to remove a capacitor then you don't know enough about circuits to work inside a tube amp.

http://www.geofex.com/tubeampfaq/tube_amp.htm#Section0

You could die or get the living shit shocked out of you if you don't know what you are doing.
 
For your hum problem, I'd guess that somehow you changed the grounding scheme of the circuit, and you've got a ground loop somewhere. Maybe one of the switches you added :confused:

The cap you removed, did you need it to block DC? Perhaps you should upgrade the cap with a higher quality and larger value rather than just leaving it out?
 
You must log in or register to reply here.
Back
Top