stumped with this amp problem

[Ang3lus]

This is an amp i purchased from ampclones.com when it was new and i didn't know what it was, the amp was working fine for a while, died for a bit, now after a year i sat and found a bad jack shorting to gnd.

this is a bogner uberschall clone, the schematic is from ampclones.com

problem is with the preamp, i'm getting really low volume (like a 0.5watt amp), replaced preamp tubes, plugged into return jack and getting massive volume (like a 100 watt should give me)

voltages are fine, i will post mine, but didn't bother checking the PI tube since the return jack was supplying me with the required volume, so i assume power amp is fine.

HEATERS ARE DC HEATED and i'm getting 11.6v on each preamp tube heater.

V1:
1: 181v
3: 1.2v
6: 230v
8: 1.9v
9: 6.29v

v2:
1: 269v
3: 2.1v
6 242v
8: 2.1v
9: 5.63v

v3: (cathode follower)

1: 180v
3: 1.1v
6: 327v
7: 181v
8: 182v
9: 5.75

v4:
1: 332v
2: 20v
3: 50.8v
6: 274v
8: 1.9v
9: 5.69v

i've also added a japanese site schematic (twin jet, added gain stage on clean) to compare voltages, but i think voltages are fine.

my only thought is that it can be a bad input jack ? because of the low volume on both channels ?

gain/tone/master volume on all channels respond fine and tonally the amp is fine, no weird noises/ossicilations or what not. just low volume.

[JazzGuitarGimp]

It might also be an issue with the effepcts send jack / circuit.

[xtian]

I'd put a sine wave at input, and trace it. Shouldn't be hard to identify the gain stage that's not working right.

[JazzGuitarGimp]

Xtian is right: I just had a look at the schematic, and there are a lot of opportunities for the signal to be interrupted between the input jack and the effects send jack. Tracing a signal from beginning to end will be the quickest way to find the issue.

[Ang3lus]

i don't have anything but my multimeter

basically when i tap pin 6 on all preamp tubes, v1,v2,v3 are the same volume and v4 seems to be loud (master at the same level of course).

I put a jumper between fx loop send and return, shoulden't that bypass the circuit ?

[xtian]

That shouldn't stop you. Use a CD player, smart phone or signal generator as your sine wave source. Use your multimeter set to AC to measure the signal.

[SilverFox]

You show 5.x V on pin 9 of the preamp tubes? Is this amp using a 12 volt filament scheme like the Bogner? They may not have used a 12 volt supply and that would explain the 5.x volt filament reading. If it is supposed to be a 12 volt supply check the output of the regulator with preamp tubes pulled out; measure preamp pins 4 and 9 for 12 volts, if it is a 12 volt filament supply.

silverfox.

[gui_tarzan]

Disregard, you already tried what I was going to say.

[Ang3lus]

You show 5.x V on pin 9 of the preamp tubes? Is this amp using a 12 volt filament scheme like the Bogner? They may not have used a 12 volt supply and that would explain the 5.x volt filament reading. If it is supposed to be a 12 volt supply check the output of the regulator with preamp tubes pulled out; measure preamp pins 4 and 9 for 12 volts, if it is a 12 volt filament supply.

silverfox.

it is a 12 volt supply, the regulator gives me (checked a week ago with tubes pulled out around 12v~.

i did measure pins 4+9 on all tubes,

pin 4 gave me 11.44v on all tubes and pin 9 game approx 5v~ on all tubes, should i be concered regarding that reading ?


Xtian, how do i measure the signal with meter set to AC, i check between the pin and ground ? what readings should i look for if it's working/not working ?

sorry for the n00b questions, I'm really stumped by this.

[bal704]

You might try an audio probe if you don't have an o-scope. Just start at the input jack and work your way into the amp.

It's worked for me in the past.

[labb]

Audio probe: http://www.diystompboxes.com/pedals/debug.html

Instead of plugging into amp and listening, plug into a cd player and you can inject the signal anywhere you want to..Works pretty good

Last time I had this problem it turned out to be a whisker from the shield of coax from the tone stack being in contact with one of the pot terminals. This is the way I found it.

[xtian]

^^^ audio probe.

[Structo]

Man, that amp must have a ton of gain!

[Ang3lus]

im ashamed to say i wired the fx loop wrong

fx send was connected to gnd return )()()

cant believe i missed it, i forgot i replaced the wire when tje amp had a jack problem, so i replaced all jacks, stupid stupid stupid

[gui_tarzan]

We've all been there, done that. At least you found the problem, and this place is full of guys that act as a second, third or fourth set of "eyes" which helps tremendously.