Troubleshooting first build

[Codymobley]

I just wrapped up my first build after dragging my feet on it over the past year. This is a scratch built Gibson GA-40 clone using a late 1950s schematic (pentode preamp, 12AX7 phase inverter, 6SQ7 tremolo, dual 6V6, and 5Y3 rectifier). All the solder points are shiny, nothing is loose, heaters are lit (though the 12AX7 is dim if lit at all but warms up to the touch)..... but no sound.

I've checked all inputs/output jacks against my 1958 GA-20 to ensure I wired them correctly, replaced the tubes, traced the schematic, prodded the circuit to see if anything was loose or a poor connection and no dice.

I'm hopeful that it is a simple fix and not a bad OT,

Any thing that I may be overlooking?

[Stevem]

First off make sure the secondary side of the OT has a ground to the chassis, as many times this is overlooked.

Here's a simple way to troubleshoot with just a volt meter, but we are not looking for voltage, we are listening for a pop sound.

Take the meters hot lead and starting in the output section and touch the plate of each output tube.

You should hear a very light pop out of the speaker if that stage is working .

If not then that's where you need to look closely at for problems.

If you do hear a pop then move back up stream to the PI section and touch that plate and now you should hear even a louder pop out of the speaker.

Keep moving back up stream until to get to the gain stage where no pop is heard and around there will be your issue.

I did not look at your schematic, but don't forget that you volume and tone controls needs to be open , because your volume pot may be after your first gain stage.


Please report back with what you find along with more detailed pictures if you still have a problem.

[martin manning]

The above “pop test” is useful for finding where the signal is lost, but I would start with a DC voltage survey. Measure voltages at each of the power supply nodes (at the positive end of each filter cap). Then, measure voltages on the tube pins, plates, grids, and cathodes. Record those and post them. The problem area will be often be revealed. Sounds like the heater circuit is working, but measure AC volts to be sure that it is in the right range.

[jabguit]

This schematic?

Gibson_ga40LP.pdf

[Codymobley]

This schematic?

Gibson_ga40LP.pdf

Yes that looks like the schematic. Here is the diagram I drew out to assist with wiring while working through it:

[Codymobley]

First off make sure the secondary side of the OT has a ground to the chassis, as many times this is overlooked.

The Secondary is grounded at the output jacks, is this sufficient?
I'll try the pop test when I get home this afternoon.

[Stevem]

Yes grounding like that is fine

[Codymobley]

Pop test resulted in no sound on any of the plates. The plates themselves were measuring voltage in millivolts (just under .4 volts at each tube). The B+ at the powertubes measured .4 v, the preamp B+ measured . 38v at each node.



The circuit is failing somewhere, maybe I didn't hook the power transformer up correctly. Here is the spec sheet, All leads are attached except the blue lead which is heat wrapped and out of the way.

[Codymobley]

Here are the specs on the output transformer. I have it wired for an 8ohm and 16ohm output jack. The white, gray, and green leads on this transformer and not being used in the circuit.

Am I Overlooking anything here?

[maxkracht]

Is there any DC voltage on pin 8 of the rectifier tube? Double check your wiring on the rectifier, choke, and standby switch. With the rectifier tube removed, you can more safely test AC voltages on the rectifier tube socket from the outside. You should have 5VAC across pins 2 and 8. Measuring from ground, you should have a bit over 330VAC on pins 4 and 6.

[Codymobley]

Is there any DC voltage on pin 8 of the rectifier tube? Double check your wiring on the rectifier, choke, and standby switch. With the rectifier tube removed, you can more safely test AC voltages on the rectifier tube socket from the outside. You should have 5VAC across pins 2 and 8. Measuring from ground, you should have a bit over 330VAC on pins 4 and 6.

Pin 4 and 6 of the rectifier have 360vAC and pin 8 has 6vAC and Pin 2 has 8vAC with the rectifier tube out and it hovers around 1100vAC with the tube in.

[maxkracht]

Was just trying to check if the power transformer was wired correctly. 360vac from ground to pin 4 and 6 is good. The filament for the rectifier should be 5vac, across pins 2 and 8. Do not measure this from ground. One probe in pin 2 the other in pin 8, ideally measuring with the tube out so you can keep your hands outside of the amp for safety and to confirm you are measuring the socket and not the wire or solder joint.

With the rectifier installed, you should have a high DC voltage from ground to pin 8. If you don't have 300 something volts on pin 8, the rectifier isn't working.

Not sure what measurement you were referring to that's reading 1099vac. Measuring from pin 4 to pin 6 you should get 720vac, but I didn't ask for that because your meter probably can't read it accurately. My guess is 1kv is a meter error.

[LOUDthud]

The rectifier tube should have it's own 5VAC winding (usually yellow wires) on the power transformer.

Missed the data sheet you posted,

[jabguit]

Is the red wire coming from the recto tube socket the CT of the OT?

[Stevem]

When you do get it up and running that speaker jack right next to the 6SQ7 might make for a oscillation problem.

Also unless I missed it I only see one speaker jack, so how as you have stated do you have it wired for 8 and 16 ohms?

Also in your post from Friday you seem to be confusing mill volts and milliamps .