Tube Amp Archive

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 t...

Attached all of the B+ points and we have sound (sounds like a grounding issues, but still...), there is an issue in the preamp that the pop test revealed that I'll chase down but for now I'm happy with sound.

On your schematic, the 3H directly after the rectifier is a 3 Henry choke. If you do not have this in place, nothing but the power tube plates are getting power. If you do not have a choke, you could replace it with a resistor, somewhere around 2-5k 5W is probably ok. Edit: be sure you are measurin...

Where’s the power supply section of your hand drawn schematic? I don’t see how any of your power supply filter nodes are reviving and voltage from the rectifier where you said you have the red wire of the OT hooked up. There needs to more then that one wire there. If I'm looking at the schematic an...

I've attached the data sheets here for ease of reference.

Where’s the power supply section of your hand drawn schematic? I don’t see how any of your power supply filter nodes are reviving and voltage from the rectifier where you said you have the red wire of the OT hooked up. There needs to more then that one wire there. I only drew out the diagram to ass...

jabguit wrote: ↑Sat May 31, 2025 10:42 am Is the red wire coming from the recto tube socket the CT of the OT?

Yes, it is the CT of the output transformer.

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 a...

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...

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?

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 transf...

Stevem wrote: ↑Fri May 30, 2025 9:45 am 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.

jabguit wrote: ↑Fri May 30, 2025 10:57 am 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:

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 ...

Thank you for clarifying that. I'll look at the 5C3 a little closer.

jabguit wrote: ↑Wed Feb 09, 2022 12:01 pm

Codymobley wrote: ↑Tue Feb 08, 2022 2:54 pm

The V2 would be a 12AX7 acting as a para phase inverter just as in the 5E3, the rest of the circuit is unchanged.


Thanks!

5E3 has a cathodyne PI. A 5C3 has a paraphase PI.


cheers,