Amp breaking up later after rebuilding?

[Yoda]

Hello, everyone.

Like the title says. I handwired a ‘68 Custom Princeton Reverb reissue for a customer and they are complaining non-stop that the new handwired amp breaks up much later than the stock reissue amp did. I even gave them a NFB on-off switch.

The only thing that was odd about the completed build was the first preamp tube plates being at 310v rather than 160v per the schematic. I only had the Fender schematic to compare voltages against as I never had the 68 CPR operational in stock form to begin with. Could the high plate voltages be causing this?

The amp was rebuilt using a Mojotone AA1164 kit and reusing the OEM transformers.

Any ideas?

Thanks in advance.

[ViperDoc]

That'll do it. Check your power string drop resistors, the 18K is likely hooked up wrong or not at all on the cap can.

[Yoda]

Interesting because I did in fact test all resistor values and check them off the list before installing as part of my QC process for every build. But you never know, I am human after all.

Voltages were higher to much-higher than the schematic all across the amp, I can provide them if it would paint a bigger picture.

I no longer have the amp with me so here are some pics of the inside, no cap can because there was no hole for it on the Fender reissue chassis since they mount the filter caps in the main PC board:

[maxkracht]

Maybe you fed the screen supply to the preamp instead of the reverb?

[ViperDoc]

Looking at the AA1164 schematic, the C-node shows 320VDC and the D-node (preamp supply) shows 240VDC. Your layout photograph makes it a bit hard to distinguish whether the red or the yellow twisted wire is connected to either the Reverb driver or the Preamp plate resistors. Judging by your voltages, the preamp must be seeing the reverb supply voltage. Switching these two wires would fix that if that's the case.

[maxkracht]

the preamp must be seeing the reverb supply voltage.

That's what I said.

[Yoda]

Those are very plausible explanations guys, thank you.

I’ll look through the rest of my hi-res potato pics to figure out what power supply nodes are feeding what. I took pics of the front and back of the board after wiring before installing for my records.

[WhopperPlate]

Looks pretty clean and tidy either way , nice job

[Yoda]

Thanks for the kind words.

The more eyes the better:

[maxkracht]

hRCAnQ4flip.jpeg

Don't see anything wrong at first glance.

Could you post voltages? Were they loaded or unloaded? If unloaded, maybe everything is right and the owner is noticing a difference in bias?

[Colossal]

Looks pretty clean and tidy either way , nice job

I agree. Very nice work and the color pallette appeals to me. Good job.

[Yoda]

So these are the voltages with tubes in the amp, Variac set to 120.0v.

Exactly 6.3v on heater winding.

V1:
Pin 1: 310v
Pin 3: 1.5v
Pin 6: 310v
Pin 8: 1.5v

V2:
Pin 1/6: 465v
Pin 3/8: 9v

V3:
Pin 1: 200v
Pin 3: 1.6v
Pin 6: 208v
Pin 8: (didn’t write this one down apparently?)

V4:
Pin 1: 292v
Pin 3: 2.9v
Pin 6: 240v
Pin 8: 65v

V5 & V6:
480v on both power tube plates and 470v at both screens.

Where the original Fender schematic shows 420v/400v/320v/240v coming in off pin 8 of the rectifier and through the power supply nodes, my readings in this amp were 480v/469v/389v/309v. Hopefully that makes sense.

[martin manning]

The supply node voltage error grows (doubles) from power tube plates to preamp. Why is B+1 so high? What rectifier are you using?

[Yoda]

The rectifier is a JJ GZ34 that came with the reissue amp. The customer has some kind of NOS or ANOS 5Y3 coming in the mail to drop the voltages.

[sluckey]

As others have suggested, you have your board B+ wires connected to the wrong filter cap nodes.