1966 Fender Deluxe Blackface has me stumped?

[capndenny1]

I got this in a couple weeks ago. It had some mods that the owner wanted removed, and just generally fixed up for use. Had bad sockets that I managed to clean up, and tighten.

The power tube idle currents were way off from each other. One was 15mA and the other was 44mA. I put in another set I had for testing, an dthey were better, 25mA and 19mA. I got a new set of matched tubes and they are the same.

I have two remaining issues. The amp is pretty noisy, and it just isn't generating full power. I had to set the bias voltage to -45V to keep the currents down. If I set it to the recommended -34V, then my currents go well over 50mA.

It seems everything is working up to the phase splitter. It has a 12at7 there, and it tests out weak, so perhaps that is my issue? I have a 12at7 on the way.

What voltage should I see on the PI output circuit?

I may be fighting bad power tube sockets? All parts ring out correct, including the OT.

Kind of at a loss here. My customer is calling me every couple of days, and eventually he will get tired of, "I'm working on it!"

Any ideas are welcome.

[Dave H]

The phase splitter tube shouldn't matter for setting the bias (unless it's oscillating). One of the coupling caps from the PI to the power tube grid could be leaking. Try removing the PI tube then measure the bias voltage right on the grid pins of the 6V6s.

Edit: Also check the 6V6 screen voltages and screen resistor values.

[capndenny1]

Screen Grids are 470 ohm. Voltage is 420V.

Control grid resistors are 1.5K. Voltages are -44V the same as the bias voltage.

I will try using the bias current extender to make it possible to measure right at the tube pins. I chased an open socket pin before. measuring voltage on the bottom of the socket doesn't mean the tube is seeing it. Not so easy to find though.

[mytco]

Is the bias voltage consistent at the tube side of the socket? I have seen sockets broken internally where the soldered side of the socket was fine but the g1 pin of the tube saw no bias voltage at all.

Measure the voltage on each pin with the tubes out to check.

[capndenny1]

Thanks for the help.

I am ashamed to admit the problem was some sort of internal short in the Fender Telecaster I was using to test the amp. I used a signal generator and a dummy load to show that the amp was working just fine. Then I tried a different guitar. Load as heck, like a Fender should! It would take a long time to fix the amp, whenit ain’t broke!

I did a lot of other work too. I replace a weak PI 12at7 tube from Telefunken with an Electro Harmonix 12at7, and the amp loves that tube there. It just sounds fabulous now.

I think there was a wire rubbing on a screw that was part of the issue, but the guitar had me keep looking.

Anyway, I learned something new, I am a MORON!

[Jazz P Bass]

Don't be so hard on yourself!

My favorite 'moronic moment' was when a test jumper lead broke at the alligator clip.