'78 Princeton(non-reverb) Humming

[cyborg_stew]

The schematic is a B1270, but I was surprised to find that the earlier blackface AA964 is exactly the same. Makes me wonder what all the "blackfacing" hype is all about, especially for a Princeton.

I didn't know that voltage on the grid was normal for this type of circuit. I still have alot to learn.

And the power tube theory that Dave mentioned makes alotta sense to me. Maybe it is just a power tube problem. They're only a year old though, so I wouldn't expect them to behave that way. I don't push the amp too hard (usually play at lower volumes), and they're not biased much over 70%. Maybe there is a slight problem in the bias, tremolo, or power section that is causing them to wear out faster than they should?

[Helmholtz]

Makes me wonder what all the "blackfacing" hype is all about, especially for a Princeton.

Well the BF uses a GZ34.

Can you post the B1270 circuit?

[cyborg_stew]

This is a photo of one that was being sold on reverb.com. I couldn't find any online. The file name has "edited" in it, because I altered the photo, so it would print better.

I didn't notice the rectifier was different in the AA964. lol. Still, the voltages and component values are the same as far as I can tell.

[Helmholtz]

the voltages and component values are the same as far as I can tell.

If B+ is the same the PT secondary voltage must be higher with the 5U4. The difference is the slow heat-up of the GZ34 (as opposed to the 5U4), which would prevent the turn-on hum surge.

Also the BF had a lower value bias filter cap which allows the bias voltage to build up faster.

[cyborg_stew]

Could the problem be something as simple as a bad rectifier? From what I understood, rectifier tubes either work, or they don't.

[Enzo]

They're only a year old though, so I wouldn't expect them to behave that way.

ANY tube can fail at ANY time. New or twenty years old, doesn't matter. New is no guarantee of anything.

rectifier tubes either work, or they don't.

Well, two elements can short together. One side can quit, the heater can die. It can just get old and weak.

[g1]

they're not biased much over 70%. Maybe there is a slight problem in the bias, tremolo, or power section that is causing them to wear out faster than they should?

That sounds pretty hot for a Fender to me. Especially with that kind of trem. I'd think the factory set-up closer to 50%. I'm assuming you put a bias pot in place of the 27K? What kind of % dissipation do you get with bias pot set for 27K ?

[DrGonz78]

Could the problem be something as simple as a bad rectifier? From what I understood, rectifier tubes either work, or they don't.

Had a 5Y3 in an old 50's Silvertone amp that was faulty. I recapped the amp, replaced dropping resistors, and some plate resistors. I got rid of most of the bad noises I was hearing but I still checked the 5y3 on my tube tester on a hunch. All the short lights were flickering and it was obvious that tube was getting ready to die. Funny thing is that the tube was still working in the amp. Voltages were good and the amp sounded pretty good, although better with a new tube. So I guess they can work, not work and work too much.

[cyborg_stew]

That sounds pretty hot for a Fender to me. Especially with that kind of trem. I'd think the factory set-up closer to 50%. I'm assuming you put a bias pot in place of the 27K? What kind of % dissipation do you get with bias pot set for 27K ?

That's a good question. I've never measured the resistance of the trim pot; just adjusted it to get the current to where I like it. I'll have to check.

[cyborg_stew]

ANY tube can fail at ANY time. New or twenty years old, doesn't matter. New is no guarantee of anything.

That's true. I'm just wondering if there is a problem somewhere in the circuit that's putting more stress on the tubes in some way, and maybe that problem is causing the amp to hum.

[cyborg_stew]

Well, I installed a CE cap can in place of the original, and the hum is completely gone now. However, I was checking B+ voltages at the can, and 2 out of the 4 capacitors are reading significantly low. (They're numbers 3 and 4 in the circuit. The first 2 are fine.) The 4th filter cap supplies the plate voltages to V1. It should be getting 190VDC according to the schematic, and I only read 50VDC.

The voltage is normal (420VDC) after the 1k resistor. Then, after it passes thru the 18k it drops down to 241VDC. Much too low for this point in the circuit. I checked the resistor value, and it checks out at 17.8k.

What could be causing this? I can't imagine that I did anything wrong installing the new can, and I have no guesses as to what's going on with the voltages. Any help would be greatly appreciated.

[52 Bill]

What could be causing this? I can't imagine that I did anything wrong installing the new can, and I have no guesses as to what's going on with the voltages. Any help would be greatly appreciated.

Bad preamp tube? Bad section of the new filter cap?

[Chuck H]

There is another 18k resistor following the first 18k in the rail. What is the voltage drop across THAT resistor?

[cyborg_stew]

There is another 18k resistor following the first 18k in the rail. What is the voltage drop across THAT resistor?

It drops all the way down to 76V after the last 18k resistor.

[cyborg_stew]

Bad preamp tube? Bad section of the new filter cap?

Yeah. It makes me think that one or 2 of the new caps is bad. I can't think of any other possibilities. But I'm new to this stuff, and I still have alot to learn.