Help identify new hum on 2 1/2 year old 5e3 build

[d. spree]

A 5e3 I built two and a half years ago has developed an annoying hum. She's perfectly playable I just don't like the hum.

Here's the thing. She's built from a tube depot kit with a PCB. She has had no problem until a week or two ago. The hum is a low hum and is present regardless of anything being plugged in and is not affected by the volume or tone controls. When they are all set to zero and nothing is plugged in (except the amp ha ha) there is that annoying hum.

I switched out each of the tubes and it made no difference. I swapped out the V2 socket as it seemed a bit loose and then I rewired the filaments and ensured that they are neatly twisted and are placed away from the signal wiring. I tried it with a different speaker. Nothing has changed the hum.

By the way I also checked for any bad grounding and any cold solder joints or bad connections. Nada.

So I am wondering what my next line of offense should be. I have built two 5e3 amps since - using Mission parts - and they both work beautifully. I could ignore this but its bugging me. Any advice will be really welcome.

[52 Bill]

If you've already changed the tubes, bad filter cap?

[Enzo]

At least start pulling tubes to see where it stops.

Make sure all jack and pot nuts are snug. In fact, I'd probably loosen then and retighten so the washers get a fresh grip on the chassis. Make sure the transformer mounting hardware is snug.

Pull the phase inverter. Hum remain or go away?

Chek the voltages on the power tubes. Is there B+ on BOTH pin 3 and pin 4 of each? Is the cathode voltage on pin 8 about what it should be? And is there about zero volts DC on the grids, pin 5?

The 5E3 schematic shows one side of 6v is grounded. Did you build it that way, or did you use a center tapped 6v and grounded the tap? And is it still?

[d. spree]

At least start pulling tubes to see where it stops.

Make sure all jack and pot nuts are snug. In fact, I'd probably loosen then and retighten so the washers get a fresh grip on the chassis. Make sure the transformer mounting hardware is snug.

Pull the phase inverter. Hum remain or go away?

Chek the voltages on the power tubes. Is there B+ on BOTH pin 3 and pin 4 of each? Is the cathode voltage on pin 8 about what it should be? And is there about zero volts DC on the grids, pin 5?

The 5E3 schematic shows one side of 6v is grounded. Did you build it that way, or did you use a center tapped 6v and grounded the tap? And is it still?

Thanks guys - will start checking tonight and I will respond.

[d. spree]

Well I've begun my checking in response to Enzo's comments. all the jack, pots and transformers are snug and properly attached. Pulled the phase inverter - the hum went away. Haven't checked B+ on the power tubes pins 3 and 4 will do that next. I'm not sure about the center tap grounding question. The filaments are grounded from pins 5 and 9 of V1 each one to ground with a 100 ohm resistor and grounded through a grounding lug attached to the screw that mounts the baord to the chassis - as per the tube depot instructions. This ground connection is solid.

[Enzo]

If pulling the PI tube stops the hum, then all the power tube stuff probably doesn't matter. The hum is coming either from the PI itself or from before it.

100 ohms to ground from each side of the heater string is what we call a "virtual center tap." The schematic for 5E3 does not include that, it is a latter day thing on Fender amps, that your clone has incorporated. I couldn't assume that.

What brand/type tube is your PI tube? That phase inverter has a fairly high cathode voltage, and SOME 12AX7s are not happy with that. You tried different tubes in the amp you said, but if the "wrong" 12AX7 was in that socket, and you tried another of that same type, it could still be at fault. Note this is not a defect in the tube, the tube would work fin in the input stage. It is just you can't use certain tubes in PI or cathode follower stages.

[d. spree]

If pulling the PI tube stops the hum, then all the power tube stuff probably doesn't matter. The hum is coming either from the PI itself or from before it.

100 ohms to ground from each side of the heater string is what we call a "virtual center tap." The schematic for 5E3 does not include that, it is a latter day thing on Fender amps, that your clone has incorporated. I couldn't assume that.

What brand/type tube is your PI tube? That phase inverter has a fairly high cathode voltage, and SOME 12AX7s are not happy with that. You tried different tubes in the amp you said, but if the "wrong" 12AX7 was in that socket, and you tried another of that same type, it could still be at fault. Note this is not a defect in the tube, the tube would work fin in the input stage. It is just you can't use certain tubes in PI or cathode follower stages.

Thanks again Enzo. The V2 tube is an elecro harmonix 12AX7 - and yes, I did swap it out for the same type and make. The V1 is a 12AY7. Tonight I will swap it for a different make - I have some JJs and probably a couple of other makes. Of course the weird thing is I've had this make and tube type in the V2 position for a year or two.

[Enzo]

It is only a possibility, could still be something else.

[pontiacpete]

I don't know if others have found this to be a problem, but the new !2AY7's that don't make noise seem to be hard to find. I've seen them go bad in a short period of time, less that a year.
I ended up putting in 12AX7s for V1 in the couple 5E3's that I put together.
If you can get some NOS AYs you probably will be better off, but they cost a bit more.

[d. spree]

I don't know if others have found this to be a problem, but the new !2AY7's that don't make noise seem to be hard to find. I've seen them go bad in a short period of time, less that a year.
I ended up putting in 12AX7s for V1 in the couple 5E3's that I put together.
If you can get some NOS AYs you probably will be better off, but they cost a bit more.

I like the electro harmonix 5AY7s and also have some NOS. No problems with noise in either. Problem here is in V2 or related circuit however not V1. Same tubes in smae positions sound fine in my other tweed deluxe.

[d. spree]

It is only a possibility, could still be something else.

Yes, tried 3 different makes of tube in V2 - an EH, JJ and tubestore premier 7025. No noticeable difference. I am wondering whether I start to swap out capacitors in that section of the circuit. On a visual examination there are no visible signs of a problem with the caps or resistors, but I have not pulled the PCB yet.