New 5F8A HUMS!

[ellum68]

After weeks of assembly and much tension before fire up, I am NOT playing the new 5F8A Fender High Power Twin that I assembled. Everything was checking out fine until I dropped in the power tubes and flipped the switch. I was greated by a very loud hum out the speakers. I would say it's a similar to the hum you get if you plugged in an instrument cable and grounded it out in the palm of your hand, but loud.

I'm by no means a guru. I built a 5F2A, and rehabbed a few old Kustoms amps prior to this, but thats about it. The amp is a stock circuit with the exception of a virtual center tap on the filaments, resistors on the cathode grounds for setting bias, a push pull volume pot to jump the input channels, and shielded cable from the jacks to the preamp tube. Any place I should start at?

[Enzo]

Sure, start at the start and isolate the problem.

Pull the power tubes. Power up, it better now be silent or you have a failed or miswired output transformer.


Check the power supply. Measure the B+ on pins 3 and 4 of all the power tube sockets. SHould be several hundred volts DC. Now flip the meter to AC volts. Should not be very many volts AC.

Pin 5. Check for DC there on all power tube sockets. There should be about -40v or so. Andn flipping to AC volts, about none. What you get? If that -40 seems very low, did you remember to wire in the bias filter caps "backwards" with + end to ground? Remember the bias is a negative power supply.

All that OK? Install the power tubes but leave out the phase inverter tube. Still hum or not?

With all tubes in and the amp humming, do ANY of the controls have ANY effect on the hum sound?

[ellum68]

I really appreciate your reply on this. It made my day knowing I had some help. The amp is dead silent with no power tubes. All voltages checked out great. I've got ~470v where it should be, no more than 500mv AC, and -48v on the bias supply. I haven't pulled the phase inverter because something is bugging me. This is the first tranny I've had where it has a real center tap for the filaments. I have that grounded along with a pair of 100-ohm resistors off the pilot light going to ground.

[MWJB]

Do one or the other, 6.3VAC CT or virtual ground via 100ohms from each secondary, not both.

[ellum68]

Center tap removed with no change. I tried pulling pre-amp tubes on at a time. When I pulled the 12AX7 closet to the output jack, she fired up dead quiet. With all controls set to zero there is no change if I have all tubes in. I just checked all my pin connections and they sure look correct. Tried the tube in my 5F2A and it's fine.

[MWJB]

So the one next to the output jack (phase inverter) plus one of any of the other preamp tubes causes hum? Looks like it is coming from the preamp, any chance of some pics of the pot wiring, board and tube sockets. Describe your grounding scheme.

[ellum68]

I tried a few combinations and I came down to all tubes in but the phase inverter it will be quiet. It's LOUD as soon as the phase inverter is installed. Everything is on a common ground with the exception of the output jack, cathode grounds, and the shielding for input lines. Think having my controls wires close to the cap board wires would make for such a massive hum?

[ellum68]

I have moved the signal wires clear of the four wires that lead to the cap boards. Nothing has changed. Also, forgot to mention I added an adjustable bias setup and have a multi-tap (2 and 4 ohm) OT wired up to switchcraft 13a jack. Plug into the left jack and you get 4 ohm, the right jack 2 ohm, and you get 4 ohm if you plug into both.

[MWJB]

You have the main, screen & PI filter caps all grounded to the buss wire, with the bias supply ground, I would start by grounding both ofthese to a PT bolt. You appear to be grounding things to the buss wire just "as they fall".

At the power tube socket it looks ike your plate wire at pin 3 sticks straight up inthe air? Wires liking the plate connections should hug the chassis floor.

Do you have any pics of the preamp tube wiring?

[MWJB]

Aha! I see now that the "stright up" wire is the B+, still this shouldn't be hanging around at the front of the chassis.

Plate & grid wires at the PI are running very close to each other and crossing over each other, some separation would be good, keep th grid wires short.

[ellum68]

I moved the B+ over, spaced my PI leads better, and started messing with grounding setup. I got brave enough to let it run for a second or two rather than instantly switching it off. I accidently left a volume pot on full blast and noticed it made the hum much louder. However, turning the presence control back off from being wide open adding the most god awful sound to my hum. It's hard to describe. There's almost a squeal to it. I don't like having the thing running when I do that. I've got the pot out now and it seems to test fine on my meter. The body of the thing is some kinda zinc colored material thats god awful to solder too. I'm going to sand a patch on the back and try again. Lemme know if this narrows things down.

[MWJB]

Squeal? This suggests something more serious than hum.

Swap the OT primaries at 6L6 pin 3, you mighthave to OT polarity reversed.

[ellum68]

I didn't hear it again. I think its because I'm so brief with the time I have it on. I'm basically scared to death I'm causing some kind of damage if I turn it on. I'm not hurting it do you think? I got my nerve up and made a recording. All controls are wide open with the exception of the volume. Hum Sound The change in sound you're hearing is me turning the presence knob from 12 down to 1 and back. Should I be doing something to ground the PT transformer itself. Mercury has plastic bushings on the the screws that makes the tranny sit away from the chassis slighty. So frustrasting and depressing. I just broke the center plastic nipple of a 6l6 too. Regardless, I took my time and made I got it back in correctly.

[MWJB]

You need to check the Mercury legend, does it have a shield that requires grounding?

Disconnect the NFB loop from the speaker jack, what happens?

I think you have an oscillation issue.

You can't cure it if you don't have the amp on & listen to what effect the changes you make have.

[ellum68]

Sounds like a normal valve amplifier sitting two feet a couple computers and a flat screen TV now. I just raised an eyebrow and let out a "Fascinating" ala Mr. Spock. Sounds like we're zeroing on something finally.