Massive hum - B-15N

[Enzo]

I look forward to the results either way, but do get that B+ return out of the bias suply wire.

As to the bias control and bias tap ground, I'd say offhand not to forget it, but since almost no current flows through that circuit, I doubt it contribues much ripple current. SO you are probably fine leaving that part alone.

If it were mine and I was feeling especially official, I think I would have run the bias winding ground up to the bias filter and pot ground as now, and then run my bias ground wire to the bolt from there - from the bias filter ground instead of at the transformer. In other words, now there are two Y/G wires, from bolt to trans, and from trans to pot. I would have replaced the trans to bolt wire with a longer pot/cap to bolt wire. I doubt it matters though.

[txstrat]

No luck. The hum is still there and worst of all, it hasn't changed a bit.
When I turn up the volume the hum increases up to a certain level and then the amp starts motorboating, like it has a hickup.
I'll go through the schematic again, to see if I overlooked something.

[txstrat]

Checked the amp against the schem and it looks ok to me.
Found a difference between the schem drawn by Joe Piazza and the Ampeg schem (bleeder resistor on the bass pot wiper). Took it out to no avail.
I've added more pics. Maybe someone could reveal a fault.
Only thing I can think of now would be a bad component since the hum doesn't change at all when I move the wires.

1. bad preamp tubes (those are the only 6sl7 I got right now so I had only replaced one by the other to see if it makes any difference).
2. bad PT (although it ran with no issues in the amp it was in before)

[tubeswell]

So it hums with no tubes in at all? (sorry for my perpetual slowness)

[txstrat]

No no, if I pull the preamp tubes, it's quiet. Well, there's a little hum left, but if it was only this, I could live with it.
With preamp tubes and volume down it's to some degree louder but starts to get real noisy if I turn up the volume.

[tubeswell]

Hmmm... (no pun intended )

So if changing the pre-amp tubes out didn't improve things, then it could be maybe in the grounding of the pre-amp or some unwanted signal coupling perhaps?

Or maybe if there is some hum there even when the pre-amp tubes are out and putting them in increases the hum, then maybe the hum is coming from the heaters?? Is the output tubes-only hum the same sort of hum as the pre-amp tubes-in hum, only quieter? Have you tried elevating the heaters or using a humdinger? (stabbing in the dark)

[txstrat]

Heaters is what keeps coming back to my mind.
Hum is the same kind like preamp tubes in. I haven't considered elevating the heater yet, cause I think the hum is too bad to be cured by elevating.
It sounds (with the preamp tubes in and turning the volume up) like if you touch the tip of a plugged in guitar cable.
Have already measured the 100ohms resitors (if one of them went out of spec) but they're ok (each shows 50ohms in circuit).
Thought about changing the heater wires on the preamp tubes vice versa. Don't know if it makes a difference. On a 12AX7 it certainly does - so far I've read.
I even consider to change the sockets for noval and test some 12AX7 tubes, just to be sure.

[loudthud]

Listen carefully as you switch the amp from play to standby mode. If the hum slowly fades out, it's from the heaters, bad tube(s) or a wire too close to the AC wiring. If the hum stops immediately, it's a grounding issue with the filter caps or input jacks.

[txstrat]

The hum fades out in a second or so. It surely doesn't stop immediately. Sort of like when you play and switch the amp off.

[tubeswell]

Well that probably narrows it down a bit (and dare I say confirms earlier suspicions) How are the heaters grounded?

[txstrat]

Have two 100 ohms resistors as virtual tap. This has worked well in all other amps I built so far. All my PTs had no CT on the 6.3v winding. I never had any issues with this method.

[tubeswell]

Maybe try a humdinger pot?? Or is one of the 100Rs bad under load? What is the heater voltage (on each side of the artificial ground reference)?

[jrfrond]

I suspect three things:

1) Bad 6SL7: Just saw one not too long ago in a B15N. It was replaced by the owner prior to coming in for service.

2) Jack Grounding: you may have a ground loop. Ampeg used to float the input jack grounds while grounding the speaker jacks. Too many techs have gotten burned with hum after replacing bad input jacks and not including the isolation washers.

3) Since it is a scratch build, just make sure that your heater center-tap is NOT grounded to the same point as your signal input. In this case, the hum is induced as current fluctuations at the ground point.

[txstrat]

The heater voltage pin to pin is exactly 6.3v. The voltage from each of the virtual tap resistors to ground is exactly 3.0v.
The input and speaker jacks are all grounded, but this is like the first copy I built of this circuit. The first one showed no issues.
At first I had no grounding wire for the preamp. The input jacks were the only ground. Changed that and put a wire to the star ground but it didn't make any difference.
When I pull the preamp and PI tube there is absolutely NO hum (at most very faint when you put your ear right next to the speaker).

[Tom Phillips]

The heater voltage pin to pin is exactly 6.3v. The voltage from each of the virtual tap resistors to ground is exactly 3.0v.

I'm not saying that this is causing your problem but the math doesn't add up. Where are you loosing the missing 0.3V?
Tom