Massive hum - B-15N

[txstrat]

Copied a B-15N and the thing hums like crazy. When I pull the first tube it gets way better but won't go completely (could live with that). Changed tubes to no avail.
Plate voltage is 433vdc, grids 426vdc.

Measured the quiescent current over one 1ohm 1w 1% resistor and the DMM said around 30mv (= 30ma) for both tubes (should be around 80 IMHO).

When I turn the volume up the quiescent current goes up to aroung 55ma. Changed the OT primaries and it got a little better (can turn it more up before the thing starts to squeal). Same behavior of changing quiescent current.
Anyone have an idea where to poke around next?

Heres the schem: http://www.schematicheaven.com/ampeg..._portaflex.pdf

[booj]

If it basically functions correctly, but just hums, it could be a layout problem. Where are your ground points, and what kind of grounding system are you using? Got any pix of the build?

[Enzo]

"Quiescent" means idle, no signal. Tube current may well increase during play, but it is no longer a quiescent reading.

Now if it is doing this with no signal applied, then you probably have a ton of RF you cannot hear coming through the amp. That could explain doubling the tube current.

Try pulling the mains fuse and clipping an AC ammeter in its place so you can monitor mains current draw. When you turn up your volume, does the mains current go up?

[Robert M. Martinelli]

Hi all,
I'd try to isolate the problem by pulling the preamp tubes first, then the PI tube...

Is the bias voltage OK? ( correct voltage/residual ripple?) as Matt stated, 30mAmps for both tubes is way too low, I'd expect something in the 80-90 mAmps too.

Another thing I'd check is the OT/output stage's symmetry - instead of using only one 1 Ohm resistor for both tubes, I'd try to use one 1 Ohm resistor on each output tube's cathode, this way you will be able to determine if the two tubes are "contributing" with the same amount of quiescent current...remember push-pulls are hum-canceling by design when working properly ( symmetrically ). If the P-P is not behaving symmetrically, this would be a good reason for the amp to hum.

Hope this helps

Best regards

Bob

[txstrat]

The hum is a deep low frequency kind of hum. It might be from the heaters (virtual tap with two 100ohms resistors).
Have poked around inside and moved every wire, with no change of hum at all.
I'm using a star ground on one of the transformer bolts. No isolated jacks. Inputs not on star ground.
That and the layout shouldn't be a problem, since I built this amp once before with almost the same layout and it doesn't hum at all (ok at least nothing to worry about). Pics are to come.

"Quiescent" means idle, no signal. Tube current may well increase during play, but it is no longer a quiescent reading.

Now if it is doing this with no signal applied, then you probably have a ton of RF you cannot hear coming through the amp. That could explain doubling the tube current.

Yep, that would explain the motorboating when I turn up the volume.

Try pulling the mains fuse and clipping an AC ammeter in its place so you can monitor mains current draw. When you turn up your volume, does the mains current go up?

Gonna measure this afternoon. If the wife gives me the time.

Originally Posted by Robert M. Martinelli

Is the bias voltage OK? ( correct voltage/residual ripple?) as Matt stated, 30mAmps for both tubes is way too low, I'd expect something in the 80-90 mAmps too.

Yes, the voltage is about -54 v. The schematic says -50 but I don't expect the additional -4 volts to be the culprit for the too low current.

Originally Posted by Robert M. Martinelli

Another thing I'd check is the OT/output stage's symmetry - instead of using only one 1 Ohm resistor for both tubes, I'd try to use one 1 Ohm resistor on each output tube's cathode, this way you will be able to determine if the two tubes are "contributing" with the same amount of quiescent current...remember push-pulls are hum-canceling by design when working properly ( symmetrically ). If the P-P is not behaving symmetrically, this would be a good reason for the amp to hum.

That's what I thought about too. Next thing I'm gonna do is using two 1ohm resistors.

One more thing that comes to mind is the OT. It's got two separate taps for the grids that I'm not using. I'm feeding the grids from the B+ after the first 1k resistor. Plus, I x-mounted the OT so there is no shielding between it's windings and the preamp.
I'll draw a layout and post it.

[txstrat]

OK here's the layout I'm using. I didn't draw every wire, but I think this is OK for an overview.

[Robert M. Martinelli]

Hi Matt,
shouldn't the probe resistor on the output tubes' cathodes be a 1 Ohm one? On the layout you posted it's indicated as a 100 Ohm ( 100R ) one....

If that resistor really is a 100 Ohm one, that would change the bias setup to a sort-of-mixed "fixed plus cathode bias" arrangement, elevating the cathode(s) from GND, thus making the grid(s) even more negative with respect to the cathode(s), and this would at least explain the low quiescent current IMHO.....

Hope this helps

Best regards

Bob

[txstrat]

Hi Bob,

no no, it's a 1 ohms resistor. I even changed one 1 ohms resistor for another. I just copied the piece in the layout software (Corel Draw) and forgot to rename it. Sorry.

[Robert M. Martinelli]

Oh, I see... so I'm just another victim of the "copy and paste" function!

Cheers

Bob

[txstrat]

OK here we go.
I monitored mains current with the DMM instead of the fuse. No change when I turned up the volume.
The idle current indeed changed while turning up the volume but I blame that on the hum driving the power tubes like a signal.
Changed the cathode resistors to one for each cathode. First one measures 14mv (ma) second one measures 18mv (ma).
Tried a different OT (this one not x-mounted but standing on brackets). Same hum.
Again poked around with a chopstick moving ALL wires. No change.
Took some pics of the wiring.
I even unsoldered the treble pot wiper and fed the signal to a separate power amp. No sound at all !?
Any ideas?
BTW the green wire with the black tube over it is the one I unsoldered. Hadn't soldered it again before I took the pics.

[loudthud]

How are all the grounds between the boards and near the front panel controls grounded? I see wires on some of the points but don't see how they get to the star. It's probably better if those all ground at the front panel jacks.

[booj]

On the illustrated drawing, I didn't see a ground wire from the IEC connector. The pix don't show the IEC connector. Is it grounded?

[txstrat]

Originally Posted by loudthud

How are all the grounds between the boards and near the front panel controls grounded? I see wires on some of the points but don't see how they get to the star. It's probably better if those all ground at the front panel jacks.

There is one blank silver wire from the input jacks to the volume pots ground tap. All preamp grounds go to this wire as well as the preamp filter grounds. The filters had a separate wire to the star ground before but the hum was the same.
For better understanding: all ground wires are yellow/green. Cathodes wires are yellow. Grids are green. Plates are blue. Heaters are white and brown. B+ is red. Others are black.
The preamp circuit is grounded at the input jacks. The filters, heater virtual tap, OT ground, power tube cathodes resistors, bias filter and stand by switch are grounded at one of the PT bolts.

Originally Posted by booj

On the illustrated drawing, I didn't see a ground wire from the IEC connector. The pix don't show the IEC connector. Is it grounded?

Yes, it's grounded. Thanks for asking.

One more thing came to my mind. I'm using pin 1 of the octal socket as terminal for the screen grid resistor. Could this induce the hum? The 6L6 (I'm using) layout shows no using of pin one so I thought I'd use this. I usually use pin 6 for this purpose but this time pin6 is the helping terminal for the control grid stopper resistor.

I took several more pics, if anyone needs to see more in detail.

[Enzo]

Well what I see, but I could be seing wrong, is in the first photo the main ground on a transformer bolt. But also it appears as if the standby switch ground wire runs to the bias control ground wire which then runs to something on the transformer and THEN all that runs to the ground lug on the bolt.

If that stop at the power transformer is the center tap, then the bias supply is sharing a few inches of ground wire with the B+. That means any B+ ripple will be added to the bias supply. I would give the bias supply its own wire to that bolt. If the standby switch grounds something as it looks like to me, then that switch ought to have its own wire to that bolt.

I just finished fixing up an old B15N a couple hours ago, took me a moment to understand yours looked so very different because you built it. The one here is just absolutely cherry on the outside. Good inside too, except for someone's replacement main filter flopping around inside on long wires. Thing sounded great but for some hum. I found this latter day main filter grounded to the speaker ground instead of the big can cap ground. Moved that over and hum gone.

[txstrat]

Aah, that's quite an information.
The something on the transformer is the ground tap for the 50v winding.
I suppose if I give that ole stand by switch (which grounds the bridge rectifier) a separate lead to the star ground that could be it.
Does it make sense to also use a separate wire from the transformer tap (50v ground) to star ground and from the bias pot to star ground?

[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]

Originally Posted by 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.

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

[txstrat]

Yeah, I though so too, but I was hoping they got lost over the resistors.
Can anybody explain this?

I changed the input jacks for isolated ones and split the grounding of the preamp and PI tube (these shared the ground before).
Put the ground for the preamp (input jacks) to the star ground at the PT bolt.
Can't say that all this helped a lot.

[loudthud]

Originally Posted by 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.

When you switch the AC mains off in the play mode or simply un-plug it from the AC mains, it better stop humming or the hum has to be coming from your guitar or your amp is possesed!

[txstrat]

I haven't put it right. I mean when I throw the stand by switch off play mode.

[txstrat]

I changed the preamp tubes (and sockets) to 12AX7 and finally found that the hum was indeed lead dress related. Changed the OT from x-mounting to outside the chassis and shorended the B+ to the OT and the wires to the output tube plates. The amps is now playable even though it hums when the volume is full open (doesn't motorboat or howl anymore). The hum is more high pitched now. Might be the preamp tubes which now have an amplification factor of 100 instead of 70 (6SL7).

[Tom Phillips]

A wire routing suggestion.
I noticed in one of the photos you posted that the primary wires from the OT were routed under the parts board and they were in close proximity to the input signal leads. This is a potential cause of undesirable coupling between the input and output stages. This may have been fixed during your latest changes. If not, it is an area for potential wire dress improvement. Did the lead routing change much when you remounted the OT? With your layout there is no easy clean rout for the primary leads between the OT and the power tubes but I think there is room for improvement.
Regards,
Tom