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.

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.

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]

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.

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?