Transformer hum?

[txstrat]

In my never ending story of my latest B15N project I have an issue I've never encountered before. Amp plays well so far BUT... there's a hum (low pitch) which starts as soon as I turn the power switch to "on". With the amp still in stand by mode I can hear the hum from the speaker.
When I switch into "play" mode with the volume full down the hum stays at the same level and increases with turning up the volume pot over 5.
Because of the hum being present even with the stand by in "off" mode I somehow assume it might be because of the transformers beeing too close together (induced hum from the PT to the OT?).
They are about 3-4 inches away from each other and in 90 degrees angle (no end bells).
Any ideas?

Thanks

Matt

[Tom Phillips]

Moving the transformers farther apart may help as they are on original B-15. However, the first thing I would try if to remove the OT mounting screws and change the orientation with respect to the PT, Listen while you are doing this and determine if you can null out the hum. Hopefully the wires are long enough to allow this. Let us know if that helps and we will go from there.

[MWJB]

Does re-routing the heater wires/primaries from the PT affect the hum?

[Robert M. Martinelli]

Hi Matt, Hi Tom!

I agree, that most likely looks like unwanted coupling between the two transformers, so as Tom says, moving them farther apart could minimize/cure the issue.

If you can't move them, maybe you could try to shield them (e.g. putting a mumetal sheet in between).

Cheers

Bob

[txstrat]

Moving the transformers farther apart may help as they are on original B-15. However, the first thing I would try if to remove the OT mounting screws and change the orientation with respect to the PT, Listen while you are doing this and determine if you can null out the hum. Hopefully the wires are long enough to allow this. Let us know if that helps and we will go from there.

The OT has open taps to which I can solder the wires to. I could lengthen them wires if necessary. I'll try moving the OT and come back to you after that.

Does re-routing the heater wires/primaries from the PT affect the hum?

I've moved the heater wires inside the chassis and it don't have any effect on the hum. The PT is mounted outside the chassis with the wires leading "into" it. I'll post some pics as soon as I'm back home.

And Bob,

yes, I thought of the shielding metal too but wasn't quite sure if this would be of help (also a little scared to put a piece of metal in between the live wires of the PT nd OT - as I said, no end bells, just open solder taps).

Thank you all

[Amp Kat]

Does the heater winding have a CT and if it does is it grounded good ?

[txstrat]

No CT but a virtual center tap (two 100 ohms resistors) "elevated" to the -50v bias supply like in the Ampeg schematic.
Don't make any difference if I use a +50v supply fed from the B+ via voltage divider.
Does make a difference to the worse if I just ground the virtual CT = BAAAAD HUM

[txstrat]

I've moved the OT as far as the existing wires would allow.
That was about double the distance to the PT. Turned the OT as much as possible too. Absolutely no change in hum.
Here's some pics.
Red and blue: primary wires (230vac) to the power switch
Black: primary wires (230vac) from the power switch to the PT
Red: secondary wires (300vac) from PT to rectifier
Red: B+ after rectification (423vdc)
Brown and white: heater wires
Yellow/green: ground wires
The PT has an internal shield which should be grounded via a copper bolt outside the PT (see pics 1 and 2).
I've used the ground wire for the bias supply for this. Should I use separate wires?

BTW the hum is even there when I pull ALL tubes.

[EFK]

Octal preamp!!! Humor me - wrap some aluminum foil completely around the preamp tubes and ground it w/ alligator leads.

I have a early GA20 w/ 6SJ7 preamp tubes and I swear that was the only way to get this crazy hum to shut up. SJ7s are supposed to be more sensitive in that respect than SL7s, but couldn't hurt to try and it doesn't cost anything.

Oops, just saw you said you pulled all the tubes. Oh well.

I can use all the help I can get here, but I would definitely ground a PT shield all on it's own to a PT bolt or to the first filter cap ground - that's always worked for me.

[txstrat]

Octal preamp!!! Humor me - wrap some aluminum foil completely around the preamp tubes and ground it w/ alligator leads.

I have a early GA20 w/ 6SJ7 preamp tubes and I swear that was the only way to get this crazy hum to shut up. SJ7s are supposed to be more sensitive in that respect than SL7s, but couldn't hurt to try and it doesn't cost anything.

I believe that's why Ampeg elevated the virtual CT of the heaters. I'll try your proposal anyway. Thanks.

Oops, just saw you said you pulled all the tubes. Oh well.
I can use all the help I can get here, but I would definitely ground a PT shield all on it's own to a PT bolt or to the first filter cap ground - that's always worked for me.

I'll do that and see what happens.

[EFK]

My GA20 had elevated heaters as well, run off the cathode resistor. Didn't make any difference - there was always this background hum that got louder as the volume was wound up. I completely rebuilt the amp, re-grounded, rerouted heater lines, moved the OT around the entire cab etc. I even put the three preamp tube heaters (2 6SJ7 and a 6SL7 PI) through a DC bridge rectifier, which quieted it down considerably, but the hum was still there when the volume was raised. The silly aluminum foil trick was just on a whim, and suddenly - sweet silence! 6SJ7 (the glass ones) have an external metal base shield that is internally grounded through the socket, so all I had to do was wrap the foil tightly and clamp it with a twisted wire to make sure it made good contact through the shield. But it does need to be grounded. I'm at a complete loss to explain *why* this works or where the hum is coming from. I have heard of other GA20 amps having this hum problem (not all of them), so it wasn't just me being over-sensitive to hum - it was quite a noticeable noise.

[MWJB]

Copper shield from PT can be grounded to the same point as PT CT's/main filter & bias supply.

Have you tried separating the heater primaries from the other PT primaries...run them through a separate chassis hole & grommet to the Pilot, run the heater wires right around the edge of the chassis.

If it hums with the preamp tubes removed...how can the preamp tubes be responsible for the hum?

[txstrat]

Have you tried separating the heater primaries from the other PT primaries...run them through a separate chassis hole & grommet to the Pilot, run the heater wires right around the edge of the chassis.

That's my next step.

[txstrat]

I have heard of other GA20 amps having this hum problem (not all of them), so it wasn't just me being over-sensitive to hum - it was quite a noticeable noise.

Sometimes I think it might be a hum which turns up occasionally like the hum in Fender amps when the heater wires are not all on the same tube pin. But there must be a way round of it since my first build was quiet (at least I can't recall it humming in a way which would make me wonder).
So here I am still trying to find the culprit.

Thanks for all your suggestions

[txstrat]

Still haven't changed the heater wire routing but what I did was disconnecting the heater wires and there was still the same hum from the speakers. Put 'em wires back on and disconnected the secondary HT wires. Still hum. Did the same with the bias supply wire. Still hum.

???