So... after biasing the cathode for the 64 mode and the fixed one for the 66 mode I cant get rid of a hum. It's definately grounding related. It is dead quiet with both volumes at zero and gradually increases with the volumes turned up.
I used the grounding system with all preamp going to the ground at the input,using a ground buss for the board and one for the 2 volume pots.. straight to the input ground. I used an isolated jack input and a dedicated ground lug near the input. PI and power amp grounds, speaker ground, filter cap ground and bias all go to a star ground at the power transformer side. Plus a dedicated ground for the IEC mains inlet.
So preamp ground and power amp ground are as far as possible away from each other.
This is the method I always use abd never gave me trouble.
Now I read that the Ampeg B15 has only one ground point, apart from the mains ground. Otherwise there will be a ground loop. Is this making sense? Is there something special about the B15 that requires the grounding like that? If so... is it a good idea to lift the star ground at the power TX from the chassis and run a wire from there to the input ground?
Sorry for the lengthy post, but thanks in advance!
Ampeg B15N build .. hum
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Re: Ampeg B15N build .. hum
I run a ground bus for all preamp stages and one for the power stage. And the dedicated mains ground of course. Dead quiet, never fails.
Try jumpering the pot ground to the preamp ground. If not successful connect the board ground also. That should be conclusive.
Try jumpering the pot ground to the preamp ground. If not successful connect the board ground also. That should be conclusive.
Tube junkie that aspires to become a tri-state bidirectional buss driver.
Re: Ampeg B15N build .. hum
I'm not exactly sure what you mean.TUBEDUDE wrote: ↑Fri Jul 22, 2022 12:22 am I run a ground bus for all preamp stages and one for the power stage. And the dedicated mains ground of course. Dead quiet, never fails.
Try jumpering the pot ground to the preamp ground. If not successful connect the board ground also. That should be conclusive.
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ChopSauce
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Re: Ampeg B15N build .. hum
Something like that - but the "Power Cord Green Wire" bolted appart...
... maybe?
EDIT (as I look at it more carefully:) isolated speaker/output jacks, really...
... maybe?
EDIT (as I look at it more carefully:) isolated speaker/output jacks, really...
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Re: Ampeg B15N build .. hum
That's how i did this one. I always do it this way.
Maybe it's those 6SL7 tubes that hum. The hum pot is doing its job well enough though.. there's another hum on top of that..and that's the one I'm trying to eliminate
Maybe it's those 6SL7 tubes that hum. The hum pot is doing its job well enough though.. there's another hum on top of that..and that's the one I'm trying to eliminate
Re: Ampeg B15N build .. hum
aren't the pot touching the chassis already before you add the ground buss?!
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Re: Ampeg B15N build .. hum
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Dr Tony Balls
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Re: Ampeg B15N build .. hum
Have you checked the frequency of the hum? Easy to do with a tuner app for a smart phone. The immediate assumption is that its grounding related but i'd check if you've got 60 or 120Hz hum. For instance it could be something as simple as forgetting to reference the filaments to ground.
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Re: Ampeg B15N build .. hum
Do you have test leads (short wires with alligator clips at each end)? If not temporarily solder wire instead.bieworm wrote: ↑Fri Jul 22, 2022 1:42 amI'm not exactly sure what you mean.TUBEDUDE wrote: ↑Fri Jul 22, 2022 12:22 am I run a ground bus for all preamp stages and one for the power stage. And the dedicated mains ground of course. Dead quiet, never fails.
Try jumpering the pot ground to the preamp ground. If not successful connect the board ground also. That should be conclusive.
You have several grounds.
Connecting the grounds together can reduce the hum, showing where a ground may have a small resistance allowing a hum voltage to develop. Test leads make this an easy way to test the grounds effectiveness.
Tube junkie that aspires to become a tri-state bidirectional buss driver.
Re: Ampeg B15N build .. hum
Hmm. That particular grounding scheme will have issues with rectifier-induced hum to some degree. The power transformer HV CT needs to go directly to the negative side of the first filter caps to keep charging-pulse induced ground voltage shifts off the signal ground.
Isolated speaker/output jacks are sometimes a good idea, depending on where the rest of the grounds to. In an ideal world (the one we never get to...
) the metal case/shell would have one and only one ground connection, that being a wire to the global star ground point. It's possible to make the input jack ground be the one-and-only-one for the chassis, but then you have to be careful about making sure no currents get onto the chassis from other places - including from magnetic induction from the PT. Low noise grounding is a game of knowing where the currents will flow, and the wire resistances connecting them.
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Stevem
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Re: Ampeg B15N build .. hum
In terms of buzz type of noise, have you tried laying out a big sheet of foil on a counter top and sitting the amp open side down on it?
The volume controls in each channel are after the first two gain stages so that nails it down to something common to both preamps.
You stated you have one hum on top of another, is the louder one close to 120 hz?
When your in fixed bias mode how are you lifting the two 270k resistors off ground?
The volume controls in each channel are after the first two gain stages so that nails it down to something common to both preamps.
You stated you have one hum on top of another, is the louder one close to 120 hz?
When your in fixed bias mode how are you lifting the two 270k resistors off ground?
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Re: Ampeg B15N build .. hum
No iI didn't yet. I'm aware of this possibility. I will have to examine that some more.Stevem wrote: ↑Fri Jul 22, 2022 9:47 pm In terms of buzz type of noise, have you tried laying out a big sheet of foil on a counter top and sitting the amp open side down on it?
The volume controls in each channel are after the first two gain stages so that nails it down to something common to both preamps.
You stated you have one hum on top of another, is the louder one close to 120 hz?
When your in fixed bias mode how are you lifting the two 270k resistors off ground?
As for the switching to fixed bias.. the dpdt switch switches the 270k resistors to the same ground where the 50uf cathode bypass cap and resistor would go to ground in cathode bias mode. I also have added the trinity trip top circuit for the 50V heater elevation before the choke of the Ampeg circuit.
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Re: Ampeg B15N build .. hum
Ok I made some progress with the hum.
Turned out to be a short in the shielded wire from the wiper of the heater hum pot. I attached the shield on 1 side to the back of the pot and clipped the other side + shrinked it off. On the 50V DC elevation I kept reading 0 V. Now I cut the shielding from the back of the pot and read 44V, so that's good.
Strange thing now is i can eliminate the hum when i turn the humdinger pot completely CCW. In the middle it hums, in ouder CW it hums even louder. I can't feed the heaters with 1V on one side and 5.5V on the other??? What is this telling us?
Oh, and i first pulled V1.. still hum. Put it back and pulled V2.. hum. With V3 pulled (PI) it makes no sound anymore..
Turned out to be a short in the shielded wire from the wiper of the heater hum pot. I attached the shield on 1 side to the back of the pot and clipped the other side + shrinked it off. On the 50V DC elevation I kept reading 0 V. Now I cut the shielding from the back of the pot and read 44V, so that's good.
Strange thing now is i can eliminate the hum when i turn the humdinger pot completely CCW. In the middle it hums, in ouder CW it hums even louder. I can't feed the heaters with 1V on one side and 5.5V on the other??? What is this telling us?
Oh, and i first pulled V1.. still hum. Put it back and pulled V2.. hum. With V3 pulled (PI) it makes no sound anymore..
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Stevem
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Re: Ampeg B15N build .. hum
What happens if you remove the heater bias up voltage altogether?
When I die, I want to go like my Grandfather did, peacefully in his sleep.
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!