Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Bieworm on September 24, 2025, 02:14:30 am
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Hey folks!
I have recently finished a clone of an Ampeg B15N Heritage. All works fine except there's a nasty static noise very prominent. I have built this amp before and that time I followed my usual grounding principles (preamp and pot grounds to a dedicated ground lug near the input / PI and power amp grounds near the power transformer and of course a dedicated ground for the AC mains)
This time I followed the advice of people who build these amps more often. The grounding advice is to make a ground buss for all grounds except the mains AC. This ground buss should be tied to the input ground point as single grounding position. and so I did...
But since I started her up there's a static and scratchy noise.
- I checked all tubes and I'm 99% sure it's not tube related. (swapped them for known good ones,..etc)
When I pull the PI tube the static is gone. That static noise is there when both preamp tubes are pulled and the pots have no influence on this noise.
- I checked all solder connections and reflowed nearly all of them, firmly pulling the leads so they are in contact with the turrets and lugs they're soldered to.
- I chopsticked throughout the entire amp and find no anomalies. Chopsticking and tapping components don't show any oddities.
Here's what I'm thinking... I think having a ground buss where the high current power amp and bias grounds is the starting point and ending at the input ground, with PI ground and preamp ground tapping in to this buss is contra intuitive. My gut feeling says it should be the other way around, so that the power amp grounds are the last tapping into the buss and from there to the single point of ground.
I could go and separate the preamp grounds and power amp grounds where the preamp will be grounded at the input ground point and the power amp at the opposite side of the chassis, near the power transformer. But I'm worried this will induce the ground loop this amp is notorious for.
I'm not going to manipulate the current ground buss until I get some advice or other input. The build is too nice to start changing things on the loose.
Side notes:
- almost all wires from the board to the post are shielded, with only one side of the shield soldered to the back of the pots.
- there's a heater elevation tapped from the 50V tap of the HT secondary winding (see schematic). This yields a rectified DC voltage of 68V. This connection is attached to the wiper of a hum balance pot on the heater wiring (not shown on the schematic)
- changes I made after taking these pictures:
-->the CC 0.5W (270k) resistors on the PI are now CF 2W (270k)
-->the cement brick 5W 220R resistor on the PI socket is swapped for a 1W CF 220R one.
--> for the eagle eyed folks: the large 220k resistors on the tone circuit and the bias dropping resistor are changed to the correct 22k ones
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What kind of static noise? Soundbyte?
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What kind of static noise? Soundbyte?
I'll make a recording tonight. It's the kind of static that makes you look at the tubes, but it's very unlikely that the 3 preamp tubes I tested in the PI show exactly the same problem. I have had this problem in other amps before and the solution resided frequently in changing some grounds. I suspect the grounding trajectory, but this being following the exact advice of people who are more experienced in building B15's made me do this layout. I have had an annoying hum problem in my previous B15 build that I had to tackle by turning the hum balancing pot completely in one direction (wayyy off center). I thought I would avoid hum problems when following that described grounding advice. Well, there is NO hum.. but static noise there izz
here's where I got the info from: the post of "Beans On Toast" : https://www.talkbass.com/threads/help-with-ampeg-b15n-project.708820/page-3
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Here’s a sound clip:
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I think I might be onto something. When I tap the ground buss of the pots it’s schratchy and audible. DC? But how do I check that?
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I think I might be onto something. When I tap the ground buss of the pots it’s schratchy and audible. DC? But how do I check that?
Inspect it carefully for cold/bad solder joints
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Solved! Turns out that the 3 brand new JJ 6SL7 are not working as they should. All 3 are microphonic!
I made some octal to noval adapters and used known good Voskhod 6N2P glass and the noise is gone.