Ampeg B15N heritage build: noise issues

[Bieworm]

Hey folks!

In advance, sorry for the long winded post. But I'm trying to give as much info as possible.

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... (this appears to be the preferred grounding layout on the Valve Wizard pages by the way)
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.
- I have noticed there is a spot on the chassis that is intermittently microphonic when tapped.
- I also found that when I tap the ground buss of the pots it makes popping sounds, i.e. scratching the buss is audible at the speaker

Here's what I'm thinking...I'm pretty sure it's a grounding issue / DC on the ground buss of the pots?
- 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.
- before I start taking out wiring and components I would like to know the origin of the problem. Time to use the scope I guess.
- I also have noticed that when I play the 2nd channel the sound is getting warbly with the volume at 8 a further (possibly oscillation? might be a good idea to add some CC grid stoppers?)

any other advice or experience feedback is welcome

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 (proves you still MUST check the value of components before installing)

sound clip: https://youtube.com/shorts/jYrIOZxbrik? ... yjGSj4LtUZ

[JMPGuitars]

Highlighter verify the amp (I'm assuming again ) and check values while also looking for shorts. Especially look for shorts on and/or across capacitors.

Check for DC voltage on the pots.

My laziest assumption is that there's a shorted or miswired capacitor going to the pots. You might also have an oscillation to deal with, but start with that stuff.

Thanks,
Josh

[Bieworm]

Highlighter verify the amp (I'm assuming again ) and check values while also looking for shorts. Especially look for shorts on and/or across capacitors.

Check for DC voltage on the pots.

My laziest assumption is that there's a shorted or miswired capacitor going to the pots. You might also have an oscillation to deal with, but start with that stuff.

Thanks,
Josh

Thangz! Good advice

[Bieworm]

Ok I might be onto something. After reflowing all connections to no avail I altered the grounding to our usual configuration (PA near the PT and Preamp near the input)
I also checked all components for shorts.
Before swapping out components I needed to be sure it isn’t the tubes. How big is the chance that all 3 new 6SL7 are microphonic? Well, I have a feeling they are.
Important note: when 2 preamp tubes are pulled the amp still gets the static noise. It repeats this with each 6SL7 in that slot. Now I made an adapter from noval to octal and put in a good known 6N2P in the PI slot. No preamp tubes. Noise gone. I will make an extra adapter and check one of the preamps if it will give me a noise free sound.
Tube brand of conflict: guess? Yup!!! JJ.

[JMPGuitars]

I think we need to add this to troubleshooting basics:

Step 1: Remove and discard all JJ tubes.
Step 2: Highlighter test.

EDIT: In my signature now.

[Bieworm]

Well just as I thought… after testing with 2 noval to octal adapters and known good tubes.. 3 brand new JJ 6SL7 tubes ARE the culprit!!!no noise whatsoever witj these Voskhod 6N2P glass!!! I declare officially all JJ preamp tubes as crap, and additionally the EL84 too… which were already banned. Never have problems with EL34, 6L6 and 6V6 tho

[JMPGuitars]

so far.jpg

[Bieworm]

Still waiting for the 6SL7 tubes to arrive from Ukrain, but in the meantime I have finished the head cab.

[Bieworm]

Ok, the order from Ukrain arrived today. Immediately plugged the NOS Russian 6SL7 tubes in and behold: no noise, only wonderful tones. I knew that noise wasn’t my doing! Mostly I am pretty confident when I finish a build this simple that there is a big chance it works as it should. Unless there are components with hidden flaws in the circuit.
Cranking this amp with my SG is very, very Marshallesque!!! If you ever get the chance to play a B15N you must try cranking that with a humbucker guitar.

I will post a video soon! I Promise!

[Daviedawg]

Meantime, I was refreshing a lovely original Dominator Mk 2 for a friend. It had all original Mullards still in it which were past their best apart from the rectifier. He supplied a set of 5 JJs so he could keep the Mullards aside.
Guess what - nasty noises erupted after about fifteen minutes of playing it. Traced to the ECC83 in the phase inverter. First time I have had an issue with a preamp JJ. I had spare NOS ECC83s so fitted one. Now what a great amp it is. That duffer affected the whole tone. The others are fine.

Dd

[Bieworm]

Meantime, I was refreshing a lovely original Dominator Mk 2 for a friend. It had all original Mullards still in it which were past their best apart from the rectifier. He supplied a set of 5 JJs so he could keep the Mullards aside.
Guess what - nasty noises erupted after about fifteen minutes of playing it. Traced to the ECC83 in the phase inverter. First time I have had an issue with a preamp JJ. I had spare NOS ECC83s so fitted one. Now what a great amp it is. That duffer affected the whole tone. The others are fine.

Dd

Go figure.. a once highly regarded tube brand is now the least reliable of them all ime. The power tubes are perfectly fine up til now though..for me at least

[JMPGuitars]

That video I posted 12 years ago was EL84s. I think I returned 20 tubes or something like that from that order.

Were they all bad? No. Did I want to gamble? No way. It was basically half of all the JJ power tubes I received were junk (purchased wholesale from a reputable supplier).

[Bieworm]

Here’s the clip as promised

https://youtu.be/57JZ_Y3vbXs?si=hxypAUW8gpGgkKru