Ampeg B15NA clone with 60Hz buzz HELP

[jaydawg]

Howdy Folks,
I've built a single channel B15 clone that I need some help with. I have a 60Hz buzz that I'm struggling to identify. When I pull the preamp tube (V1) there's no change. When I pull the phase inverter tube (V2) the buzz goes away entirely. When I ground pin 4 of V2, the PI input, the buzz goes away entirely. When I ground the the low voltage side of the coupling cap coming off pin 2 of V2 the majority of the hum goes away. When I ground the coupling cap coming from pin 5 of V2 there's very little change in the level of buzz. When I run the heater's off a dc power supply the majority of the buzz goes away. So, to me, it seems like the buzz is being generated in the PI and is somehow associated with the heater supply. With that in mind, I've tried different configurations on the center tap of the heaters. I've chopsticked this thing to death. I've moved grounds around. I've rerouted high voltage wire. I've attached pics of the build, a pdf of the schematic, a graphic representation of the buzz and a pic of my heaters on my cheapy scope. I'm running out of ideas and could really use another set of eyes and probably some more experienced minds. If anyone can see anything in the pics or could suggest some troubleshooting methods, I'd really appreciate the help!

V1 Preamp 6SL7
2 121
3 1.4
5 169
6 2.64
7 34.8
8 34.8

V2 PI 6SL7
2 237
3 2.72
5 223
6 2.48
7 34.8
8 34.8

V3 6L6 Bias- 68.4 mA @ 409
2 34.8
3 440
4 442
5 0.12
6 446
7 34.8
8 34.8

V4 6L6 Bias- 67.6mA @ 398
2 34.6
3 441
4 438
5 0.228
6 443
7 34.5
8 34.6

Heaters 3.23ac / 3.23ac

Filter nodes (starting from first filter cap)
1 445
2 443
3 385
4 419

[Stevem]

Do you have the heater center tap biased up with DC by having it hooked up to the positive end of the output tube bypass cap like the stock amp has .

[jaydawg]

Yes, I do. I've tried it to ground and tried eliminating it and using two 100 ohms as an artificial center tap. It's currently tied to the positive end of the power tubes cathode resistor / bypass cap. Looks like it's elevated my ground ref to about 34.8 volts DC.

[jaydawg]

I feel like it's either a ground issue or the heaters are picking up some AC somewhere. Just struggling to figure out where.

[sluckey]

Disconnect the NFB resistor from the speaker jack. Any better?

[jaydawg]

I just disconnected the NFB resistor and did not notice a change. I also tried using an alligator clip to bring it in and out of the circuit, to compare, and there's not discernable difference.

[Stevem]

If you turn the amp off by the power switch while leaving the standby on does the hum immediately stop or does it fad away as the heaters cool off.

How much AC millvolts of ripple do you read with your meter at all the filter nodes?

Could you post up a picture of the top side of the eamp and in specific the relationship of the OT to the PT.

[jaydawg]

There is no standby.

Here's the AC measurments at the filter nodes-
1 4.7 V AC
2 40 mV AC
3 5 mV AC
4 4 mV AC

Is the 4.7 volts AC at the first node a clue?

[Stevem]

Something is not right with your power supply.

I just opened up my B15n and at the first filter node with with 1K 10 watt resistor I have only .246 mv of ripple.may be your filter is there is bad or that filter has a high resistance to ground due to a poor solder connection, or maybe one of your diodes is leaky?

Also I did not look at your gut photos yet, but the proper and best low noise way to hook up that first filter node is for the B+ center tap to go right to that first filter and then a ground should be run to the chassis .

[jaydawg]

Cool! We're on to something! I'm pretty sure I've got another two section cap can in my stash. I'll try swapping that suspect cap out tomorrow and take a closer look at the power supply in general. I'll report back with my findings. Thank you for taking to time to check out your B15 for comparison. I appreciate it!

[Astronomicum]

Couple of questions to clarify:

Jaydawg, I see a choke in your pics which is not used in the schematic you posted. Is the choke in your amp providing power to the whole amp or after the power tube tap?

Stevem, Was your power supply ripple measurement before or after the 1K supply resistor?

[jaydawg]

I used the choke (7H @ 150 ma DC) in place of the 1k resistor that connects the first and second nodes. The center tap for the OT is connected to the first node, Every thing else comes after the choke.

[Stevem]

I was right on that first filter node I believe because I was reading 480 vdc because I have a 5AR4 recto in mine.

I will check again later today in the afternoon.

Also check your filament wiring.

On the output tubes for example pin 2 should go to pin2 of the other and preamp tube wise all pin 7s should go to pin 7.

[jaydawg]

So I double check the filament wiring. Disconnected it from the 6.3v bulb and used continuity to confirm that they were correct. They are.

Then I replaced the two section cap can that contained nodes 1 and 2 and replaced it with another I had in the parts bin. Unfortunately, same results. Still have 4.7 vac at the first node followed by very low millivolt readings at the remaining three nodes.

Question, can I use my scope (max 400vdc rated) with the probe set to 10x on that first filter node to view the ripple and confirm that's what I'm chasing?

[Astronomicum]

Then I replaced the two section cap can that contained nodes 1 and 2 and replaced it with another I had in the parts bin. Unfortunately, same results. Still have 4.7 vac at the first node followed by very low millivolt readings at the remaining three
nodes.

You can calculate the expected ripple voltage at the first node. I like using calculators.

https://www.electrical4u.net/calculator ... lculation/

If you plug your numbers in (0.068A, 120Hz, 40uF), you will find that your reading is fine. Be aware that your reading is likely AC RMS so you need to multiply by 1.414 to get peak volts to compare to what you get from the calculator.

I would also think that if there was hum on this node which feeds the power tubes, it would most likely still be there when pulling the PI.

In your write up, you did not mention trying a different tube in V2? You mentioned that you grounded the feed to V2 and the amp went quiet. To confirm the origin of the hum, remove the grid wire from the socket on V2. If the hum disappears, the hum is coming from the preceding stage. If it remains, then concentrate on V2 as you had originally suspected.