More amp hum

[Blooze]

Sorry, no. Stupid autospell! I just received the parts today. I did move the OPT and removed the impedance selector. I will rebuild the PS during an evening this week when I get a chance. The last drawing I posted is the layout as it will be when I’m done.
I think there is going to be some hum from the OPT proximity to the PT. It’s just going to be unavoidable with these transformers. It should be slightly better than it was, but it won’t be silent. I’ll be pleased if it drops to half as much.

I really do appreciate all your and everyone else’s input. It’s been a learning experience and even though I can sound discouraged, It’s just disappointment at not getting it better the first time. At least. Don’t have $1K wrapped up in the thing!

[Willabe]

Well, would have been nice to know how much if any adding the extra B+ filtering stage and fixing the grounding would have helped.

I was under the understanding that you did add the extra filtering stage and fix the grounding and it did nothing with lowering the humm.

[Blooze]

I’m pretty sure I’ve listed everything I’ve done step by step and the outcome. There wasn’t a good way to add the extra filtering besides just hanging off of clip leads. I did try everything Stratocaster asked me to try including testing all the capacitors and paralleling a 47uf with the one that was already there to no effect.

So since the original build I’ve redid the grounds as suggested, referenced the heaters to the output tube cathode, unhooked the impedance selector and rerouted the 8 ohm and ground from the OPT away from the input, tested the PS capacitors, and placed shielded cable to the volume control from the power tube. Nothing made a difference in the hum except moving the OPT to 45 degrees from the PT (and I’m not certain that will make a change as I haven’t rebuilt the power supply I tore out for the second time, I only did the headphone test while I had the OPT disconnected).

[Willabe]

Paralleling another cap will get you nowhere near adding a 2nd filtering stage. A second filtering stage will do at least 10x what the || cap would do.

It would have been fine to carefully just hang the extra cap and dropping R off gator clips temporally as a test.

Will see what happens when you do add the extra filtering stage now.     

[ac427v]

The wiring and pots for the volume and tone controls are very close to the OT and PT. You have not wanted to move them because the whole Idea of this amp is to use the cool radio chassis. But you need to find out the source of the hum and those controls are obvious suspects.

For starters, why not just unsolder the tone control wires and see what happens? If the hum stops, we can talk more about alternative solutions.

[Blooze]

The wiring and pots for the volume and tone controls are very close to the OT and PT. You have not wanted to move them because the whole Idea of this amp is to use the cool radio chassis. But you need to find out the source of the hum and those controls are obvious suspects.

For starters, why not just unsolder the tone control wires and see what happens? If the hum stops, we can talk more about alternative solutions.

I can try that once I get all the PS rebuilt.

[stratomaster]

Paralleling another cap will get you nowhere near adding a 2nd filtering stage. A second filtering stage will do at least 10x what the || cap would do.

Paralleling the cap was a troubleshooting measure in the interest of gauging the contribution of ripple to the hum experienced.  If halving the ripple by doubling the capacitance reduced the hum dramatically then that would be the rabbit hole to follow.  However I've suspected transformer placement as the root cause since the beginning. 

There's no reason not to add another filter node.  I was just trying to see how much of an effect ripple was having--that quick and dirty test told me it was a secondary contributor to the hum. 

[labb]

I am surprised that he has to place the OT 8” away from the PT to reduce the OT /PT hum.

[Blooze]

I am surprised that he has to place the OT 8” away from the PT to reduce the OT /PT hum.

Note what I did was remove all the tubes and powered on the amp with the OPT disconnected completely and headphones clipped onto the OPT 8 ohm output. It basically had to be at least 8" away from the PT to be completely silent and I did a circle completely around the PT, lol. The quietest it was when on the chassis was if I placed it to the end (not the top plate, but the side of the chassis farthest away from the PT (if that makes sense). It hums no matter where it's at on the chassis regardless.

[stratomaster]

I wonder if in retrospect a toroidal PT or lay-down OT, or both, would have been more suited to this chassis. 

[Blooze]

Maybe, but I have enough into this thing at this point for what it is  I think I'm probably just polishing a turd, but lets get the buffer up and running and make 'er shine! Next one I'll follow a definite prescribed layout/design and not just wing it.

[Blooze]

Rebuilt the power supply.
B+ 259V
B+1 250V to the OPT
B+2  245V to the 6F6 screen
B+3  245V to the 6SJ7

6F6 cathode 20.0V
6SJ7 cathode 1.395V
6F6 plate 249V
6SJ7 plate 86V
6SJ7 screen 32.3V

These voltages are with a 1K between the 2nd 47uF and he 22uF cap, and 470R between he 22uf and 10uf. Basically a 1K in place of the 10K originally there and then the 470R at the end. I’ll need to solder those two resistors in because I was just using clip leads to try them.

And, no hum. Nada. Not unless the volume AND tone are all the way up. Any normal settings and it’s good.

My pliers slipped and I chipped the 330R resistor (1st dropping). Do I need to replace this? I’ve never chipped one.

[stratomaster]

Your resistor is fine. Glad you've got a quiet amp now.  Go enjoy it. You've earned it.

[Blooze]

Your resistor is fine. Glad you've got a quiet amp now.  Go enjoy it. You've earned it.

Thanks for all your help. I would have still been floundering around with it!

[bmccowan]

Nice! Please share some pics when you get 'er all buttoned up.

[Willabe]

And, no hum. Nada. Not unless the volume AND tone are all the way up. Any normal settings and it’s good.

Glad you got it fix.

When testing the OPT position the best I could get was maybe about 1/2 the hum I was.

Going by your reply #92, ^^^,  I still think it's the extra B+ filtering stage. We've seen it happen many times here.

Your transformers are both shielded, end bells. And you had their cores at 90 degrees from each other. Should be fine. 

Leslie speakers amps have their transformers, their shielded, right next to each other and so do other amps and they don't humm.