More amp hum

[Blooze]

Looking at your schematic, compared to the original, you have a 220k resistor to ground before your 1st filter cap.
Others may have different ideas, but that could explain your ground hum, as well as your strange AC readings. 
I'd try lifting one leg of that resistor, and see if that helps.

It’s just a bleeder. I’ve used it on several other amps without issue, but I could test it.

[AlNewman]

It’s just a bleeder. I’ve used it on several other amps without issue, but I could test it.

I get the purpose, I just don't think I've ever seen a bleeder circuit installed before the 1st filter stage. 

[Willabe]

Since you moved the preamp filter cap where it belongs, you should have room for 1 more B+ filter cap and a 470 ohm dropping R. Add it to the power amp B+ filter cap terminal strip. And then take the OT B+ feed from the 2nd B+ filter cap. Feed the power tube screen from the 3rd filter cap. Leave the 1st filter caps grounding as it is.

We've found that this makes a big difference on SE amps. Many guys have done this with great results.

[Blooze]

I’ll have to see what I have on hand. Most of my electronics is lower voltage so I don’t tent to have many high voltage caps around. I know I have a couple more 47uF, but I don’t think I have any more below that. I’ll check tomorrow. If not I need to order some grill cloth so I could pick up some then. I should have left the original 10uF there, but had to cut it off and the leads are too short to reuse.

[Willabe]

The 10uF is too small, it's fine for the preamp.

For the power tube, you want at least a 20uF, 47uF would be better.   

[Blooze]

Thanks again. So 47uF, 22uf to the  then add a 47uF for the OT B+, and 47uF to the 6F6 grid. Gotcha. I’ll have to look at that strip and see what I might need to rework. That 10K resistor is in my way. I could hang another small strip off the other side on the one end, cap would be hanging over the rectifier tube though.

[bmccowan]

And if still suspecting the impedance selector,

Lets hope the output impedance selector is far enough away from the input jack. Input needs to be away from output.

you could easily remove the selector from the circuit and take the 8 ohm lead directly to the jack, taping off the other OT leads.

[Blooze]

And if still suspecting the impedance selector,

Lets hope the output impedance selector is far enough away from the input jack. Input needs to be away from output.

you could easily remove the selector from the circuit and take the 8 ohm lead directly to the jack, taping off the other OT leads.

That would be an easy thing to try. Just move the 8ohm lead to the jack and disconnect the common from the jack to see if that helps before removing everything.

[Willabe]

I get the purpose, I just don't think I've ever seen a bleeder circuit installed before the 1st filter stage.

Doesn't matter if it's drawn before or after that 1st filter cap. Makes no difference electronically.

You could put that bleeder R anywhere in the B+ power supply string. It will still work the same, it will bleed off the B+ dcv to ground after the amp is turned off. 

[Blooze]

is this correct? I'd be taking the screen B+ after the 470K?

[Willabe]

No.

The order should be ; 1. 47uF, 2. 47uF, 3. 22uF.
 
The screen grid R is only 470 ohm, not 470K = 470,000.

And there has to be a dropping R after the 3rd cap.

And 10K might be too large between the 2nd and 3rd cap. 10K might drop too much dcv. It might only need to be 1K.   

[Blooze]

Sorry, got lost for a second. The "missing 10uF 

You know, I should just stick to kits.

[Willabe]

That's still wrong.

Go from the 10K to the 3rd cap. Then from the 3rd cap to the 470K to the 10uF. 

And the order is still wrong; 1. 47uF, 2. 47uF, 3. 20uF.

[Blooze]

Thank you for your patience, although I’m sure I’m trying it at this point
I was misunderstanding what you said earlier.

[Blooze]

 This has been a shit day. I did remove the 8 ohm tap and put it directly to the output jack (removing the selector common from the jack first). No change in hum volume.

So, I'm probably going to have to order a handful of resistors and caps as I don't have enough to rebuild what I have and if I kludge it together it'll probably make it even harder to fix anything if needed. So I've tried to redraw the whole PS strip. I have them up on nylon standoffs at the moment that are screwed into the transformer bolts. There is just not any real estate to mount them anywhere else down there. I've tried to redraw it so I can go to metal standoffs instead. I used 3W resistors on everything currently, but I do have some 2W of all of them as well.

So this is what I have, noting that the 10K may need to be 1K instead. I'll just have to tack one in and see. So I have the B+ going to the first 47uF cap. A 330R to the 2nd 47uF cap and a tap there to the OPT B+. Then a 10K to the third cap (22uF) with a tap to the 6F6 screen. Then a 470R to the 10uF cap. 220K bleeder is back on the 1st cap as that's where it's convenient to fit.

[Blooze]

I getting ready to place another order for parts. I have all the high voltage caps 10uF, 22uF, and 47uF (5ea-400V), all the PS resistors including 220R, 330R, 470R, 1K, 10K, 220K (10ea all 2W). Is there anything else you might suggest I have on hand? Like I said, my stock of higher wattage resistors/caps is pretty limited, although I have a pile of 2W kiwame resistors from who knows when, but figured MOX would be better in the power supply.

[bmccowan]

I did remove the 8 ohm tap and put it directly to the output jack (removing the selector common from the jack first). No change in hum volume.

But you did not remove the 4 and 16 ohm leads from the selector? Likely does not matter, but easy to do
My suggestion is that you redraw your full layout as you intend to rebuild it and have Willabe have a look at it.
Stick with it - you'll get it.

[stratomaster]

Have you confirmed the caps currently installed are functional?  A while back I suggested lifting the bleeder resistor and measuring the resistance across each cap.  If you have a way to measure capacitance do that as well. 

I'm not convinced a new node beyond what is in the schematic entirely necessary without first confirming the power supply is installed correctly and functioning as designed.  That you measured 79vAC peak at every node implies it isn't.  Best I can figure is that the caps either have gone bad or aren't completely referenced to ground. 

[Blooze]

Have you confirmed the caps currently installed are functional?  A while back I suggested lifting the bleeder resistor and measuring the resistance across each cap.  If you have a way to measure capacitance do that as well. 

I'm not convinced a new node beyond what is in the schematic entirely necessary without first confirming the power supply is installed correctly and functioning as designed.  That you measured 79vAC peak at every node implies it isn't.  Best I can figure is that the caps either have gone bad or aren't completely referenced to ground.

I have not yet, but will try to find some time this evening. Note that my meter is not an RMS meter so I'm not confident that the measurement I took is validly in the ballpark.

[Blooze]

I did remove the 8 ohm tap and put it directly to the output jack (removing the selector common from the jack first). No change in hum volume.

But you did not remove the 4 and 16 ohm leads from the selector? Likely does not matter, but easy to do
My suggestion is that you redraw your full layout as you intend to rebuild it and have Willabe have a look at it.
Stick with it - you'll get it.

The only thing I was planning on rebuilding was the power supply at this point, but if the consensus is that the whole thing needs rebuilt I will consider it. The main reason I built the amp was that I had a handful of the tubes about.

Here is the power supply as I think will work like Willabe suggested. Please feel free to correct my butt again if necessary. Like I said earlier old and hard of learning sometimes.

[Blooze]

Have you confirmed the caps currently installed are functional?  A while back I suggested lifting the bleeder resistor and measuring the resistance across each cap.  If you have a way to measure capacitance do that as well. 

I'm not convinced a new node beyond what is in the schematic entirely necessary without first confirming the power supply is installed correctly and functioning as designed.  That you measured 79vAC peak at every node implies it isn't.  Best I can figure is that the caps either have gone bad or aren't completely referenced to ground.

I pulled the ground side of the bleeder and the CT lead and measured the 47uF cap resistance. It and the 22uf and 10uf all start at a low resistance , say 1K, then climb to above range on both of my meters within a few seconds. I shorted their leads and tried several times with the same results.

[Willabe]

The only thing I was planning on rebuilding was the power supply at this point, but if the consensus is that the whole thing needs rebuilt I will consider it.

Try the test stratomaster suggested 1st. Then if good, try adding that extra filter stage. Then if still too much hum, try what bmccowan said about lifting the 4 and 16 ohm OT secondary leads. I'd move that Z selector to the other side of the output jack, then it's away from the input.     

[Willabe]

I did remove the 8 ohm tap and put it directly to the output jack (removing the selector common from the jack first). No change in hum volume.

But you did not remove the 4 and 16 ohm leads from the selector?

The point of this is to get those OT secondary wires away from that preamp input tube socket.

Like I said above, I'd move that Z selector to the other side of the output jack.

AND I'd run those OT secondary wires over too and under the heater 220K faux CT R's. That way their away from the preamp tube socket.   

[Blooze]

Sounds like a plan. I’ll possibly need to place an order for more parts. Not sure I can get the caps off without making the leads too short.

On a side note I did add a shielded wire from the 6F6 to the volume pot, grounding the shield at the volume pot ground that goes to the input. No difference. Figured why not.

I think I’ll ditch the impedance selector all together and add some wire to the 8ohm lead to bring it under the heater CT and around like you suggested. Cap off the others and wrap them up someplace. At that point I might as well disconnect the OPT and try rotating it to see about hum. Can I just pull the tubes and check for hum, or do I need to disconnect all the PT secondaries? I’d really not like to do that.

[Willabe]

On a side note I did add a shielded wire from the 6F6 to the volume pot, grounding the shield at the volume pot ground that goes to the input. No difference. Figured why not.

That was good to try and take it off the possible problem list.

I think I’ll ditch the impedance selector all together and add some wire to the 8ohm lead to bring it under the heater CT and around like you suggested. Cap off the others and wrap them up someplace.

You can do that, or just move it along with the OT secondary fly leads to the other side of the output jack. 

At that point I might as well disconnect the OPT and try rotating it to see about hum. Can I just pull the tubes and check for hum, or do I need to disconnect all the PT secondaries? I’d really not like to do that.

I'd try moving that OT Z selector and it's wires 1st. See what happens, easier that rotating the OT.

[Willabe]

When you build an amp, either from scratch or from a known plan, you need to use known good amp layout, wiring and grounding schemes.

Like, keep input away from output, turn transformer cores 90 degrees from each other, use tightly/evenly twisted wire pairs where you can, including signal wires, use a good known wired grounding scheme, don't use multiple random chassis grounds. 

That way, you have many/most possible problems taken care off. It shortens the list of things to check. 

[stratomaster]

Have you confirmed the caps currently installed are functional?  A while back I suggested lifting the bleeder resistor and measuring the resistance across each cap.  If you have a way to measure capacitance do that as well. 

I'm not convinced a new node beyond what is in the schematic entirely necessary without first confirming the power supply is installed correctly and functioning as designed.  That you measured 79vAC peak at every node implies it isn't.  Best I can figure is that the caps either have gone bad or aren't completely referenced to ground.

I pulled the ground side of the bleeder and the CT lead and measured the 47uF cap resistance. It and the 22uf and 10uf all start at a low resistance , say 1K, then climb to above range on both of my meters within a few seconds. I shorted their leads and tried several times with the same results.

That should mean they're functional. What's the resistance to ground from their negative end?  I'm really having a hard time with why you have so much AC voltage at every node. I think the problem will persist until we figure that out.

[Blooze]

Because I only have two inexpensive meters (Innova 3320 and a Centech P35017) I’m not sure I’d trust either. They both will zero on a 9V battery onAC to 0.00VAC. Both are auto ranging, but the Centech lets me change ranges manually as well, so I tried it. On the 47uF I got 1.2VAc, 22uf 0.35Vac and on the 10uf 0.3Vac. It doesn’t bounce back and forth like the Innova I used before.
Honestly I’m not sure I’d trust either for this, but the Centech sure doesn’t bound around, I just have to set it to the range it likes.

[stratomaster]

Because I only have two inexpensive meters (Innova 3320 and a Centech P35017) I’m not sure I’d trust either. They both will zero on a 9V battery onAC to 0.00VAC. Both are auto ranging, but the Centech lets me change ranges manually as well, so I tried it. On the 47uF I got 1.2VAc, 22uf 0.35Vac and on the 10uf 0.3Vac. It doesn’t bounce back and forth like the Innova I used before.
Honestly I’m not sure I’d trust either for this, but the Centech sure doesn’t bound around, I just have to set it to the range it likes.

Ok, good. Those numbers look more reasonable.  1.2V ripple on the output plates isn't terrible on a push pull, but you don't have that benefit. 

For testing purposes you can temporarily install whatever cap you have with the appropriate voltage rating (10-47μF) in parallel with the first cap and see what happens to the hum level.  If it drops significantly--and you measure much less ripple--then the additional filtering pole will help. Don't leave that cap there long term or your amp will eat rectifier tubes.  You can flip to diode rectification for this test to be extra safe and spare your rectifier tube some stress and potential flashover.

[Blooze]

I paralleled a 47uF cap, no difference in hum, but the ripple cut in half.
I completely disconnected all the OPT secondary wires, routed them under the heater CT, reconnected the 8ohm and 0V connections to the output jack with no discernible difference in hum. Turned all the lights etc off just in case of hum from them (I always do this).

I can either order all new parts to rebuild the power supply, try reorienting the OPT, rebuild the whole thing from scratch with a new layout (maybe a board layout, but I would like to reuse the chassis and it's small), or call it a day I think.

[ac427v]

I would hate to see you redo all that beautiful solder work for uncertain benefit. I don't disagree with having two nodes of filtering for the OT plate to get the last bit of hum out of a single ended build. But I think your problem is more extreme than that solution. I suggest you revisit Stratomaster's efforts to reduce interaction between the Output Transformer wiring and the preamp circuit.  You are limited by the small chassis. One alternative is to drill a new OT wiring hole between V1 and V2. Run all the high voltage OT wires on the outside surface of the chassis. Protect the exposed wires in heat shrink or protective split loom tubing if you plan to have no cabinet.


The pilot light between the Volume and tone control... Not my preferred location because the heater/pilot wires close to preamp wires may add to your hum issues. How about moving the Tone control closer to the Volume and having the Pilot lamp at the high voltage end of the faceplate? Possibly using the current Tone Control hole?
Please don't "call it a day". You are too close to having a cool, unique amp!

[Willabe]

The pilot light between the Volume and tone control... Not my preferred location because the heater/pilot wires close to preamp wires may add to your hum issues. How about moving the Tone control closer to the Volume and having the Pilot lamp at the high voltage end of the faceplate? Possibly using the current Tone Control hole?

I would try this 1st, easier then messing with the OT. 

... reduce interaction between the Output Transformer wiring and the preamp circuit.  You are limited by the small chassis. One alternative is to drill a new OT wiring hole between V1 and V2. Run all the high voltage OT wires on the outside surface of the chassis. Protect the exposed wires in heat shrink or protective split loom tubing if you plan to have no cabinet.

If you could screw the OT to a small board for stability, then use gator clip leads, and tape the clip connections so they can't short out to each other or anything, to hookup the OT, this will tell you if it's the OT's location.

That preamp tube does have a steel shield, that should be stopping/helping shield out any stray magnetic fields. 

[Blooze]

FWIW, there is no pilot light currently. I can run it wherever. Things going into an old Bakelite radio cab so the controls are kind of where they need to be.  I think trying a different location for the OPT and rebuilding the power supply to my last layout may be the best bet.

Would I be able to do the “headphone” test without being all the PT leads? I can also clip lead like Willabe suggested, but I figure if I’ve got the whole thing n hooked anyway…

[stratomaster]

I, too, would recommend against going scorched earth and rebuilding.  Here's why--with this layout we've corrected/improved

• grounding
• lead dress
• heater elevation

We've eliminated as potential sources of hum

• supply ripple
• the entire preamp
• ground loops/contamination

That really only leaves layout (transformer coupling and the aforementioned pilot wiring) and environmental.

Re: environmental - are you completing the shield on the amp by covering the opening with something while testing?  I keep a baking sheet around my bench for such things.  It isn't perfect but blocks enough environmental noise for a quick test and is much safer than turning out the lights with an open live amp. 

[Blooze]

I, too, would recommend against going scorched earth and rebuilding.  Here's why--with this layout we've corrected/improved

• grounding
• lead dress
• heater elevation

We've eliminated as potential sources of hum

• supply ripple
• the entire preamp
• ground loops/contamination

That really only leaves layout (transformer coupling and the aforementioned pilot wiring) and environmental.

Re: environmental - are you completing the shield on the amp by covering the opening with something while testing?  I keep a baking sheet around my bench for such things.  It isn't perfect but blocks enough environmental noise for a quick test and is much safer than turning out the lights with an open live amp.

I have set the bottom plate on it.  My light switch is about 8 feet away from the bench, so I'm not near it when the lights go off. I usually turn the amp on/off through a lighted power strip as well.

[Blooze]

I ordered new parts for the PS, and most of the amp because, well why not have some extra 2W resistors on hand. Paid for 2 day shipping and they arrived at my local UPS hub this morning, but didn’t get delivered, so it’ll be Monday before I get them. In the meantime I might try to disconnect the OPT and do some rotation tests for hum.

[Willabe]

I have set the bottom plate on it.  My light switch is about 8 feet away from the bench, so I'm not near it when the lights go off. I usually turn the amp on/off through a lighted power strip as well.

There was a member here not too long ago that couldn't figure out what was causing noise in his amp.

Turned out to be a wireless router that was too close. 

[Blooze]

Contrary to what I’ve been told by experts, wind turbines cause issues too. We have several hundred surrounding our little town and I’ve had those things cause issues. They told me it was something in my house on the grid, but I used a battery powered amp outside and it was even worse. Short of a faraday cage I’m not sure you could stop it. It’s a very slight buzz/hum noise. Barely there, but it is. Messes with my hifi on quiet passages.

My routers are at opposite ends of my house, easily through 4-5 walls each way before they hit my little room.

[bmccowan]

Hey Blooze,
Wondering about those experts. Both the US DOE and WindEurope, both wind energy supporters, have for years published that wind turbines produce varying amounts of electromagnetic interference, as do all forms of electric generation. So do cell towers and other communication systems. It seems we are stuck with it. Maybe the tin foil hat people have got something, eh?

[Blooze]

Pulled the OPT. I’m pretty sure this is the cause of the hum. I have to move it at least 8” away from the PT to eliminate all the hum using headphones.  Best I can get is a 45 to the PT as far into the corner as it will go. I think I can utilize one of the grommetted holes. It will be slightly closer to the input tube by 1/4”. This was the lowest hum spot.  Actually, it was the only spot where there was any drop in hum. The b+ leads will be longer as well.

[HotBluePlates]

Contrary to what I’ve been told by experts, wind turbines cause issues too. ... I used a battery powered amp outside and it was even worse. Short of a faraday cage I’m not sure you could stop it. It’s a very slight buzz/hum noise. ...

I would say it as "high-voltage power lines cause issues."

Drive with your car radio on AM tuned in-between stations.  When you pass under high-voltage transmission lines, you'll hear tons of static/hash/etc.

People living near high-voltage transmission lines often have difficulty avoiding that noise, regardless of the power-generation-technology used to create the juice transported on those lines.

[Blooze]

I would say it as "high-voltage power lines cause issues."

Drive with your car radio on AM tuned in-between stations.  When you pass under high-voltage transmission lines, you'll hear tons of static/hash/etc.

People living near high-voltage transmission lines often have difficulty avoiding that noise, regardless of the power-generation-technology used to create the juice transported on those lines.

The worst part is we get none of the power from them. It all goes 350 miles northeast of us. The main transmission lines are all at least a mile or two outside of town. They did wreck our skyline for us though. That’s something.

[Blooze]

Ok, so hopefully get my new parts delivered today. This is what I am proposing for a layout. The OPT has already been moved to a 45 degree angle. I removed the impedance selector because it will just barely fit between the rectifier switch and the output jack. So I extended the leads on the 4 and 16ohm so I could coil them in the corner of the chassis along with the 5K plate lead. The drawing reflects the new PS component layout as well.

When testing the OPT position the best I could get was maybe about 1/2 the hum I was. It was hard to tell even with sealed earbuds. So I'm resigned to the fact that this is going to have some hum regardless. Like I said earlier, I had to have the OPT a good 8" from the PT in any location surrounding it for it to be completely silent. The chassis is only 9.5" so.... And this is a fun build going into an old radio chassis for the novelty of it all. The only other amp I've been considering building besides this one would be a Princeton of some flavor. At that point I just don't need any more and I'd probably go with a kit of some sort.

[Willabe]

I don't think you can put the volume and tone controls there. 

[Blooze]

They can't go anywhere else and still work with the radio chassis. Probably should of had the rear and front panel swapped so they would be closer to the tubes maybe.

See a few posts down from the first one.

https://el34world.com/Forum/index.php?topic=31742.msg354098#msg354098

[Willabe]

You'll have to try it and see what happens then.

[Blooze]

Yep. Technically the OPT maybe moved about 1/4" closer to the volume pot than it has been. I don't think it will end up any worse. I'll rebuilt the PS section and then it just is what it is I think at that point, noise and all. The fact that I can't get the OPT any closer than 8" from the PT in any direction without it humming isn't good in my opinion.

[bmccowan]

Does it get any better if you take the tone control out of the circuit?

[Blooze]

Does it get any better if you take the tone control out of the circuit?

I have not tried.

[Willabe]

.... I'll rebuilt the PS section...

Wait, I thought you said you already rebuilt the power supply and it's grounding?