More amp hum

[Blooze]

I finished up my little 6F6 build (only my 2nd guitar amp) and of course it hums like crazy with the volume off. Not sure if it's heater hum or a ground loop or what. My layout kinda sucks I know and lead dress is somewhat non-existant, but if someone could start pointing me in some sort of direction I'd appreciate it. I've shopsticked all the wires to no avail. Tried multiple preamp and power tubes. Hum happens whether using SS or tube rectifier (actually pulled tube rec to see). All the controls seem to work fine and the amp plays OK for what it is, just that hum.

I've redrawn the schematic to show exactly how I have connections made.

[Blooze]

..

[AlNewman]

What were you using as an original schematic?

The suppressor grid (pin3), of the 6SJ7 hooked directly to ground seems strange.

I may be wrong, but I'd try hooking it up to the cathode of that tube.  (Pin 5)

(Edit...It's weird that it has 31V...  I also wonder if there's a bad connection to ground.)

[Blooze]

What were you using as an original schematic?

The suppressor grid (pin3), of the 6SJ7 hooked directly to ground seems strange.

I may be wrong, but I'd try hooking it up to the cathode of that tube.  (Pin 5)

(Edit...It's weird that it has 31V...  I also wonder if there's a bad connection to ground.)

Was using this schematic. D-Lab and Blackcat Amps build them. I tied pin 1 to pin 3 because if you use a metal shell tube you’ll want the shell grounded.

Black cat has a video series building one.

I have the 31V on the wrong pin. Should be pin 6. I redrew the schematic and forgot to move it. Pin 3 is 0V. I've corrected it on the original posting. I just rewatched the video and they are both definitely putting pin 3 to ground.

I wonder if it's a ground loop because I have my input jack wired to the 6SJ7 cathode ground and it's grounded to the chassis. I could be the wire to the volume pot from the 6F6 I guess too, although I get the hum with the volume turned all the way off and there's no increase in the hum when playing.

[stratomaster]

If you remove the preamp tube do you still get hum?  I'm thinking proximity coupling between the OT and PT.  Did you perform the "headphone test" to establish the transformer placement and orientation?

[AlNewman]

I see 2 differences between the original schematic and yours.
1st is the tone stack.
2nd is the extra set of diodes at the rectifier. 

[Blooze]

I see 2 differences between the original schematic and yours.
1st is the tone stack.
2nd is the extra set of diodes at the rectifier.

The Blackcat version had the tone stack. He basically took the original D-lab version and added it. I added the safety diodes on the tube rectifier via Rob Robinettes advice, especially since 6ax5’s can be flakey.

[Blooze]

If you remove the preamp tube do you still get hum?  I'm thinking proximity coupling between the OT and PT.  Did you perform the "headphone test" to establish the transformer placement and orientation?

I did not. I just put them 90 degrees to each other as far apart as I could get them.

Removing the preamp tube does not change anything. Power transformer definitely vibrates slightly (you can feel it) at what feels like the same rate as the hum.


Definitely not the ground wire from the input jack either. Disconnected and no change.

I suppose it might just be the power transformer. I tried a different outlet on a different circuit in the house and no change either.

[stratomaster]

If you have a large steel sheet of some kind try placing it between the transformers while the amp is running. Make sure the sheet is grounded either by contacting the chassis or using an alligator clip.  If this affects the hum then you have magnetic coupling.  You'll need to alter the layout. 

Otherwise, the only other thing I can think of is excess ripple.  You can measure AC voltage at each power supply node.  Since you don't have common mode rejection with an SE amp, ripple and filtering have a greater effect on the hum.

[Blooze]

I will see if it’s the transformers interacting. Thank you for all the help and suggestions.

[bmccowan]

hums like crazy with the volume off

Whenever I have had that situation, and I have tried different tubes, as you have, it always seems to be that something that was supposed to be grounded is not, or something that is not supposed to be grounded is. Have you checked that out?

[Blooze]

hums like crazy with the volume off

Whenever I have had that situation, and I have tried different tubes, as you have, it always seems to be that something that was supposed to be grounded is not, or something that is not supposed to be grounded is. Have you checked that out?

I will recheck the grounds again and check every connection against ground to see if it's something that shouldn't be this evening. I did trace the schematic wire by wire with the physical layout and that appears correct. The only grounds connections I have that are different from the Blackcat builds are tying pin 1 of the 6SJ7 and 6F6 to ground. I figured with the possibility to use metal shell tubes it would be prudent. If it comes down to layout of the transformers I'll just live with the hum because I really don't want to try and rework the chassis and wiring for that. It's a fun little build and I wasn't expecting much from the amp so my expectations are pretty low. LOL, but reducing the hum would be nice. It's definitely 60Hz as it sound just like my soldering station or fluorescent lights humming.

[Jonas]

what is the "headphone test"

[Blooze]

what is the "headphone test"

My understanding is that you bolt down and hook up the power transformer to the mains (cap off all the other leads -SAFETY FIRST!) and then hook a pair of headphones to the output transformer secondary and move the OPT around until the hum is gone or at it's least. You can use a scope as well, although I don't own one except for the little handheld ones so not sure on that.

I probably should of did that with mine, but I'm not sure if I'd have to unhook all the power transformer secondaries to do it at this point. The folks here would know.

BTW, this is the hum I'm getting.   

[stratomaster]

Do you get hum from the speaker before the tubes warm up (especially the rectifier)? If so, then that's a fail for transformer coupling.

You can also pull the output tube and listen.

[Blooze]

Do you get hum from the speaker before the tubes warm up (especially the rectifier)? If so, then that's a fail for transformer coupling.

You can also pull the output tube and listen.

No hum until the tubes are warmed up. I pulled the rectifier tube and used the diode rectification with no difference. I have not pulled just the output tube yet. I’ll try that this evening when I get a minute.

[stratomaster]

The hum will be greatly diminished when on vs once the tubes warm up, so you'll really need to listen.  The comparison you want to make is off vs on before tubes warm up (or on with no output tube). 

[Blooze]

The hum will be greatly diminished when on vs once the tubes warm up, so you'll really need to listen.  The comparison you want to make is off vs on before tubes warm up (or on with no output tube).

There is no noise when first powering on, 5-10 seconds later it ramps up over 2-3 seconds to full hum volume and stays there. I'm assuming it's increasing in volume as the tubes are warming up. It's not a little hum you have to be within 1-2 feet of the speaker to hear, you can hear it across the room no problem. I will try again with the power tube pulled. If it magnetic, wouldn't it do it will all the tubes pulled as well?

[Blooze]

I pulled the power tube and no hum. I also pulled the ground to pin 1 and no change, so put it back. I also put two steel plates between transformers, grounded, for no effect. Maybe I need to use shielded wire to the volume pot? I have some Mogami W2944 console cable (26awg 2 conductor with a shield).

[stratomaster]

Try strategically grounding the signal along the path to identify where the hum could be coming from then.  Shielded wire could be the answer, but it's better to investigate first before just swapping things.

[Blooze]

Wouldn’t that be essentially what I’m doing when I’m checking voltages at the input grid, 6SJ7 plate and 6F6 grid?  Being fairly new to this and still learning I just want to make sure I’m not burning something up accidentally.

[trobbins]

As suggested, add a temporary link from 6F6 pin 5 (input grid) to local ground of 750R cathode resistor.  This checks if hum is before or after this point.

I don't see any comment about the hum level when changing the volume pot setting ?

Similarly, add a temporary link from 6SJ7 pin 4 (input grid) to local ground of 1k5 cathode resistor.  This checks if hum is before or after this point.

I can see your comment about pulling the input pentode, and the hum still remains.

[Blooze]

As suggested, add a temporary link from 6F6 pin 5 (input grid) to local ground of 750R cathode resistor.  This checks if hum is before or after this point.

I don't see any comment about the hum level when changing the volume pot setting ?

Similarly, add a temporary link from 6SJ7 pin 4 (input grid) to local ground of 1k5 cathode resistor.  This checks if hum is before or after this point.

I can see your comment about pulling the input pentode, and the hum still remains.

Thank you for clarifying for me. Yes, him is independent of volume control. Pulling the preamp tube had no effect nor did the rectifier tube. Pulling the power tube eliminated it. I’ve tried several different pre and power tubes to no effect as well, so it’s definitely something else than tubes I think.

It’s getting late here so I’ll try the latest suggestions tomorrow evening and report back.

Again, thanks everyone for your patience and willingness to help.

[trobbins]

As well as checking the shorted 6F6 pin 5 (as that would eliminate the volume pot and wiring grounding), can you measure the voltage on pin 8, as a way of confirming the idle bias current, and measure the dc resistance of the output transformer primary winding?  That can then confirm the 6F6 idle dissipation using the 250Vdc B+, and voltage drop across the primary winding.

One aspect of a single ended output stage is that any ripple voltage on the B+ feed is reflected to the speaker as hum (compared to a push-pull stage where the ripple voltage can be somewhat suppressed by balancing cathode currents).  If the speaker voltage has a dominant 100/120Hz hum signal then that may well come from the B+ ripple voltage.  You may be able to record or input the speaker signal to a soundcard and use software to look at the hum signal (ears are not good at discerning the difference between 50/60 and 100/120Hz).

In the old days there were some hum neutralising techniques used to suppress such a source of hum, but nowadays most people would add an LC filter stage between the first 47uF filter cap and the B+ feed.

[Blooze]

It's definitely power supply then. I captured the hum as best I could this morning with my iPhone and it's up around the 120Hz area. Compared to a test tone online and they seem to be similar.

https://on.soundcloud.com/LEUSsWyCTkh1pYYe9

I shorted pin 5 of the 6F6 and pin 4 of the 6SJ7 to their respective cathode grounds with no change in the hum. Pin 8 voltage across the 6F6 750R resistor was 20.8V with 123VAC on the mains. I'll have to desolder the B+ OPT primary wire to check resistance later, if necessary.

So, if you look at the schematic I drew the connections are exactly like that. For the power supply I have all the cap grounds tied together on the terminal strip with the PT center tap, then it goes to chassis at the heater divider ground.

[SEL49]

Easy to do... Move the OPT red wire to the junction of the 330Ω and 10K. This should provide better filtering (reduced hum) for the output tube B+. If you like this then consider replacing the 330 with a choke for even better filtering.

Many small SE amps have the OT and screens powered from the same B+ node. The Champ 5E1 is a good example.

[stratomaster]

Where is your HT center tap connection made?  I don't see it on the photos. But I could have just missed it.

Check the transformer datasheet to confirm the green/yellow wire is the heater CT taped off in favor of the 2x100ohm artificial CT and not your HT CT.

EDIT
Nevermind! I see it now

[Blooze]

Where is your HT center tap connection made?  I don't see it on the photos. But I could have just missed it.

Check the transformer datasheet to confirm the green/yellow wire is the heater CT taped off in favor of the 2x100ohm artificial CT and not your HT CT.

EDIT
Nevermind! I see it now

HT centertap is an orange wire in the photos.

There is no heater centertaps on the PT. There is a 6.3V/3A winding and a 6.3V/5V/2A winding.

[stratomaster]

You seem to be looking at ripple now as a potential cause.  I referenced earlier taking AC voltage measurements at the supply. Have you done this yet?  If not, then please take these measurements.

Also with respect to grounding I suggest moving the 10μF cap to the location indicated and removing the connection indicated by the X.  There's still potential for a ground loop with this change, but it's minimized in comparison with what you currently have.  Plus the change is easy to make. 

[Blooze]

You seem to be looking at ripple now as a potential cause.  I referenced earlier taking AC voltage measurements at the supply. Have you done this yet?  If not, then please take these measurements.

Also with respect to grounding I suggest moving the 10μF cap to the location indicated and removing the connection indicated by the X.  There's still potential for a ground loop with this change, but it's minimized in comparison with what you currently have.  Plus the change is easy to make.

I'll check the AC voltage measurements this evening. Removing that ground wire removes the ground for the 6F6 cathode though, but I could move the 10uF filter cap there for sure.

[stratomaster]

Then swing the wire over to re-establish the output tube ground.  The point of the exercise is to separate the preamp ground from the power amp ground. 

Better yet, swing it all the way to the CT ground connection and break the ground connection at the heater CT and reference that instead to the cathode for heater elevation and hum reduction.  Make sure the junction of the 100ohm resistors is not at ground before making the connection to the cathode. The fastener for that terminal strip may be grounding that terminal. Float them above if that's the case.

[Blooze]

Thanks again. I had thought about elevating the heaters, but figured it was my poor grounding scheme that would be causing the issue in the end.

I could do this: it includes moving the 10uF cap, the 6F6 cathode ground, and putting the 100R resistors on the last tab of the terminal strip (easier to add a wire than desolder the cathode cap/resistor), and moving the OPT HT primary wire to the second filter cap with the 6F6 grid.

Edit: now that I look I’d have to leave the ground wire from the HT centertap to the ground point where the heater resistors currently go, or drill and put a solder lug to chassis right there as the standoffs on the power supply terminal strip are insulated as they’re screwed on the transformer mounting bolts.

[AlNewman]

I agree with Stratomaster to try different grounding schemes, I was looking at the bus ground earlier and was suspicious if that could be a potential for hum.
One thing I would suggest when experimenting is to try one thing at a time, and then you (and us) can know the appreciable differences in hum between say a bus bar or a star ground, or with elevated heaters, (or both).
Your signal is also referenced to chassis through your pots, (according to your layout), which may be another possible source for a ground loop, although in my experience hum is more likely to be introduced by a poorly implemented bus bar, than a poorly implemented chassis return. 
Anyways, good luck.

[Blooze]

stratomaster, AC voltage at all the B+ nodes bounces between 0 and 80V.

I think I try elevating the heaters first, although all I have left at 100-220R at 2W Kiwame carbon films.

[Blooze]

I moved the 10uF cap. No change.
I elevated the heaters with 220R resistors. This cut the hum some.
I then moved the OPT primary to the 2nd filter cap. Maybe a slight decrease, but it’s hard to tell.

All my voltages have pretty much stayed the same except B+2 which dropped 5V due to the OPT on it now.

I am still gettting AC jumping between 0-79V.

Primary on the OPT measured 98ohms.

The only thing I haven’t done is use shielded cable or reposition the transformers and I don’t care enough to tear it apart to do that. This has been a great learning experience!

[stratomaster]

79vAC is a LOT of ripple.  You should be getting less than 10v on the first node then dropping off precipitously at each node thereafter. 

Shielded cable will not help you until you get your ripple under control.

That measurement is high enough to make me question how you measured it.  What meter did you use?  Does it have an RMS feature for AC voltage? Did you measure referenced to chassis ground? 

If the measurement is real then it's indicating a bad filter cap.  With the power off, lift the bleeder resistor (220k) then try to measure resistance across each cap.  If you find one with low resistance then I'd suspect an issue with that one. 

[Blooze]

Measured to ground. I don’t believe my meter does RMS.

[Willabe]

I then moved the OPT primary to the 2nd filter cap. Maybe a slight decrease, but it’s hard to tell.

No, move the OT PT CT to the 1st filter caps ground lead. And the 6F6 K R/C gets grounded at the 2nd filter cap/screen grid cap. That should help.

You want the 1st filter caps ground lead and the OT PT CT grounded together with NOTHING else grounded there. Then you run a wire from there over to the 2nd filter cap. 

And yes with a single ended amp, you want an extra stage of B+ filtering before the OT feed connection. Either CLC or CRC will work. But the PT CT still goes to that 1st B+ filter caps ground. 

But I think you also have too many random chassis grounds. You don't want to use the chassis as a ground wire, random chassis grounds. You want a wired ground scheme. Each filter cap is wired as a local ground star, then each local star is daisy chained/wired together.

You can ground the power amp local star at the PT/power tube end of the chassis and the preamp local star at the input jack. So, 2 circuit signal chassis grounds total. The power cord gets it's own chassis ground for the 3rd safety ground wire. Nothing else with that chassis ground. And not under a PT bolt chassis nut.   

Read this on grounding, NOT the star ground, you want the 'multiple star ground' it's right after the star ground drawing;

https://www.valvewizard.co.uk/Grounding.html
Edit; Sorry, it was late, I meant PT CT, not OT CT.

[Blooze]

Thanks for the reminders. If it comes down to tearing this whole thing apart I won’t because well, it’s not like this is a tone monster as much as a learning experience for me, of which I’m extremely grateful for all the folks that are helping me out.

There is no center tap. I attached a pdf of the transformers in an earlier post. There are the 5K, 7K plate connections and B+ lead on one side and the 0,4,8,16 on the other.

It was recommended above to try taking the B+ off the second node instead of the first.

I have three main ground points and safety ground, unless you count the mechanical grounding of the pots and jacks.  The safety ground is to its own lug. The PT CT and filter caps are to their own ground point. The preamp and input are to one point. The only other ground point is in the tone control. The large terminal strips are insulated from chassis.

The amp I copied of this had as many or more than that with no issues. I’ve read Valvewizards book, but obviously it didn’t sink in. Layout and everything is so much easier on a board than this.

[Willabe]

There is no center tap. I attached a pdf of the transformers in an earlier post. There are the 5K, 7K plate connections and B+ lead on one side and the 0,4,8,16 on the other.

Sorry, it was late, I meant PT CT, which I did say later.

The PT CT and filter caps are to their own ground point.

No, that's a problem.

You say you read Merlins book, then you should know that the PT's CT charging loop with the 1st B+ filter cap is the largest charging cycle in the amp, which means it's the nosiest. That large charging cycle can/will modulate the small preamp B+ filter cap ground. That's why you don't ground any other B+ filter caps with that ground connection. AND you don't ground the preamp filter cap(s) with the power amp caps.   

The only other ground point is in the tone control.

That should have a wire run over to the preamp ground. It shouldn't go through the chassis.

You want the least chassis grounds as possible. The ground signals running through the chassis can cross each other and modulate each other. Using a wire that can't happen.   

The amp I copied of this had as many or more than that with no issues. I’ve read Valvewizards book.

So you read Merlins book on grounding and used random chassis grounds and did not isolate the PT CT and 1st filter cap connection anyway? And now your build hums and you can't seem to fix it? 

[Blooze]

[

The PT CT and filter caps are to their own ground point.

No, that's a problem.

You say you read Merlins book, then you should know that the PT's CT charging loop with the 1st B+ filter cap is the largest charging cycle in the amp, which means it's the nosiest. That large charging cycle can/will modulate the small preamp B+ filter cap ground. That's why you don't ground any other B+ filter caps with that ground connection. AND you don't ground the preamp filter cap(s) with the power amp caps.   

The only other ground point is in the tone control.

That should have a wire run over to the preamp ground. It shouldn't go through the chassis.

You want the least chassis grounds as possible. The ground signals running through the chassis can cross each other and modulate each other. Using a wire that can't happen.   

The amp I copied of this had as many or more than that with no issues. I’ve read Valvewizards book.

So you read Merlins book on grounding and used random chassis grounds and did not isolate the PT CT and 1st filter cap connection anyway? And now your build hums and you can't seem to fix it? 

Yep. I’m hard of learning. That’s why I came here. To learn, not get a lecture. Not everyone is an expert at first, but people sure think you should be on your initial attempts. Like I said, if I can’t fix the hum, oh well, it’s not the end of the world. I have 9 other amps I can use. So you can be helpful without being snarky, or let it go.

[Willabe]

It's not a lecture, your dancing around what I'm trying to show you. So I re-wrote it in response to your reply.

You can lead a horse to water......

[Blooze]

Well, this horse is old. And stubborn 

Here's what I doodled up.  I am a visual learner so that's what I go with.
I've redrawn what I have done (moved the 10uF filter cap to the preamp section and elevated heaters).

I drew the PT CT to the 1st filter cap ground. I drew the tone control ground to a wire going all the way to the preamp ground (I'll have to try and figure the best way to do this, but I just added a section to the terminal strip in the drawing for ease of seeing it). Moved the OPT B+ back to the first filter cap (although I don't know that this would be absolutely necessary). That leaves one ground at the three position terminal strip where the heaters come from and one ground at the preamp/input.

[Willabe]

That's much better. And it didn't take much to do.

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.

[Blooze]

Thank you Willabe. Sorry if I came across wrong.


That’s all easy enough. One concern I have is how to possibly accommodate the ground at the tone control circuit without rebuilding it, although that may not be possible. I don’t believe I have any more of the .0047uf caps in case I destroy one trying to remove it if I need to move everything to a new terminal strip, but I’ll have to check.

[Willabe]

That’s all easy enough. One concern I have is how to possibly accommodate the ground at the tone control circuit without rebuilding it, although that may not be possible. I don’t believe I have any more of the .0047uf caps in case I destroy one trying to remove it if I need to move everything to a new terminal strip, but I’ll have to check.

The tone control ground looks fine in your last drawing.

But, you have a ground loop set up with the input jack and the terminal strip chassis ground.

Either the input jack or that terminal strip needs to be isolated from the chassis.

Once that's set right, hopefully the hum will be gone, or greatly lessened.

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

[Blooze]

I’ve moved the PT CT and the ground for the .0047uf tone control cap (I didn’t have any terminal strips that would work so I just cobbled a two tab one on there - doesn’t look any more janky than the rest of the amp which looks like a 2nd grader built it). No difference in hum.
Could be the input jack but I don’t have any shoulder washers right now. I did lift the ground on the 6JS7 terminal strip though so the only ground was at the input jack and it made no difference.
Could the reservoir cap is bad, or the impedance selector to close, or transformer orientation, or the need for shielded wire to the volume pot. I have a few extra reservoir caps left so I could swap that out. I don’t have a reliable way to check AC ripple to see if that’s it though. I did attempt to read the capacitance of it in circuit and get 53uF. Not ideal and not sure it really says anything anyway.

[AlNewman]

Your amp looks really good, don't be disheartened if things don't work out immediately.
I know you're getting hum with nothing plugged in, but have you put a shield on the chassis, or has it been open on your bench?
Have you tried plugging it in to different outlets, or unplugging things on that circuit? 

[Blooze]

Your amp looks really good, don't be disheartened if things don't work out immediately.
I know you're getting hum with nothing plugged in, but have you put a shield on the chassis, or has it been open on your bench?
Have you tried plugging it in to different outlets, or unplugging things on that circuit?

I have set it upright sitting on the bottom cover with no difference as well as taken it in another room on a different circuit to effect. I do have a fluorescent light over my workbench so I’ve experienced some hum from it, but typically turning it off is very noticeable. That doesn’t have any effect. My guess it’s it’s probably the impedance selector being too close to the input or the transformer placement. I should have placed them by testing but I didn’t. I wish I had a good way to test the ripple at the reservoir cap to see if it’s bad. Tearing it out will suck as I’ve tried to make good mechanical connections on everything I could.

[AlNewman]

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.