Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Blooze on October 01, 2024, 03:25:28 pm
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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.
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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.)
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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.
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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?
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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.
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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.
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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.
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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.
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I will see if it’s the transformers interacting. Thank you for all the help and suggestions.
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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?
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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.
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what is the "headphone test"
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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.
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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.
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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.
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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).
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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?
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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).
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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.
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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.
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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.
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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.
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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.
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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 (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.
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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.
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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
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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!
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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.
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Measured to ground. I don’t believe my meter does RMS.
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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 (https://www.valvewizard.co.uk/Grounding.html)
Edit; Sorry, it was late, I meant PT CT, not OT CT.
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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.
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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? :think1:
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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? :think1:
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.
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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......
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Well, this horse is old. And stubborn :icon_biggrin:
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.
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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.
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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.
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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.
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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.
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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?
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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.
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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.
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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.
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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.
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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.
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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.
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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. :icon_biggrin:
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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.
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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.
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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.
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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.
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is this correct? I'd be taking the screen B+ after the 470K?
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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.
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Sorry, got lost for a second. The "missing 10uF :wink:
You know, I should just stick to kits.
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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.
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Thank you for your patience, although I’m sure I’m trying it at this point :BangHead:
I was misunderstanding what you said earlier.
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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.
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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.
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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 :dontknow:
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.
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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.
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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.
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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 :dontknow:
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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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!
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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.
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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…
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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.
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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.
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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.
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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.
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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.
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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? :icon_biggrin:
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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.
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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.
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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.
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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.
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I don't think you can put the volume and tone controls there.
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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
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You'll have to try it and see what happens then.
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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.
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Does it get any better if you take the tone control out of the circuit?
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Does it get any better if you take the tone control out of the circuit?
I have not tried.
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.... I'll rebuilt the PS section...
Wait, I thought you said you already rebuilt the power supply and it's grounding?
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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!
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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.
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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).
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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. :dontknow:
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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.
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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.
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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.
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I am surprised that he has to place the OT 8” away from the PT to reduce the OT /PT hum.
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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.
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I wonder if in retrospect a toroidal PT or lay-down OT, or both, would have been more suited to this chassis.
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Maybe, but I have enough into this thing at this point for what it is :icon_biggrin: 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.
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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.
(https://i.postimg.cc/Z9ZcNN3V/IMG-4988.jpg) (https://postimg.cc/Z9ZcNN3V)
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Your resistor is fine. Glad you've got a quiet amp now. Go enjoy it. You've earned it.
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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!
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Nice! Please share some pics when you get 'er all buttoned up.
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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.