Loud hum in an AC30 : help !!!

[archaos]

Hi friends,

                I have one issue with one AC30/4 Top Boost I've just finished up rewiring : I get a loud 100 Hz hum (G note ; 3rd fret, 6th string on a guitar) when the treble pot is turned CW past the 12:00
                position. I can get either that kind of hum tweaking the bass pot in the same way, but it's far less loud.
                Annoying, even if "I had planned to be wrong about something", w/o quoting a famous forumer here  .
                No   for the moment...
                Attached, a gutshot, & here's the valva data chart : http://www.el34world.com/charts/valve/ValveData.php?e=view&f=11008, in case of, for the voltages seem OK...

                However that may be, both treble & bass pots work correctly, I mean they increase/decrease high & bass frequencies. Furthermore, the other pots : volumes, cut, work well, & don't produce hum.
                Hence my 1st question : do I have a grounding/heater supply issue, or a PS filtering issue ? The frequency of the hum make me lean toward the latter solution, but I'm not an expert @ all.
                I've already checked the solder joints, chopsticked the wires, tube sockets, components : no results.
                Likewise, I have checked the tubes, swapping both preamp & power tubes with other sets, & they're OK. Yet, the hum remains.
                Therefore, I've pulled the tubes one after the other & :

                  - hum goes away if :

                      * the PI tube is pulled
                      * all the power tubes are removed

                  - hum remains when :

                      * the 1st preamp tube is removed
                      * the Top Boost tube is removed

                That's why I was wondering if my issue concerned the preamp section. And if so, could it come from the cathode follower section or the PI section, or both ?

                Guys I need your help, because I just don't know where to search anymore.
                Thanks !
                  

[bobmegantz]

sounds like it may be a grounding problem - are the tone pots grounded to the chassis?

[archaos]

sounds like it may be a grounding problem - are the tone pots grounded to the chassis?

No they aren't, sorry that can't be seen on the pic. The Treble pot isn't grounded @ all, whereas the Bass pot has a wire connected to the ground buss of the volume controls.

[plexi50]

I was looking at the pots yesterday and was wondering if they were grounded with a buss wire. The hum your describing though could be a number of things. I would definatley go back over the power supply caps and make sure all your PS Nodes and resistors are where they should be. Ruling out any PS problems first gives a safer feeling that there is not lethal voltages connected where they should not be. From there retrace your layout with all the preamp tubes and finally the grounding you have used including the input jack grounding. An ungrounded input on either jack will have a very loud and deep hum to it.

(I get a loud 100 Hz hum (G note ; 3rd fret, 6th string on a guitar)

The hum as you are describing it sounds like there is high voltage where it should not be. Or a cap that is not wired or grounded correctly. Then again heater wire noise can sound like that

[archaos]

The 4 input jacks are grounded together with a buss wire connected both to a star ground system near the PT, & to V1's cathode bypass cap. The volume pots + the bass pot are grounded to another buss wire itself linked to the preamp ground (of the main components board). The hum I can hear doesn't sound as a the very loud hum you can hear in case of a ungrounded input jack.
However, I have large doubts as regards the node of the Cathode Follower PS cap, a 40 µF Sprague whose positive side may not be correctly wired to both PIN 6 & to the 100 K resistor of V2.
Indeed, while checking Doug's amplifier current flow, I've noticed my own DC plate path around V2 could be an issue. I'll do some tests tonight, & let you know.

[Leevi]

Some tips:

I think you should first localize the problem.

Disconnect the preamp from the power amp (before PI)
and solder a jack to the PI-input and plug a guitar to it.

If the problem is not there then it must be in the preamp.

Disconnect the tone stack from the signal way and bypass it.

Use similar bypassing method for preamp stages.

Measure all the critical voltages around the tubes like
- plates
- cathode
- filaments

Check that the filter caps have been correctly wired (polarization).

If possible separate the PT from the chassis and check if that has some effect.

/Leevi

[archaos]

The hum as you are describing it sounds like there is high voltage where it should not be.

I did think so, but after having checked all the voltages, everything is normal, no DC on the chassis or on the pots for instance.

Or a cap that is not wired or grounded correctly.

I have checked all of them & they're OK. BTW, I thought my Cathode Follower PS filtering cap wasn't wired correctly, but no, it does is good too in so far as I've desoldered the suspect wire, then soldered it in a different way, & it wasn't less noisy.

Then again heater wire noise can sound like that

I've moved the heater wires but it didn't change anything. Besides wouldn't they rather cause a 50/60 Hz hum instead ?

I think you should first localize the problem.

Sure, that's exactly what I am intending to do.

Measure all the critical voltages around the tubes like
- plates
- cathode
- filaments

Check that the filter caps have been correctly wired (polarization).

If possible separate the PT from the chassis and check if that has some effect.

Job done, nothing's unnormal.

Disconnect the preamp from the power amp (before PI)
and solder a jack to the PI-input and plug a guitar to it.

If the problem is not there then it must be in the preamp.

Disconnect the tone stack from the signal way and bypass it.

Use similar bypassing method for preamp stages.

Yes, certainly, yet how this can prove something ?
I should add one fact first : I get the hum with or w/o a guitar connected, & say again that it occurs only when either the Treble pot or the Bass pot are turned CW past the 12 o'clock position.
Therefore, if I connect a guitar/wave generator directly to the PI, how could I check something ? See attached the layout I've used.
Same thing with the tone stack bypassed.
Nevertheless, with the 1st preamp stage bypassed would a remaining hum mean that the issue does take place in V2, or not ?

P.S. : if it could be of any help, I have a VICTOR VC2002 signal generator + an OWON PDS5022S digital scope + a dummy load, so if someone got a tip or something else, he'd be the welcome !

[plexi50]

First do not put your preamp or input jack grounding to or near the same ground area as the PT. You can and probably are getting PT eddy currents injected into your preamp. (Thus The Humm) I made this mistake a couple years ago because the ground lug on the chassis was convenient and so i used it to ground the pots buss to it. I had a hum that baffled me. (Im Baffled ) At the direction of another forum member i removed the preamp pot buss at the PT lug and moved it completley to the opposite end of the chassis where the input jacks are. Problem solved. Hum gone

Star grounding has been a long topic over many years but i do not use the star grounding method.  Keep it simple and try to avoid and ground loops that can take place. I ground the pot buss and input jacks to one lug at the input jack chassis area. JMO

The power supply grounding and CT all ground to the PT chassis area. Ground all cathodes as far away from the PT as possable within reason. You dont have to run them all the way to the input jack area. Just the farther away from the PT the better

At first schematics were hard to figure out. Now there just about all i use except when building a board to drop in where i prefer a layout if i am unfamiliar with a new amp circuit build / Layouts also make it a little easier to understand the build process and where everything goes visually

[archaos]

I've indeed got the same hum issue you're describing when having connected the pot buss directly to a star ground near the PT. Moved it to the cathode bypass cap, & hum was gone too.

Be that as it may, I don't understand why I'm able to increase/decrease the hum when tweaking the Treble/Bass pots. I mean when I created the hum time ago since the grounding scheme was poor, the hum was permanent & even louder.

Here's there no hum if I use the normal channel for instance, or the brilliant channel w/o tone controls.

[plexi50]

Then you have one or more of the EQ pots miswired. It is a simple error. Your almost there. Recheck the grounding on the pots terminals and also the bass and treble cap wiring on the board and to the pots. Treble peak cap as well /

[Leevi]

I see in the picture that you have lot of unshielded wires crossing the components on the eyelet board.
Normally that could cause oscillation but hum as well. The components and wirings are very critical around
the PI.
/Leevi

[archaos]

Hopeless...

Yesterday, I spent the whole day trying a lot of tips around the Cathode Follower, & everything was unsuccessful :

I've swapped  :

                      - the peak cap
                      - the PS filter cap
                      - the bypass cap
                      - the slope resistor
                      - the plate resistor
                      - the cathode resistor
                      
I've moved :

                      - the wire running from PIN 8 where run 185 VDC to the cathode resistor (where AC signal rides on top of the DC cathode voltage)
                      - the wire running from PIN 6 to the PS cap

I've changed :

                      - the wire running from the Treble pot to the 50 pF cap
                      - the wire running from the Treble pot to the coupling cap

I've checked all the grounding points.

I still can't figure out what's happening.
Moreover, what is very weird to my eyes, is that I'm getting that f_cking hum with :

    - V1 + V2 + V3 installed
    - V2 + V3
    - V1 + V3, and there, I do not understand anymore.

How getting hum while tweaking the Treble pot is possible whereas V2 is pulled ?  

My heater has an artificial centre tap (2 x 100R), the wires (18 GA) are twisted tightly, & kept in phase.
I'm feeling that I'll swap the soldering iron for a hammer this afternoon...

[Leevi]

How getting hum while tweaking the Treble pot is possible whereas V2 is pulled ?

The PI sees the wire from treble pot as an open circuit. If you connect the treble wiper to the ground does the hum
disappear?
/Leevi

[archaos]

How getting hum while tweaking the Treble pot is possible whereas V2 is pulled ?

The PI sees the wire from treble pot as an open circuit. If you connect the treble wiper to the ground does the hum
disappear?
/Leevi

Yes it does, actually it bypasses the EQ controls section. I get a very low power Brilliant channel w/o EQs.
Do you have an idea ?

[Leevi]

I would temporarily move the power tube cathode resistor (cap is not essential) to somewhere else
at least faraway from the rectifier tube. Be sure that wire going to the cathode resistor is not crossing with
the wires around the rectifier.
/Leevi

[archaos]

I would temporarily move the power tube cathode resistor (cap is not essential) to somewhere else
at least faraway from the rectifier tube. Be sure that wire going to the cathode resistor is not crossing with
the wires around the rectifier.
/Leevi

Thanks for your input ! I've done the job but unfortunately it hasn't changed anything.
Yet, I'll get a 6V lantern battery next week to check if the heater supply could be the culprit. BTW, what would you recommend to supply 4 x EL84s + 3 x ECC83 ? Total is 4 x 0.76 A + 3 x 0.3 A = 3.94 A, right ? And will I just have to disconnect the heater wires from the PT, & connect the battery instead ?

[Leevi]

The total heater current is as you stated.
Regarding the heaters you could try the following which will probably not solve
the problem but just check if it has any impact to the hum level:

I suppose you use two balancing resistors in the heater circuit. Disconnect the resistors
from the ground and connect them to a positive voltage instead. One good place is the power
tube cathode where the voltage is a bit more than 10VDC. If the hum decreases significantly
put more focus on the heater circuit.

/Leevi

[jojokeo]

As nice of a job to the amp to this point, there's multiple lead dress issues here. All of those preamp wires running down along w/ each other and crossing near the heat wires, then directly under other components should be corrected or discouraged. The pi wiring is a mess too. These are very sensitive and it appears there's a lot of long length grid wires throughout. These should all be short and/or shielded. The grounds look suspect. Are the power tubes' heater wires all in phase with each other? There's many of the same issues as this thread - http://www.el34world.com/Forum/index.php?topic=11494.msg106654#msg106654
Try to focus on one thing at a time...check those heater wires phasing on power tubes, and get them away from all of the preamp wires before anything else.

[archaos]

The total heater current is as you stated.
Regarding the heaters you could try the following which will probably not solve
the problem but just check if it has any impact to the hum level:

I suppose you use two balancing resistors in the heater circuit. Disconnect the resistors
from the ground and connect them to a positive voltage instead. One good place is the power
tube cathode where the voltage is a bit more than 10VDC. If the hum decreases significantly
put more focus on the heater circuit.

Actually I do believe that the heater supply is not an issue as far as the wires are twisted & tightened firmly ; they are kept in phase too from the first power tube until the last preamp tube.
The gut pic isn't updated, sorry : I've already applied (& not stated) a DC reference by connecting the artificial centre tap, the 2 x 100Rs to the EL84s cathode. My bias votlage is 11.2VDC.
I nevertheless just want to be sure that the heater supply is OK.
Do you think it'd be useless to purchase a 6VDC battery to be sure, since the DC reference didn't improve anything ?

As nice of a job to the amp to this point, there's multiple lead dress issues here. All of those preamp wires running down along w/ each other and crossing near the heat wires, then directly under other components should be corrected or discouraged. The pi wiring is a mess too. These are very sensitive and it appears there's a lot of long length grid wires throughout. These should all be short and/or shielded. The grounds look suspect. Are the power tubes' heater wires all in phase with each other? There's many of the same issues as this thread - http://www.el34world.com/Forum/index.php?topic=11494.msg106654#msg106654
Try to focus on one thing at a time...check those heater wires phasing on power tubes, and get them away from all of the preamp wires before anything else.

Thanks for your input jojokeo, you besides saved me last year from drowning (as regards another AC30 with a burning bias...)
I know that there are both serious & multiple lead dress issues concerning the preamp in particular.
Worst of all : I had formerly wired everything here in such a manner that neither any of the plate wires nor any heater wires were running near/along either the grid wires or the cathode wires : it was horrible to see, but the amp was dead quiet, & you would hardly noticed it when switched on.
I had just followed Doug's instructions :

"...You may want to keep the red wires, going to PINS 1 and 6, away from the black and white wires. You can suspend the red wires above the other wires or the other way around but they should not be laid next to or parallel to the black and white wires. The red plate wires contain the high voltage and may induce noise into the other wires..."

Nonetheless, I assume you know that : "Better is the enemy of good", therefore I've rewired it in a neater way, which is not so neat @ all to hear now...
Still can you confirm that it is recommended or at least acceptable to twist the wires of each individual triode together, as said by Merlin ??? That seems rather astonishing compared to what suggests Doug, no ?
I have a growing feeling that I'll rewire it once again.
At last, I think my grounds are OK although it's not so evident or even confusing on the pic :

  - both poweramp & preamp ground bus are linked together
  - the ground bus of the pots is connected to the cathode resistors & bypass caps of the preamp
  - the PS caps are connected to the grounds of the audio circuits they feed
  - then the preamp ground is connected to the input jacks
  - next input jacks are connected to a star ground system (receiving the PT centre tap + the reservoir cap ground) itself connected to a humloop block network made of 1 x 10R 12W + 1 x .1 µF 630V + 2 x 1N4007s all in parallel

[jojokeo]

Archaos, you're taking things in stride very well despite frustrations.

Some of those old "rules to follow" are only good practice to maintain along the way. Every build should get better in some way. Many times wiring can be unruley but things are quiet, no oscillations, etc... but it's when extra high gain stages are added and/or w/ multiple switching, reverb, etc...that things on the order of magnitude seem to get more touchy and sensitive. Or as Murphy's Law says the more expense, time, and effort applied the more things are likely to go awry?

I was thinking along the same lines a Leevi when I saw the 100r artificial CT too. It's always good practice to put that on the power tube's cathode no matter what biuld. I'm wondering about your resevoir cap neg lead and PT center tap are going straight to each other and grounded directly to the PT lug at the same point? (with no wire being grounded somewhere else and then going to the ground?)

It appears that your OT wiring is very close to your preamp wiring - is that the blue/green/black taped wires coming from the grommet are? Is it possible to shorten and go over everything to the el84's grids? Those wires are best kept as short as possible and away from everything. Since they're under the board it's hard to see where they're coming from. Same thing with the two white grid wires of the PI. Don't run them below all of those components along the board. Fly them over and away from everything else keeping these as short as possible too. In both of those instances those grid wires should be short and separated from everything but the pi especially. That's about all I can see at this point from your picture.

[archaos]

Thank you Jojokeo ! You're giving me hope with these suggestions.

Tonight I'll jump again into the amp to try your tips, & will report.

[archaos]

I have the huge pleasure to tell you that I've finally got rid of that unbearable hum.

Thanks a lot for your inputs & support. 

I'll post a pic with explanations tonight.

[archaos]

In the long run I've found the culprit : the author. 
There was no bad component, it was just multiple lead dress issues.
Actually, I've changed the EL84's plate wiring, I've put shielded wires on the grids of the PI, but in short most of the job has been done on the preamp section & you can compare the 1st pic of that topic to the first one of this post.
During the battle I've killed one Sozo vintage, but fortunately I had one Xicon with the same value for replacement in stock.



BTW, Jojokeo, Murphy's law is funny. 
Now let's  again !

[TIMBO]

Well done mate I am in the process of my own BUZZ KILL, so hopefully today will be the day. 

[archaos]

Well done mate I am in the process of my own BUZZ KILL, so hopefully today will be the day. 

Thank you TIMBO !

What's the link of your buzz kill ?

[TIMBO]

http://www.el34world.com/Forum/index.php?topic=11494.0 I thought i did a good job on the layout and trying to keep things short and neat but it still was untidy. So i'm in the process of the BUZZ KILL  .  

I just red your post again.Could you explain the HUMLOOP BLOCK as i'm not familiar with it and its location. Thanks

[archaos]

I just red your post again.Could you explain the HUMLOOP BLOCK as i'm not familiar with it and its location. Thanks

Sorry Timbo, I didn't notice you had edited your post.
Here's the direct link : http://valvewizard2.webs.com/Grounding.pdf where you'll find explanations concerning grounding + the humloop block network.
It'll certainly be far better explained than I'd do myself, as far I'm neither English nor American.   

[TIMBO]

Thanks for that, some of the other guy's have supplied me with that info.Thanks to Geezer we found the pest being the relay and was a simple fix

[archaos]

Thanks for that, some of the other guy's have supplied me with that info.Thanks to Geezer we found the pest being the relay and was a simple fix

Excellent ! 

Congrats !