Vintage amp hum - cap replacement

[Erik Aa]

Hi. I am working on an old tube amp with two 12ax7 and 2 el84. The amp works but I struggle to get rid of hum. I have replaced the electrolytic filter caps and quite a few (but not all) coupling caps in the preamp stage. Still hum.

I am planning to change the old electrolytic bypass cap for the cathode resistor on the output tubes. I hope changing this will help?

Also, there are still some rather cool looking coupling caps still in the circuit (green ones in the picture). Are these ones prone to failure you think and should be replaced? Or can they stay? Thanks! Erik

[tubeswell]

Have you also tried a tube swap with fresh tubes (esp V1 preamp tube) in case you have a hummy tube?

[PRR]

> ...coupling caps ..... bypass cap for the cathode resistor on the output tubes

Won't be those.

[Erik Aa]

I have tried to swap out the 12ax7s. Still hum.

The hum is almost inaudible at very low volume, and then the hum increases with increased volume. At close to max volume the 120 cycle hum seems to subside and be replaced with a darker, milder and weaker sounding hum.

What to check next? Thanks.

[Erik Aa]

Oh, and the on/off switch is on the volume pot.

[tubeswell]

I have tried to swap out the 12ax7s. Still hum...

What to check next? Thanks.

Chassis signal ground connections  - how are these attached?

[Erik Aa]

Soldered to chassis by input jack and the first preamp tube.

[tubeswell]

Soldered to chassis by input jack and the first preamp tube.

So are there 2 preamp chassis grounds?

[PRR]

hum increases with increased volume. At close to max volume the 120 cycle hum seems to subside and be replaced with a darker, milder and weaker sounding hum.....and the on/off switch is on the volume pot.

Wudda been nice to know about that switch.

Is the amp make/model mentioned here?

New symptom suggests hum/buzz comes in on the wire to the pot wiper. Where does it run? Near any AC wiring? (And now we know it does, a very unusual thing on a large amp).

Plastic chopstick. Move that wire around. Worse? Better?

[Erik Aa]

Here is an image of the preamp stage. There seem to be multiple ground points.

[Erik Aa]

Indeed the hum reacts somewhat to chopsticking the AC leads going into the volume pot (in the black plastic tube in the photo).

I did a quick check with alu foil around the volume pot wiper wire (and tone pot viper wires). The hum is noticeably reduced, but still there.

As you can see in the photo there is also a cap running from the AC lead on the pot to ground. The AC is 3 prong, and grounded on onther side of amp closer to the PS.

[Erik Aa]

I noticed one more thing now. When I touch any of the knobs for the pots (volume or tone for instance), the hum subsides considerably. The knobs are made of alu. I have of course previously checked for continuity between pot and chassis, and I get continuity signal on my multimeter. But there still seems to be a grounding problem?

So I have grounding to chassis from the pots, but still there seems to be a grounding issue here? What to do? Any suggestions? Thanks!

[PRR]

Turn the wall-plug over.

[Erik Aa]

I live in Europe and here the direction of wall socket does not matter.

[sluckey]

Tell us the manufacturer name and model number. And post the entire schematic.

[Erik Aa]

EKO Herald 1.

[PRR]

Schematic shows separate symbols for wall-ground and audio ground. Your picture seems to show everything solder to chassis or pot-body. Is that really how it was built? There's also two holes in chassis near power switch. Missing shield or safety-ground lug?

[Erik Aa]

I don’t know about missing shield or ground lug. Here are the ground connections in the amp aside the pots.

1. wall ground and ps center tap.

[Erik Aa]

2. filter cap ground

[Erik Aa]

3. ground by input jack

[Erik Aa]

4. ground by bypass cap for bias resistor

[Erik Aa]

5. ground by first 12ax7

[sluckey]

Disconnect the black and white wires from the power switch. Pull the wires and tubing back and let dangle over the PT. Now solder the black and white wires together and tape to prevent touching anything. Does the hum improve?

[Erik Aa]

Interesting. I desoldered the black and white power wires from the switch and moved them to the pt side of the amp just like you suggested.

Absolutely no difference. Still the same identical hum. Still hum subsiding when touching the pots.

[kagliostro]

I've find an old thread on DIYItalia about problems on the Heco Valet amp which is very similar

see the different components the author signed on an Helald schematic respect a Valet


the author of the thread referred about a Hum problem that was present only if the line was noisy
(however the thread was about other things referred to the amp, not the Hum)

My council is to give a try to a line filter (and pay attention if there are neon lamps or other noise sources)

Franco

[Erik Aa]

I have tried with variac and without variac (but with current bulb limiter). And with as well as without ceiling lights on. Still no difference.

[Erik Aa]

Thanks for the updated schematic Franco. I will check it out.

[kagliostro]

line filter I mean something like this



or this



Franco

[tubeswell]

A number of possibilities comes to mind:

1) The existence of multiple signal-ground return points within the chassis is probably contributing to ground loop hum. See Merlin Blencowe's suggestions for grounding arrangements on the Valve Wizard site.

2) Also, it could be over time, that oxidation has built up between surfaces where components such as pots and jacks are bolted to the chassis. These components may need to be dismantled from the chassis and the surfaces thoroughly cleaned and then remounted.

3) In addition, if you (or someone who has worked on the amp previously) has done further (or replacement) soldering to the chassis (esp when replacing components such as filter caps), it may have produced poor electrical bonding to the chassis - which also would contribute to hum. (When soldering grounds to a steel chassis, you need a totally clean surface, a powerful (hot) iron capable of heating up sheetmetal (preferably a flat tip iron 200W or so), and sufficient solder flux (e.g. separately applied solder paste). (They had big irons in the Fender factory for this). But this also depends on what work has been done on the amp previously  - some well-intentioned modifications may not have worked out previously, resulting in poorly considered signal ground returns - in which case go back to 1).

[kagliostro]

I've find a clean image of the EKO Herald 1 schematic

here it is



Franco

[Erik Aa]

Thanks tubeswell. I will read up on the tube wizard site tonight.

Thanks Franco. The EKO Herald is a cool little amp. Made in Italy.

[Erik Aa]

I read the entire valve wizard chapter on grounding. Very interesting. However, it will be hard to apply some of the principles advocated (star ground, bus) without completely changing the amp layout. It would be nice to keep it as close to original as possible.

[Erik Aa]

The preamp section is grounded through a bus wire, but the 12ax7 cathodes have a separate grounding point. Is this a ground loop issue?

The hum also changes character if I bend and pull on the the wire from the mid lug on the volume pot. Maybe the pot needs attention or replacement?

[dude]

Is that buss wire grounded at the input jack only, picking up the preamp cathodes on the way? I believe that buss wire should be hanging and only grounded at the input jack.

[Erik Aa]

The bus wire is grounded in the chassis here (see pic) and provides ground connection at various places on the eyelet board. After ground point it first provides ground to the power amp section, then to the preamp section. The cathodes for the 12ax7s have a separate ground soldered to the chassis.

The shielded wire from the input Jack has a separate ground for the shield next to the input Jack.

There is connection from each pot to tve chassis. But hum reduces considerably when I touch the metal knobs still.

[dude]

The power section should be grounded separately from the preamp, power amp section is star grounded near PT. The buss picks up the pot grounds (even though they are grounded at chassis) and the other preamp grounds but not connected to the power section, the buss should be free hanging and not tied to power ground. End of the buss should only be grounded at the input jack, the preamp cathodes should be grounded on the buss (not separate grounds).
Hard to see where the grounds are in pic.
Basically two ground points, power section, bias, speaker ground and second ground point at the input jack (for the buss). With preamp cathodes grounded to the barr. Good practice to run a wire from input jack ground (where  the buss is grounded) to a separate ground close to the input jack. (I don’t rely on the input jack’s ground to the chassis).
Someone correct me if l’m wrong. I always have a star ground for the power section and a separate preamp ground for the buss at the opposite end of the chassis that picks up all the preamp grounds including cathodes and preamp filters.

[acheld]

Someone correct me if l’m wrong. I always have a star ground for the power section and a separate preamp ground for the buss at the opposite end of the chassis that picks up all the preamp grounds including cathodes and preamp filters.

That's how I approach it too.  Basically this is what Doug taught me through his layouts some years ago. It works well.

[dude]

Someone correct me if l’m wrong. I always have a star ground for the power section and a separate preamp ground for the buss at the opposite end of the chassis that picks up all the preamp grounds including cathodes and preamp filters.

That's how I approach it too.  Basically this is what Doug taught me through his layouts some years ago. It works well.

Right, but Erik Aa has a vintage amp that the schematic shows the grounds totally different, not saying the schematic is wrong and perhaps at one time the amp had no issues. Over time as Tubeswell mentioned, oxidation and other time related factors might have interfered with those grounds. If it were my amp and I still had an unwanted hum after all the info tried, I'd use a lot of shielded leads, especially on that on/off pull switch, even though Suckley bypassed that pull switch and still the hum. The tin foil wrapped around some leads is a band aid done that way, should use shielded wire properly grounded if needed.
Even though the schematic shows the grounding different as we suggested, it wouldn't be hard to change it since all else failed. I don't like the preamp cathodes having a separate soldered ground to the chassis in the middle of the amp, I'd put them on the free hanging buss bar, along with all other grounds that are not involved in the power supply. And I'd either chop stick or use shielded wire on sensitive  signal wires if the change in the grounding scheme didn't solve the hum.

[Erik Aa]

Progress! I moved the 12ax7 cathode ground to the bus wire. This removed the sensitivity of the pot knobs. Clearly there was some kind of ground loop issue there.

 I separated the preamp buss wire from the power tube stage and wired the preamp buss directly to the input Jack. I connected the power tube stage to the ground for the filter caps next to the pt.

So now the amp is quieter. It still hums a little 120 cycle. Last mile now.

I think I will rewire the shield of the signal wire from the input Jack to the same buss connected to the input Jack.

I am also considering reattaching the ground lugs from the pots and instead connect the lugs to the preamp buss wire.

Any thoughts? Thanks guys!!!!

[tubeswell]

I think I will rewire the shield of the signal wire from the input Jack to the same buss connected to the input Jack.

I am also considering reattaching the ground lugs from the pots and instead connect the lugs to the preamp buss wire.

Any thoughts? Thanks guys!!!!

Grouping all the low current ground returns on the ground buss bar (away from the high current returns) and having these near the buss attachment to the chassis should help reduce the effect of high current returns affecting the stability of the low current ones. (I prefer to keep the returns grouped according to which filter cap node those components source their power from.) Whereas if everything is randomly attached to the chassis, all the current in the ground returns gets mixed up - with the tendency to cause hum loops. At least keep the low current returns grouped together near the input Jack sleeve ground.

[dude]

And use shielded wire to the volume pot’s wiper and 12ax7’s grid, grounding the shield on the pot only, don’t let any of the strains touch the center wire in the shield.

[Erik Aa]

Thanks. I moved the input signal wire shield to the grid to the input Jack ground with the rest of the preamp grounds. No real diffeerence.

The amp is now quiet on low volume, and increases some hiss with increasing volume. At Max volume there is no hiss.

I wonder whether I should rewire the grounds of the pots to the input Jack and shield the viper? Or should I look at resistors? Only a slight ti medium hiss left now.

Thanks!!!

[dude]

Don’t know what “viper” is, shield the wiper wire on volume pot, put the cap directly on the tube grid (take it off the board), ground the shielded wire to the pot case only.
Don’t worry about pots grounded at chassis, just take any ground wires coming off the pots or preamp side of the bd. to the buss bar, you did disconnect the buss near the PT l hope. That buss should just pick up the pot grounds, a filter cap ground for preamp (not HT filters)  and any other grounds soldered to the chassis that don’t go to power amp side and be grounded to the input jack only.

[Erik Aa]

Ok. The buss is now disconnected between pt and power tubes on one side and the preamp on the other side. The preamp is connected to the Jack. I will Connect the pots to the buss going to the Jack as well. And ground the wiper wire shield from the volume pot to this buss going to the Jack.

[dude]

It you disconnected the buss from the HT filters, you might have to do nothings else. Try the amp

[acheld]

Don’t know what “viper” is

 
1. An F-16,
2. A snake,
3. A shielded snake is likely a cobra,
4. Wiper in false German accent,
5. An auto-corrected wiper.

Take your pick!   

[tubeswell]

...
4. Wiper in false German accent,..

Ich weiß auch nicht, was die Viper ist.

[Erik Aa]

Ok, so now all the pots are also grounded on a buss wire going to the input Jack. So I now have two ground circuits. One is for preamp section including the pots, all grounded at input Jack. The other ground circuit is the filter caps and power tube section grounded by the filter cap away from the input Jack and preamp. The pt centertap is grounded separately together with earth ground.

There is almost no annoying hum now, except on full volume with Tone control off, there is a low weak hum. Not really a problem.

However I do have some hiss now when turning up the volume. It is clearly in the section of the amp between input Jack and Tone control. The Tone control is before volume control but after the first tube.

The hiss goes away at full volume with Tone control off. It is then replaced by the aforementioned slight weak hum. The hiss gets more pronounced when I turn up the Tone control. When the Tone control is turned all the way up, the hiss is also at full volume.

What is the likely culprit now? Resistors? Or bad soldering? Or is it still a grounding issue? The soldering connections look ok. Can bad quality buss wire induce hiss? I am contemplating going after the resistors in this stage next.

Any advice? Thanks!

[thetragichero]

the hiss is coming from the viper 

[tubeswell]

Hiss is likely to be resistance noise from a combination of resistors - including the pots. You can reduce it somewhat by changing the plate resistors and grid leak resistors in the preamp with metal film resistors and adding 10k to 33k grid stopper resistances right at the grid pins of the preamp tubes